Bassman AB165 huge hum

[rpbale]

I have an old blackface AB165 Bassman with a terrible loud hum. Totally un-usable. I think it's more of a 120hz buzz, but it could 60hz. Sounds just like when you accidentally touch you finger the the guitar cord end.

When I first received the amp turned it on it smoked the original filter cap in the dog house. Since the whole head is 100% original I replaced all 5 filter caps, the bias cap, and the 16MFD/450v cap (schematic shows 8MFD). I also replaced the 470 ohm plate resistors and the caps on pins 3 & 8 of all the preamp valves. All tubes are confirmed to be good. 6L6's are matched.

I thought I didn't have a CT on the heaters so I added the two 100ohm 1/2 watters but then realized the PT does have two grounds coming out of it. The extra balance resistors didn't seem to affect my bad hum problem. (Should I remove the resitors I put on?)

The PT has only 331AC on the mains just before the diodes. But only 325DC at the power valves. The hum is there with all the preamps valves out and any combo of power tubes in or out. The PT has measures 14 ohms to ground on the secondary mains.

Of course the scope shows a nasty AC signal pattern on the OT but at low voltage. No problem with the pattern in the preamp section.

I never heard this amp before the filters smoked. So I don't know if I caued it during the recapping. But I still triple checked all my cap orientations. By the way I am able to run an audio tone through it and it responds to all the pots. I did check all the pot and input jack grounds. But I don't think that would have caused a problem because it still hums with the preamp tubes removed.

I thought I had a diode problem because all but one measures 5M ohms from end to end. But cumulatively they all have balanced resistance.

I'm stomped. Does anyone have any ideas. I'm not really sure how to chek the OT.

Thanks

[Enzo]

The bias supply cap - was it replaced too? I see it was.

Measure the negative DC voltage on pin 5 of each power tube. is there about -45v. or more like -25 or -30? if it is low, flip your meter to AC volts. Clean DC voltage will register as zero volts on the AC scale. If you have 20 volts of AC, your bias is not filtered.

Look at the bias cap. remember the bias supply is negative, so the POSITIVE end of the cap is GROUNDED. is that the case here?

Turn the amp on but leave the standby off. Now measure DC voltage on the standby switch. One terminal will of course read zero, but what DC voltage is on the hot side? That is your unloaded B+.

[rpbale]

The bias cap was replaced and reads -41VDC. Bias pot can run it up or down fine. VAC is zero. But using that same logic, the unloaded B+ is 301VDC, but the 125 VAC. That tells me it's not clean DC on B+.

One thing I forgot to mention. When I originally installed the big main 20MFD 450 caps in series I hooked one up backwards. It blew the 2 amp fuse a couple of times before I found it. Could that have fried the cap?

[pdf64]

Yes, that cap is dead, or at best walking wounded. It might well be causing all this hum. It needs replacing.
You're lucky it didn't explode.
I think you should confirm that all the rectifier diodes for the B+ are good. Not sure of what your test was
'all but one measures 5M ohms from end to end. But cumulatively they all have balanced resistance'
Can you explain? But you will need to take them out of circuit to check properly. Might be best just to replace them, given them stress from a reversed cap.

'unloaded B+ is 301VDC, but the 125 VAC'
Is there a reservoir cap in circuit when you make that measurement, or is it the other side of the standby switch? If there's no cap to smooth things out, you can expect a lot of Vac, with all the ripple.
Peter.

[Steve Conner]

Did you say that it hummed even with all the power tubes out?

[rpbale]

I should have suspected the reversed cap but I was thinking that the fuse took all the hit. I will pull it.

Here's the diode testing that I did. By cumulative I mean I'm measuring multiple diodes in series and am comparing one series to the other series and they match in total ohms. But individually one of the diodes had much less than 5M. I may have not measured it carefully.

| 9MΩ |
| 5M | 5M | 5M |
|--Di--|--Di--|--Di--|

Once I pull the diodes what is the proper way to test them?

I did say that the hum was there when the power tubes were out. I haven't studied the schematic to see what would cause that.

[rpbale]

Diode bench test

Actually, when the diode is on the bench I don know how to test it since my meter supports diode testing.

[rpbale]

120Hz vs 60 Hz

Because the hum sounds like a 120Hz hum does that indicate that the problem is post the diodes?

[guitician]

120hz could leak through the OT w/o the 6L6's. There is a path to ground through the center tap w/o tubes. Change that reversed polarity(almost shorted), cap and see if things improve.

Shorted diodes will beep the meter continuously, good ones beep once, and show a .6-.7 volt drop.

[guitician]

Are your series input filter caps 20uf? My schematic shows 2) 70uf 350v in series, paralleled by 2) 220k 1w resistors. That's a lot smaller than stock.

[rpbale]

I miswrote it. Here's what I originally replaced.

30mfd/500V (3)
100mdf/350V (2) for series input caps
100mf/100V (1) Bias Cap
22mfd/40V (5) Cathode Bypass Caps
470 ohm/1W (2) to replace screen resistors

I did not replace the 16mfd on the far right side in the dog house. A new one just arrived in the mail. But of course not another 100mfd/350v

I also through in a 3 cord plug and the heater balance resistores.

[guitician]

So, your getting 125vac across these two 100uf's in series? W/o tubes installed. And, was the 100uf one that was reversed? An "un-loaded" B+ of 310vdc sounds low, it should be over 400vdc.

[rpbale]

Yes it should be 425V. It was one of the 100mfd's that were reversed.

Could the one screwed up cap be pulling down the B+ with the tube in our out? The 120VAC seems to indicate that the cap is bad too. right?

[guitician]

A shorted 100uf cap will short the 220K resistor too resulting in more load. Also, something bad may have happened to the other as a result of reversing one. You will have to test them both.

[rpbale]

It will take me a few hours before I can back to it. I hope you're online this evening.

Question for you...what is the 8mfd/450V cap for in the first stage of the preamp? All the other filter caps are 20/525V mfd in the schematic. The 8mfd was actually an original 16mfd in my chassis.