Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on March 22, 2020, 02:19:15 pm
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80,s Champ 12 red knob, I came across some mods to make this amp sound better, link is below of the schematic with the mods. Amp played before the mods and "played after the mods". I noticed the reverb didn't work so I pull the knobs and checked if I knock a wire off. Couldn't find anything but re-soldered the reverb wires to board anyway. I broke the reverb pot, a mini pot, I had a spare and installed it with a working 1MA (what broke). Then with amp board hanging over chassis as pictured, no go, no sound what-so-ever, maybe a slight hum, very slight. I think it was 60 cycle hum. The hum does not increase with vol pots.
All "voltage are there", both preamp and PI tube both have good connections, the wires are super thin, a few broke doing the original mod but I fix and resoldered them to board, continuity checked out on all preamp tubes wires to the tubes. (The amp played after replacing the tube broken wires before I notice the reverb not working).
The plates on the V1A, 161v, cathode 1.2v, V1B 169v and 1.8v, plates, the PI tube had voltage too, maybe a little higher (I didn't write them down). The 6L6 had 475v plate, screen 377v. I tried know working tubes all of them, different speaker, made sure all tube connection pins were tight, check out input jacks with continuity all are good to the input grids on V1A.
There were no issues after the mods, actually sounded very good. It was after I took it apart a second time to check the reverb, checked everything I did to the reverb, nothing. I'm stumped, when I touch the plates on V1A and V1B to get voltage, no signal noise, same with PI tube plates, both 12AX7s, no signal noise. I figured the input jacks, no wires off, used meter on traces continuity to the grids. Seems I have no signal to the amp, ? Side of board with components is well marked and numbered according to schematic.
https://www.effectrode.com/wp-content/uploads/2018/08/fender_champ_12_mods1.pdf (https://www.effectrode.com/wp-content/uploads/2018/08/fender_champ_12_mods1.pdf)
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It was after I took it apart a second time
:laugh:
yup
pcb's revenge :icon_biggrin:
trace it against schematic, find gremlin #1, kill, move to gremlin #2, kill...........
fixed, sell, buy turret board, build again :icon_biggrin:
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The replacement pot appears to extend too far forward to fit back in the cabinet. But that should not affect the amp while on the bench. Can you check the pot for defects or solder drops inside? Measure the resistance from pin to pin at different pot settings to verify that it works correctly. If the silence is not due to the repair itself then look for a loose wire. I have had the switch on the plastic headphone jack short out so plugging in headphones may give a clue. Good luck!
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Getting somewhere, the reverb pot will fit in the face frame, they bend a bit and look off centered, all pots move some. That's not it. But I found the culprit, a wire from the single end OT came off the board. But didn't see soon enough, I think because of that I blew the OT. With that sec wire soldered back I now get a loud humming, buzzing noise from the speaker, with a faint and very thin guitar sound, not playable at all.
Also, the B+ very high, like 550v along with the plate on the 6L6 550. The rest of the voltage down rail are OK. I think leaving the amp on too long without that sec connected, blew the OT. Would A blown OT measure excess voltage at the plate, cause a high B+ ?
I have another small 15 watt OT but it's a push and pull. I believe I can use it but not sure about the wiring, secondaries stay the same but it's the Primary side I'm unsure about, I think one of the plates wires goes to the B+ and the red (that used to be the B+) goes to the tube plate? the other plate taped off. Please tell me if I have this right. And if a bad OT causes very high B+...
Very much appreciate some help, thanks in advance.
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Looks like it just took some time for the voltages to bleed down. After half hour amp played with original OT, looks like that secondary speaker wire being off caused the voltages to spike everywhere. The wire was hard to see and hanging on by a thread, one strain of 18g wire and when it broke it stayed next to the spot. A broken wire was first thing I looked for, still can't believe I couldn't find it. Goes to show, look twice and then more, ha.
Back together I have to say those mods posted in that schematic really opened up the terrible overdrive channel, no more mud with gain past 1, nice and punchy overdrive. Before the OD channel was useless. The reverb works too, kind of weak but you can hear the difference, these amps are know for weak reverb, works off the OT, no ta tube. The clean channel is now as loud as the over-driven channel, that's a nice bonus with this mod. Clean channel sparkles like a Fender BF deluxe, I have tried several tech's mods to these amps and I have to say this mod takes 1st prize. Highly recommended. I was fixing this amp to sell but now I'll probably keep it.
Here' the link to mod and some a clips.
https://www.effectrode.com/knowledge-base/how-to-transform-a-fender-champ-12-amp-into-a-champ-deluxe-with-a-few-mods/ (https://www.effectrode.com/knowledge-base/how-to-transform-a-fender-champ-12-amp-into-a-champ-deluxe-with-a-few-mods/)
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I 'm pretty sure I blew this OT, it did come back to life for ten minutes, next day loud noise when turned on again, looks to stay now.
Is it possible for an OT (after I left the sec ground off with amp on for an extended period) to play OK for fifteen minutes. Later back on and the loud hum again? Now same loud hum all the time.
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put 1-5vac on it and see, if it's good, save PT, OT, tubes, recycle the rest :icon_biggrin:
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Save , PT, chassis and ... trash the PC board..., I hear ya. :laugh:
But after several hours doing this mod (I'll never do a mod like this on a tough to get at PC board again, especially one with tiny 1/4 watt R's that are very hard to get too) but the amp sounded so good after the mod, I can't convert it.
I ordered a new OT but the problem is the OT is inside the chassis on it's side and the Fender replacement is too thick. So I'll probably have to mount it under the chassis and drill holes. Fender doesn't have the exact size available anymore. Fender #24038 OT, chassis depth is only 1 3/4", and the OT is about 5.5K @ 8 ohms, 10 watts. One 6L6 running about bout 12 watts.
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trash the PC board
I found a cool amp store, asked about a tech position, He already had a good tube guy He was happy with, I asked about the pcb, ss stuff, the owner chuckled, said he takes them on trade then throws 'em in the dumpster, take ALL you want, I went fishing :icon_biggrin: