SF Twin Reverb hum problems

[el_goodo]

Hello all,

Thanks for having a great forum. I recently purchased a 70's 100wt MV twin Reverb chassis (actually a Super Six, but same thing). It appeared to be mostly stock. It hummed a bit when I got it, but had original caps so I wasn't surprised. I have since replaced the filter caps in the doghouse as well as the 25uf caps on the board. I have done a lot of testing and have the following symptoms:

-Pretty loud 120hz buzz, and slightly lower 60hz hum.
-Pulling tubes has only stopped the hum when V4 or V6 is pulled, leading me to believe the problem is in the preamp section.
-The 120hz buzzs "nulls" (greatly reduces) when I put either channels volume around 4, leaving mostly 60hz hum. 120 buzz is loudest when both channels are at 0. When both channels are at 4, the hum does not reduce - only reduces when 1 channels is at 4ish. I think I head that the channels on a twin are out of phase with each other?
-Master volume turns down the both hums. The pull boost greatly increases 120 hz.
-I have swapped out every tube with no reduction. Hum balance also set to lowest hum, even with only 1 pair of output tubes.

Other notes:

-I have done reverb/trem to both channels mod, but disconnecting has no effect on hum.
-I currently have the NFB wire disconnected as I like the clean sound better. Reconnecting has no effect on hum.
-B+ on power tubes is 465v on all.
-Rectifier puts out 467v and -66v. Bias at power tubes is -53.

As you can probably tell, I know just enough to be dangerous (tho I do drain my caps, etc), but I dont really understand all that's going on in tube amps. I usually try troubleshooting by swapping things out, looking for obvious problems, etc. When that fails, I used to bring it to a tech. I would like to get better at this, so any help would be much appreciated.

Pictures posted soon.

Thanks!

Robert

[el_goodo]

[Jazz P Bass]

You replaced the caps but have you measured the Vac ripple on the power supply nodes?

[stokes]

Check that you have the caps properly grounded.The first or main filter cap in that "doghouse" is a totem poled arrangement,a common mistake inexperienced techs do is to mis-orient those first 2 caps.Check that they are installed properly.

[pdf64]

Is it 'pure' hum or buzzy?

[Jazz P Bass]

Check that you have the caps properly grounded.The first or main filter cap in that "doghouse" is a totem poled arrangement,a common mistake inexperienced techs do is to mis-orient those first 2 caps.Check that they are installed properly.

Yeah, that's a good one.
The end result is usually a cap that blows its guts.

[el_goodo]

You replaced the caps but have you measured the Vac ripple on the power supply nodes?

I dont yet have an o-scope, but I believe when I check the power supply with my DVM, that there was no AC on the DC lines. I will double check, but is this a valid way to check for ripple with the DVM?

Thanks!

[el_goodo]

Check that you have the caps properly grounded.The first or main filter cap in that "doghouse" is a totem poled arrangement,a common mistake inexperienced techs do is to mis-orient those first 2 caps.Check that they are installed properly.

I double checked when I installed, but will check again. Off the top of my head, I remember one was installed opposite polarity to the others.

[el_goodo]

Is it 'pure' hum or buzzy?

A bit of both. What I refer to as "60hz" in my post is more hum. "120hz" is more "buzzy". They both occur, but at different times and symptoms as above. I edited my post to make things more clear (I hope).

Thanks for all the responses, all.

[Jazz P Bass]

A DVM set to read Vac is fine.
The raw B+ going to the output tube plates is going to be just that. Raw (ripply).
That's o/k.
The nodes further down the line should be pretty stable.
They will not read 'zero volts ripple' .
What you do not want to see is hundreds of millivolts Vac riding on the Vdc.

[pdf64]

Depending on which model it is, there may be an output tube balance control (output tube bias) and a hum balance control (on heaters).
As the problem is occuring in the pre-amp, it can't be the output tube balance.
With buzzes, often the heaters have lost a proper ground reference, due to the hum balance control failing in some way or balancing resistors failing.
Suggest that you check that the heater voltage is swinging evenly above and below ground / that both sides of the heater have the same resistance to ground.
Pete

[überfuzz]

Did you mention anything about the hum/buzz status before you started thinker with it? The noise you describe would have me suspect a gound issue or the very caps you say you've changed already.

[el_goodo]

A DVM set to read Vac is fine.
The raw B+ going to the output tube plates is going to be just that. Raw (ripply).
That's o/k.
The nodes further down the line should be pretty stable.
They will not read 'zero volts ripple' .
What you do not want to see is hundreds of millivolts Vac riding on the Vdc.

I am seeing hundreds of Vac on the preamp tubes. V1, pin 1 has 247Vdc and 230Vac for example. Am I measuring correctly? I switch from 1000 Vdc range to 700 Vac range on my BelMerit DL21 meter.

Thanks, this is great stuff!

[el_goodo]

Depending on which model it is, there may be an output tube balance control (output tube bias) and a hum balance control (on heaters).
As the problem is occuring in the pre-amp, it can't be the output tube balance.
With buzzes, often the heaters have lost a proper ground reference, due to the hum balance control failing in some way or balancing resistors failing.
Suggest that you check that the heater voltage is swinging evenly above and below ground / that both sides of the heater have the same resistance to ground.
Pete

I believe we have a winner! One of the 100 ohm resistors to ground at the pilot light has a broken lead! I am off to rat shack. Thanks pdf! I will report back.

[JoeM]

If I'm seeing your photos right, it looks a 500uf;50v cap is in the bias circuit. I would suggest replacing that with a 100v;50uf cap.