Silverface Twin Reverb issues

[sirshackleton]

Good day, all. A few months back I acquired a lovely '74 Twin Reverb.

Overall, it's a terrific-sounding amp that I can't wait to gig. There are some (non-deal breaker) quirks with reverb that I'd like some insight/advice on, though.

When the reverb is set at around 5 or below, it functions normally and provides more than enough effect for me. I usually leave it parked on 3 or so. However, once you start pushing into the higher ranges a couple of things happen. Around 6-7, a lot of high-end seems to go away and the amp darkens and seems to start breaking up...not a lot, but noticeably. Once you get to 7 and beyond, it's just self-oscillation/low-pitch feedback.

V3 had a recent-production 12AT7 installed. I swapped that out with a NOS JAN 12AU7 I had lying around. That improved things noticeably; the oscillation is slower to come on and I still had plenty of reverb effect for my needs despite the lower gain available. I also popped a lightly used new production 12AX7 into V4 (there was an old Fender-branded USA-made tube there that may be original to the amp) for the heck of it, but didn't notice any marked difference in the reverb performance.

I'm not desperate to fix the issue (unless I find out it's critical), but curious as to what might be going on. I haven't checked the tank and don't really know what to look for anyway but I'm guessing that's original. Thanks!

 

[Pete Farrington]

When the reverb is set at around 5 or below, it functions normally and provides more than enough effect for me. I usually leave it parked on 3 or so. However, once you start pushing into the higher ranges a couple of things happen. Around 6-7, a lot of high-end seems to go away and the amp darkens and seems to start breaking up...not a lot, but noticeably. Once you get to 7 and beyond, it's just self-oscillation/low-pitch feedback.

It may be due to one or more bad ecaps. Are they still the original ones fitted at the factory?

If there’s a master volume, does its setting affect the point at which oscillation manifests?

To narrow things down, how about if the reverb is switched off, at the footswitch?
If no footswitch, how about if the return RCA plug (ie not the red one) from the tank is unplugged, at the amp’s rear panel?

 

[sirshackleton]

Not sure if the caps are original or not. The amp was serviced a couple years ago according to the previous owner, but I don't know what precisely was done on it.

Master volume setting does not affect the oscillation point. It consistently happens right about 7 (pitch is C# played on the B string, if anyone's curious) regardless of where that or anything else is set. It also happens whether the reverb is on or off using the footswitch and when any combination of the RCA plugs are disconnected (red, black, both).

Interestingly, I did not notice the darkening/break-up I mentioned before this time around.

Otherwise, it sounds and performs great. No other hum or extraneous noises, controls are responsive, takes pedals fine, etc. No hint of a problem when reverb is set below "7."

Thanks for helping me diagnose the issue!

 

[Wally]

Not sure if the caps are original or not. The amp was serviced a couple years ago according to the previous owner, but I don't know what precisely was done on it.

Master volume setting does not affect the oscillation point. It consistently happens right about 7 (pitch is C# played on the B string, if anyone's curious) regardless of where that or anything else is set. It also happens whether the reverb is on or off using the footswitch and when any combination of the RCA plugs are disconnected (red, black, both).

Interestingly, I did not notice the darkening/break-up I mentioned before this time around.

Otherwise, it sounds and performs great. No other hum or extraneous noises, controls are responsive, takes pedals fine, etc. No hint of a problem when reverb is set below "7."

Thanks for helping me diagnose the issue!

‘Serviced’ could mean it had the dust knocked off of it. When I get a new-to-me used amp, it goes to the bench immediately. I want to know what I have…..and usually I have already looked at the details and know what has and has not been done.
got pics….including under the doghouse?

 

[Lowerleftcoast]

All we can do is guess. As Wally said pictures might be helpful.

Many times reverb issues can be solved with good RCA cables. It amazes me how many odd behaviors can be tamed with properly working cables.
Another guess is the caps.
And still another guess is the 3.3M resistor. Resistor values over 1M often drift horribly. Loss of the high frequencies made me think of this resistor and it could explain the oscillation/feedback, as well.

 

[Pete Farrington]

Master volume setting does not affect the oscillation point. It consistently happens right about 7 (pitch is C# played on the B string, if anyone's curious) regardless of where that or anything else is set. It also happens whether the reverb is on or off using the footswitch and when any combination of the RCA plugs are disconnected (red, black, both).

From the above, my most likely suspect is the V4 cathode bypass cap.
The V4 triodes are arranged in cascade. The shared cathode of the common cathode stages allows a positive feedback path to develop if its not well decoupled.

 

[sirshackleton]

Thanks all, I've got a lot to go with here. Appreciate the insight as to what might be going on!

 

[Wally]

In short, an amp of that age needs to be recapped and given good general service. Many times, that work cures a great number of problems.

 

[sirshackleton]

In short, an amp of that age needs to be recapped and given good general service. Many times, that work cures a great number of problems.

Oh for sure, and it's totally on me that I didn't get more info about what sort of maintenance had been done by the prior owner. I suspect some of the preamp tubes are original to the amp or have at least been in there for a long time (the power tubes are definitely later production) so it's a good bet the caps might be original or fairly old, too.

I just wasn't sure if what I'm experiencing had a known cause/solution or was indicative of a potentially larger issue.

 

[Wally]

Tech time, imho….
I have seen people replace every tube in an old amp when the only problem was that the amp needed proper service. Yes, occasionally a particular situation like yours can be cured with a tube, but that does not make the amp correct and proper. Making sound is not truly making music when the electrolytics are old.

 

[glenlivet]

It amazes me how many odd behaviors can be tamed with properly working cables.

These are wise words. Especially when it comes to reverb in/out cables (the old wrapped / shielded ones that came with that amp can get pretty flakey...after all these years. )

 

[Refugee]

All we can do is guess. As Wally said pictures might be helpful.

Many times reverb issues can be solved with good RCA cables. It amazes me how many odd behaviors can be tamed with properly working cables.
Another guess is the caps.
And still another guess is the 3.3M resistor. Resistor values over 1M often drift horribly. Loss of the high frequencies made me think of this resistor and it could explain the oscillation/feedback, as well.

I concur. New reverb leads can make a huge difference. I had mine swapped on on the '76 I have. Also a new MV pot replaced. purrs like a kitten. But that kitten can turn into a sabertooth tiger with a couple knob twists.

 

[sirshackleton]

THE UPDATE YOU ALL HAVE BEEN WAITING FOR:

Took to a well-regarded local tech who has worked on a lot of Fender amps from all eras. He popped open the chassis and we found some "graffiti" scribbled in Sharpie on the inside that noted the last service performed was in 2021. The original caps were long gone and the ones in there appeared to be basically brand new; it seems pretty likely they had been replaced at that last service. He also found a couple of other components inside that looked like they had been replaced/refreshed at some point in the recent past.

I mentioned some of my internet research (including here) suggested that V4 is a good place to start in terms of diagnosing and remedying the problem, and he pointed out a couple of spots he was going to focus on that seem to follow that path.

Thanks again for everyone's help!!