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Dirty-South Blues Harp forum: wail on! >
Capacitor kits?
Capacitor kits?
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LIP RIPPER
91 posts
Jul 23, 2009
11:27 AM
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A friend has given me an old Gibson GA-5. I have cleaned it, repaired the speaker and I'm compiling a set of tubes for it. This will be my first amp restoration. The way I understand it is that one should replace all the caps since they give up over time. Are there kits offered or do you just buy them individually?
LR
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isaacullah
267 posts
Jul 23, 2009
1:39 PM
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One should replace the electrolytic capacitors. Ceramic and film caps last pretty much forever. The electrolytic caps have a liquid filling (basically battery acid) that can leak or change ph over time. This is especially true for old "can" type electrolytics that are found in vintage tube amps. Be very careful, as the high capacitance high voltage can caps used in the power supply can hold lethal charges that will zap you if you make inadvertent contact with the leads. Make sure you bleed these to ground first (google search how to bleed capacitors). Also, old rolled film capacitors (they look a little like tootsie rolls!) can dry out over time and get brittle. If the break, then they don't work, so you should consider replacing all of those with new poly film or tantalium caps. You can get capacitor kits for various popular vintage tube amps from many sources on line. E-bay is a good place. Also try the weber amplifiers www.tedweber.com site or the antique electrical supply www.tubesandmore.com site. I'm sure you can find some kind of kit by google searching for your amplifier. A kit will definately save you time, and even though you probably pay more for each individual cap, you'll save money becasue you won;t have to bulk order a bunch of caps that you'll not end up using. If you think that you are going to be doing a lot of amp work, however, you might try just ordering a bunch of the common values of caps, and keep them on hand.
Cheers,
Isaac
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The magnificent YouTube channel of the internet user known as "isaacullah"
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Jim Rumbaugh
69 posts
Jul 23, 2009
8:12 PM
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As a general rule.
It's OK to use too large a value for replacement.
If the capacitor is rated for a higher voltage, that's OK. Also you can use a higher value of capacitance and it should work fine. Example, you couild replace a 5 microfarad with a 10 microfarad and you should be safe, (larger capacitors allow for more filtering and less hum) The place you do not want larger capacitors is in any tone circuitry, but it would be rare to find an electrolytic in a tone circuit. They are usually in the high plus voltage supply or in the cathode bypass circuit.
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mrdon46
5 posts
Jul 23, 2009
10:42 PM
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In general what Jim says is true. Only caveat is that the rectifier tube limits the value of the first capacitor, after that you can go as high as you want, though I don't think there's anything to be gained by going too high. I believe there were actually a number of versions of the GA-5, it may have either a 5Y3 or 6X4 rectifier--in either case I don't think you'd want to go above 20uf for the first cap. Also, electrolytics are generally used as cathode bypass caps--if your amp has these, changing the value will effect both the gain and the tone of that stage (not necessarily a bad thing).
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mrdon46
6 posts
Jul 23, 2009
10:55 PM
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Another thought--since you're most likely restoring the amp to be used as a harp amp, one of the most common mods is to up the value of the coupling caps, usually to .1uf
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Andrew
464 posts
Jul 23, 2009
11:57 PM
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I agree mainly with Isaacullah.
In the Seventies I restored half a dozen very old tube radios dating from the Thirties and Forties, but I never found a single capacitor to have blown. The only capacitor that I ever saw fail was in my friend's Celef loudspeaker cross-over unit!
Nevertheless, I would still replace all the electrolytic capacitors, since usage of an amp is totally different from usage of an old radio (you don't want the amp failing on stage, do you!)
When Jim says "too large a value" he's only right about voltage. The value of the capacitance is important - it's a functional parameter - change it and you change the way the circuit it's in performs.
(It may be that he's expecting the only electrolytic capacitor in the amp to be in the power supply's rectifier, in which case you could feasibly raise the capacitance slightly. I'm trying to think what would happen if you put a ridiculously big one in. I think you'd just get a very slow build-up of voltage, and the tubes would seem to be slower to warm up than before. But when you switch the amp off, you'd get the residual DC voltage hanging around for a long time. I don't know if that would be dangerous.)
