Hey, I know the ax84 forum is a more appropriate place to ask about this, but I feel like there are more knowledgeable people here, and more intelligent advice/comments, etc.
So I built the firefly, and it worked the first time I fired it up! It was my first project that did so the first try (out of 20-25 pedals and other amps), so needless to say, I was pumped. After rocking out on the non-boost setting, I flipped the switch to cascade. It works too, but when the GAIN is set to about 50% and the BOOST GAIN is anywhere past 15%, it squeals like crazy. I want to max out and jam my pants off, but I can only turn the GAIN up high with the BOOST GAIN low, or the BOOST GAIN high with the GAIN low. This is very frustrating, especially after hearing Doug's latest soundclips. I WANNA ROCK!
On my AX84 P1, I had the same squealing problem only when the volume was completely maxed out, and the treble was over 60%. I ended up replacing the controls with long-shaft pots because of the old tube radio cabinet it's in, and after doing that, the problem was gone. I have no idea what I did while swapping the pots, but it was fixed.
I tried shielding lots of wires, on the input, wherever the signal wires cross, between pots, and it didn't change a bit from when I had no shielding. I believe (and hope) that this is an easy fix for someone who knows anything about tube amps (which many people here do). I know I've read about this problem, but I can't remember what caused it or how to fix it. And I was pretty sure that shielding wires would fix some of it, but if changing the pots fixed the other problem, what's the deal? I'm just rambling, it's pretty late. I'll probably read this tomorrow and wonder what I was thinking. Sorry to you and my future self. Thanks for reading.
Alex
First, Congratulations! :D
The squealing is quite possibly simply due to layout. Tube amps are very fussy about this and in fact the coupling between unconnected components is used by some of the best amp designers to create a signature tone. Generically, all I can say is that something is too close to something else. Can you post any pictures?
You can try moving wires and nudging components with a chop-stick or some other very-dry piece of wood. Once you find the place where you can affect the pitch of the squeal, you can start to address it.
Layout is a tricky game. Just be patient and you'll get it all worked out eventually.
Good luck,
-Peter
Hey Peter, thank you very much for the reply. I might be able to get some pictures up, but not for awhile. I'll try to describe the situation. I have two sections of perfboard, one for the power supply filter components and one for other components to ground. I used the Revision 3 schematic found here:
http://home.cfl.rr.com/dbhammond/Firefly_sch_R3.gif
Here are to components on the "components to ground" board:
R1,R5,C3,R8,C4,R11,R12. The rest of the components are connected between the tube sockets and the pots, no boards. The problem might be there, as some of the components ( R3,C7,C8 ) are wired directly over the tube socket, and are maybe too close to other connections or components. I have read about warnings about cathode resistors (I probably don't remember correctly) needing to be "as close as physically possible" to something (I don't remember what) to prevent noise or hum. I am obviously uninformed about a lot of the basics, could this be part of the problem?
Anyway, thanks Peter, I'll try your suggestions, and if I can't get pictures soon I'll draw a layout in Paint (I spent so much time working in there I can remember everything still). Thanks!
Alex
My guess is that it's a plate connection that gets too close to a grid. Ideally those wires should be far apart as you can make them and if they cross, ideally it should be a right angles.
Grids and cathodes are in-phase so no direct coupling problem there, but if you get the stage 1 grid wire picking up audio from the stage 2 cathode, you can have squeal.
As you may have guessed, tube amps and small layouts are somewhat mutually exclusive.
I have also had squealing problems from the speaker leads and the speaker coil itself. Just something to watch for. I was building a combo and it just wouldn't behave until I figured the direct-to-speaker feedback was happening. I added a foot in-between the amp chassis and the speaker and suddenly everything was great.
Take care,
-Peter
Make sure your output transfomer primary wiring is nowhere near any tube grids.
I would advise asking over at ax84.com. They have helped a lot of people solve problems with oscillation, which is what you are experiencing. Nine times out of ten it is some sort of problem with the layout.
Doug