Blackface Deluxe (non-verb) problem

[dhodgeh]

Got a '65 Blackface Deluxe that I've had while. Was supposedly re-capped prior to me buying it.

It's doesn't get played much, but I decided to get it out and play it for a while. Seemed to be working fine, then volume cut in half. Both channels affected.

A new set of power tubes was suggested on another site. Went with Tung-Sol reissues (not wanting to spend big bucks with an outstanding problem). Put them in and attempted to bias using an Amp-Head bias tester. Readings where way high, around 150ma and the tube were red plating.

Let me say here that I have built a several pieces of tube based equipment; a 5E3 clone and a 6G15 reverb clone, plus a couple of tube based stereo amps. Guess I'm good at following instructions because everything I have assembled so far has worked from the get go and I have not had to dig into troubleshooting until now. I'm also aware of the dangers of poking around in these things and do take the necessary precautions.

Will also say that the only test equipment I have is my trusty Fluke DMM.

What I have tested and know so far is as follows:

AC from the transformer to the rectifier is good. Heater voltage is good. Measuring the voltage at the positive side of the first filter cap I get 470 in standby. But when I flip the standby switch, the voltage drops to 290-300 volts. I also see much lower voltages (from what the schematic says it should be) at positive side of the remaining three filter caps.

It was getting late and I stopped here.

What should I be checking next? I would love to take the amp in, but, alas there is no one within 100 miles that I can take it to.

Thanks in advance for any and all suggestions.

Don

[pdf64]

With the amp power on but in standby / non-operational mode, take voltage measurements of the bias supply, from the bias cap to the power tube grids terminal 5.
Don't leave the power tubes drawing massive overcurrent whilst you take voltage measurements - switch if off, eg to standby, asap!
The VB+ is sagging so much because of the overcurrent issue.
Pete

[g1]

The term "recap" to many means just the main filter caps. Bias filter caps should be done at the same time but often are not. They could be causing your issue. Also leaky coupling caps from the PI.
Get your proper voltage at pin5 of power tubes with power tubes removed, before you run it with power tubes again (or keep in standby like pdf64 said).

[dhodgeh]

OK, think I found the problem. Thanks for pointing me in the right direction.

I was getting nothing from the bias cap. Check the voltage from the PT to the bias circuit, which seemed OK, checked the resistor which was also good. Went to check the output of the diode and discovered that the negative lead of the diode was broken.

I was able to solder the lead back in place, and whoa-la, the other voltages fell into place. Set the bias, and all seems good now.

One final question, just for future reference. If I ever needed to replace that diode, what should I replace it with?

Thanks again for the assistance. Happy camper now.

Don

[The Dude]

I use 1N4007's, but I buy them in bulk so I always have plenty around the shop.

[Enzo]

Same here.

ANy basic 1A diode is plenty, the bias supply has to provide almost no current.

We tend to stock the 1N4007 because it is the highest voltage in the series. But if you had a lower voltage one like a 1N4003 or something, that would be fine too.