6G3 clone x2 Ken Fisher-ized

[blodviol]

I have completed half a bachelor in electronics engineering, and have finally gained some basic understanding of triode circuits. I've built a 5E3 before, and got it to work through luck and good help from this forum.

My dream since that was completed, was to up the ante and go for a self-designed amp. And 40-60 watts. I love the tone of the brown deluxe and pro amps, and got the idea that my dream amp would be a 4x6V6 6G3 Deluxe.

The KF school of thought, less is more, appeals to me, and now that i know what a long-tailed pair is, the simple Trainwreck designs really appeal to me. So what i did was ripping the 6G3 schematic from FFG and fix it in photoshop.

I removed the bias tremolo circuit, the normal channel, the low input and the 68k series resistor in the input. All in all one tube down. Now i want to add two 6V6's, and from other (6L6-) circuits it seems there needs to be 1,5k resistor to one tube in each pair.
What's the function of this?
I don't fully understand that.

And also, should the LTP be modified for this?

I realize the PT and OT should be swapped, too, maybe for a Brown Pro type? I'm thinking the GZ34 can handle 45 watts, as it does in the SR...

Thnx, Tristan

[MWJB]

The 1.5K resistors are grid stoppers, they aid stability, fit them.

The LTPI is OK as it is. You might want to tweak the 56K NFB resistor to taste?

A Brown Pro runs a B+ of nearly 500v, far too much if you want "classic" 6V6 tone. You want a 40-50W Fender style PT with a B+ secondary winding of around 320-0-320VAC to 330-0-330VAC with a GZ34. 200mA current min. It would probably be easier to use a repro 5F6A type PT like the one at www.hoffmanamps.com and zener down the B+ with 50V 50W reverse polarity zener diode.

[blodviol]

should I mount a 1.5k resistor to each grid, or just the middle tubes? I checked out the Marshall 1959 schematics, which has a 5k to each grid, and even the 1987 have these fitted.

I guess they lower gain somewhat, as they form a voltage divider with the grid. probably not too much though, since the grid should be in the Megohms range, right?

If i lower the NFB resistor, the gain in the LTP would go up, i gather.

how about the OT? Would the Brown Pro type be a good choice for this?

thx

[MWJB]

Mount the 1.5K on each grid (pin 5).

The Marshalls used EL34 tubes, you should be OK with 1.5K on 6L6/6V6s...or the smallest size that keeps the amp stable, if you're layout is good.

If you're using an 8ohm speaker, then yes you could use a '2x6L6 in to 8ohm' OT (4K:8ohms). You could try a brown pro OT if you have one handy, but whether it could take the 50-60mA per leg idle current that you'll end up with, without burning up is another matter. The 18343 type OT would be a better bet, or a Twin OT if you can scrounge one?

No, gain goes down if you reduce the NFB resistor value, this is the resistor that connects to the speaker tap. If you increase this gain goes up. If you decrease the LTP tail resistor value, gain also goes up.

[blodviol]

Thanks, I don't have any trannies lying around. I was thinking maybe Mercury or look if i could part an amp. Would AC30 trannies be in the ballpark? I'm not really good at reading spec sheets if i could find them, and i can't.

I understand that lowering the "tail" resistor draws more current through the cathodes, and thus increases gain. But the way i reasoned with the NFB resistor is that a lower value would drop less voltage, and make the difference in amplitude between the grids bigger thus more gain....
How DOES it work?

[MWJB]

Lowering the tail decreases voltage at the cathodes, the tube plates see a greater voltage as they only see the difference between the plate & cathode.

A smaller NFB resistor allows more voltage to be fed back. Let's say yo have a 10:1 NFB ratio, for every 10 volts at the output, one volt is fed back through the NFB loop. If you have a 100:1 NFB ratio, only one hundredth of the voltage at the output is fed back = more gain. Typically Fenders have feedback ratios of around 8:1 to 18:1 (higher in SF Champs & tolex Princetons) depending on output impedance.

I think an AC30 PT would be a bit low voltage-wise for what you want, it wouldn't have a bias tap either (not the end of the world in this respect, but as you are buying a PT you may as well buy one that has the features you want).