Super Squirrel Monkey - First from-scratch tube amp build, first guitar amp build

ericj

Member
2008-12-08 10:24 pm
so i've been working on a tube practice amp for my 7 year old niece who bought her first strat a few months ago. She's been using her dad's practice amp, which is ok but not special. She's a pretty neat kid (what kid saves up their pennies and buys musical instruments?) so i figured she needed a special amp.

I don't play. I picked up a First Act for $15 at a thrift store so i can test it.

Anyway, it seems to work but i am getting some screechiness if i turn up the volume. Some kind of bad feedback?

The tubes haven't had enough time for the getter to get rid of whatever gasses may have seeped in over the last 40 years, and I need to check some voltages. Also, the output transformer I am using was intended for a 50c5, so the load on the output pentode might be too great (2.5k impedance, I think the original squirrel monkey design landed on 3.5k as the ideal).

Oh, video here:

https://www.youtube.com/watch?v=NgLUP_aXQY4&feature=youtu.be
 

Thomasha

Member
2012-06-13 8:32 pm
I read at the hundred buck challenge that sometimes these compactrons have the high mu triode near the pentode and that could create feedback, but there scott, from the 100buck thread tamed this kind of feedback and removed the squeal. The schematic is buried in the 100buck thread, you just need to scroll down>
http://www.diyaudio.com/forums/instruments-amps/190738-hundred-buck-amp-challenge-159.html

As for the screechiness, i had same thing with subminiature tubes, volume and gain decreased and the sound was horrid. I used a big grid resistor and that solved the problem, but I'm not sure if that was really the problem.
 
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Thomasha

Member
2012-06-13 8:32 pm
Now that I looked for a schematic for this amp I see that the 6Af11 triodes are the first stages, with lower signal levels, and there is no feedback resistor.

First thing check the wiring, squealing noises are mostly lead dress problem.

Second, you could try the feedback resistor and see if that tames some of the noises.
 

ericj

Member
2008-12-08 10:24 pm
Well, this is my first point-to-point 'air wired' tube amp, and some of the wiring is a little, eh, horrifying.

Particularly my attempt at a star ground, which is fairly hideous. I think I'll rip it out and go with a 12ga solid copper wire as a ground bus.

i'll see what improvements i can make.

Thanks
 

Printer2

Member
2010-04-02 6:34 pm
Tried to tame a compactron way back when, finally gave up on it. Squeal at anything greater than low gain. Anyone looking for some 6T10 or 12T10's? Maybe set up the pentode as a triode and add another stage. But then why bother when I have other better tubes to use?
 

ericj

Member
2008-12-08 10:24 pm
Revised layout. Less buzz, still squeals



I think the output iron may be a problem. It was labeled as being for a 50c5, which would, i guess, mean it has a 2.5k impedance. Which is probably too low for the 6af11.

If i can make head or tail of how to hook up my universal stancor transformer, I'll try that.

That big sandstone resistor is a 1r dropping about 1.5v from the heater supply, to get it down to 6.3v. Got a pair of 0.5r 2w on the way from mouser along with some theremin parts.
 
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Printer2

Member
2010-04-02 6:34 pm
Sure, but people do build these successfully.

Here's the original amp: https://www.youtube.com/watch?v=8YFqkp_JocQ

And some have problems, do you feel lucky today?

Just looked at the schematic and using an added 12AX7 takes some of the heat off the 6AF11. Using a 4.7k and 15k resistor in the cathodes of the 6AF11 triodes probably helps to tame the tube. Just using the one tube to build the amp might be harder.
 

ericj

Member
2008-12-08 10:24 pm
My thoughts on taming oscillation after experimenting with layout:
1. Add grid stoppers
2. Add anode bypass caps
3. Add 10k resistors to the top of gain/volume pots.
Combinations of these have helped with many high gain pentode circuits I've built.

Thanks. I may or may not have time to tweak this again before, say, thursday night. Cleaning up the wiring today seemed to help a bit. I'm gonna try and learn some more before putting a soldering iron to it again, and i am gonna be busy for sure wednesday and probably tuesday.
 

ericj

Member
2008-12-08 10:24 pm
I don't remember exactly where the voltages are but i recall that the voltage for the first two stages was a bit high and i need to adjust the power supply.

as for where the output pentode is biased? i haven't checked other than to verify that my build matches the schematic. I actually have a clamp dc milliammeter that i can check a few things with.
 

Printer2

Member
2010-04-02 6:34 pm
I don't remember exactly where the voltages are but i recall that the voltage for the first two stages was a bit high and i need to adjust the power supply.

as for where the output pentode is biased? i haven't checked other than to verify that my build matches the schematic. I actually have a clamp dc milliammeter that i can check a few things with.

Just measure the cathode resistor resistance when the amp is off and the caps are discharged. Then when powered up measure the voltage across the resistor. Ohm's law is your friend here. I usually check all my voltages on the tubes when starting up a new build. It can show obvious problems like a bad tube or wrong part in the circuit. I took a look at the curves on the pentode, looks like you only have a few volts to play with when biasing.
 

ericj

Member
2008-12-08 10:24 pm
Just measure the cathode resistor resistance when the amp is off and the caps are discharged. Then when powered up measure the voltage across the resistor. Ohm's law is your friend here. I usually check all my voltages on the tubes when starting up a new build. It can show obvious problems like a bad tube or wrong part in the circuit. I took a look at the curves on the pentode, looks like you only have a few volts to play with when biasing.


160R, 4v at idle = 0.025A

B1 = 178v
B2 = 175v
B3 = 170v

Thanks again for the help.

I've added a grid stopper on the input, 12k from the input jack and 470pf from grid to ground. Didn't make a difference. Also behaves the same whether the case is buttoned up or the bottom half is off.