In general, if you are replacing caps in another part of the amp, a small percentage change would be OK, but only if new caps don't have exactly the same values as old caps.
Last Edited by on Jul 24, 2009 12:00 AM
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LIP RIPPER
92 posts
Jul 24, 2009
3:42 AM
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Thanks for the input. Their is a large cardboard tube that isn't marked. It's maybe 1" in diameter and 4 inches long. Isn't that a filter capacitor too?
There aren't any of the canister type caps on this chassis. It is amazing to how simple this circuits are.
Last Edited by on Jul 24, 2009 3:44 AM
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Kingley
275 posts
Jul 24, 2009
4:11 AM
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"A friend has given me an old Gibson GA-5"
I could do with some friends like that! :)
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tookatooka
314 posts
Jul 24, 2009
8:03 AM
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Before you go buying loads of caps. let us know which ones you need first. I have thousands of the blighters here and may have just what you need. I don't have any of the larger older electrolytic type for mains voltages, but have many of the more modern types including tantalum and poly film.
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When I'm not blowing, I'm drawing.
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Jim Rumbaugh
70 posts
Jul 24, 2009
10:04 AM
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I'll qualify my answers by saying my electronics experiences were during the late 60's and early 70's
Lip Ripper : Physical size can be missleading. The old filter capacitors would have 2 terminals on the bottom for connecting wires. They may be the size of a small or large vacuum tube. The last time I saw a replacement for them,(about 10 years ago) I was amazed at how small the new capacitor was.
Andrew : My "theory" background says that it's hard to get a capacitor too big, though I will admit, Theory also says yes you can get one too large. I would not hesitate to double or quadruple the value of a capacitor if that's all I had on hand. The old standard for calculating the value a bypass capictor was to figure what would pass %70 of the lowest desired frquecy. So a larger capacitor would just mean less attenuation on low notes (lower than most harps play). Now keep in mind, I the kind of guy that would use solid state rectifiers in the power supply and NOT a tube.
Last Edited by on Jul 24, 2009 10:15 AM
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isaacullah
269 posts
Jul 24, 2009
12:00 PM
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A quick word on power supply caps. In general, power supply caps are t here to do TWO things: 1: filter the power supply to reduce mains hum 2) as a consequence of this, they act as "reservoirs" of power that the circuit can call on when it needs it, thus preventing too much "sag". Therefore, you need a few properties to make good power supply caps. You want them to be able to handle a lot of electricity without blowing. Therefore they need to be rated for high voltages! (this makes them big). Seecond, you want fairly high capacitance. For Solid State amps (what I build) you'll want capacitance between 2200 uf and 10000 uf, depending upon the power supply configuration (if you have a couple of caps in parallel, their values are reduced). Any thing more is over kill. I'm not sure about tube amps, anyone else know? Also, you want to buy quality caps with fast recharge rates. This will help the "speed" of the amp (ie. it's ability to react quickly to changes in the input signal)... Remember, all amplifiers are essentially large powersupplies with a small bit of specialized circuitry attached. The power supply is really the heart of an amplifier. If the power supply sucks, then the amp will suck too.
Cheers,
Isaac
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The magnificent YouTube channel of the internet user known as "isaacullah"
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isaacullah
270 posts
Jul 24, 2009
12:06 PM
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Oh yes, and defiantly consider upping the DC decoupling caps to .1uf like mrdon says. This will let more bass through each stage, and will make the amp sound much better for harp. These will be film or (gasp) ceramic caps. Search "capacitor codes" if you don;t know how to decipher the info printed on the cap itself. eg: .1uf caps are labeled "104" and then a letter (usually "K"). The current coupling caps will be lower than that: perhaps 0.047 (labeled 473) 0.022uf (labeled 223) or 0.01 uf (labeled 103).
