New Build- Intro to Amp Design

[epis]

Hi SilverFox,
you have to move input jack far away from output transformer wires.
Best would be to the front panel.
But, to be honest, I don't like your layout at all. I would suggest first to close these rectangular openings in chassis, find a nice scheme and verified layout and do a new build from scratch.
Good luck with your project.

[SilverFox]

Martin- That was the problem.

Tictac- There is a switch in the Boost circuit and that is what threw me off. The amp did oscillate as soon as it came on but pulling out the Boost switch stopped it by grounding the FB signal so it threw me off.

Epis- I'm never opposed to constructive criticism. Thanks for your observations. Remember this is only the power amp portion so the preamp issues with noise aren't there. I will have to reroute the wires or perhaps use some shielded cable to cut the slight amount of hum I'm picking up due to the crazy routing of the MV and Boost SW.

Regarding the other issues with the layout- I put as many parts on the tube sockets as possible and in two cases didn't have the correct value resistors so had to concoct the values which led to some hacking.

Other than that this amp reproduces the original signal very faithfully and takes pedals very well. The tubes soften up the signal quite nicely and it is very loud. Still haven't turned it all the way up.

Thanks for all the assistance from the Forum. I will have some other questions later as this is still an in process build, but I'm going to play through it for a while.

Regards to all,

silverfox.

[epis]

Hi Silverfox, I'm glad you didn't get me wrong.Simply when you mentioned adding 3 or 4 preamp tubes later in your project, I had to warn you.
Without nice clean layout it would be impossible to have it running without hum or oscillation problems. Cheers, Damir

[SilverFox]

I got around to taking some scope shots of the input vs. output and have a couple questions, see attached.

The input is 1khz, 2 volts, volume max, Boost (no feedback). The output is taken at the grid of the power tubes and scaled to 20 volts/cm.

The bias is currently set for -37 volts

Another point that comes to mind is: The power amp was originally designed for 6550 tubes as I recall and an UL transformer. Perhaps the grid resistors are not correct here.

I think my problem is the input without FB is overpowering the phase inverter, (JMFahey from another post). This power amp was taken from the sunn Sceptre design and I only added a 1 Meg pot as a volume control.

Questions: The output seems to have dotted line construction- Why is that?

Is this working properly given that the input is likely over-driving the phase inverter?

Even though I've got the output connected to a dummy load I can hear the 1k hz tone passing through the, output tranny?

Otherwise the amp is working great as long as I don't max the volume. Then the square wave blanks the speakers. With FB on there is no problem and even the slight hum noise is cancelled.

Thanks for any assistance on this project,

silverfox

[Jana]

Decrease the time on your scope so we only see about 2 complete cycles at most.

The "dotted" line might indicate a high frequency oscillation going on.