Amp only has sound when caps are draining

[blueamberol]

Hi All,

This is my first post. I have been searching for an answer everywhere and can't find one. I figured this is the place.

I will soon be starting my first tube amp build, and I have been ramping up by trying to fix an old blackface bandmaster. I had it working, but wasn't sounding right, so I messed with a lot of stuff, and had it pretty good, and then I really goofed.

Basically, right now the amp produces no sound unless i strum really hard, and then a really harsh, farty distorted sound breaks through momentarily.

When I turn off the power switch though, and the caps are draining, It sounds great for that one second and then fades away. I was thinking that perhaps I damaged a transformer.

Any thoughts?

[pdf64]

That sounds like too much bias voltage, probably to the power tube grids.
What is the static dissipation of the power tubes?
eg how much cathode or plate current are they passing under 'no signal' condition?
The best way to check this vital metric is to fit 1 ohm resistors between the power tube cathode and ground, 1/2 watt types are fine, eg high stability types like metal film.
Pete

[passfan]

Also check your filament power and your pre amp tubes.

[Stevem]

Check pin 5 on the output tubes and set the voltage with the bias adjust pot to -48 volts if you find that the voltage you find at first is over -52 volts .

[gui_tarzan]

What exactly do you mean by "messed with a lot of stuff"?

[blueamberol]

I recapped, and changed some resistors that had gone pretty far off spec. It was working fine after that though. I think what messed it up was that I accidentally soldered the wire from the PT on the bias board to ground off of the pos. end of the capacitor instead of to the resistor. Those two points are right next to each other. Would this have damaged my power transformer?

[martin manning]

Yes, grounding that lead that would mean shorting out the bias supply winding. If you have it connected properly now, remove the power tubes and get voltage measurements to ground at pins 3, 4, and 5 on the power tube sockets.

[blueamberol]

Alright we're getting somewhere!

Pin 3 = 325v

Pin 4 = 324v

Pin 5 range = -113v to -64v

Since my bias setting should be -48v, I'm guessing that that range is saying something.

[ampgeek]

Yep....way overbiased. That will do it as Pete points out.
What do you have for an AC voltage off of the PT bias winding (e.g., before the "backwards" diode)?
Good luck,
Dave O.

[blueamberol]

sorry, that was tubes in.

tubes out:

pin3 = 335

pin4 = 333

pin5 = -113 to -64

[blueamberol]

[quote="ampgeek"]Yep....way overbiased. That will do it as Pete points out.
What do you have for an AC voltage off of the PT bias winding (e.g., before the "backwards" diode)?

i see 0.0 testing AC

dc i see -67

[blueamberol]

sorry, is the bias winding the red leads or the single one that connects to the 470ohm resistor?

[blueamberol]

red leads show 215vac directly and 730 on the other side of the diodes

[martin manning]

This is AB763 circuit? The voltage at the tube sockets looks ~100V low for the plate and screen, and too much negative voltage for the bias.

Dave asks a good question re the AC voltage on the red/blue bias tap; you should have ~50VAC there (before the 470 ohm). The red PT leads should each have ~320VAC to ground before the diodes. These are loaded voltages from the Fender layout.

[blueamberol]

yes, it is the AB763.

The red leads are showing 210vac now

the red/blue bias tap shows -00.0 in AC test