Switchable micro to nano PP Class AB EF80 experiment

[GrayDigger]

Few months ago I built a small dual-EF80 powered push-pull class AB practice amp (<1.5W). Using it, turns out it’s still loud enough to annoy “she who must be obeyed” sleeping in bedroom adjacent to den, (although not annoying with practice during the day).

So I thought it would be fun to design a switchable one that goes down to “nano” 100mW range and see if the SPL relationship really works in halving volume. But how would tone and feel be affected?

Wanted to maintain PP Class AB pushed pentode IMD goodness at a realistic bedroom volume. (I.e. maintain “pushed” AB pentode touch sensitivity, squish, bloom/harmonic content, guitar volume control sensitivity, not sound too compressed, etc). Get the “nano” mode to tonally play/feel like “micro”, which in turn is supposed to feel like a “real” 15-50W amp you’d gig with.

What do you think?

[lonote]

Very nice build.

Watched the video as well. I appreciated the thorough explanation & thinking behind the design (as well as the documentation)

I am dabbling in the same general area; lower wattage with less-typical tubes in a compact form-factor.

[jbrrrrr]

Very cool, and applicable to my interests - I have this wild hair that building a 1W-ish power amp section to be primarily used for the impedance/saturation effects before going into a cab modeler either direct for recording or to a PA. The best "feel" I've gotten for recording digitally has involved something specifically trying to shape the power amp section, so I thought about seeing if it'd be possible to impart all the same "goodness" from a generic cathode biased PP output section into an OPT, then sunk into a reactive load with a DI out tap taken from the speaker (with an option to send the full output to a speaker as a secondary use for low volume practice/weird little recording amp). There's analog boxes that "do" something similar without all the hassle of B+ and transformers, but it'd be a fun little box that would be otherwise impractical to be sold as anything other than a quirky novelty.

All of your work designing this nano/micro practice amp is giving me really good ideas about things to be concerned with regarding this idea of mine, and I think you landed on a very clever and functional solution that looks great in a classy build. Great video explaining all the detail involved in your design decisions as well. It *would* have been nice to be able to hear the disparity between the "wife" and "baby" modes without the iPhone mic limiter evening them out, but the tones came across anyway.

For my concept, concerning space/size/realism, my first hunches are leading me to two ECF80 triode/pentode tubes, using the pentodes as a cathode biased PP output power amp, one of the triodes as a cathodyne PI, and the other triode remaining for an input buffer, assuming the input will be line level preamp signal, not direct guitar input, from either a small tube preamp or digital modeler. This would also allow for incorporating global NFB, with resonance and presence controls that I imagine would be very useful in shaping a digital signal. I'm fighting urges to start another preamp tube for CF buffering and other over-engineering before I've even vetted this idea first. Anyway. If anyone else feels compelled to share any cautionary tales around this kind of a thing, I'm always interested in the experiences of people who've gone down similar roads before.

Back to your amp - well done!

[GrayDigger]

Thanks for checking the little fella out. Your comments are very kind.

I had originally tried to figure out how to continuously vary all the power tube voltages, bias, and OT impedance proportionately with a single control, but realized I just wasn’t smart enough to make it happen. Has anybody got a solution for this they could share?

Taking the easy way out with fixed values, I also didn’t know how just increasing the resistive load on the OT secondary – for the “nano” mode – would sound/feel compared to the speaker’s impedance load alone. Although I had originally envisioned an intermediate switchable stage (about 300mW), in the end I decided just to shoot for a simple 10-fold drop in output.

jbrrrrr - very cool concept. I hear you! I previously owned a (ridiculously) expensive modeler, thinking it was the answer to my practice dreams. But although it sounded absolutely killer on a given setting, I got frustrated because it didn’t act like a real amp so I sold it. I did think about ECF80s for this project, but ended up with EF80’s for no better reason than they’re way cheaper, and the triode bit was way more horsepower than I needed for my PI. But for what you have in mind, ECF80s sound very cool.

I would be very interested to hear what you guys come up with!