Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 3jaw on July 05, 2026, 11:42:35 am
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Hello all,
This is my first post here but not my first amp build. I just finished a Hoffman Plexi 50 with 6v6's instead of EL34's. I did the hot switch gain mod, PPIMV, and a variable negative feedback pot. The normal channel is very quiet at max preamp and master volume save for an acceptable amount of low level hiss. When I switch on the added gain stage, the hiss gets very loud. I realize that the added gain stage will amplify any noise already present but I am pulling my hair out trying to reduce it.
Here are some things I've tried so far:
1. Swapping V1 tubes. Right now it has a TAD Redbase 7025. It, along with a JJ 12ax7 seems to be the quietest. I have also tried a Mullard reissue which was very noisy and microphonic, and a Tung Sol reissue which caused excessive hum.
2. 33k grid stoppers on V1, directly into the tube terminals.
3. Shielded wires from input jack to V1a grid stopper, hot switch to V1b grid stopper, and from V1a coupling cap to volume pot.
4. Increasing negative feedback helps, but too much causes high frequency oscillation which is not good for the 6v6's or the OT.
All resistors are metal film.
The only possible carbon comp resistors are the Alpha pots throughout.
Hoffman states in one of his old posts that the hot switch mod isn't noisy unless maybe you have terrible wiring skills, which I'm beginning to believe I have! :BangHead:
Any help would be greatly appreciated.
Here are a few gut shots. Note the last image was taken before I added the 33k grid stopper to V1b:
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You can try a reverse 12DW7
ECC823 = reverse 12DW7
Pins 1,2& 3 = 12AX7
Pins 6,7 &8 = 12AU7
Or try adding a volume pot to the hot switch mod. You can put the pot on the back panel of the chassis or even use an internal trim pot. You can also try a grid resistor into the 2nd gain stage. Maybe a 100k to 220k range to start with if you don't use a hot switch volume pot? IF you added a volume pot to the hot switch mod, then I'd probably try 10k - 33k grid resistor with that pot.
An advantage of using the hot switch with a volume pot is that you then can set the volume increase when using a foot switch activated relay. Instead of a giant jump in volume, you can set it as a boost for solo's.
With respect, Tubenit
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3jaw, your mailbox is full
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3jaw, your mailbox is full
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Please fix your mailbox
I am getting email notifications from the forum, if that's what you're talking about.
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Tubenit,
I haven't heard of a reverse 12dw7 before. That looks interesting. Would I have to change the value of the cathode resistor on the 12au7 side or can I leave it at 2.2k?
Also, just for clarification, the 500k volume pot on the hot switch mod would take the place of the 470k resistor circled in Hoffman's schematic below?
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I would try it as is first and then you can see IF you wish to change the cathode resistor value.
The schematic I drew for you already answers your question about replacing the 470k resistor.
With respect, Tubenit
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Thank you for your suggestions, Tubenit. I will order an ECC823 and also add the volume pot.
Any other advice is welcome.
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Try to reduce the resistance between the input jacks and the grids of V1:
What does it measure now?
Replace 33k grid stoppers with 10k.
Remove Dual input jack wiring if it is present--especially if you still have 68k grid resistors mounted on the jacks.
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Try to reduce the resistance between the input jacks and the grids of V1:
What does it measure now?
Replace 33k grid stoppers with 10k.
Remove Dual input jack wiring if it is present--especially if you still have 68k grid resistors mounted on the jacks.
Right now it is wired to Hoffman's schematic except for the 33k grid stopper I added to the switchable gain stage. There us only one input jack.
It will be a few days before I make any of the changes suggested. I will give an update then.
Thanks,
Greg