6V6 amp cutting out when guitars strings are picked hard

[cryptozoo]

I'm experiencing an odd amp issue I'm hoping to get some guidance on. I've got an early 60s off-brand push-pull 6V6 combo. The tubes are 6sc7, 6sf5, 6v6 x2 and 5Y3. It sounded weak and buzzy, so I went through and replaced the filter caps. Now it works and actually sounds very good, but when I dime the volume on the amp and pick the guitar strings hard or do fast runs, it starts to cut out. The faster or harder I play, the more it'll sputter, then if I stop playing the volume will gradually return. It's almost like a 'super sag' or something. With the amp volume down halfway or so it doesn't sputter at all. I've already replaced the electrolytics and cathode bypass cap, and in my troubleshooting I've tried the following:

- checked voltages against the printed schematic I've got (all within range)
- swapped in known good preamp (6sc7, 6sf5), power (6V6) and rec (5Y3) tubes
- checked all resistors and subbed in new coupling caps
- cleaned tube sockets, jacks, etc
- tried different speakers

I looked around online for some similar problems / solutions but didn't have any luck beyond 'change the tubes.' Worth noting is that if I remove the cathode bypass cap, the cutting out behavior happens at low volume as well. That would lead me to believe it's something related to the power tubes, but I can't find anything wrong.

Any ideas?

[Bruce / Mission Amps]

I'm experiencing an odd amp issue I'm hoping to get some guidance on. I've got an early 60s off-brand push-pull 6V6 combo. The tubes are 6sc7, 6sf5, 6v6 x2 and 5Y3. It sounded weak and buzzy, so I went through and replaced the filter caps. Now it works and actually sounds very good, but when I dime the volume on the amp and pick the guitar strings hard or do fast runs, it starts to cut out. The faster or harder I play, the more it'll sputter, then if I stop playing the volume will gradually return. It's almost like a 'super sag' or something. With the amp volume down halfway or so it doesn't sputter at all. I've already replaced the electrolytics and cathode bypass cap, and in my troubleshooting I've tried the following:

- checked voltages against the printed schematic I've got (all within range)
- swapped in known good preamp (6sc7, 6sf5), power (6V6) and rec (5Y3) tubes
- checked all resistors and subbed in new coupling caps
- cleaned tube sockets, jacks, etc
- tried different speakers

I looked around online for some similar problems / solutions but didn't have any luck beyond 'change the tubes.' Worth noting is that if I remove the cathode bypass cap, the cutting out behavior happens at low volume as well. That would lead me to believe it's something related to the power tubes, but I can't find anything wrong.

Any ideas?

Sounds like massive 6V6 grid blocking. Maybe the phase inverter stage coupling caps, to the grids of the power tubes, are charging up and shutting the tubes off...
Try some 5K6 to 22K grid stopper resistors on the 6V6 tube sockets and or lower the resistance value of the 6V6's "grid load" resistors (the ones from lugs 5 of the 6v6 sockets that are grounded on that cathode biased set up)... that could be tricky because many old amps use a "paraphase driver" which will have different value grid load (not grid stopper) resistors to steal a little signal to drive the other 6V6 out of phase. Some use two 220K to 270K with a 6K8 to 12K resistor holding up from ground one of the grid load resistors... that junction would be connected to one of the grids of one of the 6V6 sockets.... confusing huh? ha ha

[cryptozoo]

Hi Bruce, thanks for the info... that is a little confusing, but based on your description, I believe I've got a paraphase circuit here. I'm seeing an R 12k between one of the grid load resistors (560k) and ground on one of the 6v6 tubes. I took a pic of the schematic. Based on that, how would I properly add a grid stopper? Can it be connected to pins 5 and 1 like Fender did on their later blackface amps? I tried changing the value of the two grid load resistors (dropping them to 220k) and also changed the value of the 12k resistor holding up the ground. That did have an impact on tone, but didn't solve the problem.

[Jazz P Bass]

You need to identify what is failing.
From the input jack all the way through to the speaker is a long signal train.
That plus the supporting voltages can all be monitored & checked.
Is the signal holding up all the way to the output tube grids?
Is the B+ voltage sinking really hard when the problem occurs?
Is the cathode voltage taking a hit when the issue rears it's head?
Is the rectifier tube not keeping up?
Is the output transformer weak?
Time to break out the volt meter, heh.

[Bruce / Mission Amps]

Hi Bruce, thanks for the info... that is a little confusing, but based on your description, I believe I've got a paraphase circuit here. I'm seeing an R 12k between one of the grid load resistors (560k) and ground on one of the 6v6 tubes. I took a pic of the schematic. Based on that, how would I properly add a grid stopper? Can it be connected to pins 5 and 1 like Fender did on their later blackface amps? I tried changing the value of the two grid load resistors (dropping them to 220k) and also changed the value of the 12k resistor holding up the ground. That did have an impact on tone, but didn't solve the problem.

[ATTACH=CONFIG]23066[/ATTACH]

Oh, those Newcomb amps can be stinkers.....yes it is a type of paraphase driver.
Typically you can goof around with those grid load resistors but you have to understand the the paraphase uses the three resistors also as an AC voltage divider... so I wouldn't mess to much with those values.
You still could be leaking some high voltage across the .1uF coupling caps to the power tubes, which will pretty much wreck the operational parameters of the cathode biasing resistor!
Just to see if it helps, you can lift the wires from lugs 5 of the power tube sockets and insert a pretty large value resistor in series with the signal wire. Try a big jump first... 33K-47K... I know it sounds odd but it won't hurt anything and might help...

[Joe L]

I hate posting in a thread where Bruce has posted, knowing he has forgotten more than I know about 6V6 amps.

But, from what I see of the schematic, there is no way for that amp to not have blocking distortion.

I would:
Lower the .1uF caps on the grids of the 6V6s to at least .047uF, or even .022uF.
Add the resistors on the grids to the values Bruce mentioned.

That should mostly cure the problem. If it doesn't, try adding a 47K-470K resistor to the grid of the 6SF5.

Blocking distortion occurs when the grid goes positive with respect to the cathode. Grid current starts to flow, and it charges up the .1uF caps until the bias has drastically shifted. Until the caps discharge this voltage, the amp will be cutoff. So, make the cap smaller so it can discharge faster, and add some resistance so it doesn't charge up as fast, and you eliminate the problem.

I'm sure this amp was never intended to be pushed into heavy overdrive. But that doesn't mean a few well placed parts can't stop it from "singing".

[Gtr_tech]

Looks like a good candidate for a rebuild into something a little more guitar freindly. Grid leak biased preamp stages sound like crap for guitar IMO...

[Bruce / Mission Amps]

I hate posting in a thread where Bruce has posted, knowing he has forgotten more than I know about 6V6 amps. ...

Ha ha.. that's just plain silly....

[cryptozoo]

Thanks for the replies, guys. I think you're right, the amp was working as designed (which is unfortunate). After much experimentation, I finally got fed up and reworked the circuit into a single-channel 5C3 Deluxe. To be honest, I wish I would've done this to start with because it took way less time and sounds much better. Oh well, live and learn!