Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: dynaman1 on June 21, 2010, 11:59:14 am
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A buddy recently played an amp I made and was interested in getting one made with the ability to switch on the fly for a slight volume boost with no or minimal change in tone. The amp was simply four cascaded gain stages/CF into a 100W output section. It was basically just a Soldano Avenger or Marshall 2203 w/additional gain stage (volume and pre-PI master volume controls).
My bud said he'd rather not just bypass a gain stage as he wants minimal change in sound and cited Marshall's JTM900 2100/01 SL-X as an example. Schem can be viewed here: http://schematicheaven.com/marshallamps/jcm900_slx_100w_2100.pdf
It appears to have two pre-PI master volumes and the ability to switch between them.
I'm not familiar with switching circuits and I simply don't understand the schem. Does anybody know of a "pre-engineered" design for this? Could I somehow adapt some commonly available boards for this task (Mojo's mosfet assembly or Weber's channel switching kit)?
How about simply dumping signal to ground right before the PI? Maybe a switch could pull signal from the master and drain some signal to ground through a resistor or pot. This would be the "normal" setting and bypassing this setup would add a boost in volume.
Any ideas??
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relays are a route to go, but you'll need another gain stage, and you'll need to find a place (or places ) to plumb it in and out. plus the business of powering the relay, which you make need another transformer or at minimum a power supply circuit for.
he says he doesn't want to bypass a gain stage, but is it perhaps a case of him being to concerned with the details? is what he really wants to do is step on a button and get a bit more dirt from boost? or does he want to have 2 switchable channels at lower volumes (clean (non-MV) and dirty (MV))?
perhaps easier: what about a boost pedal?
if he doesn't want an SS boost, for whatever reason. it could tube, you could build it for him. a simple gain stage with a $50 transformer from triode or edcor. 12au7, or 12ax7 with a voltage divider to get the +dB boost down to +6 to +12 or so. use the other 1/2 of the twin triode as a CF for output.
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Nah, he says he doesn't want more distortion or change in tone, just a jump in volume. Otherwise, I'd opt for adding/bypassing a gain stage or cathode bypass caps.
I did recommend recommend a boost, but he claims his pedal doesn't really change the volume much. Just adds more dirt, sustain, etc. I guess he just wants to jump out in the mix more. Volume pedal maybe? I'd imagine that would just clean up his sound much like rolling the guitar's volume knob back.
Yeah, I know a relay needs power. Looks like Weber's board will run off the filament supply. I think I also have a Marshall JCM 900 100W PT laying around that has an extra winding just for relays.
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I presume then he's relying on the preamp for any and all his overdrive/distortion, and not the power amp stage?
If so, then, the idea of switching in and out two MV controls might work. Just place the center tap of the relay switch to ground, and the leg of each pot that normally runs to ground to the relay switch. When the leg is lifted from ground, it will become ineffective and the one connected to ground will control the signal levels hitting the PI.