music, guitars & other obsessions

6AF11 Push-Pull Compactron Guitar Amp

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While sorting through my tube box the other day, I came across a few 6AF11s I ordered awhile back, what were they for? After some digging around, I found a few schematics on my PC, there were two guitar amps – The Squirrel Monkey and The Super Squirrel Monkey . Both are single ended designs, the SSM just added a 12AX7 to the SM, the demo on YouTube sounded pretty decent, so I decided to do a scratch design using the 6AF11, but in push-pull…

The first order of business was to generate the SPICE models for the tube – there are three, one medium mu triode, one low mu triode and one remote cutoff pentode. The triode models were generated using Curve Captor, which were pretty straight forward. But as usual, the pentode presented a challenge, using three different tools, I came up with three different models, with large variations in the parameters. As a result, when the pentode model was used in SIMetrix, the output section was essentially useless, other than to provide some general indications… A bit frustrating, may be I have not master the craft of pentode models yet…

Anyhow, the first design I came up with was T1a -> T1b -> T2a -> T2b -> pentode PP, the phase inverter was a paraphase; once on the breadboard, its flaws became apparent rather quickly – it oscillated like crazy! No point trying to fix it, so I scraped the design and analysed what happened – I concluded that the main cause of the oscillation was the circuit topology – with the gain flowing back and forth between the two tubes, there were too many incidences for parasitic capacitance to creep up and mess with the signal… It’s the gain, too much of it caused oscillation in the compactrons, see the fixes later…

So in the next design, I tried to keep the gain build up relatively even among the tube sections as well as between the tubes, I ended up with the following topology: T1a -> T2a -> T2b – T1b -> pentode PP. I also changed the phase inverter to a split-load or concertina type. The results this time around was more encouraging. The signal stays pretty clean all the way up to about 80-90% on the gain and volume before self-oscillation set in (around 6KHz). The “clean power” output was around 3 Watts or so, still far from the target 5-6 Watts. I spent some time playing the guitar with it, but the sound was pretty anemic… got to find ways to get more gain out of it, isn’t that always the case?!

Put in some HF roll off caps to tame the oscillation, while not completely successful, they did help. Also rewired the bias off the filament supply (-6.3V) and powered up the amp using the small SMPS that I have been waiting to try out since the last built when it was used in the Otto Special.

Ran the amp at full power (before clipping) into a dummy load for about 10 minutes, everything was fine, although the SMPS switches did get kinda hot (bare TO-92 packages without heatsink…), the amp put out about 12Vp-p, so a whopping 2.25Wrms! Raising the voltage and pushing a bit more (before the bench power supply crap out), I saw 16Vp-p on the scope, so 4Wrms is doable I think, still the theoretical limit should be more than 5W, not quite there yet… I will try and put everything into a chassis and clean up the messy wiring, perhaps that will also help tame the oscillation some more.

I think even with everything dimed, it will still not have the nice sustain and overdrive sound that I am looking for, but the verdict won’t come in until I tidy up a bit… It’s a rats’ nest of wiring, since it is working, I won’t fuss with it too much unless of course, it turns out to be a killer sounding amp…


Spent the morning cleaning up the messy wiring and made some minor changes to the design – the bias is now adjustable instead of simply fixed at 6.3V, changed the gain of the last stage before the PI, it should give a bit more preamp distortion. Still debating if I should swap the gain control for a PPMV, that way I can better balance the pre vs PA distortion…

Put all the parts on the board and tested it out, now the amp just needs a nice chassis/cabinet to go into… The best idea that I came up with so far, is just to stuff everything into the mini-tweed cabinet that I have, since the existing Princeton is due for an overhaul. Checked the dimensions, it should be no problem to fit everything in, including the new 12V SMPS that I ordered.

Thinking of a name for the amp, since the tube is 6AF11, how about the “American Flyer”? It kinda goes with a cool postcard that I found:

Still trying to get the best distortion combination from the preamp and the PA, where it was way too much preamp distortion, now I think it is too little… With three triode sections at my disposal, the gain isn’t an issue but how to distribute them properly…

Finally got the 12V SMPS that I ordered, it even has a output voltage adjustment pot, so I set the voltage to 13V and fired up the amp – it worked just fine. Spent a bit of time with the plate to grid feedback caps, they should be as small as possible to get rid of the oscillation but not to reduce the stage gain too much… it was a compromise between maximizing the output volume, the amount high frequency roll off and the level of harmonic distortion. If I can figure out how to get rid of the oscillation without them, I would rather get rid of them, as they really kill the gain.

Anyway, I think I managed to get a decent crunch and usable range with the gain and volume controls, as the volume is turned up, the tone would thicken up nicely (from more distortion), perhaps it could use a bit more sparkle once everything is installed in the cabinet, I will do the final tweaking, but it is good enough for now…

Got the DC-coupled cathodyne to work, may be it is a bit better than the AC-coupled version, but it was rather subtle, not as much as I was hoping for… Now with everything packed into the mini-tweed chassis, I need to come up with a nice-looking front panel design, that I will save for another day… Now it’s time to play some tunes!


One response

  1. Gil Lavie


    I would like to try and build this kind of an anplifier for my son. Can you post the schematic?

    Thank you.

    Gil Lavie

    September 9, 2012 at 3:04 pm

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