Low Voltages on 78 Twin Reverb 133 (UL)

[caesparza]

Hi Chuck, will try the 12AX7 on my Princeton Reverb. Yes the MV has a push-pull switch to activate a boost on the signal (some call it overdrive or distortion circuit).

I removed it following this article: https://www.premierguitar.com/articl...n-twin-reverbs (see the last part). It implies disconnecting the cable that goes all the way to the reverb pan input, tape it and leave it there and lifting one leg of the resistor that connects to the .01 cap. The cable that generates sound on the speakers when tapped (on V4) passes over this resistor, when I tap directly over the resistor it doesn't generate any sort of microphonic sound.

Other thing I have noticed (haven't got much time to keep working on the amp) is that there are two types of hum on the amp, one is low frequency and its not annoying at all, that probably is the 60Hrz. The other one is brighter and it varies according to the movement of the channel volume pot (could it be filter cap leakage? I recapped the amp). At the extremes (close to 0 and close to 10) the hum seems to be attenuated. I don't know if that bit of information could be useful.

That is all I got for now.

[Chuck H]

What the article proposes seems effective enough but for one thing. The signal coming out of the reverb recovery triode is grounded at the pot, then again at the 470k/1k resistors in series just after the pot in the circuit (not sure where these are on the board). Chances seem high that to implement the different attenuation circuits that the 470k/1k series resistors are grounded somewhere other than where the pot is grounded. That means the same signal trace may be grounded in two different places in the circuit and THAT is a potential ground loop. Something like this might have been a concession for the MV circuit, with the consequence that there is some added noise. That you have two different hums isn't unusual. The one that increases with channel volume is probably 120Hz and would indeed be ripple from the power supply. Though the only reasons I can come up with for why it would attenuate at the highest channel volumes would be either phase errors (because the channels are out of phase) or some kind of instability in the amp at higher output levels causing an ultrasonic oscillation.

Hum can come from many sources and having more than one isn't unusual. Tracking it down and minimizing it can be tough though and that particular amp won't likely make it easier. Without having it on the bench in front of me and you not wanting to take much more time with it I'm not sure how far we can get. I will say that knowing more about how to troubleshoot hum, recognize tells in circuit operation, locate and minimize it are skills that are better experienced first hand. Trying to do that sort of work from a remote location through someone elses hands is not as productive. You should definitely do some searching and reading about chasing down hum here at the forum. I'm pretty sure anything you need to learn is here and not hard to find or pin down. Just know that such threads don't have to be about your specific amp. The information is about circuit knowledge rather than rote experiments.

[caesparza]

Thanks Chuck, I appreciate all your answers and the time you have put reading and writing the thread. I'll look for information on hum.

Just one final question. Do you think that using a oscilloscope I can trace the signal and narrow even more the place where the hum is entering in the signal?

All the best.
CE

[Chuck H]

I've never used a scope for tracking down hum. Maybe someone here has a different experience. So far we know that at least one of your hum sources is within the circuitry of or going into V4. That was pretty easy to narrow down. Pulling tubes and grounding signal along it's path is usually the easiest way to narrow down the responsible circuits. What causes hum is signal lead or components in proximity to high current sources of hum, poor grounds, ground loops, poor or failed filtering and maybe some stuff I'm not remembering just now. In your case with the V4 circuit I think it's a ground loop.that's where the study and learning apply. Since the amp is in front of you, you can track/trace where things are grounded. Learning where things should be grounded and just what constitutes a ground loop would be all the information you would need to change a ground lead or two and likely rid that amp of some of the hum. But those are things I can't do from here without looking at pictures, which is frustrating because the quality of the photo, the angle of the shot, leads that move out of the shot or under the board, etc. can make it a real PITA and my altruism only goes so far because even with that it's still hard to help effectively and it's time I'm never getting back So...

If the hum is acceptable now, box it up. Read about the topics I mentioned if you're interested in learning those things and the next time you get that amp on the bench you might even have some fun using new skills.

[caesparza]

Got the client to test the amp. The guy isn't bothered at all with the bright hum (the one that got me worried), he says it's normal for that kind of amplifiers. That's a relief.

Now, there is a low pitched hum, according to my cel phone tuner it's a 120 Hz hum. Know I'll have to hunt that thing down.

Thanks for your help everybody.

[Chuck H]

120Hz would be rectified power supply hum. Ripple noise. That could be a power filter issue or something in the signal chain in proximity to a transformer. If it occured with all preamp tubes removed it could be induction from the PT into the OT or an imbalance in the power amp failing to cancel ripple which (often) isn't sufficiently filtered out of the power tube plate circuit and relies on the push/pull circuit being in balance to cancel the hum.

[caesparza]

Removed all the MV related wires that were left connected to the MV Pot and Push-pull switch.

Fine tuned the BIAS to 70%-71% dissipation on all 4 tubes. Amplifier is much quieter now, and there is a little 60Hz hum according to the tuner, seems normal to me, but will listen again carefully today.

Thanks.

[Chuck H]

That "70% rule" doesn't really apply to TR's. Which are typically biased cooler from the factory and perform well as the clean amps they are intended to be that way. There is a lot of internet lore about SRV mods and such that remove the master circuit, disconnect the trem pot, etc. But a hotter bias on the Twin Reverb really isn't necessary. If the amp will be played cranked a lot and clipping sounds better with the hotter bias then I suppose it's alright. I haven't dealt with but two TR's on my bench, but most reports here put them around 50% to 60% for the stock circuit. I mention it because you would certainly experience even lower hum and better performance from the balance circuits and a tad more clean headroom if you bias the way Fender did when they designed the amp.

[caesparza]

Hi Chuck,

Yeah I read the same, but this guy plays with the amp cranked, initially he was asking for 80%, that didn't make any sense to me, fortunately he accepted the 70% number. I like the sound that the amp has right now. I installed a couple of Eminence 1218 Legend Speakers (by clients request) that sound really nice.

[Helmholtz]

...and a tad more clean headroom if you bias the way Fender did when they designed the amp.

Why would you expect more clean output from a colder bias setting? The influence of idle currents on B+ at full output is insignificant.

[pdf64]

Why would you expect more clean output from a colder bias setting? The influence of idle currents on B+ at full output is insignificant.

For continuous signals, yes; but it may be that lower idle current raises the idle HT a bit, which might lead to a slightly higher instantaneous, transient power output. Tricky to verify that kind of hunch without some high end test gear though.

[Chuck H]

Pete has it. The cooler bias sounds spankier and more dynamic. Further, if the idle current is entirely independent of signal drive then why should it be that a hotter bias is often employed to mitigate crossover distortion? In fact, why bias at all? Why have the tubes sitting there cookin' away if what matters is the drive voltage? (facetious) I'm no tech but certainly the idle current of the tube is affecting it's center of operation. Ergo class A is (ideally) center biased. Certainly there are things I don't know or don't understand. I always imagined it like a starting point above zero. There is less voltage rise and fall possible between the starting point and max the further the starting point is from zero.?.