Fargen mini plex MKII decade switch

[peckjed]

Hey All, I picked up a Fargen Mini Plex MKII dirt cheap on Craigslist recently. Awesome sounding Marshall style amp that does everything from JTM, JMP and plexi/800. However, when the decade switch is on the 80’s mode and the bright switch is on (even somewhat off), it feeds back A TON when the gain is turned up. Which somewhat makes sense, but I feel like it feeds back TOO much, almost a squeal. I spoke with Ben Fargen briefly and he mentioned he originally used a 4.7uF cap on the 80’s switch and then later swapped it to a 10k resistor. I swapped the cap with a 1/2 watt 10k resistor (didn’t have a 1 watt)... and it helped. But not a ton. Does anyone see any other sort of tweaks or issues with the amp as is?

I circled the 10k resistor I swapped in the first photo.

[frankdrebin]

A schematic could be useful

[peckjed]

A schematic could be useful

Agreed. I tried to obtain one with no luck. The response I got about the 80's decade setting was "It's called tube shot noise and honestly that's just the downside of an 800 style high gain cascaded tube stage...by standards my amps are 50-60% less noisy than a Marshall of the same gain staging."

[frankdrebin]

you have the amp,draw it yourself,otherwise its impossible to assume any cause.

[gktamps]

I have one of these, and yes, there is a lot of gain at that setting. I get feedback too in some situations, but less than my high gain cascaded Marshall style build. Try some different preamp tubes before you go crazy with mods.

[peckjed]

I have one of these, and yes, there is a lot of gain at that setting. I get feedback too in some situations, but less than my high gain cascaded Marshall style build. Try some different preamp tubes before you go crazy with mods.

That’s good to know. What pre-amp tubes dude you settle with? Did you do any mods at all?