Errors?

[Howlin' Mad Mac]

Re-tweaked my take on classic 15 watt p-p. Overall looks good to me but sometimes I tend to stare at the screen too long & can't see what's right in front of my face. Also not sure if it needs some additional coupling capacitors & if I did the math for the supply & bias right? Any suggestions?

American Girl.pdf

[Tonewood]

Cap from triode 2 to triode 3 & move load or bias for the cathode follower?

[Howlin' Mad Mac]

Thanks but I just realized I posted an earlier revision. This is the one I meant to post where I split the load going into the follower & corected the cathode resistor. Wasn't sure whether or not to put a cap between them? .0047u or bigger, .01-.022 maybe? Hmmm... Anyway here is the schematic you were supposed to see.

American Girl r1.pdf

[Gingertube]

Is that 100K pot in the anode of the 1st stage a trimmer or is it a front panel control?
With DC through the pot you will get earthquakes when you try to adjust it. The rails to the 1st 2 stages seem low. 12AX7 triode sections like around 180V minimum at the anode to limit grid current. That infers a rail voltage of say 280 to 300V. I'm a big fan of the concertina splitter for guitar amps (providing you use a high mu tube) - nice to see.
You WILL need a MUCH higher rail for the concertina splitter. With 35 volts bias on the 6V6 you will want to swing at least 35V peak (70 V peak to peak) at the anode and cathode (opposite phases) and want say 120V minimum left across the tube when anode is minimum and cathode is maximum. Since you have feedback around the power amp it needs some headroom to accommodate that. Get that power supply feed for the concertina up to say 350V.
I think you need more filter capacitance in that first filter (the series connected 32uF => 16uF total) I would be using 100uF for those caps to give 50uF effective for the series connected pair. The voltage share resistors seem to read as 220R, I'm sure you ment 220K. The wiper of the bass control connects to the top of that pot.

Also the amp has only 2 gain stages, it will be very clean. If you want overdrive sound then you will need to:
1) ditch the 100K pot on stage 1 and just use a fixed 100K
2) convert the cathode follower to another gain stage and drive the tone stack off its anode.
3) add a bypass cap to the cathode follower anode supply.
4) increase the load resistor on the second stage to 100K

For tube longevity you may also want to increase the 6V6 screen resistors to 1K - particularly at that voltage.

Just a few initial salvos at the "barn door".
Good Luck with your amp,
Ian

[sgelectric]

The power supply fuse should precede the switch in the circuit.

[J M Fahey]

You just can`t use that phase splitter as a feedback stage.
Any signal you inject at the cathode (what you do with that 56K resistor from the speaker ) will unbalance it into uselessness.
As bad as that, plate and cathode resistors must be the same for any symmetry; hard to do because you parallel the lower one with a 5K pot, plus a parallel .1 cap depending on the ¿presence? pot setting.
The variable resistor on the first triode plate does nothing the following volume pot does not better, plus adding more scratch than a Hip Hop DJ.
Sorry.

[Howlin' Mad Mac]

Ah ha! You've brought up some good points there Ian. 1st I took your advice & stiffened up the PS significantly (50u/32u/32u/16u) as well as altered rail resistor values (1k/10k/1k) & corrected share resistor values (typo, they were supposed to be 220k, not 220r.) Voltages figured using Duncan's PSU II using 20k load so kind of a "best guess" there. Still not happy with 2nd stage but not sure what course of action to take. My thoughts are I could split the triodes & place the stack between them & move the volume pot after the now 3rd stage or keep the stack as is & use the follower triode to create a long tailed pair but wouldn't a LTP shift to much gain (for a 15w amp) to the PI & throw any chance of keeping overdrive balanced between the Pre/PI/Pwr sections? Thoughts? Ideas? PT is only putting out 325-0-325 @ 70mA so I don't think I've really got enough current to add another pair of triodes.

American Girl r1.5.pdf

[Howlin' Mad Mac]

Huh? Why would that make any difference whatsoever SG?

[Howlin' Mad Mac]

Ok J M, wasn't sure if I could add a presence control to a cathodyne PI or not... guess that's not allowed. As for the 100k pot on the 1st stage, this is for static split-load adjustment only, not a front panel control. I realize it would make noise if adjusted while the amp was in use.

[Howlin' Mad Mac]

Further tweak to PS bias supply shown here... Seems all that is still needed is what to do with that 3rd triode & "fix" the PI & should be ready to build.

American Girl r2.pdf

[Howlin' Mad Mac]

Moving this to new thread... Please see "Re-tweaked Design" & leave any additional comments there. Thanks.