Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on August 02, 2023, 04:52:35 am
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Hi guys
I’ve been looking a lot at circuits with mosfets substituting triodes in tube amps. I always end up with circuits that have a mosfet cathode follower, but I would like to try an IRF840 in the recovery part of an AB763 reverb circuit. Has anyone got a schematic for such application?
Thanks
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I know an LND150 is used as a recovery in active FX loop designs. See reply #33 in FX loops from the ARCHIVES:
tube effects loop on board (el34world.com) (https://el34world.com/Forum/index.php?topic=10208.0)
I have not used one, but I'm thinking that perhaps it could be used as a reverb recovery?
Hopefully, someone more knowledgeable can offer a more educated answer.
With respect, Tubenit
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Is it an idea to test this on an amp doing the following?
Pull the recovery tube, clip in an LND150 mosfet on both triodes with each a back to back 12V zener from cathode to grid? And leave all other connections as they are. Or are there other values that would definitely need to be changed?
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Hi guys
I’ve been looking a lot at circuits with mosfets substituting triodes in tube amps. I always end up with circuits that have a mosfet cathode follower, but I would like to try an IRF840 in the recovery part of an AB763 reverb circuit. Has anyone got a schematic for such application?
Thanks
Sub the ckt at the top of the page for a triode gain stage - For reverb tank recovery, you don't need ZD1 & ZD2. An IRF840 to do the job is overkill.
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm
--Pete
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Hi guys
I’ve been looking a lot at circuits with mosfets substituting triodes in tube amps. I always end up with circuits that have a mosfet cathode follower, but I would like to try an IRF840 in the recovery part of an AB763 reverb circuit. Has anyone got a schematic for such application?
Thanks
Sub the ckt at the top of the page for a triode gain stage - For reverb tank recovery, you don't need ZD1 & ZD2. An IRF840 to do the job is overkill.
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm
--Pete
Thanks Pete! That is exactly what I was looking for👍👍👍
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This thread includes a LND150 recovery stage that has been used successfully, last post.
https://el34world.com/Forum/index.php?topic=27722.0
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In my archive I've this
(https://i.imgur.com/2vclYYf.jpg)
Franco
See attached pdf
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The Surfy Bear stand alone reverb is mosfet driven IIRC (it’s like a 6G15, but with fets instead of tubes). That’ll have a recovery stage. Schematic is floating around somewhere. If someone else hasn’t linked to it by the end of the day, I’ll attach the schematic file when I get back to my desktop thingy later.
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(Edit - attachment deleted - schematic already posted in reply #6)
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And the Surfy Bear (JFET)
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Hi I think there is a problem in the drawing of the phase inverter. The 1M resistor shoud be connected to pin 7 of the 12AX7 and the other point of the capacitor 10nF to ground?
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Hi I think there is a problem in the drawing of the phase inverter.
In reply #7? 6 Yes, there is.
Reply #6 and #8 it seems.
Typo, thanks.
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Hi I think there is a problem in the drawing of the phase inverter.
In reply #7? Yes, there is.
Reply #6 and #8 it seems.
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And the Surfy Bear (JFET)
Would be nice to know what type PNP Q1 is - Assuming BC556 or similar.
--Pete
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And the Surfy Bear (JFET)
Would be nice to know what type PNP Q1 is - Assuming BC556 or similar.
"Silicon"?
It doesn't look fussy. >6V, >0.7mA, Vbe near 0.6V. You could probably put it in backward (E and C reversed) and it would play OK.