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DIY tube amp squeals when turned up

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I just built a Real McTube preamp for my guitar:

and it worked great! It's my first tube project (I'm a real tube newbie), so I'm very happy.

Never able to leave good enough alone though, I decided to modify it, substituting components based on a Tweed Fender Princeton 5F2A schematic:

My changes can be summarized as follows:
- replaced C5 (input coupling cap) with 68k grid resistor
- replaced R5 and C7 (1st stage cathode resistor/bypass cap) with a 1k5 resistor and 22uf cap
- replaced R7 and C8 (2nd stage cathode resistor/bypass cap) with a 1k5 resistor
- replaced R4 and R6 (1st and 2nd stage plate resistors) with 120k resistors (ran out of 100k's)
- replaced RV1 (gain control) with the Princeton's gain/tone stack (2 1M pots, and 2 caps)

Everything else remains the same - the only other difference is that I'm running it with 300ma transformers, so I get about 110V at the plates (vs. the 140 of the original McTube design, or 150 of the original Princeton).

Everything sounds great until I turn the gain and/or the tone control past 9 -- then it starts squealing! With the gain up to about 8.5, it sounds clean and sparkly, between 8.5-9, it starts to get a dirty, and at 9, it immediately (and very abruptly) gets really distorted and starts squealing uncontrollably.

I did some research and came up with the following theories:
- grid resistor isn't close enough to the tube socket?
- plate resistor value is off? (try 100k's)
- not enough voltage - use larger transformers or modify component values (need larger plate resistors?)

Any ideas on what might be wrong, or suggestions on tests I could try?

Neither circuit shows grid stopper resistors on the grid of the 12AX7 connected to the wiper of the gain control. The thing is therefore most likely oscillating. Add a resistor in between the wiper and 12AX7 grid.
This resistor will work with the Miller capacitance of the 12AX7 input to form a low pass filter which will cure the oscillation. You can also trim the resistor value to get the sonics (high frequency response you want). Try 47K and adjust down (say 22K or 10K) f not enough highs, adjust up (say 100K) if its too bright. The body of the resistor wants to be directly on the grid pin of the tube socket, i.e. the resistor lead on that side to be VERY short. Make sure that you have that 68K resistor as a grid stopper on the input 12AX7 stage too. Again body of resistor as close to the grid pin as possible.
"no grid stoppers"

"therefore it's oscillating"

Gee, shoulda told that to all the RF engineers of yore. Coulda saved a lot of time :D :D

I'm betting you have too many capacitors in the circuit and too much gain. That's a good way to make a phase shift oscillator. The final cap of course probably *wasn't* intentional, but the gain allows it. Try seperating input and output leads, shielding them if necessary.

FYI, in case anyone is interested, I wanted to close the thread on this topic:

I added a 22k grid stopper on the second stage (directly on the tube socket), and this stopped most of the squealing (still had some when the volume was at 100%). After moving the grid stopper on the first stage to the tube socket, all of the squeal disappeared.

It might be psychological, but i think I did lose a little high end after adding the grid stopper on the second stage. Still sounds plenty good and bright, but I might try switching to a 10k resistor some time.

I'm interested in squeezing out a little more gain, if it's possible without affecting tone -- maybe the simplest is to switch from the 12AT7 I'm currently using to a 12AX7? If anyone has suggestions, I'd love to hear them.

Thanks again for your help -- you helped me solve the problem, and learn about the weird and mysterious world of theremonic emission!
I Built a McTube last fall as a warm up project. I nicknamed it SQUEAKY as it was a squealing monster. I had it feeding a 6AQ5 SE amp that I had scratch built. Interestingly, I did the same grid stopper fix, Made it better but not perfect. When I ran the 12AX7 in the unit it was vicious distortion wise, I followed Fred's advice and altered the caps:
. Remove C7 - you'll have less gain, but see how much cleaner the tube sounds? Short the cathode directly to ground - see how much dirtier the sound gets as the grid is driven into conduction?
I reduced the Cap in first stage to (C7) 25UF, (C8) to 50UF
I Also changed C5 & C6 to .1uf. RV1 to 1 meg pot

I was able to run the 12AX7 without excessive squealing but it was still very close to the edge.

Paul Marossy Had a pretty nice construction article with mods on this unit at his Site
DIY Guitar In The DIY Effects section.
Sadly he reports that Fred Nachbaur, has recently passed away.

Good Luck!
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