Blackfaced Fender Twin Reverb Master Vol buzzing

[kdawg]

Hello - I just completed blackfacing a Silverface Fender Twin Reverb 100W Master w/JBLs. It sounds tonally very good, but has bad buzz+hum on both channels only with guitar plugged in.

What's been done:
* Removed Master Volume from circuit, including its "extra" parts and push/pull. Circuit is now wired exactly like AB763
* Replaced all filter caps + all electrolytics, changed power resistors between caps to 1k / 4.7k. Replaced coupling caps.
* Removed Hum Balance Pot and wired DC 40v circuit to heater wires
* Bias circuit wired as AB763
* Tried playing with lead dress, no luck.
* Replaced all power tube grid 470 ohm resistors
* Replaced all 100k carbon resistors with carbon comp
* I've swapped tubes around, 12AT7s are new but 12AX7s are not.

Symptoms:
* No guitar plugged in = no hum/buzz other than normal noise floor as I increase volume on either channel
* Guitar plugged in = both channels buzz increase with volume. It sounds like I've stuck the guitar pickup close to a transformer... doesn't matter what position guitar is in. If I turn down volume on guitar, all buzz goes away. Doesn't matter what pickups and I've tried two cables and two different guitars.
* Same buzz occurs with just a cable plugged in and no guitar.
* Vibrato is off, knobs don't affect anything.
* Reverb is working

The only thing I haven't tried yet is there are two tubes that have their Cathode Bypass Caps grounded to Reverb jacks - I know on blackface twins those are mounted on the board and grounded commonly there... could that possibly an issue? There really isn't much room to move those on board so I've been avoiding it.

Any other suggestions? I want to be able to record this thing! Thanks

-kdawg

[Alex R]

* Removed Hum Balance Pot and wired DC 40v circuit to heater wires


-kdawg

This is one likely culprit. What is your 40v DC source? Are your heaters connected directly to it or referenced through resistors? (to put it another way, with what did you replace the ground reference/artificial centre tap provided by the hum balance pot?).

[Chuck H]

This is one likely culprit. What is your 40v DC source? Are your heaters connected directly to it or referenced through resistors? (to put it another way, with what did you replace the ground reference/artificial centre tap provided by the hum balance pot?).

I don't think this is the culprit. The amp is quiet unless a guitar is plugged in.

The likely culprit is that without the master volume the amp is louder at your otherwise usual preamp settings. If the amp is louder any noise from your guitar pickups will also be louder. If the noise is REALLY excessive check the input jack wiring for ground faults. It could be that the jack is grounding the input properly with no guitar plugged in but the ground is lifted when a jack is inserted. Have you tried a different cable? Never rule out the possible coincidence of something else in the chain being the problem even though you just worked on the amp.

[Prairie Dawg]

If nothing else this points to the advisability of doing major reconstructions in increments and testing as you go. I'd probably start backing down the mod chain from last to first in that order and test it on the bench at each stage. That is assuming, of course, that you did not do this work in stages.

[Ronsonic]

If I turn down volume on guitar, all buzz goes away.

This means it ain't the amp.

Okay, not unless you somehow wired the input jack backward, then there is some possibility it's the amp.

Otherwise, it ain't the amp.

[Alex R]

The input jack and the volume control on the guitar essentially silence the amp by grounding the grid of the first stage. If the heaters are hummy that stage will provide most of the noise. So I still would like to know whether the heater reference is balanced after the mod.

[kdawg]

I know gain increased with master out but I'm not even at 3 on the channel volume and the noise is loud. Grounding has changed internally because master wiring is removed, I had worse hum before I removed hum pot and replaced with dc circuit with slight improvement.

Here is hum circuit:

[Alex R]

OK. I don't much like that heater circuit, principally because you almost certainly don't need it. If you want to go with me on this and remove the possibility of stage 1 grid hum from proceedings you might lift everything else from the junction of those two 100ohm resistors and clip the joint to ground instead. If the hum goes away solder that in, or refit a hum balance pot.

If that doesn't work then I agree with everyone else that there ain't much else it could be inside the amp.

[kdawg]

I tried grounding the 100 ohms instead, definitely a big improvement! I think I can get away with that noise floor. I'll stick with the 100 ohms instead of the balance pot because the pot there seems to need replacing from years of not moving.

Any other tips for helping to reduce general buzz? Would that v4 25/25 + 820k resistor grounding to main board instead of reverb jack help at all? I will get a scope at each stage to see where the most noise is getting amped, but I'm pretty sure it's at v4 or v6 since it's common to both channels.

Thanks
-kdawg

[Alex R]

Good, I'm glad that worked. You only need a little bit of hum on stage one grid and it will get amplified through the rest of the amp.

I doubt changing that grounding would make a difference. Two things to try:

1. It seems you didn't rewire the s/f bias balance pot as some like to (good! imo you only lose a useful function that way). See how much hum you can dial out with that.
2. fit a 220R hum balance pot and do some more dialling-out.

[Ronsonic]

It is a Fender Twin, if it is noisy it is broke and in need of repair. Find and FIX what is broke or out of spec.

This is probably one of the most thoroughly engineered amps of all time. Don't turn it into a science fair project. If the goal is to produce a low-noise, loud, versatile amp well that's the way it came out of the factory. Modifications are not likely to help. FIX what is wrong, broke and/or reverse any ill-considered mods.

Do not try to mod your way out of a repair. No R&D, weird science or voodoo is needed to get a Twin to sound good and run quietly. If that isn't the case then the amp still needs repair.

One area to go over, Fender got sloppy with the lead dress on the SFs, I don't know how well the "blackface" mods were implemented. Time spent comparing what's in the chassis to an AB765 diagram might be well invested. It is more plausible that the guy doing the mods screwed something up than finding fault in the original design.