Relay for volume boost?

[dynaman]

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 ... w_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??

[John_P_WI]

Hey Chris,

The easiest way would be to "stack" pots. Put a small 25k or 50k pot under the current master, electrically speaking (between the existing pot and ground). So you would have 1 meg + 25K or 50k resistance. When the new pot is shorted out (or bypass shunted) you have the original master or normal volume. When the the "shunt" is lifted, now the second pot is in circuit adding resistance and "boost vol".

Fairly simple, I believe Splawn does it this way IIRC.

Yes, a simple relay could be used to shunt the pot.

John

[dynaman]

OK, can you recommend a simple relay layout or schem? Looks like Weber has one that will run off the filament supply. https://taweber.powweb.com/store/modkits.htm#csk

Would that suffice? I've little experience with switching and I'm weary of introducing noise/pops.

[John_P_WI]

I have not used that kit, but it pobably would work well. In the switching method above, ground is being switched from between the two pots and open, when the ground will go through the bottom pot (lead boost). There will always be a path to ground so the pop should be very little if any. With the weber kit using an opto there should be no pop at all. I think it would work well, hopefully someone who has used the kit will chime in....

John

[Jana]

https://tubeamparchive.com/viewtopic.ph ... ght=#65550

[iknowjohnny]

I always used a pot and relay at the effects loop. i always have a tiny home made rack with my DSP, wireless, and tuner. so that made a good place to mount the pot and relay. At times i've had it rigged as a actuall volume pot in the look, other times just as a variable signal ground via the pot. In my current amp i'm using the latter but no effects loop. all i did was put a jack on the back, take a shielded cable from the pre PI master's output lug to the jack. I take a cable from that jack to the rack and the rig just grounds that point via a pot/relay who's value i can't recall. But i just play the amp with the footswitch that activates the relay on and then hit it again and adjust the pot so it's a tad lower in volume. i go back and fourth till the volume boost is exactly how much i want. the footswitch for the relay has a separately powered LED too so when it's lit i'm boosted. Been doing this for probably 25 years. The slight volume difference you need for volume boost seems to make rigging it as a volume pot verses as a simple signal ground via pot negligable. I hear no tonal advantage wiring it as a volume pot. the other way also requires a simple relay...just a spst will do.