Hi guys. Back again after some months playing with various valve amp ideas.
I shelved the single valve ECL82 guitar amp concept because it didn't have enough gain - as widely predicted. However as part of that development I put a single stage FET front end on it and I was pleasantly surprised by the smooth breakup that the FET created.
So I grafted the same circuit into my twin 6V6 and twin 6AQ5 single ended amps to see what benefits I could get.
Attached is the schematic for the modified "hybrid" 6AQ5 amp. Via front panel switches I can dial in the FET stage to get some smooth early crunch, or I can switch it out to make the amp valve only for a clean sound.
Note that the FET load resistor is split in two with a switch. The lower load value operates the Drain at a higher bias point which is a light crunch setting. Switching both load resistors into circuit pushes the bias point lower and raises the harmonic distortion quite a bit. In both cases the gain of the FET also means the 12AX7 is driven hard and contributes its own crunch as well. Finally extra crunch is added by over biasing the 12AX7 with a change of cathode resistor. So I have clean, plus three stages of crunch available.
Assuming the schematic attaches OK, I will post links to some sound samples.
Pete
I shelved the single valve ECL82 guitar amp concept because it didn't have enough gain - as widely predicted. However as part of that development I put a single stage FET front end on it and I was pleasantly surprised by the smooth breakup that the FET created.
So I grafted the same circuit into my twin 6V6 and twin 6AQ5 single ended amps to see what benefits I could get.
Attached is the schematic for the modified "hybrid" 6AQ5 amp. Via front panel switches I can dial in the FET stage to get some smooth early crunch, or I can switch it out to make the amp valve only for a clean sound.
Note that the FET load resistor is split in two with a switch. The lower load value operates the Drain at a higher bias point which is a light crunch setting. Switching both load resistors into circuit pushes the bias point lower and raises the harmonic distortion quite a bit. In both cases the gain of the FET also means the 12AX7 is driven hard and contributes its own crunch as well. Finally extra crunch is added by over biasing the 12AX7 with a change of cathode resistor. So I have clean, plus three stages of crunch available.
Assuming the schematic attaches OK, I will post links to some sound samples.
Pete
Attachments
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So here are some soundcloud samples of the hybrid FET/valve amp. These were recorded with a SM57 style mic offset in front of the Celestion 10/30 speaker and low (bedroom) level.
This is off my handbuilt strat. The first section is in Tele mode (neck and bridge pickups) for a slightly twangy sound. Later in each track I switch to start mode (neck and middle) which is a slightly fuller sound and higher output.
Clean sound, no FET preamp)
6AQ5 V2 Clean1 by Pete Hanson | Free Listening on SoundCloud
Light crunch with low FET load resistor.
6AQ5 V2 Crunch1 by Pete Hanson | Free Listening on SoundCloud
Mid crunch with FET bias in more nonlinear region.
6AQ5 V2 Crunch2 by Pete Hanson | Free Listening on SoundCloud
And finally the full beans with the triode bias tweaked as well.
6AQ5 V2 Crunch3 by Pete Hanson | Free Listening on SoundCloud
Pete
This is off my handbuilt strat. The first section is in Tele mode (neck and bridge pickups) for a slightly twangy sound. Later in each track I switch to start mode (neck and middle) which is a slightly fuller sound and higher output.
Clean sound, no FET preamp)
6AQ5 V2 Clean1 by Pete Hanson | Free Listening on SoundCloud
Light crunch with low FET load resistor.
6AQ5 V2 Crunch1 by Pete Hanson | Free Listening on SoundCloud
Mid crunch with FET bias in more nonlinear region.
6AQ5 V2 Crunch2 by Pete Hanson | Free Listening on SoundCloud
And finally the full beans with the triode bias tweaked as well.
6AQ5 V2 Crunch3 by Pete Hanson | Free Listening on SoundCloud
Pete
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I just finished listening to all of the clips. Most of my amps have focused on the other extreme of the sound spectrum......they sound like if I pushed them just a little more they would explode.
This is one of the first "clean" amps that I have really liked. I suspect that it has something to do with your DIY guitar and your playing style, but I really like that "twang."
Oh, and I have a few 6AQ5's too.
This is one of the first "clean" amps that I have really liked. I suspect that it has something to do with your DIY guitar and your playing style, but I really like that "twang."
Oh, and I have a few 6AQ5's too.
Attachments
The playing stands on its own clean and the distortion just adds some texture. I was hoping to build a cleaner amp with the transformers that came from a 6AQ5 P-P console for a relative but thought maybe I should use 6V6's so he could pick up a tube anywhere. I might have to retink it. What speaker are you using?
Thanks guys. I spent a lot of time listening to YouTube clips of 1959 Princeton tweed amps and set that as my desired sound. Those amps just seem to do it effortlessly...
So the key for me is the pair of pentodes in parallel in a Princeton like single ended output stage. In this case I'm using the output transformer from a 1950s radio. Theres enough feedback to keep the output fairly clean, and there won't be enough power to make the speaker break up. (That would be far too loud for my needs anyway). So it all has to happen in the front end.
I experimented with diodes to get the crunch sound but it wasn't quite right. The mis-biased FET idea produces something much nicer, more organic. The extra gain it produces also drives the triode harder which is a bonus. Both of these are second harmonic heavy, whereas my diode clipper idea was always more third harmonic. You see when the FET preamp is engaged I switch in an attenuating resistor after the first triode to bring the signal back down to reasonable levels for the main amp.
The speaker is a Celestion 10 30 - a ten inch fitted in a stupidly small box. So I have a large acoustic absorber (filled with sheep wool, cotton layers and leather cloth) that I drape over the rear to improve the tone.
Pete
So the key for me is the pair of pentodes in parallel in a Princeton like single ended output stage. In this case I'm using the output transformer from a 1950s radio. Theres enough feedback to keep the output fairly clean, and there won't be enough power to make the speaker break up. (That would be far too loud for my needs anyway). So it all has to happen in the front end.
I experimented with diodes to get the crunch sound but it wasn't quite right. The mis-biased FET idea produces something much nicer, more organic. The extra gain it produces also drives the triode harder which is a bonus. Both of these are second harmonic heavy, whereas my diode clipper idea was always more third harmonic. You see when the FET preamp is engaged I switch in an attenuating resistor after the first triode to bring the signal back down to reasonable levels for the main amp.
The speaker is a Celestion 10 30 - a ten inch fitted in a stupidly small box. So I have a large acoustic absorber (filled with sheep wool, cotton layers and leather cloth) that I drape over the rear to improve the tone.
Pete
Your 6aq5 amp sounds fantastic. I build a similar amp with a slightly different fet circuit. I also added a 100k pot in series with a 22k resistor to adjust the nfb. I also added a dpst switch on G3 of the tubes to be able to use 6AK6 (1.1w) 6AM5 (3 watts) or 6AQ5 (4 watts). I just use pin 1 only for G1 and switch G3 pin 7 to either pin 2 (cathode) on the 6AK6 or leave pin 7 open for 6BM5/6AQ5. It gives you many tone options and power levels. I also added a Pentode/triode switch on the output tubes. Just some things i did. Your amp sounds great. Bob. S
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