Sputtering, spitting, and bad breakup past volume 6

[Contech]

Hello Gang,

Here is another problem with this late 60’s Twin Reverb I have on my bench right now and I’m not sure where to begin. Take a listen to this video.

https://youtu.be/nHEtGRbXkmc

I’ve done a bunch of work on it and it’s sounding pretty good…except at full volume.
It sounds good to about 6. Clean, bold, big, Fender clean. When I open it up to 10 it falls down. Usually I would expect to enjoy dirt, grind, and overdrive (typical tones when you crank these old Fenders) but this amp is sputtering, spitting, low-end farting, and breaking up badly. It’s also got this “distorted ticking” sound almost like fast tremolo. I suspect the power section. I did change out all the filter caps and they should be good. I’ve got the tubes biased at 40mV. There are new tubes, and it’s pretty clean up to about level 6. At idle, there is no hum, ticking, crackling, etc. I got rid of all that using the usual methods (i.e. solid ground, plate resisters, pot cleaning, etc.)

Any thoughts?

[Chuck H]

Never heard THAT before.?. I'm suspecting parasitic oscillation even though I've never heard it sound like that. You do have a lot of LF in the tone and that could be coupling via the power supply if there is any issue with the caps or wiring. That might explain the trem effect.

On another note (no pun intended) I like the visual on that vid It really demonstrates the power of the TR

[Justin Thomas]

I've heard that before, not as extreme... I fight it nearly every time I build an amp. I vote for oscillation also. Did you change any layout at all? Fresh filter caps? Then again, a wonky tube can do just about anything. Did uou check ALL the tubes, and does it do it on BOTH channels? Do any of the controls make it worse if cranked up? There is also the possibility of abad connection somewhere, which may not show up except under "appropriate" volume (FFS, it's not THAT loud!).

Pull out the scope & siggy-gen... Which I sure wish I had...

And is that the amp jiggling the phone? That's friggin awesome!

Justin

[Contech]

Yes, an odd one for sure. Yes, there is too much low end on that clip but its way worse than farty low end. I suspect something going on with the filtration and Power Supply somewhere. I'll double check my filter caps. That was the first thing I fixed on this amp; which was dead when I received it. Thanks for your thoughts. I'll keep chasing these electrons.

And...weird video isn't it. Not sure how that happened. That was my iPhone propped up on a chair. Kinda cool and disturbing at the same time.

Thanks man.

[Contech]

Hi Justin,

Thanks for your reply. I agree, there is some kind of oscillation/filter issue. As far as wiring, there was a bunch of bad modification done to this poor amp when I got it. I've undone most of them but there are still a few in there. I think I will remove them all and get it back to original wiring and specs. I added bias measuring resisters on pin 8 to ground. There a gain boost on the Normal Channel (i.e. EQ lift) and another gain boost on the Vibrator channel (cap and resister before the PI). I'll go through this carefully but I've never heard this kinda ugliness before.

ALL of the tubes are brand new. I replaced them. And yes, this happens on both channels and that's one of the reasons I suspect the power section or power supply/filtration. That said, I did not adjust any of the tone knobs to see if that affected the noise. Good idea. I'll do that.

My signal generator just died. I ordered a new one and I should have it next week. I should see more when I can use my Scope.

Thanks Justin. I'll keep you posted.

[Justin Thomas]

Get rid of any mods and restore it to stock. I'd almost bet that's what's cauaing it - added gain in a layout that was never really idealized for it, anyway.

If ypu can find out what actual circuit it is suposed to be & get a factory layput, that'd be awesome. Heck, put it back to stock & you may not need a scope.

Justin

[pdf64]

It sounds like motorboating to me, ie positive feedback via the HT. It likely gets worse as the bass control is increased.
If the HT caps really are all good, then maybe a bad 0V connection (all 0V connections to the chassis are suspect).
Or perhaps mods have just added more signal gain (especially at low frequencies) than the stock HT can adequately decouple.
Throwing the net wider, all carbon comp resistors, solid core wires and solder joints are suspect.

[Contech]

Yes, I know exactly when it was built and the schematic layout....AB763. I got some work to do to get it back to spec.

