This is my Champ amp, modded for harp. The list of mods to this Champ is quite extensive. It has lost a lot of the raspiness you get from a Champ, and it no longer has the boxiness or nasal tone. Mods include: Weber 10A125-O alnico speaker with H dustcap, NOS JAN Philips 5751 preamp tube, NOS RCA blackplate 6V6 power tube, NOS Sylvania 5V4 rectifier tube, negative feedback loop is disconnected. It has been voiced for blues harp by Bruce Collins of Mission Amps in Denver. Bruce replaced all the capacitors, installing NOS paper in oil tone caps. He also installed a killer-sounding line out. This may be only 5-watts, but I run it through the PA board and this thing just kills.
I played a Hohner Marine Band Deluxe in Bb, using a Front & Center microphone. No effects. Volume on 6, Treble on 4, Bass on 8.
The Weber speaker made the biggest single improvement in the sound of this little amp. But the Weber is not very efficient, so you will need to mic the amp or line it out.
You can find out a lot more about this amp project at The Blues Harp Amps Blog.
Sounds nice Rick. I have a Weber 8F100 in one of my champs and it really sounds great. My Champ has the Gerald Weber tweed mod done to it and is very harp friendly. With a Carbon Copy Delay it holds it own at most jams.
Interesting post. you bring a subject that has long been baffling to me. And it is great info for this audience.
Why? Here's a story. I brought my "57 Vibrolux to a noted amp tech, refered to me by a noted vintage guitar saleman. I waited a long time to fix the sucker and I wanted to trust it to only the best. He took it in, recapped one or two caps, said the amp was not the best '57 he had worked on, it was made on a friday,etc. When I played it again, it still sounded awful. Turns out it was the speaker that was shot. Now granted, I should have figure this out myself, but I am clueless, but what about the master amp man? I paid him 125 bucks to change a cap or two for what? When I asked the amp man about about this, he said he works on amps not speakers. Oh brother!
The amp man I use now laughed at this story. He said that this stuff happens all the time, particularly with vintage amps and extends to when techs say they can do all sorts of mods like "blackface a silverface." I asked him if there was any books or sites that could explain this to me. I also remarked I had read Dan Torres book on tube amps. He quipped that I should not try any of the torres stuff at home as because I would be sending it to him after I wrecked it!
I digress. Back to your champ. You have done a lot of work to your amp. A bunch of questions:
1) Why replace all the caps? What are the oil in paper caps that I have heard waxed poetically about?
2) Did you change the transformers and upgrade to a larger ones?
3) Negative feedback loop, what is that and why does the removal produced desired results?
4) You say it has been voiced for "blues harp," what specifically does that mean? Did you change the schematic?
5) Now be honest, for a mere mortal whose drummer isn't the finest amp tech/builder in CO, how much would this work cost me? More than a new HG? New tubes, New speaker, new caps, new line out, secret tweeks, sounds like a lot.
6) What is a "microphonic" tube (from a previous post.)
Thanks in advance for the answers and helping unlock the deep mystery of amps.
FWIW, it's not my Champ but since I build and modify amps as well, here are my takes on a view points:
1) oil in paper caps are to me the biggest myth I have ever encountered. Soundwise I prefer the lowly Orange Drops or Mallorys. The difference is marginal anyway but IMHO the oil in paper caps sound a bit more sterile and brighter (the later might have been a result of varying values). There are a lot of thing one wants to hear and I really wanted to like them but I could not hear an improvement at all. The oil in paper caps are more prone to leaking they say. Since I don't use them anymore I have not enough experience with their longevity so that's just what I heard. Orange Drops will last a lifetime. Finding the right values is a lot more important than switching brands.
3) the -fb loop feeds a negative signal back before the previous gain stage. This cleans up the sound a bit and makes it more defined and tighter. You lose gain and "beef" at the same time. By installing a bigger value resistor or removing it completely you decrease headroom and fatten things up. This might also result in a lower feedback threshold.
4) Again, I don't know what has been done on this particular amp but changing the coupling caps to bigger values darkens the sound a bit. You can also use different cap values around the tone stack to shift the controls' range. Some other things work as well but these are the easiest ones.
