Footswitch-able FET Boost

[martin manning]

12A. Fixed, thanks!

[Matt J]

Should the jack in the OP be 11A(?), or 11, or more likely 12A?

I was wondering the same thing! That's why I left it unlabeled in the picture I drew up. Only a 12A would make sense in that application.

- Matt J.

[martin manning]

Ok, no need to rub it in! ;^)

[dorrisant]

Martin, if that is the worst thing that happened... Still genius.

[Matt J]

Ok, no need to rub it in! ;^)

Haha! No offense intended Martin! Thanks so much for posting that original diagram! It's still miles beyond anything I can do!!

- Matt J.

[martin manning]

Here's a foot switch mod that will prevent FET boost in OD, which might result in too much gain. A switch can be added to enable/disable this feature if desired. In the end, what this does is save a step (literally) when going from clean with FET boost to OD without or vice versa.

[norburybrook]

Here's a foot switch mod that will prevent FET boost in OD, which might result in too much gain. A switch can be added to enable/disable this feature if desired. In the end, what this does is save a step (literally) when going from clean with FET boost to OD without or vice versa.

Norburybrook verified!

marcus

[jazzbass]

Here's a foot switch mod that will prevent FET boost in OD, which might result in too much gain. A switch can be added to enable/disable this feature if desired. In the end, what this does is save a step (literally) when going from clean with FET boost to OD without or vice versa.

Hi Martin Manning,

I'm building a # 183 and would like to relay the FET circuit with the front volume potentiometer, instead of the jack, excluding the PAB. I follow the discussion on Ampgarage carefully and thank you for all your shares and those of all the fans who help me understand and learn how to build an amplifier. I've built several amps but I still have a lot to learn . It is not clear to me how to bypass the PAB and relay the FET circuit. Where can I find the scheme for this modification? thanks for your patience.

Franco

[martin manning]

t is not clear to me how to bypass the PAB and relay the FET circuit. Where can I find the scheme for this modification? thanks for your patience.

Hi Franco, nice to see you around!
The FET relay schematic and layout are here: https://ampgarage.com/forum/viewtopic.p ... 08#p365408

To get rid of the PAB relay, just imagine that the two 22M resistors are wires, and erase the relay. You can still get PAB if you use an on-off-on Rock-Jazz switch. The center position is PAB.

[jazzbass]

To get rid of the PAB relay, just imagine that the two 22M resistors are wires, and erase the relay. You can still get PAB if you use an on-off-on Rock-Jazz switch. The center position is PAB.

Hi Martin,

thanks for the quick reply, here in Italy it is very late and half a brain is already asleep . Please could you clarify the above suggestion to deactivate the PAB from the relay, if I have not misunderstood I have to remove the 22M resistors and then ... sorry, maybe tomorrow I will be able to turn on the other half of the brain.

Franco

[jazzbass]

Hi Martin Manning,

good morning, the other half of the brain woke up after a good coffee but maybe I need more caffeine . I analyzed the schematic of # 183 of mdroberts1243 found on Ampgarage and realized that I have to replace the 22M resistor that connects the Rock/Jazz relay to the Treble pot with a direct connection but, what to do with the other 22M resistor ?. If I want to dedicate the ex-PAB relay to the FET I will have to disconnect it from the Bass potentiometer and from the upper contact of the Rock/Jazz relay. Will this contact be connected to the BASS potentiometer central ? I suppose so! I have never asked myself what the two 22M resistors are for but this reasoning prompted me the question. In the shops near my house the maximum value I find is 10M as well as in the Tube Town and Tube Amp Doctor catalog. What are they for?

Grazie and have a good day

Franco

[martin manning]

I analyzed the schematic of # 183 of mdroberts1243 found on Ampgarage and realized that I have to replace the 22M resistor that connects the Rock/Jazz relay to the Treble pot with a direct connection but, what to do with the other 22M resistor ?. If I want to dedicate the ex-PAB relay to the FET I will have to disconnect it from the Bass potentiometer and from the upper contact of the Rock/Jazz relay. Will this contact be connected to the BASS potentiometer central ? I suppose so!

You have it right. I've marked up that section of the 183 schematic below. The resistors are connected across the relay contacts to eliminate popping noises when they are opened or closed. I suppose the large value (22M) was found to be necessary to avoid loading the circuit in PAB mode. Using Rock/Jazz switch center off for PAB does not seem to have a noise issue.

I've also updated the first post in this thread with a new schematic and more complete layout.

[jazzbass]

Thanks Martin,

for the 22M resistors I will try with Mouser when I face another Dumble project. By the way, what's your favorite amp? up to now the constructions that, in terms of sound, have given me the most satisfaction are: Ken Fischer's three projects "Trainwreck", a Matchless DC30 (it's the amplifier with which I normally play in Pabs), Mark's Bartel "Imperial" (excellent amplifier with a great Fender voice at its highest levels), The Dumble # 004 also known as John Mayer, a great amplifier suitable for any genre especially if you modify it to have more voicings. I will insert in Ampgarage the photos of the construction of the new amplifier, the # 183. with your suggested modification.



Thanks again

https://www.youtube.com/watch?v=-5YJgGBhg6I a short demo of my latest amplifier, the guitar is played by a friend of mine (it sounds better than me ), the chassis is from Ceriatone thanks to the courtesy of Nik, the rest of the components have been bought in Europe.

[Stephen1966]

Greetings, I've posted a schematic of this FET boost in my post ODS 24 - revisited for the 21C https://ampgarage.com/forum/viewtopic.p ... 16#p425416. It is a bit of a variation on the theme and it isn't tested or peer reviewed yet, so caveat emptor!

Good health!

Stephen

[didit]

Hello --

Unclear if this adds anything. However, here goes. Variant take on the schematic & design recently captured in KiCad. The PCB in photo below is rev0.0, from prototype run that arrived late last week, while schematic is revised and rev0.1. Hope time in the next day or so allows for stuff and solder. It is expressly designed to fit within the space of a Fender style chassis end-wall, and hence bit less than 1.5" tall. The resistor mafia may disapprove given scaled-down footprints. Perhaps more detail to share once first board is built and installed.
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switchable FET preamp.jpg

PCB.jpg

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Best .. Ian