Low Voltages on 78 Twin Reverb 133 (UL)

[nickb]

I think you should have mentioned the limiter in the first post

No, popping is not normal. My first suspect would be one of the new tubes. Pull /substitute one at time to isolate.

[caesparza]

Replaced the old ones with the ones that will stay on the amp, no more popping sounds but a really loud hum coming from the amp.

BIAS Readings:

Plate voltage: 484 VDC

Voltage drop between secondary main and plates: (V7)1.14 (V8)1.15 (V9)1.23 (V10)1.21
Resistance between secondary main and plates: (V7)37.2 (V8)37.3 (V9)35.5 (V10)35.6
Plate current: (V7)30.64mA (V8)30.83mA (V9)34.64mA (V10)33.98mA
Dissipation: (V7)14.83W (V8)14.92W (V9)16.76W (V10)16.45W
Grid voltage: (V7-V8)-51.5 (V9-V10)-55.8

Dissipation went down a lot 49%-55%. Could that be the reason for the hum? I'll make the BIAS mod and then come back with the results, need to source some parts.

I noticed some heater wires are not going to the corresponding pins on successive tubes, that could be contributing to the hum.

Thanks for your help everybody.

[Chuck H]

I'm having trouble interpreting this post. Are you saying that you are now using some combination of the old power tubes and the new power tubes? This could certainly be the source of the hum. If the hum wasn't there before then don't trouble with the filament wiring yet. I think you may have a bad or dead tube from the old set in there and it's causing an imbalance that isn't cancelling hum in the output section.

[caesparza]

Hi Chuck, I'm using a fresh set of power tubes. All 6L6 are new. It's a Bugera matched quad provided by the owner of the amp. I checked preamp tubes with an emission tester (BK 620) and they read good.

Double checked the filament wiring and it's correct. Reverb is not working.

[caesparza]

I have a doubt with all this stuff.

When is suitable to use a current limiter? I mean, if the limiter affects the functionality of the amp, seems like a useless thing to add. I understand how it protects other components in case of a short but I though I could work with the limiter and expect some variation on the amp's tone not affecting how it works.

[Chuck H]

Did the reverb work before? You haven't mentioned it yet. Coincidences happen, but not often. Usually new symptoms are analogous. You replaced tubes, the popping is gone but now there is hum. If the reverb worked before then you also lost the reverb and got hum with the tube swap. Tube testers do not test for hum and, in fact, don't typically test near the stresses that guitar amps put on the tubes. They can only only really tell you if a tube is grossly bad but not if it's suitable for use in a guitar amp.

[Enzo]

A light bulb limiter is a tool for troubleshooting, for finding problems in the circuit. It is NOT intended as something you use during operation. A limiter is something you use in your shop on the bench, it is not something you add to your rig on stage.

[caesparza]

Sorry, tank cables connected wrong. Reverb is working fine. I think I should get the tubes to the proper working dissipation and come back to you with the results, chances are that the hum is reduced.

[caesparza]

Ok set the new tubes to dissipate 21.31W on V7-V8 and 21.98 on V9-V10. around 70%. I persuaded the owner to lower to dissipation to that percentage.

The amplifier sounds good. If you are not playing you can hear the hum, but you can perceive it while playing the guitar. Should the amp be dead quiet with no input?

[g1]

I like the new bias numbers better than the old ones. Fender are typically biased quite cold, so your new tubes at 50 to 55% are probably closer to the stock values than the hotter idle current numbers you were getting from the old ones (65%)
Adjust the 'matching' (bias balance) control to minimum hum. Does that get rid of it?
If the owner wants 80% idle dissipation, he has read too much internet nonsense. Make sure he at least knows that Fender never ever biased anything that hot!

[caesparza]

g1, the hum bias balance doesn't get rid of it. Could that be something related to grounding issues on the building?

I've got a 77 100W Twin Reverb here, I'll check against it to see how different they are.

[g1]

Sorry, I had the impression that the hum came in with the new power tubes. Is the hum there with the old power tubes?
There are 2 trim pots that will affect the hum, one is 'output tubes matching' and the other is 'hum balance'. Did you adjust both?

[caesparza]

I put the original power tubes and the output shows the hum among other noises, I guess that is because of the new BIAS voltage and the some of the old tubes been in poor shape.

I made a couple of additional changes requested by the owner. First I thought he wanted the overdrive circuit on the MV removed, but then he said he wanted the whole MV removed. No MV now. Removed V1 since the guy is going to be using the Vibrato channel only. The amp is really powerful.

I adjusted both the hum control and the bias balance to get the lower hum possible, still present, I'll have to try to place the amp on another room to check this, I sometimes have doubts about the grounds and wall connections of the place where I live.

I made the chopstick test to check for cold solders or issues on the board and found something weird, when I tap over the cable that goes to pin 7 (grid) of V4 (12AX7) from the junction of the 3 resistors (470K, 470K, 3.3M) and the cap (10pf), I can hear it on the speaker, I checked a couple of tubes on that socket and both were microphonic. Is it possible for the socket to be bad?

[Chuck H]

Try tapping the grid leads on the input tube. Does it make an audible similar to the mixing stage grid? I bet it does, but I'm honestly asking.

Something to know about removing V1 is that it changes the bias of V2b. Since V1b and V2b share a cathode bias circuit pulling V1 halves the current through that circuit and lowers the voltage drop. Less voltage drop means a hotter bias for V2b. This could be an intentional consequence on the owners part. It's part of what some consider to be SRV mods for Fender amps. The tube will operate fine like this but I thought you should be aware that performance will be different than it would be with V1 in it's socket.

It's also possible that any additional hum is a consequence of removing the master volume circuit, sort of. You're basically hearing more of whatever hum was in the preamp and reverb circuits that the MV may have been attenuating. I don't know how much hum there is, it may be a typical amount. IMHE all Fender reverbs hum a little so do make sure the reverb is turned down and perhaps expect more hum when the reverb is up. Also, did you replace the bias circuit electrolytics when you did the cap job? They should be replaced whenever a cap job is done "just because". Also, also... Too many new preamp tubes are noisy right out of the box anymore. When I buy preamp tubes for a build or repair I buy extra so I can cull the noisy ones, which happens more often than not. In my own work I think that at least two tubes out of five is too noisy to be in the first preamp position and often one is too noisy to use anywhere in the signal chain. So you might want to do a little signal grounding to determine if the hum is coming from the preamp or the power amp. If it's coming from the preamp it can't hurt to take that V1 tube you pulled and swap it, one at a time with each other preamp tube.

[caesparza]

Well put back V1, checked, V1 and V2 are microphonic too.

I guess I should replace the preamp tubes, IŽll put the ones on the 77 Twin just to check.

Chuck could you explain this to me: "So you might want to do a little signal grounding to determine if the hum is coming from the preamp or the power amp."?

Thanks for your help.