Cheers,
Isaac
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The magnificent YouTube channel of the internet user known as "isaacullah"
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LIP RIPPER
93 posts
Jul 24, 2009
12:25 PM
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According to the schematic and the caps it seems that they're all .022 microfarads. If I replace all nine of these what size would you guys recommend? I'm still trying to figure out what the piece is that resembles a toilet paper roll. Is this an electrolytic cap?
Hey, I really appreciate this input fella's, Thank you.
LR
I now see the recommendation of upping them to .1uf. Interesting about these values altering the bottom. Very intersting. Before I do that I want to test drive this amp. My buddy that gave it to me has a GA-5T that I plugged into and it blew my mind, his too. Hopefully I'll have the tubes this weekend and we'll see what she's got.
Thanks again everyone.
Last Edited by on Jul 24, 2009 12:33 PM
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tookatooka
315 posts
Jul 24, 2009
12:47 PM
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If the toilet roll device is wax covered, it may be an old type of foil capacitor. Is there any indentification numbering or anything on it? Are there wires coming out of it (axially) each end? Any chance you could post an image of it?
PS. Had a look at the GA-5 schematic and couldn't see anything that the toilet roll device could be. Has it been modded in the past? Maybe someone has added power smoothing caps to the power supply output?
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When I'm not blowing, I'm drawing.
Last Edited by on Jul 24, 2009 1:02 PM
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isaacullah
271 posts
Jul 24, 2009
3:40 PM
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I saw something like that "toilet roll" thingy in an old Tube radio I was dissecting once. I think it was part of the reception circuitry. I don't know if it is a cap or not. A picture or any codes you see on it may help us help you...
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The magnificent YouTube channel of the internet user known as "isaacullah"
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LIP RIPPER
94 posts
Jul 25, 2009
3:49 AM
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It has a black wire on one end that goes to the 6x4 tube. On the other end it has 2 wires, one that goes to the 6x4 and the other jumps over to one of the 6aq5 output tubes. This particular GA-5 has the 5 tube chassis. It may well have been added. On the chassis it is written "fixed April 30 66 rt." with a magic marker.
How do you put a picture on here? I'll do that.
LR
P.S. it isn't on the schematic that I found either. And, I've only found one schematic with this tube set up.
6EU7
6C4
2 6AQ5'S
6X4
Last Edited by on Jul 25, 2009 3:54 AM
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tookatooka
317 posts
Jul 25, 2009
5:11 AM
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To add an image, go here.
http://www.itriser.com/harmonica/mbh_how-to.htm
Hmmm! got me stumped so far. As Isaacullah said you could find something like this in an old radio, it would be the tuning coil, but not in an amp.
It is solid, isn't it? It's not hollow? Would it be heavy? Would it have been added as the repair you mentioned? I wonder whether someone has added a little extra bit of circuitry and put the extra components into a tube for insulation purposes. Interesting.
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When I'm not blowing, I'm drawing.
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tookatooka
318 posts
Jul 25, 2009
7:24 AM
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Just found this image. If they look like these there is a fair chance it is a filter capacitor. There are probably two caps in the one package with one lead being common and taken to ground. I can't help any further than that without a good schematic and knowing the correct values. My guess is they would be paper capacitors which do deteriorate over time and it would be wise to update them with modern equivalents if their values could be determined. Hope that is a little bit clearer now?
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When I'm not blowing, I'm drawing.
Last Edited by on Jul 25, 2009 7:29 AM
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LIP RIPPER
95 posts
Jul 25, 2009
11:58 AM
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Tookatooka, thank you. Yes they are similar and I suspected that they were indeed filter caps. I have gone to the link you posted but I'm unable to get it to work. I wanted to show a picture of it. The paper is quite worn and I don't think I can find any info on it but I'll look closer tomorrow. My buddy that gave me the amp is coming over shortly with his wife . I'm gonna smoke a 6lb pork butt, we're gonna eat, drink and then play some blues.
