Is my OT BAD???

[martin manning]

If you tried to play it very long with the OT ungrounded, there was no load on it and that could have damaged it. With the PI ungrounded, there couldn't have been much signal, though.

You should get a very measurable resistance across the primary. If you are getting zero Ohms, that is not good.

MPM

[Firestorm]

As Wayne said, definitely get the bias set right first before you start killing tubes. As to the 0 ohms readings you were getting on the primary, be sure your meter is set at the lowest end of the resistance settings (100 ohms or 400 ohms, whatever your meter has). The winding resistance should probably be 100 ohms or lower so if the meter is set too high (unless it's automatic) you might not get a reading. It is true that running the amp without the common wire connected makes it an open load, but to fry the tranny to the point where you really get zero resistance should have produced mega smoke, which you didn't report, so maybe it's not dead. (Of course, it could have been bad to start with.)

You said you pulled the presence control out and grounded the PI through a 47K -- that is awfully big for a tail resistor and could kill your headroom. Traditional values there are 10K or 15K. The other place to look for "fartiness" would be the coupling caps from the PI to the EL34 grids. Marshall always used 22ns; 100n is on the big side.

When you get it going, you may find that the EL34s misbehave a bit without feedback, so you can put your presence control back in and tap the 8 ohm tap or the 16 ohm tap on the OT and try varying the feedback resistor. With a 4K7 shunt resistor, you could try 47K or 100 K feedback. Or even make it switchable between the two.

But again, get the bias set right first.

[skyyamps]

I am having a bit of confusion on biasing with the cathode resistors. I know typically you run a 1 ohm between PIN 8 and GROUND and measure the voltage across and what not. This design used a 10 ohm cathode resistor already, so can I still use that in place of the 1 ohm, and then change my calculations accordingly?

**EDIT**

I got the biasing problem worked out. I got the tubes running much cooler now. I'm still having problems however with the actual signal. It's running at way to low of a volume. If my playing is low volume, it plays normally, but if I play hard, it makes a horrid popping noise. I actually have output, but it's just bad output.

Any ideas?

[skyyamps]

I just started a new thread. Here's the link

https://tubeamparchive.com/viewtopic.php?t=9122