[Contech]

Hello pdf64,

I'm not recognizing some of your terms. Can you elaborate?

1.) What is "HT" (i.e. positive feedback via the HT)?
2.) When you say "0V connections", are you referring to electrical ground of the power supply section?
3.) Finally, what is the issue with "solid core" wires?

Thanks in advance.

[Bloomfield]

1. HT= high tension= B+
2. yes
3. solid core can fatigue/ crack, possibly creating intermittent connections

[1ampman]

Coupling caps? Changed to a large value or leaking

[Contech]

Hello Gang, I’m still chasing down this motorboating problem on this Fender Twin. I did quite a bit of troubleshooting this morning. See below.

Set Up
• Input 500Hz signal (100mV)
• Both Channels (everything at zero)
• Connected my scope to one side of the Phase Inverter output.

Here is what happens on Channel 1 (at 100mV)

Channel 1
• Vol = 10 (with EQ knobs turned down, sinewave signal amplifies nicely)
• Treb = turn up to 10, not motorboating
• Mid = turn up to 10, not motorboating
• Bass = turn up to 10, not motorboating

Note: I increased input signal to 200mv and I immediately get motorboating oscillation. If I turn down the Volume or Bass to below 6, it goes away.

Here is what happens on Channel 2 (at 100mV)

Channel 2
• Vol = 10 (with EQ knobs turned down, sinewave signal amplifies nicely)
• Treb = turn up to 10, not motorboating
• Mid = turn up to 10, not motorboating
• Bass = turn up past 6, motorboating/oscillation begins

Note: If I turn down the Volume or Bass to below 6, it goes away.

I’ve replace a lot of components.

Decoupling Caps (Bypass Cap)
• Preamp stages on both channels (V1a, V2a, V1/2b)
• Both Bypass Caps in Tremolo Circuit (V5)

Coupling Caps
• .047uF Cap after V1b
• .02uF Cap after V2b
• .001uF and .1 Cap before Phase Inverter
• Both .1uF caps after Phase Inverter

EQ Caps
• For the hell of it, I swapped out the .047 bass cap on Channel 2. No change.

Well, I’m running out of components to swap out and check. I replaced all the primary filter caps a few months ago, but I haven’t checked them today…yet. I’m going to check all grounding as well.

Any other thoughts, ideas, suggestions would be helpful.

[Chuck H]

I think the problem is more likely in the HV rail. Possible errors in sequence and/or grounding. LF from like phase stages could be coupling there if AC on the rail isn't decoupled to ground adequately and correctly. I mentioned this in your first response on the thread in post #2 but you have gone all around it instead.

I would discontinue the practice of just replacing stuff and hoping. It can cause new problems or errors that confuse troubleshooting and repair.

[Contech]

Thanks Chuck. I'm still learning my troubleshooting skills and this is very helpful. I'll look into this next.

[Contech]

Hello Chuck,

Well, as usual, Chuck, you were right on. I looked into the HT line this morning and it appears that the motor-boating problems are stemming from the filter caps. I first checked the voltages to see if I was in range and everything looks correct. I also looked to see if there was no AC on the caps to make sure the rectifier was working properly. All good.

I tacked on another 20uF filter cap in parallel with the first filter cap (the one right after the choke); essentially doubling it to 40uF. Viola! The motor-boating went away. I also increased the input signal from 100mV to 500mV and then to 1V. Still no motorboating. A higher filter capacitance seemed to stabilize the amp and remove any oscillation.

Keep in mind that I replaced the older filter caps earlier this year so technically, these filter caps were brand new. I removed that one cap and measured it on my meter and it reads 20uF; so I don't think it's gone bad. That said, I’m not sure how to determine if caps are leaking (aside from the physical characteristics).

I suspect that I can simply increase the value of these filter caps and that should solve this issue. That said, there seems to be a lot of opinion and debate on higher filter cap values (i.e. tighter bass response or different tone). My understanding, however, is that these filter caps are here to smooth out rectified voltage ripple to keep hum and motor-boating noise minimal/gone. I think if I stayed in the 20-40uF range it wouldn’t make a big difference in the operation and tone.

What’s your opinion or thought on increasing the filter capacitance value?