6G2 Brownface Princeton Build

[bmccowan]

In a thread about the new Fender 6G2 reissue I said I would post some progress notes/photos of a 6G2 build I was about to start.
It is underway. I typically rebuild amps, or convert organ and/or PA amps, but every now and then I get the urge to do something with all new components. When I saw a 6G2 repro chassis and faceplates show up on FleaBay from Marsh Amplification I decided to jump in.
Building mostly stock to start - maybe mods later. Changes at the outset include:

• variable bias
• ground bus for preamp - single star ground for power tubes, bias & center taps
• separate filter cap for preamp on turret board
• screen resistors on power tubes
• twisted pair heater wiring

I purchased a Classic Tone PT that offered voltages in the original 6G2 range. The chassis was cut for a larger transformer so I modified the chassis cutout and rotated the PT 90 degrees. I'm using a Deluxe OT I've had for a while.
I modified a generic turret board, moving and removing turrets. I then dry assembled the caps, cut the lead ends to length, and then removed the caps, putting them on a spare turret board in order, so that they'd be handy for installing.
I decided to following Doug's recommendation to install the components on the board after the board was installed. So I soldered up the grounds and jumpers by wrapping the turrets leaving all the turret holes open.
I decided to use a ground bus, and used a drill trick I saw on You Tube to get a straight bus wire.
Suggestions are welcome.
Attached are some of the early photos.[/list]

[st]

Nice job so far. I'd like to build one one day. Tell me more about the drill trick for the straight ground bus. I always a heavy solid copper wire (tinned) for the ground bus and have bever found it challenging to get it straight.

[bmccowan]

Hi St,
Clamp one end in a vice, chuck the other end in a drill. With both ends clamped tight enough to keep it from spinning; twist slowly and it gets straight and stiff. (favorite Viagra joke goes here)
Its the second method in this video:

I also use a drill and vice to twist heater wires tight.

[PRR]

If you tie copper wire to a door-knob, the other end to a stout stick, and PULL until it "gives", it will be straight and hard.

On electrical wire (dead soft) the "give" is distinctive. Increase the pull and it hardly stretches, then suddenly it stretches with little increase of pull. On small magnet wire a few fingers does it; house-wire may need most of your weight. Wear glasses: if you keep pulling after the "give" it breaks.

It will also be smaller, maybe 1-2 gauges thinner. It will be higher resistance partly from the thinning and also because of the strained metal (why electric wire is normally soft). This should be no-difference in tube work (all our wire is oversize for mechanical robustness and ease of working). If you are wiring MegaWatt power buses.... well, you probably not using copper wire.

[bmccowan]

I finished the circuit. Had a couple of hiccups.
One forgotten solder joint (pretty good for me)
When defeating (grounding) the tremolo circuit, I got a mean buzz. Tremolo on; the amp sounded great. I had decided to connect the positive side of the preamp filter cap at a location closer to the first stage. Bad decision, as it turned out that grounding the trem at the footswitch jack was interrupting the path to the filter cap. I moved the positive lead to the location shown on the schematic and all good. I kept the ground for the preamp filter cap on the preamp ground bus.
My plate B+ is about 316, which is right on spec, but I found that with the standard -35V bias supply I could not set the power tube bias where I wanted (+/- 24ma). The highest I could get it was 13ma - pretty darn cold, so I changed the trim resistor from 27K to 22K and got a good range on the bias pot, and -28V.
Interesting to note that Rob Robinette's site states (for the 6G3 which uses the same trem circuit) that "Bias is non-adjustable because adjusting the bias will change the intensity of the tremolo which is the major weakness of bias wiggle tremolo." But I would counter that adding adjustable bias allows you to dial in the sweet spot. I found that if you bias the tubes too hot, the tremolo starts to make a flutter noise like the wing beat of an unladen African Swallow. But setting to about 60-70% of max range gets great drive and nice tremolo. So I suspect Fender biased these cool intentionally to avoid issues with the range of tubes that might be installed.
I also toyed with the negative feedback, adding a 50K linear VR pot. I found that restricting the NF too much is not good with this amp and replaced the 56K NF resistor with a 20K for a range of 20-70K. It's on the back panel, so I used a chicken head knob and straight up is the stock 56K setting.
The amp is very quiet and sounds great. As many say, much different than the blackface sound, as the amp has mid-range focus and crunch.
Now a cab - going for a head. A 6G3 head I built a couple of years ago is in one of the photos, as is a Vibro-Champ head. I like making configurations Fender never made. But I may go a different tolex and cloth route this time. Suggestions welcome. That's a late 40s Valco type SE (6SL7, 6V6) I built a while back, and a Gibson GA-9 propping up the two heads.
 

[st]

Hi St,
Clamp one end in a vice, chuck the other end in a drill. With both ends clamped tight enough to keep it from spinning; twist slowly and it gets straight and stiff.

Thanks. This is new to me. Somehow, i have always been happy bending it straight (enough).

When defeating (grounding) the tremolo circuit, I got a mean buzz. Tremolo on; the amp sounded great. I had decided to connect the positive side of the preamp filter cap at a location closer to the first stage. Bad decision, as it turned out that grounding the trem at the footswitch jack was interrupting the path to the filter cap. I moved the positive lead to the location shown on the schematic and all good. I kept the ground for the preamp filter cap on the preamp ground bus.

Congrats on a nice build. I have found, against received internet wisdom, that Fender had its grounding schemes at least 95% right. Small changes almost almost make things worse. (I guess things changed for good in the sf era, when poor wire dress obscured any good grounding practices inherited from the bf era).

[bmccowan]

Thanks St,
On the wire straightening thing; I guess I've also always been satisfied with just straightening out a length, but not understanding metallurgy, this twist thing seemed like magic, as does the stretch thing.
Grounding - I agree with being careful about changing established schemes, my mistake here was not looking at the schematic closely enough to realize that moving that filter cap lead would allow the trem circuit to shortstop the path. In the past I have tried to "correct" Gibson amp grounding schemes, only to make things worse!
My last comment: WTF, no Monty Python fans out there?