Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on November 08, 2023, 04:24:00 pm
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I’ve made my own harmonic trem based on a fender 6g2 super.
After a while, and some tube prototypes. I started subbing mosfets (LND150’s)
Works, but the mixer section (V4 in the amp, feeding the PI) exhibits some distortion due to its biasing.
This artifact is reduced if you make those two stages biased on more, of course, but you lose the lo/hi frequency wobble in the push pull mixer.
Ok, so maybe drive it with just a simple discreet low voltage fet stage, cleans up a bit.
I’m wondering if there’s a way to tackle this distortion, and still preserve the sound we all associate with harmonic trem.
If I make the bias of that stage accommodate the input signal without clipping, I lose all the mojo, and biased where I get good wobble, the wave looks/sounds distorted.
Is there a way to introduce local feedback around that stage to increase headroom.
Or possibly fix bias the grids a little to make up the headroom.
https://el34world.com/charts/Schematics/files/Fender/Fender_super_6g4a_schem.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_super_6g4a_schem.pdf)
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Yeah, tried some fix bias configurations. No dice.
Looking forward to your thoughts, but it might be a wash.
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> I started subbing mosfets
Why? Tubes are more the size and shape of nonlinearity you want in that plan.
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Good question PRR. And, those constructs are certainly what I’m experiencing.
The reason is I’m using an SMPS to derive HT and getting rid of an amp or more of heater ac, is the primary reason.
With all mosfets, and Smps, I’m only drawing 200ma off my 9v supply.
Making it modern pedal board friendly.
Square peg, round hole I guess.
Good news is that I could go back to valve for just those two stages and be likely fine.
See the tube variant, was using mosfet for oscillator, then 6112 subminis for all other stages.
Works a charm.
Just wanted to see if I could streamline it.
Thank you!
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Here is the submini schematic variant. Sounds good to me.
So I was just supplanting Mosfets for the tubes. Thanks to Sluckey and others, who have spearheaded such things.
The schemo has the SMPS, and the DC heater supply. HT is derived via SMPS. And the heaters are DC/regulated as well.
the Hi/Lo pass filters have trimmers to retain pretty flat response when LFO is off.
And theres some switching for just bias trem mode.
I've built this circuit 4 times now. Sounds authentic, as compared to my 6g4 and 6g7 amps here at the house.
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This is the tube variant demo and gut shot.
https://youtu.be/_TK4zo7d-d8?feature=shared (https://youtu.be/_TK4zo7d-d8?feature=shared)
Powered by a one spot.
Also, works great into an FX loop return straight to PI/power section.
Insta brown amp
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Wow! I'm impressed. I'd love to see the final tube version schematic.
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Thanks a ton man.
That schemo up there despite being labeled mosfet, is the exact tube circuit in the video.
It’s sorta confusing because there are a ton of pads all over it to facilitate single side etching here at home.
There’s some jumpers on the board, to make sense of it. But it’s a lot of your revibe, and some experimentation.
Ideally, that’s why I went to mosfet.
Shrinks the supply from 9vdc 1.2A to 200ma total. So I was really jazzed to do an all mosfet version.
The all mosfet proto went well, aside from the output push pull stage biasing dirty and clipping.
Those LND150’s really like source resistors of 470R or so to replicate a 1k5 cathode resistor bias in a tube.
But moving to that bias (4k7) on those mosfets, makes it clip asymmetrically super early.
If I figure that out, it would be the cats jammies.
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Welp after much trying, I’ve resorted to a standard low voltage input stage to not invite clipping at the filter gain stages.
I also paralleled another mosfet there.
Got rid of nasty clipping. Buffered lfo on this iteration.
Works fine.
Draws 90ma off the 9v power.
Sweet.
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Got proto boards.
Works well, thanks!
Some crowding edits, but just some shifting.
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Wow! Love that 'wobble' mode!