6G3 "Lite" Build

[Ledzepp007]

Hi,

First post here. I have a couple of transformers that were given to me by a buddy (ClassicTone 40-18087 and 40-18016, the Fender Deluxe transformers), as well as some AX7's, 6V6's, and a 5Y3 looking for a home. I modified the 6G3 schematic to be single channel and cathode biased, with a 5Y3 rectifier (oddly, the original listed B+ on the schematic is more consistent with a 5Y3 and not the listed, stiffer, GZ34). The schematic is attached. I also plan to use a small choke.

I believe the Stout chassis on here would work great for this project. My main concerns are: 1) Will I have enough room for a board? 2) Will my transformers fit? (Fairly certain the dimensions of my PT are inconsistent with the Heyboer 18-watt PT that fits the Stout chassis) 3) Is my filtering too soft or too stiff? (planning to use a 20/20/20 filter cap with an 8uf preamp filter) 4) Can I add a tube buffered effects loop if there's any additional room?(my modified schematic only uses two 12AX7s)

Any comments are greatly appreciated.

[sluckey]

Why not start with a blank chassis? I believe what you want would easily fit in a 12 x 6.5 x 2.5 chassis.

[Ledzepp007]

I certainly could start with a blank chassis. However, I'd strongly prefer one that's punched already, if possible. It's just a pain to do all of that with a blank chassis, especially if using a laydown transformer.

Is there a universal transformer mount out there that would allow me to use the PT I have with the Stout chassis?

[Baguette]

I would copy the Bright channel (leave off the .003 plate bypass cap which kills the treble response). Maybe change the tone control treble cut cap to .005. Maybe add the 220k to ground before gain stage 2.
Filtering looks spot on (not too much, but probably vastly sufficient for adequate hum rejection).

[Ledzepp007]

Thanks for the suggestions. I'm likely to just get the 18-watt PT and then run a GZ34. Should get me right on the 375VDC B+. Also, likely to share the cathode bias of V1 instead of biasing each triode of the AX7 separately.

[Ledzepp007]

I would copy the Bright channel (leave off the .003 plate bypass cap which kills the treble response). Maybe change the tone control treble cut cap to .005. Maybe add the 220k to ground before gain stage 2.
Filtering looks spot on (not too much, but probably vastly sufficient for adequate hum rejection).

220k to ground?

[Ledzepp007]

I would copy the Bright channel (leave off the .003 plate bypass cap which kills the treble response). Maybe change the tone control treble cut cap to .005. Maybe add the 220k to ground before gain stage 2.
Filtering looks spot on (not too much, but probably vastly sufficient for adequate hum rejection).

Would using a 40/20/20/20 cap can for the filtering be too much?

[Baguette]

The 220k to ground at the input of stage 2 to form the 220k / 220k voltage divider (which are the channels mixing resistors in the original design).
This will reduce slightly the range of the Volume pot for more controlable gain and useful range.

You can't share V1 cathodes since the 2 stages are in series. Well you could try it and it might work but I would certainly not bet on this (I'm pretty sure it's not gonna work, as stage 1 is directly fed into the next with little attenuation).

Both filtering options will work and will be safe (a GZ34 is perfectly ok with a 40uF reservoir cap). I would stick to vintage specs first and experiment later if the sound is not tight enough / the hum level unacceptable (should not be).

[Ledzepp007]

Sounds good everybody. I'll come up with a schematic and layout over the weekend!

[Ledzepp007]

Attached is a rough schematic. I changed the filtering a bit for use with the JJ 40/20/20/20 cap can. I also have an extra 5879 tube that I really want to use in this design. Unfortunately, my noobie self doesn't necessarily have the design skills. In my attempt here, I have nearly copied the preamp from a Gibson GA-40 amp and attempted to mix it with the normal 6G3 channel via 220K mixing resistor. I also chose (in this rough schematic) to mix the channels before the second half of the V1 12AX7. So, the 6G3 channel would go: 12AX7 A --> 12AX7B --> LTP. The GA-40 channel would go 5879 --> 12AX7B --> LTP. Is this crazy? Would the 5879 totally overwhelm the second half of the V1 12AX7? Should I mix in the 5879 before the LTP instead? Am I mixing the channels correctly?

Sorry for the questions onslaught and I appreciate the help.

[Ledzepp007]

Bad and/or ridiculous?

Also, anybody recommend any good, easy schematic programs for Mac?

[Ledzepp007]

Any help greatly appreciated.

[John]

Express SCH, pretty sure that's Mac friendly. If not, try googling for Jschem, I *think* that's still available as well. They are both free downloads and work great.

[Ledzepp007]

OK. How about this? Much more readable. Would this work? Any advice greatly appreciated.

[PRR]

V1B plate circuit looks wrong.

[Ledzepp007]

I believe it's consistent with the Fender schematic (I think!), but not sure.

Does anyone else notice anything else wrong?

[PRR]

> consistent with the Fender



Thought-test: how can there be +165V on the plate when there is no DC path to B+?

Gonna be a lot of un-used 15K laying around the factory after this batch ships.

Might also wonder how Fender's 100K doubled in size.

[Ledzepp007]

LOL. My bad.

Updated with stiffer filtering and fixed the DC issue. Thanks.

[Ledzepp007]

I see a mistake. The suppressor grid (pin 9) of the 5879 should go to ground, correct?

[Ledzepp007]

The layout I'm putting together is PTP. I expect a dumpster fire, but it's worth a try.

 

[Ledzepp007]

Here is another idea. A 6G3-ish build with an additional 5879 stage. Terrible idea?

[labb]

A 6G3 build will surely be a lot of fun....But there is a reason that Leo built so many amps with  the AB763 circuit. Just saying.....

[Ledzepp007]

A 6G3 build will surely be a lot of fun....But there is a reason that Leo built so many amps with  the AB763 circuit. Just saying.....

In my limited experience with both amps, the 6G3 is much, much nastier. Which suits me nicely :-)

[Ledzepp007]

Here's a more finished 6G3 Lite schematic, btw. Could be errors. I am careless if you can't tell.