Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bullkelp on October 04, 2025, 05:24:10 pm
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I have a ampeg reverbarocket built by a competent amp builder about 10 yrs ago.
Ive just recapped the filter section and put new 6v6 JJs in
Ive biased it using the output transformer resistance and voltage drop method
Left winding resistance to centre tap = 110ohm and Right 120
Approx 460 plate volts. Average Drop 2.1v plate current at idle = plate current 18mA gives me 70% plate dis for the 6v6s
The issue I have is very low output. Its clean, sounds as it should, but I have the MV on full and having to turn gain up to 9pm (3 quarters) for decent volume.
Ive chopsticked everything.
How can this be so?
OT is fine, plate current fine. All tubes heating up. Voltages on the plates of all the preamp tubes. SS rectifer diodes all test fine. Solder joints ok. Caps i replaced like for like.
I tried some 5881s I had on hand and same issue.
Its always been a clean machine, and has tended to have to be turned up a but to work as it should but with bboth volume and gain mid way up its barely audible.
Thanks :think1:
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Was it playing normally prior to the work you did?
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Yep :dontknow:
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And id have thought if the filtering wasnt right itd just be noisy.
A quick probe through and the power rails seem right.
But if the output section tests ok (plate curretn / voltage etc ) and the OT tests ok then im assuming there is a gain stage thats not working. Or an issue at the PI ?
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Go back and carefully inspect each point you touched. Look for cold/loose solder joints. Re-tension the tube socket pin clamps. Did you replace any resistors or cathode/screen bypass caps?
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You should post a schematic of this amp, showing voltages.
Also, is speaker impedance matched to the OT secondary ouput?
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Same speaker as its always had
Now a schematic I dont have and I do not think this is faithful to the original.
It has an MV and also a boost swith.
It does use the 6SL7 preamp tubes and one 6NS7 (V2).
Everything ontop of the board looks good. The filter caps are as they should be and are mounted on board also. Its a garolite board and the way the builder has done it hes used a lot of underboard connections. Im wondering if an underboard wire to a turret has dropped free.
Ill have to go through with an ampeg r12r schematic and try and check continuity.
This is one of those pesky ones.
Thanks
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Then you should create a schematic.
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I suspect if i was looking at a fender layout id have figured it out.
Not used to these tubes or this circuit.
Anyhow you are correct
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... Its always been a clean machine, and has tended to have to be turned up a but to work as it should but with bboth volume and gain mid way up its barely audible.
Thanks :think1:
I experienced such a similar issue with an amp, clean sound with volume at max and barely audible half way, I spent lots of time looking to find the problem, then it was a preamp tube ... the heater was okay but it simply wasn't receiving voltage at the plate because of a cold/loose solder.
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Check your output jack/speaker connections. You don't have an oscilloscope?
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Now a schematic I dont have
Try looking here https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm)
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Check your output jack/speaker connections. You don't have an oscilloscope?
No I dont own a scope. Im opretty sure the speaker out is ok.
Im suspicoous of a preamp tube.
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The 6SNL feels a lot hotter than the other preamp tubes
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Would you say the table on this page would be ballpark voltages im looking for if I check through?
https://www.tdpri.com/threads/ampeg-reverberocket-r-12-r-diy-project.1102984/page-2
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Would you say the table on this page would be ballpark voltages im looking for if I check through?
https://www.tdpri.com/threads/ampeg-reverberocket-r-12-r-diy-project.1102984/page-2 (https://www.tdpri.com/threads/ampeg-reverberocket-r-12-r-diy-project.1102984/page-2)
Maybe (but if you get 76mA idle in a 6V6 you’re in trouble).
Most 6V6 amps run between about 340v to 430v B+ depending on the vintage and whether they’re cathode bias or fixed bias and what sort of rectifier is used and the PT secondary winding VAC. For something like this, I’d expect a B+ between 380-400 is normal.
Reverborocket schemes start around about #395 on the list in that link I posted earlier. One of them is a more modern version that shows 382V at the plates. So somewhere in that ballpark there. All the other HT voltages etc will be in proportion to the B+. What you’re looking for is something that’s oddly way different than expected - if it’s a voltage issue (which it might be, or not)
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Would you say the table on this page would be ballpark voltages im looking for if I check through?
https://www.tdpri.com/threads/ampeg-reverberocket-r-12-r-diy-project.1102984/page-2 (https://www.tdpri.com/threads/ampeg-reverberocket-r-12-r-diy-project.1102984/page-2)
Maybe (but if you get 76mA idle in a 6V6 you’re in trouble).
Most 6V6 amps run between about 340v to 430v B+ depending on the vintage and whether they’re cathode bias or fixed bias and what sort of rectifier is used and the PT secondary winding VAC. For something like this, I’d expect a B+ between 380-400 is normal.
Reverborocket schemes start around about #395 on the list in that link I posted earlier. One of them is a more modern version that shows 382V at the plates. So somewhere in that ballpark there. All the other HT voltages etc will be in proportion to the B+. What you’re looking for is something that’s oddly way different than expected - if it’s a voltage issue (which it might be, or not)
Sorry I mean the preamp tubes
I’m happy with the voltages and currents on the 6v6 tubes as they are
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You can work out all the HT voltages from knowing the B+ voltage, and the cathode current of each tube and the HT supply resistor resistances. see example diagram at the bottom of the page in this link https://valvewizard.co.uk/smoothing.html (https://valvewizard.co.uk/smoothing.html)