Power amp woes

[ioginy]

I started a thread about another subject but ended up here so I figured it would be a good idea to break the old one off and start one a little more direct.



This is the power amp that i just built. It seems to me that my voltages are all a little high, but i was hoping on getting some in put.

Also, it seems like the driver tube is causing some major issues. With the 12ax7 out, the amp is almost silent. With it in, there is a solid hum. Also, when running a signal through the amp (I ran both a 1000hz signal as well as some music) it is very distorted, not very loud and completely cuts out when I have the volume all the way up.

Ideas?

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I got to thinking and swapped out the 100k plate resistor on V1a to a 220K. I played my ipod through it and it still was a distorted mess, however as I turned the input volume down the signal got quieter and cleaner until it went away, but as soon as I bottomed the pot to what should be all the way off, the signal came back. It was quiet, but loud enough so i could hear it standing outside the room, and clear of distortion for the most part. I am thinking of pulling the whole gain stage out and starting again from scratch. I think the buffered line out may be causing a bit of this mess, but I'm not sure how or why.
Perhaps this weekend I'll do a revamp, unless anyone has some ideas.

[Geek]

Guitar amps need guitars to test them

This looks straightforward and nice bias setup.

What I would do is make you 0.1 coupling caps absolutely no larger than 0.022 and preferably 0.01uF. It's cutting out because of a phenomenon known as blocking.

Since you have no tone stack (why is that?), I would also add another volume control on the PI input... it'll give you "Pre" and "Post" control.

As for the hum, you need to lift the DC of the filament. Float the filament supply and add something like this:
http://geek.scorpiorising.ca/images/Figure-1.png

That should cure your hum!

[ioginy]

It's true, a guitar is always a better choice, but I always get distracted when I grab my guitar for testing purposes. I'll end up heading back to the music room and playing for an hour instead of working on my project

I will try swapping out the coupling caps this evening, but I was wondering if I should do all of them, or just the ones after the PI. I was also wondering if the .1 and .01 coupling caps splitting the line out tap is a good idea. I got it from a guilt thunderbass line out design, but it seems a little odd to me. Why would they have set it up that way?

There's no tone stack because I am intending it to be a power amp only. I built a 9 band eq (based on the blonder tongue audio baton) that I want to use in front of it. Then build a single pentode stand alone preamp in front of that. Sort of a rack system that I can swap pieces in and out of as I build them.

A master volume is something I was also considering last night. I think I'll pull out the input volume control and just stick with the master.

Out of curiosity, would those .1 coupling caps also be causing the volume to be very low, on top of cutting the signal?

Thanks for the link for the DC lift. I will also throw that in tonight.