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Dirty-South Blues Harp forum: wail on! >
matched tubes & bias ?'s
matched tubes & bias ?'s
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BreezeCC
10 posts
Apr 13, 2014
9:07 AM
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Hi all,
So, after many years of harp playing, I've been lucky to get a decent collection of vintage amps. For most of my time, I just played the amps stock, maybe swapping a V1 preamp, rectifier tube or speaker from time to time. Out of curiousity, I recently started digging deeper into why the various tube swaps work and what else I can do to change or improve the amp's harp tone. In that sense, I started learning about power tube bias. I bought a bias probe and a multimeter and began checking some of my amps. I was hoping this group could help me with a couple questions:
1. My understanding is that a good baseline bias target would be about 70% of the max power dissapation for the tube type used and then adjust from there by ear. I also understand that harp players often prefer a slightly lower, or "colder", bias.
Is this correct in your experience?
When they say colder, do they mean a lower bias current in mA or a lower power dissapation in Watts? Again, if so, how much colder?
2. In checking my amps, I have found that some have well matched power tubes (within 2-5 mA at idle current) and some are way off balance (as much as 16mA difference), but the amps still sound OK to me. How important are matched power tubes and will replacing the unmatched tubes with a matched pair improve the harp tone?
I appreciate your time and any insight you guys are willing to share
Best
-Carlos
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tmf714
2503 posts
Apr 13, 2014
9:15 AM
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@Breeze-"colder" bias helps tube longevity-it also depends on the amp-some amps prefer a hotter bias point,and may actually sound better for harp biased hot.
70% plate dissipation is a good starting point. I will bias my 6L6 tube amps between 28-32 ma. They sound good there,and my tubes last longer.
16ma is out of spec-acceptable would fall in the 3-5 ma range.
Weber has a good bias calculator here-
Weber Bias calculator
Last Edited by tmf714 on Apr 13, 2014 9:21 AM
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5F6H
1758 posts
Apr 13, 2014
11:30 AM
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Hi Carlos,
What amps are we talking about here? Ideal bias (% -wise) vary will on design.
Fixed bias, push-pull amps with multi speaker set ups can sound good cold biased (lower mA, also translates as lower W dissipation - don't confuse W idle dissipation with W power out, the relationship can be tenuous). I wouldn't normally bias a 2x10" cold, but would a 3 or 4x10", maybe some single big (12", 15") speaker amps too?
Power tube mismatch can be desirable on the ear (not my ears typically, but horses for courses), as long as the lower tube draws some current (6mA+) & the higher biased tube isn't going to burn up, go by ear.
Matched tubes are good for best efficiency, often good harp tone conflicts with practices that yield best audio efficiency...I'd buy them matched (my preference), then maybe experiment with mismatching known good tubes?
Going too cold with bias will cause long, sustained notes to fade out before you finish them, it'll also take bright edges off the tone, increase crunch, I wouldn't go lower than 6mA per tube for 6L6s in Fender...maybe 10-15mA would be more a more typical range? Watch your plate voltage, especially with BF/SF Deluxes & Princetons - modern 6V6s can take higher voltages but watch filter cap ratings too.
Cathode biased amps are less likely to respond well to big drops in bias current, maybe keep to 50%+, but you may have some room to move...many make a sound people like precisely because they draw lots of current.
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www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by 5F6H on Apr 13, 2014 11:32 AM
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BreezeCC
11 posts
Apr 13, 2014
2:03 PM
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@tmf714 and @5F6H
Thanks. I was hoping to hear both of your thoughts.
Here is data from amps I checked. Feel free to correct any miscalculations.
