So I set about planning out the layout. I decided to make it a lot like the original...but not identical. For one...why would I want a ground reversal switch? I see all the repro chassis are made with this, like the original...but it seems like a useless waste of space. And as space was at a premium, I decided to move the fuse to the bottom of the chassis, where there is a bit more room. I also decided to get rid of the dog-house and mount the filter caps on the board...and try to ground them with the sections of circuit that they serve.
I also figured that a multi-tap output transformer would be of more utility to me than four-RCA-hole speaker output in the original design/layout. So I got to work. Ordered transformers from Magnetic Components (Classictone.net) and designed the control panel and placed an order with Sandy @ Precision Design in IN. (Chassis came from Terry @ Seaside.)
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I should have taken some shots of the circuit board before I laid it out...but...oops Anyway, here it is filled up. I'm using a 15K bias resistor with a 50K trim pot.
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That was version 1. Sounds real good, very much what I was hoping for...you get those big, bold gorgeous Fender cleans but a bit tweedier than blackface...with a bit o grit as you edge it up past 1:00 or so. Between 2:00 and wide open, it starts sounding more like my old tweed Harvard...but massive. You never get a great deal of gain, more like mild classic-rock crunch.
All good...but man...this is one loud amp! And I missed the sweetness, softer-edged sound and responsiveness that I'm accustomed to from my cathode-biased amps (which is mostly what I use). So I decided to mod it with a switch that enables you to flip between cathode and fixed bias. So when you want to go cathode-bias, you must take the pair of 220K resistors coming of the PI and send them to ground. The bias supply circuit needs to be disconnected. The power tube cathodes must go to the bias resistor (and cap) and then to ground rather than going directly to ground. You can do all this with a DPDT switch--you can see the switch in the last photo.
I decided to mount it between the rectifier and first 6L6. Got this done last night. Works like a charm. I can hear the value of both settings but I suspect I'll be playing it cathode-biased most of the time. It takes on a somewhat more responsive, softer-edged vibe that's a bit easier on the ears; mine at any rate.
Well, only had time to demo it over the weekend before I got the cathode-bias switch in there. But I'm diggin it; sounds good to me in both modes. Sure can hear the early Marshall roots. Mostly Les Paul in this demo, but I've got a section with my Thorn R/S (strat type guitar) near the end.
Also, I didn't realize until afterwards it was biased really cold; I've subsequently reset the fixed bias to about 32mA and it sounds/feels much better there, IMO. Finally...I tried playing it through one of my combos that's loaded with a pair of 10" greenback. MAN!!! Fantastic sound. Like them even better than the golds with this amp. They attenuate the real high freqs in a very musical way, and are much less efficient...so for my needs, they're a better choice.
I'll do another clip with the new stuff soon. But for now...
http://www.youtube.com/watch?v=FHY2uPlDlI0