5) Changing the speaker is easy. You can do that yourself if you can hold a wrench or a screwdriver. Then you can try a lower value preamp tube (12AY7, 12AU7, ...). There two mods are easy and are probably all you need. The rest is finetuning and should be done by someone who knows what he's doing. The results will most likely be a lot less dramatic than the speaker change and also a lot more expensive. You don't have to do all this to get a great sounding amp.
6) This is a tube that picks up mechanic vibrations and amplifies it. YOu can try this by lightly tapping on the glass part of the tube with your finger nail while the amp is running. If you hear this through the speaker then the tube is microphonic and should be replaced. Sometimes they tend to squeal as well.
hope this helps ... ---------- If it ain't broke you just haven't fixed it enough ...
2- I have not changed the output transformer yet. I'm looking for a good one with a 4-ohm tap.
3- Fender’s goal for its guitar amps was a sparkly clean tone, and one way to do this was with a negative feedback circuit. It was thought that power tubes introduced distortion, so Fender engineers looped an out-of-phase portion of the output signal back to the preamp section. That way the distortion cancelled itself (sort of), but it meant they were siphoning off some of the Champ’s power for the sake of clean tone. Amped harp does not need sparkly clean tone, and distortion is our friend. Many pro players swear by this modification, saying it makes the Champ louder and warmer, while a few others say it can make the tone gritty and harsh. (Heck, I live for gritty and harsh tone!) So I have decided to install a potentiometer in the NFB circuit so that I can dial down the negative feedback from the factory setting to nothing – or anything in between. Old vintage amps actually have wires and resistors and things; not just chips and printed circuit boards. This is a pretty simple mod that requires drilling a hole in the chassis and mounting the pot, and connecting two wires.
4- "Voiced for harp" means many things: The bias level of the tubes, the values of the caps, etc. If you check the Blues Harp Amps blog you can see a list of parts used in the the upgrade:
2 .068uF/400v Paper in Oil K40y caps 1 .047uF/40ov Paper in Oil K40y cap 2 22uF-50v Electrolytic cathode bypass caps 1 100uF-50v Electrolytic cathode bypass cap 1 .001uF-500v CM Ceramic cap 1 .001uF-630v Mallory 150 cap 1 .0015uF-630v Mallory 150 cap 1 250pF-300v Silver Mica cap 1 150pF-300v Silver Mica cap 1 470 Ohm 5 Watt 6V6GT tube cathode bias resistor 1 47K 1w carbon comp B+ power supply resistor 1 56K 1/2w carbon comp NFB reduction mod resistor 1 220K1/2w CF Line Out mod resistor 1 47K 1/2w CF Line Out mod resistor 1 Mono Jack non-switching Line Out Jack
If you want more info, contact Bruce Collins at bruce@missionamps.com.
5- Yes, a vintage Fender SF Champ plus the upgrades would proably cost you more than the price of a HarpGear HG2.
6- Bluefinger explains microphonic tubes pretty well. Tubes sometimes become like microphones: They transmit physical vibrations into their signal stream. Tap on them gently with a pencil to check. If you distinctly hear the tap amplified through the speakers, your tube is microphonic.
Some tubes come from the factory like that. Some tubes explode in a blaze of white light from the back of your amp five minutes after installing them. Tubes are complicated and variable.
A tube that is mildy microphonic does not need to be replaced right away. A badly microphonic tube will cause your amp to ring or to feedback, or to sound kind of shrill.
Remember, the speaker swap involved putting a 10-inch speaker where an 8-inch speaker had been originally. I had to cut out the baffle opening and position it so that the bell cover of the alnico Weber speaker did not touch the filter cap. The speaker was shifted down and to the left about an inch when looking at the amp from the front.
As I said in the original post, the Weber speaker made the biggest single improvement in the tone of the amp. But the amp did not really sing until after the voicing mods were done.
Now we are looking for a 10-watt transformer with a 4-ohm tap…
I didn't undertake this project because anything was wrong with the Champ. I did it because I wanted to know firsthand what it took to turn it into a better harp amp. I did it to learn.
The 125 DSE has taps for 4, 8 16, 32 Ohms. The size is another matter. If it can fit fine. 3.56"w x 2.63"h x 2.10d. I use a 125CSE (which also is multi tapped)on my tweed clones and have plenty of beef. the 125CSE is a tad smaller than the DSE and puts out 8 watts, which is imho plenty.
