Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Thanks!!!

Here’s the screamertube I built many years ago that might split the difference and give you something to experiment with that isn’t overly complicated. Note that there is a small error concerning the heaters - they should be wired in series, not parallel (causes excess heat). The circuit was changed, but not the blog.

http://screamertube.blogspot.com

Disclaimer: tube purists will hate it, but it was a fun project to put together!

This would be considered starved plate and the B+ supply is +15V and not internally boosted, but the design does not include biasing so the first half 12ax7 only amplifies the bottom (-) half of the waveform, which is supplied via the boost amp. The second half 12ax7 reamplifies the flipped, attenuated signal (100K/4.7K resistor divider). I ran a SPICE simulation this morning and attached the results. The lowboost.png file is the output of the two tube halves (top is 1st half, bottom is the 2nd tube output) with a small input signal (low boost knob setting), hiboost.png is the same thing with a massively boosted input, and combogain.png is a combination plot. The harmonic distortion was 38% for the most distorted output.

I am not a fan of this design and I agree with Enzo - it does *something* --- but there is no duty cycle modulation you expect out of a true high-gain cascaded tube preamp when you overdrive the input, and it's going to add noise.

Have you looked at the classic PAIA Stack in a Box (SIAB)? it used a switching charge multiplier circuit to get the plate voltage up from 15V to almost 50V or so, getting it out of starved-plate region. I never built it, but it does seem to have a following of sorts.

This popped up on my LinkedIn feed, thought you might like it. MOSFET power amp section, claims 242W into 4 ohms. No connectors, leads solder directly to traces. I have not heard it, nor tried it, nor bought it. At the bottom of the linked page is a google drive link with the schematic and PCB layout info. Purely FYI.

https://www.tindie.com/products/jojo83/242-wrms-single-channel-audio-amplifier-core/

Agree with g1 — the subbed parts are not identical with respect to the built-in base-emitter shunt resistors (which are slightly larger in the new part) and the abs max Ib, which is more than double the original (0.5A vs. 0.2A). You may need to push the bases a bit harder on the new parts. Following up on g1’s question, are your output Vbe’s healthy at idle?

The duty cycle shift is apparent. Is it 50% up to clipping point, then as you push the signal up, the duty cycle keeps getting farther away from 50%?. Or, is it always off like this, even when not clipping? Did you solve the bias issue where the voltage-drop was 10x off?

Here’s the preamp (shows letters).

Yes, picture of x-over distortion. Right at 0V or biased?