For a while now I've wanted to build a stereo vibrato amp for guitar and finally I had the time to do it. Since the kind of varistors Magnatone uses are hard to find and expensive I decided to copy the vibrato circuit and preamp from Gibson GA83-S. I added middle control, opamp driven reverb, and used LND150 mosfets for the LFO circuit. Power amp is cathode biased with zener diodes limiting how much cathode voltage can rise. Either 6V6 or 6L6 tubes can be used. With 6V6 tubes output power is 15W per channel and with 6L6 it's 25W per channel. NE5532 op amp reverb driver is powered from power tube cathodes.
Sound samples:
https://ghr.fi/tube/vibratoamp/vibratos ... ssion.flac
https://ghr.fi/tube/vibratoamp/vibratos ... ssion.flac
https://ghr.fi/tube/vibratoamp/tremolom ... ssion.flac
https://ghr.fi/tube/vibratoamp/tremolos ... ssion.flac
Schematic:
Gerber files:
Stereo vibrato amp
Moderators: pompeiisneaks, Colossal
Stereo vibrato amp
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Re: Stereo vibrato amp
An ambitious project, sounds very nice!
I will have to study up on the trem circuit as I am not familiar. Looking for it, but I don't see the MOSFET LFO on the schematic.
I will have to study up on the trem circuit as I am not familiar. Looking for it, but I don't see the MOSFET LFO on the schematic.
Re: Stereo vibrato amp
Mosfet LFO is on the lower left part of the schematic.
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Re: Stereo vibrato amp
So it is..
Looked twice at the downloaded PDF, but missed that whole corner somehow.
Looked twice at the downloaded PDF, but missed that whole corner somehow.
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martin manning
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Re: Stereo vibrato amp
Looks and sounds great, thanks for sharing! Lots of interesting stuff in there.
Re: Stereo vibrato amp
Nice work!!
If you even get into this kind of thing again, the Magnatone varistors can be replaced with neon bulbs and light dependent resistors, or even LED-LDR modules if you're willing to tinker the LFO.
... which you clearly are. Nice work on the LFO.
If you even get into this kind of thing again, the Magnatone varistors can be replaced with neon bulbs and light dependent resistors, or even LED-LDR modules if you're willing to tinker the LFO.
... which you clearly are. Nice work on the LFO.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Mark Twain
Re: Stereo vibrato amp
Very cool, thanks for sharing!
Re: Stereo vibrato amp
Nice project. Impressive range and very quiet. Well done!
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Stereo vibrato amp
Here's a circuit I'm thinking about testing. It has pretty much the same LFO with added mosfet buffers for the optocoupler LEDs.
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SoulFetish
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Re: Stereo vibrato amp
I love the idea of using those fets to drive the LFO parts of the circuit.
I just acquired super reverb which needed too much work for the previous owner, so he decided to give up on it. It was in terrible condition, but I've repaired worse. Anyway, when I was deciding what to do with it when I got it, I was flirting with the idea of incorporating a true vibrato in place of the stock tremolo effect (ala magnatone style or merlin's adaptation of Dorf's vibrato).
I was inspired because I think it would sound killer, and I let my imagination get the best of me and it began to look like a fair amount of redesign as well. So I began work out what I would need to do in order to get the results I was looking for, it would have involved drilling and installing at least one more preamp tube sockets and try and find space for the components involved. I made the call that it would have been too impractical for the project that was already in front of me. However, the use of these depletion mode fets for your LFO generator and drivers is a good idea. It makes use of the available power supply and has a small footprint.
I was wondering about a something though.. I notice you chose to use 1k as the source resistors to bias the gate voltage for both the oscillator and driver in the LND150s. When I look at the transfer curves, that seems odd to me, but I'm probably misunderstanding something. What supply voltages are you starting with after the 22k drop and 47uF cap? If it's oscillating, then you're obviously getting enough gain, so what is the Vgs ultimately at idle? does this limit to the signal swing or something? Just curious.
I just acquired super reverb which needed too much work for the previous owner, so he decided to give up on it. It was in terrible condition, but I've repaired worse. Anyway, when I was deciding what to do with it when I got it, I was flirting with the idea of incorporating a true vibrato in place of the stock tremolo effect (ala magnatone style or merlin's adaptation of Dorf's vibrato).
I was inspired because I think it would sound killer, and I let my imagination get the best of me and it began to look like a fair amount of redesign as well. So I began work out what I would need to do in order to get the results I was looking for, it would have involved drilling and installing at least one more preamp tube sockets and try and find space for the components involved. I made the call that it would have been too impractical for the project that was already in front of me. However, the use of these depletion mode fets for your LFO generator and drivers is a good idea. It makes use of the available power supply and has a small footprint.
I was wondering about a something though.. I notice you chose to use 1k as the source resistors to bias the gate voltage for both the oscillator and driver in the LND150s. When I look at the transfer curves, that seems odd to me, but I'm probably misunderstanding something. What supply voltages are you starting with after the 22k drop and 47uF cap? If it's oscillating, then you're obviously getting enough gain, so what is the Vgs ultimately at idle? does this limit to the signal swing or something? Just curious.
Re: Stereo vibrato amp
Supply voltage to the mosfets is 360V and Vgs is about -0.65V. I chose the source resistor value so that the drain voltage of the LFO is somewhere between half and 2/3 of the supply voltage and then just used the same value for the phase splitter mosfet.