1961 Gibson GA77RV Vanguard
RCA 5R4GB and two RCA 6L6GC
19mA & 35.8mA @ 435V = 8.3 & 15.6 Watts = 28% & 52% max tube dissipation
1962 brown Fender Princeton
RCA 5Y3GT and two RCA 6V6GTA
11.8mA & 9.9mA @ 439V = approx 4.8 Watts = 35-40% max tube dissipation
1962 brown Fender Pro
Two Tung Sol 6L6GCSTR
24.0mA & 24.9mA @ 498V = 12-12.4 Watts = approx 40% max tube dissipation
1963 brown Fender Concert
Two GT 6L6S
22.5mA & 22.9mA @ 461V = 10.5 Watts = approx 35% max tube dissipation
1964 black Fender Deluxe
Mazda 5Y3GB, Westinghouse 6V6G, Sylvania 6V6G
21.5mA & 23.1mA @ 402V = approx 9 Watts = approx 65-75% dissipation
1965 black Fender Deluxe
GZ34 and two Sylvania 6V6GTY
23.5mA & 24.3mA @ 449V = approx 10.8 watts = approx 77-90% dissipation
1996 Sonny Jr first series 4x8
JAN Phillips 5Y3WGTA, two JJ 6L6GC
47.4mA & 47.6mA @ 341V = 16.2 Watts = 54% dissipation
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5F6H
1759 posts
Apr 13, 2014
3:02 PM
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Brown Princeton - I wouldn't want more than 425vdc on the RCA 6V6 plates, 10-11mA is low, even for my tastes, try 17-20mA at the lowest, or the least amount of current that pulls your voltage below 425vdc. That said, I'd expect a brown Princeton with original PT to be running quite a bit less than this, even the transitional white knob BF's ran less than your amp & that's with a GZ34 rectifier. Has the PT been changed? If not, I'd run it by a tech...don't want to panic you, but something seems a little adrift here?
63 Concert - Voltage is low on this one, bias shouldn't be pulling it that low, should be close to the Pro. Original PT (125P7D...earlier PT was 67233 but less likely in a '63)? Again, once over by a tech would be good if the PT hasn't been replaced (if it has, then voltages could be OK, suply no's if so).
65 Deluxe - Bias is too cool (I know the % don't say so) but vintage 6V6 don't like these high voltages, replace with JJ, or other modern 6V6, or rebias to 30mA per tube with the NOS...the high screen voltages will kill the tubes before plate current. Or, if you like the tone of the 5Y3 in your other Deluxe, try one in here too?
SJ 4x8 was designed to take a range of rectifiers, bias resistor value was altered over the run, maybe yours has the larger value (330ohms?) if you only want to run the 5Y3 you may have scope to try a 250ohm 10W cathode resistor? Are you measuring plate voltage to ground, or from plate to cathode (20something volts less)?
64 Deluxe (5Y3 isn't stock, but fine if you like it) & 62 Pro look ball-park.
GA-77 the 19mA tube looks abnormal, but not necessarily an issue if you like the sound, try subbing sockets to see if the low current follows the tube (good) or the socket (bad).
Go more by the reading in mA than the % dissipation.
Nice collection there ;-)
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www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by 5F6H on Apr 13, 2014 3:05 PM
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BreezeCC
12 posts
Apr 14, 2014
10:20 AM
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Thanks Mark!! I REALLY apppreciate it. I'm learning with every post I read.
I am using a new Amp-Head dual bias probe along with a new (but cheap) DMM to measure bias current and plate voltage.
I looked at all the amps. As far as I can tell only the '63 Concert has had one XFMR replaced. The OT on it is 022855 606652 which I understand is nearly identical to the original 125A9A.
The PT's on the brown Princeton and Concert are original (125P1A & 125P7B).
As you said, the '64 & '65 Deluxe amps are pretty much identical. I consider them twins. I left the GZ34 in the '65 and am trying different rectifiers (5R4, 5Y3) in the '64 so I can A/B them and see which I like best for harp.
Sonny Jr amp is very early. Serial #16. Built in '96. I bought it in 2005 from orig owner. I understand there are some small mods that Gary O publicized some time back, but I don't think anything has ever been done to this amp. I am considering selling this one.
I checked the GA77 and the low current did follow the tube and not the socket - so that's good. The amp does sound pretty good now. It is generally my go to amp. I just wonder if it would sound better with a matched pair of 5881/6L6 or with a bias adjustment.