Thank you for all of your efforts. This ole world is a little smaller with this technology and I am grateful for that.
LR
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Jim Rumbaugh
71 posts
Jul 25, 2009
9:45 PM
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Let me confirm, augment, question and correct some of isaacilla's remarks.
Confirm: I agree totally on his comment on values for solid state amps. These are low voltage, high current devices (low impedance) that need to store lots of current between 60 cycle power pulses. Tube amps are high voltage lower current (high impedance) and can use lower values of capacitance.
Augment: I just checked some Fender schematics on line. The Princeton had a few 8 micro and the Bassman had a few 16 micro. I would believe anything from 5 to 50. The Princeton had a 500 ohm resistor (unknown wattage) between the capacitors, the Bassman used a choke ( also called an inductor, you don't see many of those used any more) between the capacitors.
Correction: Capacitors in parallel add their value, it is not reduced. Capacitors in series reduce their value, but this increases their voltage rating (a rarely needed method)
Question: I am not aware of any electronics theory about "fast charging" capacitors, nor how power supplies make an amplifier "fast", sag resistance perhaps, but I am reluctant to use the word fast.
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Andrew
465 posts
Jul 26, 2009
2:01 AM
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This is a weird thread because people are talking about capacitors as though they only have one function; they don't. Two important functions are as parts of low-pass and high-pass filters: -
http://en.wikipedia.org/wiki/File:1st_Order_Lowpass_Filter_RC.svg
http://en.wikipedia.org/wiki/File:High_pass_filter.svg
Notice that these capacitors are always used in conjunction with a resistor (in these cases).
The high-pass filter is an example of a coupling capacitor (these are also seen without resistors, but the theory is easier at radio-frequencies). The low-pass filter is an example of a decoupling capacitor. The power supply employs its main capacitor for this purpose - there's a diode/rectifier which chops off the negative parts of the AC cycle, which leaves you with an average positive DC voltage and also a load of harmonics (one of which is the original 60 Hz fundamental frequency). The low pass filter is designed to pass (in an ideal situation) only frequencies lower than 60Hz, i.e. only the DC.
But it's not very efficient. It has a knee in its frequency/output curve called the cut-off frequency. This is a calculable combination of the resistance and the capacitance. It is TUNED - it is like a coil and a capacitor combination but the bend of the knee is less sharp (resistors pass all frequencies evenly, whereas a coil passes the low frequencies and blocks the high frequencies, the opposite of a capacitor. In these high-pass and low-pass filters you could replace the resistors with coils, but at audio frequencies the coils would have to be physically huge to have enough inductance). The bigger the capacitance, the lower the cut-off frequency. In a power supply this is no bad thing - the lower it is, the more it attenuates the 60Hz mains hum.
But in a high-pass filter (i.e. a coupling device), if you make the capacitor too big, it will start to let the "mains hum" through. However, in an audio circuit, you may have a cut-off frequency of 100HZ, lower if it's a bass amp. so it's more likely that such a capacitor is just used to block the DC from the previous part of the circuit. In such a case, making the capacitor bigger would just be a waste of money.
Jim is right about capacitors in series and parallel - they behave oppositely from resistors and coils in this respect.
"solid state amps...are low voltage, high current devices (low impedance) that need to store lots of current between 60 cycle power pulses. Tube amps are high voltage lower current (high impedance) and can use lower values of capacitance."
I don't like this paragraph. I think it is confused. The frequency is the same for each - 60Hz. The cut-off frequency is a function of resistance and capacitance, but there are multiple combinations of each which give the same cut-off frequency. The absolute values do have an effect of some kind on the "efficiency":- there is an optimum combination for each frequency, but it certainly doesn't depend on the voltage. As an exaggerated example, 100 ohms and 10 microfarads might give you the same cut-off frequency as a millionth of an ohm and a few farads, but you can see that 100 ohms and 10 microfarads would be the only sane choice (the relationship is not actually linear - I think it depends on square roots, or squares).
