Rebuilt Amp- A couple problems

[martin manning]

Yes, always connect a load before powering up with output tubes installed! There is the possibility that an oscillation will occur, and it may be ultrasonic, i.e. it may be there even if you can't hear it. In this case I strongly suspect that is what you had going on. Swapping grid wires or swapping primary leads will have the same effect with regard to flipping the the phase of the signal on the secondary, but the paths that the in-phase and out-of-phase signals take will be different, and as a result there may be more or less opportunity for undesirable coupling to occur. Your flaky bias pot may be fine too, it's just that when you bring on the current the oscillation starts and the grid voltage shifts.

[SilverFox]

How does the improper impedance affect the power section when it is much higher then the design specifies. In this case there is a good chance the OT is around 10 or 11K while the 420 V plate B+ should probably be about 6600 ohms. I know that will lead to a higher voltage but what does that mean to the design? Just had a thought- Distortion and not the good kind?

silverfox.

[Structo]

Were you powering the amp with power tubes installed and no speaker load?

[SilverFox]

I was adjusting the bias so there wasn't a signal going into the amp. But yes, I had no load connected then at some point I connected a resistive load. Anyway, the amp is set up and working now except for what may be a mismatch between the OT and output tubes.

While trying to adjust the bias with no load I couldn't get it to stabilize at a setting and I found I had a bad tube, and the primary taps had to be swapped, so all things considered, just wanted to make sure I had correctly deduced that the amp should be biased with a load on the OT.

silverfox.

[ToneMerc]

How does the improper impedance affect the power section when it is much higher then the design specifies. In this case there is a good chance the OT is around 10 or 11K while the 420 V plate B+ should probably be about 6600 ohms. I know that will lead to a higher voltage but what does that mean to the design? Just had a thought- Distortion and not the good kind?

silverfox.

For example, let's say it's 10k, 4/8,16, that would be the same as 5k, 2/4/8. Thus, treat it as a 5k OT.

TM

[SilverFox]

I decided to change out the OT and the replacement sounds much better. The bias isn't wandering all over on lower frequencies any more either. Very stable. It is also much louder but I have to wait until the AM to check that feature out. The house is asleep.

ToneMerc, sounds like you were suggesting I change taps on the previous OT to get a reflected impedance of 5K?

I've learned quite a bit on this project and for the first time I'm very satisfied with the sound of a completed project.

Thanks everyone for assistance past and present. Now I can start a preamp section...

silverfox.