John Suhr Provided Details Regarding the SL68 (JMP SL clone)

[johnnybgoood]

So... like most people that have a day off, I spent it in front of the computer instead of either playing my guitar or enjoying the outdoors. By dumb luck while searching for "PEC even taper pots" I eventually ended up on "The Gear Page" forum. There I found posts by John "Husky" Suhr giving his expert opinions on pots and capacitors as well as posting a picture of the inside of an SL68.

I hope I'm not breaking any rules here by linking to another forum but the information he provides should be really valuable to any music hobbyist that solders once in a while.

Here is the link. http://www.thegearpage.net/board/showth ... st13247530" onclick="window.open(this.href);return false;

Regarding pots he wrote, "PEC have the most even taper out of all of the pots, second best is CTS...charted them all with plots."

Regarding caps Suhr states that "(he) did some impulse testing on caps and actually found the Mallory 150's to be very close to the Mustards." This confirmed my own use of Mallory caps. I've been ordering them from Mouser and Digikey for years along with other caps from Cornell Dubilier.

The real pleasant suprise was the inside chassis shot of his SL68 that Suhr himself posted. http://www.dropbox.com/s/5og36yddsy63bsi/SL68.jpg" onclick="window.open(this.href);return false;. The circuits used are present-day production. However, the values he used must have been matched to a stock '68 plexi.

The amp has a solid grounding structure where he uses shielded cable from the inputs that eventually lead to pins 2 and 7 of V1. Also the electrolytic caps for V1a and V2a seem to be identical (250uf possibly http://mouser.com/ProductDetail/Vishay- ... 2e3paxw%3d" onclick="window.open(this.href);return false;) For V1b and V2a he also uses radial film caps (.68uf value?? http://www.digikey.com/product-detail/e ... 4-ND/56374" onclick="window.open(this.href);return false;). The radial film cap is soldered in parallel with the electrolytic cap on V2a. This picture confirms what Friedman and Suhr stated about Ed's primary plexi.

The high-pass treble peaking cap and the tone stack treble cap are both mica caps. The presence cap is a radial film cap.

The biggest surprise is that there is no 0.005uf bright cap on the volume I pot.

With the exception of the voltage reduction option and the improved grounding scheme the amp's architecture is virtually identical to a plexi from '68.

The preamp, main, screen and PI filtering are probably inline with the 12 series plexi (i.e. low filtering).

Awesome info by John Suhr himself and a great thread!

[vanhalen5150]

PEC pots?

[johnnybgoood]

PEC pots?

Precision Electronics Corporation. They're based in Toronto, Ontario. Galco is one supplier in the U.S. for these potentiometers.

http://www.precisionelectronics.com/0-40a.html" onclick="window.open(this.href);return false;

[mightymike]

Wow that's some twist on those heater wires Love the PEC mil spec pots too. Nice workmanship, and interesting choice of current production and NOS parts that speaks volumes on his component tonal refrence bank

[SteadyEddie]

I'm going to have to revisit the Mallory 150s. I've liked them in Fender amps for a while now. I'm just not feelin' the love for Sozos, just too dark and meh.

[VelvetGeorge]

Great info!

I love the PEC pots too, but they don't sound as right in a '68 circuit to me as smaller, cheaper pots. I do prefer them in anything pre '68 specs though. Just my opinion.

Friedman and I did some listening to caps while I was at his shop this past Summer. I had to fight off my anti Malory bias, but once I did I had to admit they sound very good. Closer to the mustards he had than Sozos. That was hard to swallow let me tell you.
Mallorys definitely changed at some point. Maybe when they switched to white. They don't sound like the veiled, muddy, one-dimensional old Mallorys any more. There was more detail than the Sozo Vintage we compared to.

I also learned that Dave and I hear things a little differently, or rather have different preferences tonally. What he hears as "furry and smeared" I tend to hear as "warm and complex". I like what the Sozo and Allen Bradleys do to a new amp, while he leans towards vintage voicing but with attack and definition. The difference is quite small and I doubt you'd hear it at full volume, but very interesting how we each had our own preferences.

Lately I've been trying combinations of Sozo and Mallory to hear what sounds best in each position. Less NOS AB's in the '68 circuit too.

george

[bill bokey]

That's very interesting info. I prefer Alpha pots to PEC but have never tried the Mallory 150s so I guess I'll give them a go now. Are these the ones you're talking about ? http://www.tube-town.net/ttstore/index. ... 630-V.html" onclick="window.open(this.href);return false;

[vanhalen5150]

Interesting how the heater wires to pin 9 are routed. What is going on at v1? Are those some type of mounts for the 68k's.

[BrownSound1]

Very interesting! I would have never believed that George would say Mallorys sounded closer to Mustards than Sozo caps. Maybe 12/12/12 is the opening of the Bizarro universe or something.

[rgorke]

Interesting how the heater wires to pin 9 are routed. What is going on at v1? Are those some type of mounts for the 68k's.

I've looked at that before. It appears that instead of having the input caps on the board and running unshielded wires from the inputs to the resistors to the tube, he has the resistors mounted right by the tube. My inputs seem to have developed some noise, so I am thinking of trying this setup. Just as long as the top input sees 33-34k and the bottom input sees the full 68k, you are good. Shielded inputs seems smart to reduce noise.

[bmf5150]

can anyone see how he is dropping the heater voltage with the variac switch?

[vanhalen5150]

can anyone see how he is dropping the heater voltage with the variac switch?

I don't think it drops the heater voltage. That stays the same. Only the B+ drops directly from the PT secondaries.

[bmf5150]

I think Peter thorn said it did drop heater as well,but I don't see anything to do so???

[BrownSound1]

He says in the demo it drops all the voltage. The indicator light dims when he flips the switch.

[julkke]

This just confirms it's a "stock" amp. Some improvements here and there but no voodoo. And no NOS parts!