A power supply is always (comparatively) low impedance for best performance, otherwise when the circuit, whatever it is, draws current the power supply's output voltage will drop too much. It drives tubes, so it's high voltage, and this is all the capacitors really care about - it's simply a safety issue.
You speak in general about an amp's impedance.
An amp has various parts, such as the input, the output and the power supply. The parts have different functions and necessary impedances. e.g. the output is to a speaker, it's always lowish voltage, lowish impedance (typically 4-300 ohms). The input impedance is generally high, (but not because the voltages are high - operational amplifiers have theoretically infinite impedance, but this is so that negative feedback can be arranged more easily and the final gain can be easily calculated. I have no idea about less esoteric stuff, but I did notice that this Gibson amp has what looks like a negative feedback resistor - the 47K one going from the speaker to the cathode of the second tube).
This was just some general theory.
I'm not going to answer the question of what's going on in this particular amp because I've seen the schematic and it's a mess. It looks like no-one ever designed it - I think its design just evolved over decades of trial and error starting from very early tube amps.
Actually, there are lots of sources of schematics on the web. The first one I saw was pretty badly drawn. This site looks very good: -
http://www.freeinfosociety.com/electronics/schematics/audio/gibsonga5.pdf
The way this is drawn makes it very clear that there's a whole series of low-pass filters going from the power supply back to the first tube (the one where the voltages are lowest, so you'd want the smallest amount of mains hum possible there)
Last Edited by on Jul 26, 2009 3:24 AM
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Jim Rumbaugh
72 posts
Jul 26, 2009
4:31 PM
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I admit, I am obsessive compulsive.
After reviewing a dozen GA5 schematics, I believe this is the one. It was described as a GA5 Crest. It uses the 2 6AQ5 tubes in a push pull circuit. I would expect the output to be about 10-15 watts. This schematic shows only a volume control, no tone, no tremelo.
http://www.freeinfosociety.com/electronics/schematics/audio/gibsonga5_crest.pdf
LIPRIPPER: It looks like 20 microfarad capacitors are used in the power supply. I just saw some pictures of new caps, and they are SMALL compared to the old ones like in the pictures posted by tookatooka. The schematic says the circuit voltage is +240 volts. 450 volts is a common capacitor rating. If you find a 350, I say OK. You described a capacitor with 1 black wire on one end and 2 on the other. That should be a double capacitor (2 in 1). I would replace it with 2 of the new smaller ones. The black wire should be going to chassis ground. You said the black wire went to the 6X4. Check that lug on the tube socket and see if it goes to the chassis ground. The other end of the cap that goes to the 6X4 is the input capacitor. This is the main power input capacitor. The other wire going to the 6AQ5 tubes is to filter the screen voltages.
Now here comes electronic nerd stuff………
ANDREW: Yes, capacitors with resistors are used in high pass and low pass circuits. I usually see them in tone control circuits. I was fascinated by your description of the power supply capacitors and resistors as a low pass circuit, (which it is). When you said," The way this is drawn makes it very clear that there's a whole series of low-pass filters going from the power supply back to the first tube ", I mildly disagree. I doubt if the circuit was designed as a TUNED circuit with a relationship between resistance and capacitance values to block 60 cycles, nor as an intentional low pass circuit. I say the values of capacitance are related to the current drawn from the circuit. For example, in a no load situation (pull the tubes out), no current would be drawn. Even a small capacitor would charge to peak value, and there would be a pure DC voltage between 60 cycle power pulses. But practical circuits do draw power, so as the capacitor discharges, the voltage to the circuit drops until the next 60 cycle power pulse recharges it. A larger capacitor discharges slower and causes less voltage drop between power pulses. Isaculla described capacitors as a reservoir of power. I add that they are a reservoir of power between 60 cycle power pulses. This is also where I say low voltage circuits need larger capacitors due to a higher current drain (faster discharge) caused by lower impedance circuits.
