Funny how the discussions turn... I just finished the preliminary tweaking and voicing on this 2 stage volume-only amp, which I started building almost 2 years ago. It does cost more than $100 but it's just what I've needed for a small amp.
The idea was to re-do the basic Champ circuit using parts I had on hand; EF804 input, 807 output, 500V power transformer, and a Doberman OPT. It ended up about 2X the output capability of the Champ and I'm using a 12" speaker so I call it Da Crusher.
I switched to a choke input to get about 440V B+ for the 807, which results in a super quiet amp. Just a little EMI buzz with the gain wide open!
I started with values based on the Champ and got really lucky on the voicing first time out. Currently using a 12" 50W ceramic speaker in an open baffle (Fender HRD box) and it sounds nicely balanced. The guitar controls and the second input jack at -6db give me a wide range of tone. It's mostly clean with a nice smooth crunch as I dig into my hot pickup. I can't think of anything constructive to do with the voicing now, but plan to try a low wattage alnico 12" in both open and sealed cabs. I also have an Altec 421 15" I want to try for grins.
Tons of fun!
Michael
The idea was to re-do the basic Champ circuit using parts I had on hand; EF804 input, 807 output, 500V power transformer, and a Doberman OPT. It ended up about 2X the output capability of the Champ and I'm using a 12" speaker so I call it Da Crusher.
I switched to a choke input to get about 440V B+ for the 807, which results in a super quiet amp. Just a little EMI buzz with the gain wide open!
I started with values based on the Champ and got really lucky on the voicing first time out. Currently using a 12" 50W ceramic speaker in an open baffle (Fender HRD box) and it sounds nicely balanced. The guitar controls and the second input jack at -6db give me a wide range of tone. It's mostly clean with a nice smooth crunch as I dig into my hot pickup. I can't think of anything constructive to do with the voicing now, but plan to try a low wattage alnico 12" in both open and sealed cabs. I also have an Altec 421 15" I want to try for grins.
Tons of fun!
Michael
Attachments
I pulled some random transformer symbol out of the lib. If someone someday would like to make Eagle parts for some popular Hammond transformers that would be ... just ... great.
There is a 6.3V winding and heater connections which are not shown. I thought that bit was not as important to show. the 6.3V string is simply one side grounded as in the Champ.
Thanks,
Michael
I just reworked this amp to produce a wider range of tone.
Gone is the pentode input, replaced with a 2 stage 12ax7 front end and a TMB tone stack.
The tone stack is in the post-gain position so as to allow more overdrive and be able to shape the resulting tone.
Gone is the 807 for a functionally equivalent but beefier 6L6GC with a higher bias point from 45 mA up to 60 or thereabouts.
Now using a bit of cathode NFB in the output stage to get a better damping factor into the speaker (now using a nice alnico Weber), and now using the 4K primary OPT tap, down from 6K with the 807.
The result is exactly what I was looking for; a range from clean, to chimy, to gain-ey, to thick overdrive with good tone shaping running the tone controls approximately mid-range.
The output stage adds a sweet second harmonic at louder levels and gets strident when opened up. It seems to be a great match with the 12" Weber alnico speaker.
BR
Michael
Gone is the pentode input, replaced with a 2 stage 12ax7 front end and a TMB tone stack.
The tone stack is in the post-gain position so as to allow more overdrive and be able to shape the resulting tone.
Gone is the 807 for a functionally equivalent but beefier 6L6GC with a higher bias point from 45 mA up to 60 or thereabouts.
Now using a bit of cathode NFB in the output stage to get a better damping factor into the speaker (now using a nice alnico Weber), and now using the 4K primary OPT tap, down from 6K with the 807.
The result is exactly what I was looking for; a range from clean, to chimy, to gain-ey, to thick overdrive with good tone shaping running the tone controls approximately mid-range.
The output stage adds a sweet second harmonic at louder levels and gets strident when opened up. It seems to be a great match with the 12" Weber alnico speaker.
BR
Michael
Attachments
Reminds me of the AX84 P1-eX but with CFB, which you never see in guitar amps.- Status
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