Leslie 25 Guitar Amp

[scstill]

Looking for some feedback...
I modified this Leslie 25 Booster amp into a guitar amp, it's quite loud and sounds "decent" but has a few quirks that I'm hoping to fix.
  1) too bright; added 6L6 grid stop seemed to help. Other thoughts?
  2) too much overdrive even though all stages are balanced. I experimented with removing the 2nd stage but gain was way down. Thoughts about taming the overdrive?
  3) too noisy with guitar connected (HB or SC). Could the grounded heater leg be a culprit? cleaned up the wire dress a bit and used star grounds. All remaining components test to spec. filter caps replaced. somewhat stumped...

The following mods to convert to guitar amp
   1) Added a two stage 12AX7 preamp with guitar input.
   2) New filter caps and an added B+ node for the 12AX7.
   3) added on/off switch, AC input, LED pilot, and isolated speaker output (OT had grounded sec CT)

I wanted to leave the Leslie parts as stock as possible but changed the following, thoughts?
   1) replaced 10k vol pot with 1M (not sure why but more guitar amp like)
   2) originally came with 5881 but plate was high at 440v so replaced with higher spec 6L6GC; notice one plate is 6v higher would this be considered unbalanced?
   3) screen was 20v higher than plate so added a screen resistor to bring it down a bit.
   4) added 6L6GC grid stops (3K mellowed harsh treble); but 5C3 and Supro 1696 are a similar paraphase design w/o grid stop
   5) weird center tap OT secondary; tried lifting with standard guitar speaker jack (shown in pic) but seemed less noisy keeping w/isolated jack (see schematic)  NOTE pic shows lifted
   6) weird cap between 5881 plates (no real change when lifted)
 
   

[PRR]

> too much overdrive even though all stages are balanced.

Balanced how? I see a power amp with good gain, and a preamp with LOTS!! of gain in front. Much too much to play in a house or small club.

Add a Fender 3-knob tone-stack somewhere. Even with fixed resistors like a Valve Jr.

Change V1 to a 12AU7.

[Ronquest]

Like PRR said.   Tone stack or just a one knob treble circuit and volume after V1a (replace the 1M to ground) and you'll have treble control and gain reduction. 
A hundred ways to do it.  Adding the additional volume pot (or a voltage divider / 470k/470k typical) will lessen your noisy guitar, currently V1a is sending full signal gain to V1b.
Right now, your also getting all the low-end one could use on a guitar amp and that will make your amp distort/flub too soon IMO

[scstill]

Thanks I tried both 12AT7 and 12AU7, will definately use 12AU7 for playing at home; at idle B3 drops to 145v,Vplate to 45v, Vgrid to -1.8v.  Still would like to get the 12AX7 to work well.

I will experiment with the tone stack with fixed resistors in place of the 2nd stage grid leak. Also like the voltage divider thought before V1b.

The noise is before the volume pot as it goes up with volume. even with guitar unplugged. Still present (albeit less) with 12AU7. Maybe I'll try coax signal line from V1b to PI. Since its a volume thing could I assume the grounded heater side is safe for now?

Always learning so appreciate any wisdom...
By balanced I mean the idle grid voltage is in the center of the load line so the voltage swing from idle is equal (previously suggested by this forum to me in the Phirty6 post). At idle the design has -1.2v grid @ 140v plate, but this ratio is not the gain correct? Isn't the gain measured relative to this point as on Merlins Page 10 http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
Following that,  this design, at Vgrid =0, Vplate = 75 and at Vgrid = -2.5, Vplate=210. Which would be gain of 54 (210-75)/2.5-0) which seems reasonable for a preamp; although there are two of these.

Right now, your also getting all the low-end one could use on a guitar amp and that will make your amp distort/flub too soon IMO

Are you suggesting a bass tone stack in addition to treble?? Or a lower coupling cap?

[Esquirefreak]

I'd probably try to lift the K bypass caps on the first two stages. Also, it might be worth a shot to try a split plate load on V1B to attenuate the signal. Possibly lowering the total plate load resistor values in the preamp could help bring the gain down as well.

/Max

[scstill]

I'd probably try to lift the K bypass caps on the first two stages. Also, it might be worth a shot to try a split plate load on V1B to attenuate the signal. Possibly lowering the total plate load resistor values in the preamp could help bring the gain down as well.
/Max

There is a diminishing return though, currently with 100k the 12ax7 gain is 54 and even with 30k the gain is 40. And then you have two stages at 40.

With the 12au7 the gain with 100k is 10, Vplate is 168v,
but with the 1.5k cathode resistor the swing is not balanced and it idles at 50v plate
However the 12au7 sounds really nice even with uneven swing so I might just stay with that.

As a ref point 1, at 30k the 12au7 bias is more balanced (on paper) with only a slight gain reduction to 9. Idles at 85v. Sounds about the same as above

As ref point 2, increasing plate voltage for the 12au7 to 300v (from B2) allows idle at 90v and Grid at -3.5v. I cannot hear much of a tonal difference between this and just running it in the 12ax7 original spot.