I say the resistor's values were chosen for a desired voltage drop (voltage loss) to supply the needed lower voltages for the output tube's screen grid voltage and pre-amp voltage. We called them voltage dropping resistors. The value of the capacitors used would not change the voltage to these circuits, only the amount of filtering.
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isaacullah
272 posts
Jul 26, 2009
8:26 PM
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Hey guys... This is very cool! Who knew we could get some serious discussion of audio electronics on this forum! Jim, thanks for correcting me, you are right, I had confused the series/parallel math for caps with that of resistors. Also, perhaps I should not have use the word "fast" the way I did. The reference for that tidbit of wisdom about power supply filter caps comes from this website http://www.tnt-audio.com/clinica/ssps1_e.html. Like i said, ALL my experience is with solid state amps, which have different power supply requirements from tube amps (especially in terms of voltage). I think Jim is right when he says that SS amps need higher capacitance in the powersupply filter. The main reason I see for this is the secondary function of these caps as a reservoir. It's not technically correct to discuss caps like this, but it's a useful analogy. SS amps have do have fairly high current drain, but more importantly the actual amount of current drain is very variable. SS amp powersupplies need to be able to provide "surges" of power on demand, and large caps in the power supply add this functionality. You can see this visibly by using a lightbulb wired in series with the hot mains supply (I use this as a current limiter when testing new amps). In general, the light bulb only glows barely bright when playing through the amp, but when you blow hard into the mic, the light bulb gets brighter very quickly because the amp is drawing more power....
Cheers,
Isaac
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The magnificent YouTube channel of the internet user known as "isaacullah"
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Andrew
471 posts
Jul 27, 2009
2:37 AM
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OK, well this is an interesting discussion and I think we’re homing in on the truth, and for me it's useful revision/learning, although obviously I have to be very careful when revising in public. Also, I have to be very careful because I’m aware that I know some things about ideal or theoretical circuits, but possibly not enough about the practical side of things.
Mathematically, what’s left when you rectify AC is a Fourier series with a bias (you'd see it on a spectrum analyser), and you eliminate the Fourier series, leaving the bias, which is your DC voltage. How do we translate that into reality? Low-pass filter, as I said.
But what we seem to be discussing is short-term (surges) and long-term loads.
I don't think I mentioned that all combinations of resistor/coil/capacitor have an impedance (which is the AC equivalent of resistance and depends on frequency, except for a purely resistive circuit, whose impedance equals its resistance and is frequency independent). When we draw a general purpose diagram of an amp, we draw an “ideal amp” (zero-impedance) in series with a resistor (its effective impedance). The circuit supplied presents its own effective impedance and the two impedances are a potential divider. If the amp’s is too high, the voltage across the driven circuit is too low. This is the long-term load. Strictly speaking, all power supplies should have the same low-as-possible impedance, without consideration for the kind of circuitry they are supplying (tube or solid state), but if a tube amp's power supply is a compromise because the tubes amp’s combination of high voltage low current presents a high effective impedance to the power supply, as Isaac said, then it may be the case that a larger decoupling capacitor would lower the impedance, so that Isaac and I are describing the same phenomenon but in different ways? My way is quantitative and therefore can be used as a design tool; his is qualitative, which was probably the kind of thinking that led amps like the Gibson to evolve in the way that they did. Also my potential divider explanation would explain the long-term problem with the solid-state amp, whereas the capacitor reservoir explanation only explains how the short-term loads or power-surges are dealt with.
I’m aware that response times haven’t been covered here. I'm also aware that there's the unexplained statement that power supplies have (frequency-dependant) impedances (represented by a resistor? i.e. I'm as puzzled as you!) rather than (non-frequency-dependent) resistances, which begs the question, which frequency, 60Hz or audio?
Last Edited by on Jul 27, 2009 2:47 AM
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isaacullah
274 posts
Jul 27, 2009
10:02 AM
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Mn, this is making me want to hit the books now! I must admit that all I've learned is from a bit of trial and error experience and from the collected wisdom of various forums on the interweb... I'm an archaeologist by training, and have no formal classroom experience in electrical engineering. I do have a few technical books however, that I've used as references while teaching myself this stuff. I think I need to go back to them and see if I can get my head around this. My curiosity has been peaked! Now I must have some satisfaction!
