Sunstone

As I’ve probably mentioned before, I’m sort of hooked on garage sales, flea markets and thrift stores. I rarely buy anything because I have a very narrow category of things I’m looking for. One of the things I do look for is solid state guitar amps that can be converted to tubes.

Six months ago at the Alemany flea market in San Francisco I came across a god tube conversion candidate in an old Peavey Studio Pro 40.

It’s a fairly compact 1×12 combo with room in the back for tubes.

So I bought it for $30. I figure 30 bucks for the amp chassis and cabinet is a really good deal.

I spent a little bit of time opening the chassis up and taking stock on the size and what I could realistically fit in there. I figured 5 tubes would be about tops.

I went back and forth on what exactly to build in there but eventually remembered that I wanted to build something sort of inspired by a trip my wife and I made to Memphis. We had a great time. We did Graceland, Sun Studios, Beale Street etc… At Sun Studios they had an old 5E3 and I thought it would be cool to build something along those lines or something that might have been played in a club on Beale street back in the day. I didn’t have a old Fender tweed type amp like that so it seemed like a good choice.

I decided to go for something sort of like the 5F11 but with a couple tweaks. Instead of the standard Fender tweed tone control I used a slightly modified tone control from the Princeton 5F2 (no A). This is the schematic. This is a really nice sounding tone control but it requires one extra resistor compared to the more common one built off the volume pot so maybe it fell out of favor.

I also added a switch to turn off and on negative feedback. One position has the feedback defeated so it’s very much like a 5E3. The other position add NFB and a presence control. I thought NFB and a persence controlo would be the ticket for taming the low end. For more on this presence control see here.

Over the years I have a few things I usually do to a cathodyne PI to make it behave better. On this amp I only added the PI grid stopper because I didn’t want to change the tone too much. For example, there is a reedy, raspy quality to the overdrive that can usually be overcome with bigger (say 47k) power tube grid stoppers but I kept the 1.5ks in there instead.

So here is the schematic:

I used the same tremolo I have before that is slower than the usual Fender designs. The tremolo is also the main reason I went with fixed bias rather than cathode bias.

The OT is a MPS OT17PP which I think was made for a very short time.
The PT is a Hammond 270EX. With a 5Y3 rectifier and the tubes in I’m getting 363v at the first node which seems high to me for a 275-0-275 PT but I’m not complaining. I think that’s a good voltage for 6V6s.

The speaker has since been swapped to an Eminence Swamp Thang since these pictures were taken.