Modded Brownface Princeton 6G2 Circuit

[TubeJunkie]

Hi all, just found out on Sunday that me and my family are going to be quarantining for the next 14 days (cough, cough, sniff, sniff).  So as not to go stir crazy, I drew up a schematic of a Modded Brownface Princeton 6G2 circuit I've been thing about for a while now.  I paralleled both triodes of V1, used V2-A as a second gain stage, and V2-B as the PI.  I removed the Trem and NFB, and converted the Fixed Bias to an Adjustable Fixed Bias.  Looking for more sets of eyes to see what I overlooked, and/or got wrong.  As always, thanks in advance for your expert knowledge and insights.

[bmccowan]

I'm interested in what your goals are for this project. Looks like a lot of gain, and my guess is you'll have to play with resistor and K bypass cap values to keep things under control. I built a 6G2 and had to play with resistor values in the bias circuit to get into an optimal range, but part of that is the tremolo circuit, which you will not have. Nice sounding amps!

[Latole]

I'll put a 470 ohms 1w limiting resistor on each 6V6's pin 4

[TubeJunkie]

I'm interested in what your goals are for this project. Looks like a lot of gain, and my guess is you'll have to play with resistor and K bypass cap values to keep things under control. I built a 6G2 and had to play with resistor values in the bias circuit to get into an optimal range, but part of that is the tremolo circuit, which you will not have. Nice sounding amps!

To tame gain, if necessary, I was thinking an NOS 5751 or 12AY7 in V1.

[TubeJunkie]

I'll put a 470 ohms 1w limiting resistor on each 6V6's pin 4

Haven't decided yes or no on these, and if so, what size . . . possibly a 1k 3W.

[bmccowan]

To tame gain, if necessary, I was thinking an NOS 5751 or 12AY7 in V1

That should work well - I have a 5751 in V1 of my 6G2. I might suggest not deleting the NFB circuit and instead trying different resistor values. I wound up with a somewhat higher than stock value, reducing the NFB. I tried removing it on my amp and also on a princeton reverb I rebuilt. In both cases the bass became flabby. I did an adjustable NFB on another amp, but found I liked one setting the best, so it stays there.

[TubeJunkie]

That should work well - I have a 5751 in V1 of my 6G2. I might suggest not deleting the NFB circuit and instead trying different resistor values. I wound up with a somewhat higher than stock value, reducing the NFB. I tried removing it on my amp and also on a princeton reverb I rebuilt. In both cases the bass became flabby. I did an adjustable NFB on another amp, but found I liked one setting the best, so it stays there.

Haven't decided yet, may put a pot in there just for more versatility, but trying to keep it as simple as possible.  I believe that too many knobs makes it harder to be happy with the tone you get out of an amp.

[vampwizzard]

IMO fender put the NFB in there for a reason.. to maximize headroom for where the amp is at. If you want more gain out of the circuit, do you want it preamp or power amp? Youve designed yours for both. May be worth experimenting with it on the power amp side.. maybe not a knob per se but with different resistor values and a bypass switch.

[Latole]

I'll put a 470 ohms 1w limiting resistor on each 6V6's pin 4

Haven't decided yes or no on these, and if so, what size . . . possibly a 1k 3W.

Why 1 K ? more than 470 ohms may change tone
Size ; 1 W is enough, some 5 watts cement resistor are often cheaper and easy available at may local electronic store

[TubeJunkie]

I'll put a 470 ohms 1w limiting resistor on each 6V6's pin 4

Haven't decided yes or no on these, and if so, what size . . . possibly a 1k 3W.

Why 1 K ? more than 470 ohms may change tone
Size ; 1 W is enough, some 5 watts cement resistor are often cheaper and easy available at may local electronic store

From Rob Robinette: "A screen resistor will accelerate screen voltage drop and increase distortion and so quicken the power amp's transition from clean to distortion. Increasing the resistance value of the screen resistor will increase the overdrive screen voltage drop and therefore increase power tube distortion, compression and sustain . . . time has tested and proved the 470 to 1.5K screen resistor value . . . my suggested screen stopper resistor values are 6V6: 1k (3 watt or higher)."

As mentioned, haven't decided yet, but somewhere between 470 and 1.5k if I do.