~Isaac
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The magnificent YouTube channel of the internet user known as "isaacullah"
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tookatooka
328 posts
Jul 27, 2009
11:34 AM
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Isaacullah, I knew you were an archaeologist and was always surprised when you were talking electronics because it seemed odd that you were proficient about virtually anything you were discussing. I couldn't see how an archaeologist should have so much electronics knowledge. I thought hey we've got a genius in our midst. I used to be a designer of digital telecomms systems before I decided to bow out and take it easy. Didn't want to burn myself out to keep my government afloat.
But in digital Electronics, not much theory was required only 5v and 0v 1 and O, off and on but the complexity grows with logic circuits getting so large with so many logic gates.
However, most of my knowledge is home spun as you say, trial and error and I can see you are the inquisitive kind who likes to know what makes things tick. I take my hat off to you sir for being so determined to learn about these things.I know the theory can get a bit daunting. I've followed your i-mic adventures and look forward to seeing what you get up to in the future.
It's great to think that, as a forum, we can count on some highly experienced members with wide ranging skills to help us when we need it.
Sorry LIP RIPPER didn't mean to take your thread off topic.
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When I'm not blowing, I'm drawing.
Last Edited by on Jul 27, 2009 11:39 AM
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walterharp
1 post
Jul 27, 2009
11:40 AM
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when working on this with power, one hand in pocket at all times. Also be careful with the filter caps to hook new ones up with the + where the right thing is. if you are in doubt go to the lone wolf harp amp forum and try to get one of the guys there (Mark Burness) to comment, he is tremendous. new to this board, he might be here
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LIP RIPPER
96 posts
Jul 27, 2009
1:07 PM
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Gentlemen, thank you again. This sharing of information is wonderful. Jim you are right, it does go to ground. I need to find out what size the two filters should be to replace the 2 in 1 cap. I haven't gone to the links some of you have posted just yet but I will. Hell, it may be in working condition. I'll know when the tubes get here Wed. The schematic I have sucks. Now I don't know much about this stuff but I do remember bits and pieces from childhood when I was playing with this stuff. All of you are so helpful and I can't thank you enough for taking the time to share what you have learned.
Adam, I want to thank you again for "Making Things Happen". This is the motto of my Real Estate company and it is certainly applicable here!
I'll get this thing rockin thanks to all of you!
LR
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isaacullah
275 posts
Jul 27, 2009
1:08 PM
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Tookatooka: Hey man, that's really very nice of you to say! You've made my day! This forum really is the best there is, and it's all because of the cool folks that hang out on here!
Cheers,
Isaac
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The magnificent YouTube channel of the internet user known as "isaacullah"
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Jim Rumbaugh
73 posts
Jul 27, 2009
6:44 PM
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Lip ripper: I say get 2 capacitors of 20 microfarads at 450 volts to replace the 2 in 1 capacitor.
more electronic nerd stuff next
Andrew: You said,"power supplies should have the same low-as-possible impedance". My teachers described that as the internal resistance of the power supply. The more power you draw, the more the voltage goes down. Therefore, there must be an 'internal resistance (or impedance)" Even though the power supply may have a low impedance, it can be used to power either high impedance or low impedance circuits. The impedance of the power supply does not directly correlate to the impedance of the circuit being powered. Perhaps that was why my original statement seemed "confused",
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Tuckster
210 posts
Jul 27, 2009
7:25 PM
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With all due respect,let's not reinvent the wheel.
Jim & Andrew: you're talking about something that only circuit designers need be concerned with. Even a simple resistor has capacitance and inductance.Capacitors have a parameter called ESR Equivalent series resistance.I'm more concerned that LR doesn't get his ass smoked.If the amp has been sitting awhile,you're probably OK. If not,as issac said,you need to discharge it.Tube amps need to be treated with the upmost respect. I've worked on amps that were sitting for 3 days and still got nailed. Not fully charged and it still got my attention! Be careful! If you don't feel confident about what you're doing,find someone who does.
