Re: 6SL7>Concertina>6SN7


"RichL" <rpleavitt@xxxxxxxxx> wrote in message
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Phil S. <psymonds@xxxxxxxxxxx> wrote:
"RichL" <rpleavitt@xxxxxxxxx> wrote in message
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Phil S. <psymonds@xxxxxxxxxxx> wrote:
I belive it's done! There was lots of good advice here. Some of it
was better advice, but, really, it was all great. I learned some
things along the way, and I found out (maybe) that I actually did
have a vision of what would make this amp turn out the way I
imagined.

So, the line up is a Fender tone stack, volume, treble, bass with
one section of a 6SL7 at the beginning and the end. Feeding
through a .022 cap to the concertina PI, using half of a 6SL7.
Outputs are a pair of 6SN7's, wired in parallel to permit pulling
one, essentially a "4 tube" power section. Aside from the third
gain stage, which was eliminated, this was pretty much my original
idea. BTW, I scored some very pretty, new looking, chome dome
Sylvanias for the amp at an average cost of about $6 per tube from
eBay.

Running the volume wiper in to the floating paraphase PI sounded
like poo. I decided that the tonestack needed a recovery/gain stage
following it and that the gain provided by the floating paraphase
was the wrong kind of gain.

I didn't have .01 caps, so I used .015 caps to couple the concertina
to the finals. This amp has the kind of volume you'd expect from a
10W amp, loud, but certainly not overpowering. It has plenty of
good clean tone and sound great with both guitars (p90's;
h/buckers). It is the only homebrew amp I've got that doesn't hum
with the p90's set for only bridge or neck. The Gilmore Jr doesn't
count as homebrew in my book. The amp is quiet (hiss, hum). I
think it's a keeper.

Special thanks to all who contributed, even if your suggestions
didn't make it into the final. It was a gas to try something a
little different.

Now, I'll see if I can put together some clips. As I've said
before, I'm not a very good guitar player, but I'll see what I can
do.
Please give me a few days. I'll take some pictures, too..

Phil

Did you wind up using the idea of the resistor to drop the plate
voltage at the power tubes?



This really was a must do, as plate voltage was above the 450v rating
of the caps, as well as the spec *** for the 6SN7.

Yes, I put a 1K-7W between the rectifier diodes and the first filter
cap. It produced a fairly significant drop, IIRC about 35v across the
resistor, but it also has a dropping effect on voltage comming from
the rectifier by 25-40v depending on which configuration it is
benchmarked from. From 460-0-460, B+ is ~420, plate voltage is ~380,
B+ to the concertina is ~340 and after the 56K plate load resistor it
is ~276, and preamp B+ is ~338 and voltage after the 100K plate load
resistor is ~205.

Plate voltage on the PI is about 25v over spec and I'm not exactly
losing sleep over it but it might be possible to tame it without
fooling with the plate load resistor, as the messes with the whole PI.

What I'm getting at is that I think, actually, this amp would benefit
from a change in the supply ladder which is presently as follows:
Diodes > 420v > 1K > 380v (plate supply) > 10K > 340v (PI supply) >
1K > 338v (preamp supply). There is one 20-20-20 can cap, so it is
1K>20u>10K>20u>1K>20u if this makes sense.

I am thinking along these lines...yes, I lied, projects are never
really finished, but this one is nearly so...:
Diodes > 680ohm > 15K > 100 ohm (3x 20u caps omitted, I'm bad at ASCII
art)..
I think this will let plate voltage on the power tubes rise a little,
while lowering the voltage on the PI. It is hard to leave well
enough alone.

Too bad there aren't variable high-power resistors that you could pop in
there to play around with to find the sweet spot! But if you knew how
much current draw there was on each plate and you knew what voltages you
wanted, you could eliminate the guessing game.


Actually, they make them, and I only need a 5-watter, but not needed here.
It is just math I am too lazy to do after a glass or two of wine with
dinner, or too pressed to do earlier in the day when fooling with amps isn't
the priority. I'm pretty sure where this stuff will settle out.


.

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