Basically all the amps sound pretty good. I rotate and use them all. However, since I have have been recently learning about bias I started down this kooky path of checking my amps and wondering if I could tweak the bias on any and improve the tone.
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5F6H
1763 posts
Apr 14, 2014
10:35 AM
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The SJ still might benefit from going a little hotter on the bias *as long as you do not change to a hotter rectifier*, HTownfess (Stephen Schneider) did some tweaks in this direction, maybe try a 200ohm 10W cathode resistor (if it has the original 250ohm)? Be a shame if you sold it, only for a new owner to change one part & hit a sound you really like?
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www.myspace.com/markburness
http://www.facebook.com/markburness
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Tweedaddict
148 posts
Apr 14, 2014
6:14 PM
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@Carlos "The amp does sound pretty good now"
C'mon man... That amp sounds Killer Diller!
That Ga77 A/B we did a while back was FF-FUN.
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BreezeCC
13 posts
Apr 15, 2014
12:23 AM
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@tweed HAHA ... careful Mike, you'll blow my cover and everybody will find out I'm a never satisfied, detail obsessed, amp hoarding, harp playing super-nerd masquerading as a cool bluesman. Oh well, too late. Now that you are back from your "working vacation" we'll have to do another amp shootout.
This all started a few months back with me trying to get the brown Pro sounding better for harp. I connected with @tmf714 and he suggested some tube swaps and I used a different mic than my usual one and it made a BIG improvement. Then I started reading about power tube matching and amp bias and I fell into the rabbit hole. I bought a meter and probe and started testing all my perfectly good harp rigs but got unexpected/confusing results. Started reading old stuff on this forum and found a lot of great info from @5F6H. I decided to post my ?'s directly and luckily both Tom & Mark answered.
Now I'm really deep in it!!
In other news, today the stars have aligned in such a way that another old amp may come into my possession. This time a mid sixties Ampeg Reverberocket II. (1x12", 2x7591, 2x12AX7) Anybody ever used one successfully for harp? I tried one years ago and did not have good results. Clean, feedback prone and not a lot of beef to the tone. Maybe the one I tried was a dud. Anybody have any advice?
Last Edited by BreezeCC on Apr 15, 2014 12:59 AM
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5F6H
1765 posts
Apr 15, 2014
4:30 AM
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I had a 6SL7 version, as far as mods goes, try clipping the bright cap off the tone control - there'll be 2 caps on the tone control, a large value to ground & a smaller value shunting highs around the unused portion of the volume pot - clip out this one (easy to replace if you don't like it.
Lots of preferred old 50's amps only had hi-cut pots, which is what you will be converting the Ampeg to.
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www.myspace.com/markburness
http://www.facebook.com/markburness
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HawkeyeKane
2472 posts
Apr 15, 2014
8:28 AM
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I've not tried a Rocket, but my band's bass player has a '65 Jet J12T that's similar. Same 7591 output tubes, but with 6BK11 compactron preamps. It sounds bitchin for harp! I'm probably one of the few harp players out there who's a nut for the 7591/7868/6GM5 tube sound. :-)
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Hawkeye Kane - Hipbone Sam
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htownfess
288 posts
Apr 15, 2014
11:29 PM
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@breeze: I like the SJ1's tone a lot better with the bias running in the 55-60mA range, though it will feed back more easily. Not all current tube brands, though: JJ, TAD and SED are good that way, as is the Sovtek 6L6WXT+ (aka Electro-Harmonix 6L6EH). As Mark says, dialing the bias in toward the warm end tends to lock you into using a given rectifier, and with the 5Y3GT rated at 125mA total, it's working pretty hard with two main tubes at 60mA apiece. However, they did originally voice that amp with a 5Y3 and I think the preamp sounds best with one, so aim for 55-57mA; a 200 ohm main cathode resistor ought to be about right to try first, as Mark suggested.
With a low serial number # like that, I can see why you might just sell yours instead. Really good platform to modify, though, because the circuit is so simple and it's built with premium parts.
