Blackfaced SF Twin Reverb issues

[drfunkphd]

Hi All,

I just converted a SF Twin to the BF circuit...a controversial move, I know...and did my initial power up with my light bulb current limiter with no issues. Powered it up without the limiter...no issues.

Next, I popped my tubes in and there was a nasty hum with all controls zero'd out and I noticed serious redplating. Obviously, I turned it off in a hurry.

I tried pulling the tubes and the hum disappeared. On measuring my screen voltages, they were 440 volts (pin 4) on all the tubes and my bias voltage -59V on all tubes. (With the tubes all removed.)

Anyone have any ideas? Is my screen voltage too low without tubes in? I know it should be closer to 460V according to the schematics, but that is with tubes in.

Any input would be appreciated.

[drfunkphd]

I am considering parasitic osscilation...the circuit matches up to the schematic, but when I decided to take this on, it was something that I had considered and I know can be a problem.

[Albert Kreuzer]

Measure the bias voltage with one power tube installed. Repeat one by one with the other tubes. Maybe you have one shorted tube that pulls the bias down.

[drfunkphd]

I am actually only using a pair of tubes as opposed to a quad and they are KT66's. I just don't want to kill a nice new pair of tubes.

I should also mention that all the power tube sockets are brand new with new 470 and 1500 resistors on them all. As well, the bias circuit is all new components - diodes, resistor and electrolytic cap on the bias board (all high quality).

[52 Bill]

Either do as Albert suggested and power up with one output tube and see if the bias dies out or use a set of old tubes until you work out the problem.

Parasitic oscillation would cause the amp to lose audio power and perhaps overheat, but I don't think that it would cause the bias voltage to die. When you rebuilt to bf specs did you replace the bias supply filter cap?

[drfunkphd]

OK, but if the tube redplates and shorts, I run the risk of taking other stuff out with it, don't I? I understand what you guys are saying and that may be the only way to solve the issue, but I don't want to cause other problems in the process.

I guess I'm in a bit of a catch-22 here, huh?

[52 Bill]

You only run the risk of damaging things if you let the amp run until it melts down.

You only need to turn on the amp long enough to see if the bias voltage remains steady or if it is pulled down by a bad tube or some other cause.

[drfunkphd]

OK, I will give it a shot and hope for the best. Thanks!

[drfunkphd]

OK, is this weird or does this happen to anyone else...I tried installing one tube and had no hum, no issues with redplating. "Sweet," says I. So I approx set the bias.

I try putting in the other tube. No problems, bias holds up fine, no redplating. Adjust the bias closer...all good.

Try turning up the volume with nothing plugged in...no problem, no hum, tubes still A-OK.

It's like the problem just went away on it's own. Why do I feel like I should be worried?

[Albert Kreuzer]

Some tubes have intermittent shorts - they only act up from time to time.
Could also be a bad solder joint anywhere in the bias supply.

[pdf64]

Re-tension the tube socket contacts - one for the grid may be loose?