trobbins wrote:It may be worth trying a large'ish cap between grid circuit and input socket switched ground (instead of a link) as a way of AC grounding the grid with no input plug - that shouldn't change the DC grid bias, or affect performance. Even the larger 0.1uF as used by Fender is equivalent to 26k ohm at 60Hz.
Bingo, Awesome. Happy Joy. Worked a treat. Only large’ish value/ small size caps I had were 4.7mf/50V e-caps. I think that’s an appropriate value, I plugged it into the calculator got 500 ohm at 60Hz (but let me know if somehow it’s a bad choice). The bias charge stays put now and the plate voltages of the triodes stay high and the same no matter what switching is going on. Really nice when you find a fix that doesn’t seem to screw something else up.
trobbins wrote:There seems to be in and out heater cables - although of no significant issue, its good to wire the input stage heater by itself (not part of a daisy chain).
Yes, but my daisy chain was good, from power valves down to last input tube. I wanted to try DC heaters and even a separate tranny for V1-V3 but VERY glad I didn’t bother or waste money. It’s as quiet as any other well made, well running vintage circuit after a few proper tweaks. (BTW I have the heaters elevated at 60V but I’m betting I could disconnect that now and it would be just as quiet.) The really big deal here was converting to 6SL7s from 6SC7s. Grounding the V1 grids was the clincher.
trobbins wrote:It looks like the input valve socket is rubber 'sprung' - but the leads from the socket are 'hard' with no flex (eg. the pin 1 grid stopper).
You got me there - Duh slap! Yes, not at all useful if wiring PTP and teflon wire sure doesn’t help. Tits on a bull. But anyone looking from the outside doesn’t know that
martin manning wrote:I think it will be cleaner and louder after separating the triodes, and you may not like it, so yes experiment using expendable parts. I haven't forgotten about doing the grid leak bias test for 6SL7... I'll probably have some time tomorrow.
You called it Martin. Perked up, sounds snappier. Sounds great actually. I’m thinking you can hear a tube chart, maybe even sing a tube chart! Not sure what the best plate resistor value is but 470K seems to work well. I’m now preferring the grid leak side to the cathode biased side. Has a great Richard Thompson bouncy Straty twang to it. Sounds real solid too, grid leak without any of the bugaboos (except for the eventual tube ageing issue).
trobbins and the EFK M-E forum guy, solved any grid leak issues people might have and if they want to try it seems this is the best way to do it, at least on a dual triode. The only thing in the signal path that wasn’t on the original circuit is just the 2.2K stopper. I set up one channel as .02mf to prevent any overloading and the other remained as the schematic w/ .1mf. Every tweak I tried before I never felt the grid leak channel was ‘solid’ - like plug in and let rip solid. I think it was that open triode mucking up the sound, even unstrapped. A lot of noise came in as you turned up though it wasn’t apparent while playing. I had seen a THD amp schematic w/ grid leak set up normal Fender style 33k/1m before the input cap and tried that, and 68k/68k low side too, and it just seemed to take a bit of the life out it, some of the immediacy that grid leak has. The 68k/68k was dark and lifeless. Good riddance to all that, this set up works.
I think this amp is finally buttoned up and not all that far from stock
Anyone down the road who looks inside it will think I actually know what I’m doing - Yikes! Time to start my own half-assed amp business!
So: anyone looking up grid leak guitar amp, something above an SE Gibson, Supro or Champ and with a dual triode to deal with, look here first. Too bad TAG might be the first product of the digital era to prove that data ain’t forever
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not bad for Bumfucco. Must be an old pile of parts, 99% of their business now is satellite TV and security - entertained and safe, welcome to the modern world. 