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tmf714
2516 posts
Apr 16, 2014
7:20 AM
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From Sonny to Harp-l Jan 15,2005:
Keep in mind is it a Class A, 20 -22 watt amp with 12AU7 12AX7 tube setup
with 5U4 rectifier. Any SJ 1 owner with serial number less than 100 should
check the large grey resistor on far left of circuit, we upgraded to a 330 ohm
15 watt from the 220 ohm 15 watt due to moving from the 5Y3 to the 5U4 stock.
The 5U4 needs the larger resistor or it will get the other one too hot.
Getting information out is much easier with the website and HarpL. Once the new
site is up all updates will be posted, along with me having the ability to
modify it myself. We went for more Chess authentic LW recorded sound with our
first production model.
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tmf714
2517 posts
Apr 16, 2014
7:21 AM
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Thank you all for responding. I am all set with amps. Since so many
responded with #1's saying they won't part with them, there is an upgrade to the amp,
using a larger core output from David Allen, gives the amp another 10-15%
all around ooomph. This is for SJ 1 as it only has a 4 and 8 ohm tap.
_allenamps@xxxxxxxxxxxxxx (mailto:allenamps@xxxxxxxxxxxxx) Ask David for the Sonny Jr
1 beefy upgrade. Will run you around $100 installed. Good investment. If
they were available when I was building, I would have used them. Keep looking on
our site, we will have some updates for current owners and a new site
hopefully within a few months. I am gathering a small but rare collection of harp
amps to view, have some discussions about amps and upgrades.
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tmf714
2518 posts
Apr 16, 2014
7:32 AM
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Transformer upgrade notes from Don Zeller-
As promised, here's the results of my output transformer upgrade on my Sonny Junior 1. Thanks muchly to Gary Onofrio (Sonny Junior) and David Allen of Allen amps for your help and advice on the wiring. In a quest for more output, and as posted here by Sonny a few weeks ago, I switched the original transformer for the heavier(by several pounds)/ beefier Allen output transformer model TO40MT, which is a 40 watt 4/8 ohm upgrade transformer for Vibrolux amps that retails for $59.00 plus shipping.( http://www.allenamps.com/parts.html )
The conversion was fairly simple and took under 1/2 hour( after removal of chassis and the VERY IMPORTANT BLEEDING of the CAPS which can deliver a dangerous shock from an unplugged amp if not shorted out to the chassis first: hire a qualified amp technician to do the work if you have no idea what I'm talking about here!):
The results are very good: the amp is substantially louder (could I estimate 20 % by my ear?) and preserves the tone of the circuit well. Like a wild stallion, it is a bit more prone to feedback, as one would expect, but using a 12AY7 and a 12AT7 as the phase inverter and preamp tubes respectively I control feedback adequately. (Going 12AU7 to 12 AT7 or 12AY7 eliminates the feedback almost completely but to my ear cuts the snarl factor a little more than I like. Sonny suggested trying 12AU7 to 12AX7 but to my ear this wasn't as good as the above).
So, for the cost of a couple of power tubes, the upgrade gets my double-thumbs-up rating and endorsement for those SJ1 owners seeking more stage volume. Any SJ1 ownwers who need more details on this are welcome to email.
Ahhh, the smell of solder resin!
Zig
Last Edited by tmf714 on Apr 16, 2014 7:34 AM
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BreezeCC
14 posts
Apr 17, 2014
3:44 PM
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Good stuff guys. Thanks to all.
Despite gigging it for years and recording two CD's with it, I kinda stopped using the SJ1 around '05 for various reasons, but mainly cause I needed a bit more muscle on stage. Maybe the mods above make it worth a revisit.
By the way, we've mentioned the 5Y3 and 5U4, anyone try a 5R4 in the SJ1?
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tmf714
2523 posts
Apr 17, 2014
7:40 PM
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I have a 5AR4 in there now-but I am going back to the 5U4 with a 12AU and 12AT with a Boss GE7 Equalizer at Sonnys suggestion-I will have time to play with it this weekend.
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