Problems with charge pump amp after positive ground pedal

[evirob]

I recently built a low wattage tube amp based on the ax84 firefly schematic, but powered with a 12V charge pump somewhat similar to Rick Holt's "Murder One" power section. That is, the charge pump is identical to Rick's design except I added one extra stage to get the voltage up to ~75V. I did this because I found the 12AX7's really came alive if you can get them closer to 80V, and previously I was only getting about 65V. To my surprise, the charge pump is putting out enough mA to power both pre amp tubes and the power tube.

Other than the power, I built the firefly exactly as per the schematic except I switched the 12AU7 for an ECC91 because;

a) You can pick them up pretty cheap, and
b) The ECC91 is intended to operate at a lower voltage

The other difference between the 12AU7 and the ECC91 is the latter is a 7 pin miniature tube with a common cathode, but since the design hooks up the cathodes anyway, I figured it ought to work out. I'm no expert, I just try things and see if they work. It did, and the amp sounds pretty nice. The cascode gives a huge gain boost, a little noisy but apparently that's normal for the design. Disengage the boost and the amp is super quiet, i.e. noise-free, but actually pretty loud in terms of volume.

Anyway, please excuse the lengthy preamble. My question is, has anyone built and used one of these charge pump powered amps (such as the Murder One) and experienced weirdness with positive ground pedals? My range master clone makes the amp cut out. I can drive the amp hard as you like with other boost/overdrive pedals e.g. SHO or a tube screamer, and I can even use the range master in front of another pedal such as a SHO as long as the SHO is engaged. As soon as the range master is the last thing in the effects chain, the amp starts cutting out. Does anyone have any idea what could be going on?

Cheers,
Chris

[R.G.]

Measure the output of the rangemaster clone for DC. If there's DC on the output, the typical DC coupled tube amp input will be biased into cutoff.

Do the schematics of the clone you built and/or the amp have capacitors blocking DC from the output and input respectively? If so, they are either shorted, wired wrong or inserted incorrectly if there is DC getting to the first tube grid from the input.

[evirob]

Thanks for your reply.

The firefly is like this:



I made the range master from someone else's vero layout so I will have to check it tonight. It works fine with a different tube amp I built however.

[R.G.]

Try this:
- Change R1 to 2.2M
- Add a new resistor R1A = 2.2M between the UN grounded contact of S1a and ground
- insert a 0.1uF film capacitor between the junction of R1/ input jack and the new R1a/S1a/S1b.

If this cures it, it's DC coming out of your Rangemaster.