Hybrid Amp

[printer2]

Not really a pedal but about the size of a large one. The idea is to have a Marshallish preamp into a Class D amp. I want to use a 19V laptop brick as a power supply for the power amp. For the tubes I want to use a voltage booster bought online to run from the brick.This is just at the back of a napkin stage and have yet to put part into breadboard.

The preamp in one switch position (S1) is like running two Plexi channels from one input. Switch S1 cascades the channels and change the bias on the second stage giving you a JCM800 more or less. Next is the normal triode with a cathode follower afterward, found in that mess of switches. A four pole switch, it converts the CF to a gain stage as seen on some Trainwreck amps. The switch positions are in TW mode. Is it a good idea with the tone stack behind it unlike the Wrecks with the stack after the first stage? Who knows? Also doubt I will have the two input triodes cascaded, but you never know. After the phase inverter, I am sort of making stuff up. I rarely play with IC's, it is suppose to be a differential amp to sum the outputs of the PI. The diodes are there hopefully to protect the input from something strange happening with the triodes and high voltage frying stuff. Someone might even say 'that won't work'. I won't take it badly, really I won't.

[PRR]

Effect loop recovery stage won't work biased as drawn. I think you know the answer, just didn't work it through.

The rest of it "might work". Sound good? Many uncertainties. I suggest you find a way to breadboard before you invest a lot into a "Finished Product".

[Rob Strand]

Interesting idea.  I haven't seen a configuration that keeps the front-end of the power amp before.

What it doesn't have is the output impedance of the power amp working with the impedance of the speaker.   This normal gives some bass boost and some treble boost.

You might consider emulating this at the output of the first opamp.   You would need a filter at the output of the first opamp emulating the amp/speaker interraction (with a fixed speaker).   You could then take the output of that and feed it back to the last tube stage.    The other option is to make the output impedance of the D-class amp non-zero, like most guitar amps do, by using current + voltage feedback.

Anyway definitely worth pursuing with what you already have.

[printer2]

Oops, need a reference voltage. Drew up the schematic and thought it was done. Then I saw and corrected a mistake. Created an image of it. Darn, another mistake, fix that, create image, damn another...

Yeah I am guessing I'll be spending a fair amount of time swapping and twiddling to see what works. I have a breadboard arrangement for tubes that has been collecting dust for a while. Haven't really messed around with electronics for a few years. The CF transformation is more of a what if exercise. The amp was to be a lightweight grab and go thing, I have a decent enough sounding speaker without a heavy magnet. Have some lightweight pine that I need to glue together, was originally intended to be a combo but then thought why not keep the amp separate.

Started out thinking of doing a solid state preamp and looked around at what other people have done to make them tube like, or at least 'warm' sounding. Then I thought what the heck, it is not like I am short of tubes to use, why not use the real thing? But will it all fit in the enclosure I have? I have a limit of three tubes given the 19V supply. If the PI section does not turn out I might go for a pentode channel. Way back when I was going to use the enclosure, think it was a video switcher or something, and do a Bassman with a pair of 6AK6's for power tubes for a 2-3W amp. Almost thought of doing it and taking the signal off the output transformer into the switching amp. But I would loose one preamp tube to supply the 6AK6 heaters. 

Mind you, If I went with a Mosfet as a PI then I have four triodes to work with. So first stage, volume, second stage+cathode follower, tone stack, effect loop, gain stage, Mosfet, outputs. I would loose a few watts to supply the outputs, maybe 25W rather than 30W. It would be a tight squeeze. See what happens when I get thinking? For now I'll just play with the above tube circuit and see if it is viable. What I stick in this box can wait.

[PRR]

> limit of three tubes given the 19V supply

Or six or nine.

Or 12V heaters with 7V wasted in resistor.

Or 16V-19V heater tubes.
http://scottbecker.net/tube/sheets12.html
http://scottbecker.net/tube/sheets13.html
http://scottbecker.net/tube/sheets14.html

[imJonWain]

 
  While I'm not super knowledgeable on the topic I will say that I think keeping the PI tube into a solid state power amp may be worth it.  The early musicman amps had a 12ax7 PI which changed to a solidstate PI in the later models and I think the earlier models had a much nicer distortion sound vs the later ones.  I'm also a fan of post phase inverter master volume setups as I think distortion in the PI sounds good and is a fair part of what people commonly think is power amp distortion. 

[printer2]

> limit of three tubes given the 19V supply

Or six or nine.

Or 12V heaters with 7V wasted in resistor.

Or 16V-19V heater tubes.
http://scottbecker.net/tube/sheets12.html
http://scottbecker.net/tube/sheets13.html
http://scottbecker.net/tube/sheets14.html

Trust me, I thought of other combinations. Then it becomes a packaging issue. But I can make a different case to fit around the guts. Might as well add reverb. Growing even farther from the idea of a small package to get the job done. I still think of dumping the tubes and going all SS to cut the size in half. Or go the other way, find a 24V brick. Too many options sometimes is not a good thing. Hard to nail down a moving target.

And that is another thing I thought of. Use a on-off-on switch to give different tail and bias to the PI. But the schematic was getting too busy as it was. Watch, in the end I'll probably have a low gain preamp and stick a pedal in front of it. But I doubt too many people here would have a problem with that. 

[printer2]

A change in direction on the amp. Thought about circuits I liked and the only reason I was thinking of a high gain one is I didn't have one, well the high gain lost out. Doing more of a 5F6A Bassman now. Also finished the cab and with a Yamaha 12" it comes in a hair over 15 lbs. Probably start on the amp once I finish the pine Telecaster I am building. Neck carved and almost ready to finish on the above cab.



Body with finish on it and pick guard and hardware just thrown on for the picture.

[printer2]

Thinking of doing this preamp with the FV-1 for reverb into an amp module. Comments welcome.