Fender Showman Help- LOUD Hum

[Telebuckers]

This amp was brought to me not working at all- save for the heaters which were lighting up. It turns out someone had been working on it before and had wired the entire output section wrong. I've fixed all of that and replaced the filter caps- all other caps seem to check out fine- now it fires up but has a LOUD hum which I can't track down. It goes away when I pull the PI in V4. I'm getting some strange voltage readings particularly on pins 2&7 of V3. Here are the voltages and a link to the schematic:

Preamp-
1 2 3 6 7 8
V1- 200 0 1.49 208 0 1.61
V2- 200 0 1.58 196 0 1.60
V3- 331 -18 0 280 -18 0
V4- 184 46 75 188 46 75

Output-
1 3 4 5 6
V5- -21 335 336 -21 339
V6- -21 336 337 -21 340
V7- -21 337 337 -21 339
V8- -21 336 337 -21 339

Schematic:
http://www.schematicheaven.com/fende...a763_schem.pdf

Also IIRC- the voltage reading I was getting on the hum balance pot was somewhere around -20 instead of the -52v as per the schematic.

Any help on this issue would be greatly appreciated.

[Don Moose]

If the hum can not be controlled with the volume knob, you may have Positive feedback instead of negative, and the first thing to try is swapping the OT leads (or the runs from the PI plates to the coupling caps).



Hope this helps!

[Enzo]

The grid bias is -20 instead of -52?

There is the problem, your bias supply has no filtration. I bet if you flip your meter over to AC volts and measure that bias you'd find about 20vAC as well. Hum indeed.

You have lost the filter cap on the bias supply. Replace.

Make sure to note that as a negative supply, the POSISTIVE end of the cap goes to ground. Or caps.


And since that bias is less than half what it should be, your tubes are running extra hard, dragging your B+ down. I bet with clean -52vDC of bias, your B+ will rebound from that 330v level.

[Telebuckers]

Thanks guys. I'll try this tomorrow. I did notice that the output tubes were running extremely hot, but there was no redplating or anything. All 5 filter caps were oriented with the positive and negative ends all matching up- but aren't two of them supposed to be reversed? I'm guessing Enzo that that's what you're talking about in regards to the bias supply?

[Enzo]

How about the cap on the little board behind the pilot light up front? The bias caps are not up in the cap vault on the outside of the chassis.

[Robert M. Martinelli]

Also IIRC- the voltage reading I was getting on the hum balance pot was somewhere around -20 instead of the -52v as per the schematic.

I'm with Enzo, something is wrong with the bias section, most likely the filter cap, as he wisely says, but the above statement of yours has got me puzzled a little..... Are you sure you took those measurements at the HUM BALANCE pot?

The voltage across the hum balance pot should be about 3,15 V(AC) between each external lug and the wiper ( GND ), so I assume you were referring to the BIAS pot. |20| or more DCV at the heaters would be overkill.

Cheers

Bob

[Enzo]

Bob, Fender used the heater balance control on many amps, as you are aware. On others just a pair of resistors, but they also did the bias balance pot on many models. A lot of guys modify that balance control to a bias level control. The adjustment for that balance control is for minimum hum. It does that by balancing the tube currents across the push pull.

In the case of his linked schematic, the bias pot is not a balance contol. SO either his amp has been wired that way contrary to the drawing, or he is mistakenly calling the bias pot a hum balance pot. Either way there is an adjustment pot on the chassis with the bias supply on it.

[Robert M. Martinelli]

.....or he is mistakenly calling the bias pot a hum balance pot.

That's what I thought, having the bias voltage on an "heater" hum balance ( wired between the two legs of the heaters' 6.3 VAC supply and GND ) pot seemed to me weird to say the least....but, then again, as you say, the pot he was taking the measurements on could also be wired as a "bias balance" pot...

Anyway....as you said,the goal is to bring the bias voltage back where it belongs and to eliminate hum, and I bet you' re right on the target about the bias filter cap....

Cheers

Bob

[Telebuckers]

Hey thanks a million guys- you guys nailed it. Replaced the leaky bias cap and it's working perfectly now and all the voltages are back to normal.