Rectifier tube arcing.... fuse blows

[Garydean]

I did replace the cathode bias cap. Is the cathode resistor the 250ohm 10w wirewound resistor that goes to ground?

250ohm 10 w

sorry

[Bruce / Mission Amps]

I did replace the cathode bias cap. Is the cathode resistor the 250ohm 10w wirewound resistor that goes to ground?

250ohm 10 w

sorry

OK... I didn't read this the first around so if you have the cathode biased version the first thing you need to do is remember that the idle current is determined by using the voltage from plates to cathode not to ground.
Assuming you know how to measure all this with that in mind, if that is still 400vdc, then I would replace the 250 ohm cathode resistor with a 330 ohm 10 watter.
I have run the Chinese TAD 6L6 tubes at around 48ma-54ma in my Tweed 6L6 amps with fixed bias and plate voltages of around 400vdc-410vdc.
They seemed to be OK with it.

[Garydean]

I just measured the voltage from plate to cathode and it is 355vdc. The line was at 120vac drawing 1.25amps. Does all this sound OK? The tubes look fine. I am going to suggest new filter caps.

Side note on the rectifier arching. I read that these amps tend to blow fuses on a warm restart, which is what happened earlier. Correcting this problem involves installing a 10w resistor off the cathode of the rectifier, to the rest of the amp. I forgot the value but it was low.

comments welcome!!

[52 Bill]

Did you check the value of the 250 ohm 10 watt cathode resistor? They will drift.

The rectifier arcing was a bad tube, does it blow fuses when you warm restart it now?

[Garydean]

Hi Bill, Yes that resistor checks out fine. I did another warm restart with the NOS rect tube in, and everything seems stable, fingers crossed. I played thru the amp again this morning for about a half hour at all volumes and everything is fine so far. Plates are fine and the amp breaks-up when it should. The plate voltages on the output tubes are about 10% lower than what's shown on the schematic. That I can live with.

With a self biasing amp, how reliable is that design, with respect to the tube setting/finding it's own bias point, once it gets cranking? Am I understanding this correctly?

Thanks again,

Gary

[52 Bill]

Very reliable, but it allows for less power output, which I think was the main reason for switching over to fixed bias designs.

[stokes]

Now that it seems you have the fuse blowing sorted out,I'd like to offer a suggestion.I recently built a bass amp for someone and used the B-12 and B-15 as a base.I put a switchable cathode/fixed bias set-up in it.I found the cathode bias mode broke up too quick,even the fixed bias mode was not to my liking,so I upped the filtering and it helped a lot.Unless you are concerned with keeping the amp stock for collector value,using 100uf on the main and screen taps really tightened things up a lot.That 10% lower plate voltage could be due to old caps as well.