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Andrew
473 posts
Jul 28, 2009
12:56 AM
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Jim
"Even though the power supply may have a low impedance, it can be used to power either high impedance or low impedance circuits."
It's not a case of "even though" - it's a case of "because". Low (internal) impedance is the ideal - it'll drive anything.
"The impedance of the power supply does not directly correlate to the impedance of the circuit being powered". No, we're saying it may be high because the tube amp doesn't draw much current, then that's a compromise design power supply. I've never been sure why we're talking about solid-state amps. Was the original point that I said "don't increase the size of the caps at random", and someone implied that "they are probably too small anyway because it's a tube amp which doesn't have the same hit on the power supply as a solid-state amp" (this resulting in my distinguishing the different functions caps have)?
Tuckster
We're not re-inventing the wheel, we're clearing our thoughts, revising, learning.
"Even a simple resistor has capacitance and inductance."
Come on, these would only matter, if at all, at microwave frequencies!
If you're afraid of the caps, get a screwdriver and a lead with a crocodile clip on each end and connect the screwdriver to the earth and touch the ends of each cap. It won't hurt 'em.
(Someone pointed out electrolytics have a positive end and a negative end, so if you replace them, you need to wire them in the right way around, and my memory of that was hazy. I'm pretty sure that's right, but when I thought about their innards I couldn't think of any way in which they'd be asymmetrical, cos in theory they are just two aluminium sheets in parallel rolled into a cylinder in some electrolyte, so I ignored the point. I haven't Wiki'd any of this because that would be time-consuming.)
(I mostly use English spellings, except that I've said tubes instead of valves. I refuse to convert aluminium to aluminum!)
Last Edited by on Jul 28, 2009 1:05 AM
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LIP RIPPER
97 posts
Jul 28, 2009
3:58 AM
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Thanks again. Jim, I will head up town today in search of the 2 caps. my tubes will be here tomorrow so I'll let you guys know what happens. And thank you for the warnings of shock. I do know my way around electricity and understand enough to be safe. This amp has been on the shelf for years so there isn't a chance of any capaitors holding any charge. Thanks again for sharing gentlemen.
LR
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LIP RIPPER
107 posts
Jul 31, 2009
4:27 PM
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Gentlemen, I would like to thank you again for sharing your knowledge. I got the caps in the mail today, soldered them in place, plugged in the new tubes then plugged her in. The jewel light came on, I let the tubes warm up and everything was quiet. I'm thinkin oh yeah! I plugged in the bullet and this old amp rips! Volume wide open and she's puttin it out.
LR
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Greg Heumann
112 posts
Jul 31, 2009
6:02 PM
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Lots of valuable discussion here. However I want to take exception to the statement:
As a general rule.
"It's OK to use too large a value for replacement.
If the capacitor is rated for a higher voltage, that's OK. Also you can use a higher value of capacitance and it should work fine. Example, you couild replace a 5 microfarad with a 10 microfarad and you should be safe, (larger capacitors allow for more filtering and less hum)"
It is true this is SAFE - but in tube amps, especially my beloved little Kalamazoo's, power supply sag is part of the character of the amp. I've had several 'Zoo's sent to me with double-the-original-capacitance replacement caps in the power supply section. They sound OK, BUT THEY DON'T SOUND LIKE KALAMAZOO'S.
I recommend the following strategy: always use the SAME value replacement components as a starting point. If you KNOW what the component does and want a different sound, THEN experiment with changing it. It is perfectly safe to use a higher voltage rated component than originally called for - if the capacitance, or resistance (rated in power dissipation, so watts, not volts) is the same they'll sound like the original. But don't juts use higher value caps in the power supply section for the hell of it.
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/Greg
http://www.BlowsMeAway.com
http://www.BlueStateBand.net
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