AO-44 conversion (Sluckey ???)

[Cobbler]

I have a Hammond AO-44 reverb amp conversion project in my future and after much research on the interwebs, I'm liking Sluckey's PeeWee conversion best.  A clone of the Marshall 1930 would be cool.  I like the sound clips I've heard of this amp but I don't have need for 2 channels or vibrato on this one, plus I've read that the 1930 is notorious for having quite a hiss at low volume because of how the pre-amp circuitry is done.  I'm looking for something a little bit "gainy" but wanting to be able to play at low volumes as well as what the 2 ECL86's can produce at full throttle.

My question is regarding a couple mod options I am wondering what the end result would be...one or the other, not both.
1) Add Post-PI MV?
2) Add Presence control to the negative feedback.

[sluckey]

I don't think either of those mods would be worthwhile in my peewee. Not even sure how you would add a presence control to that cathodyne PI. I did consider adding a raw control under the tonestack but there just isn't enough room on the front of that chassis.

Even with an AB763 preamp, there's plenty of signal to overdrive the power amp. That's why I removed the bypass caps on the preamp cathodes.

[Cobbler]

I finished up the conversion today, following the Peewee schematic. It was a bit noisy, I added an artificial center tap and that seemed to help a bit. I suspect that the small chassis and cramped point-to-point has something to do with this.

Is there a way to tame this beast?  Here's what I get from 1-10 on the volume knob....
1-2.5 nice Fendery clean
2.5-3.5 nice grit when I dig in, clean when I pick lightly
3.5-5 very aggressive dirt, very loose, have to back out most of the bass to make it useable
5-10 loud, loose and raunchy. Might be usable through a quality 4/12 but very flabby in an open back, single speaker can.

Would a 12a_7 with a lower gain factor tame things down?  Would this also cut into the SPL that this thing is currently capable of producing?

[sluckey]

Here's what I get from 1-10 on the volume knob....

Your results sound similar to what I remember. My PeeWee is long gone. It became a donor for a Magnatone M2 (L'il Maggie). Using a lower gain tube will probably help. I'd try a 5751 or 12AY7. Probably won't notice much SPL change with a 5751, but the 12AY7 will probably be quieter.

You don't have cathode bypass caps on the 12AX7? I removed them from PeeWee to reduce the bass and gain.

[jojokeo]

Adjust your signal voltage by creating a voltage divider as seen on the attachment. The 100K gives 82.5% output, 220K gives 68%, and 470K gives 50%. This should give you what you're looking for while keeping the 12ax7 in place.

[Cobbler]

Could I use a 500k pot and have it adjustable?  This thing does behave very differently with different guitars/pickups.

[jojokeo]

Could I use a 500k pot and have it adjustable?  This thing does behave very differently with different guitars/pickups.

Yes, I just used a resistor to make it easier to insert into the circuit for you. You could also use a switch if you wanted. You would leave the lower 470K resistor though.

[Cobbler]

I want to get inside and do some tweaking but I just can't stop playing long enough to do it. This thing sounds glorious.

[sluckey]

   That really looks good! That little AO-44 is pretty powerful. Cute too. Enjoy...

[Cobbler]

I took some time to tinker inside this thing today. I tried the voltage divider and didn't care for what it did to the tone... seems like even slight changes started bleeding off "chime". Then I replaced the 68k resistor with a 47k on the negative feedback. That gave me a bit more headroom. I think what I'm after will require a speaker purchase. The speaker from the organ donation just doesn't seem willing to perform its new task.

[jojokeo]

I took some time to tinker inside this thing today. I tried the voltage divider and didn't care for what it did to the tone... seems like even slight changes started bleeding off "chime".

Try it again but also put a cap across it so that the high end that passes through is preserved. Try a .001uF and work around till you find what you like.

[Toxophilite]

Very cool! and inspiring
I'm going to start building one of these as I have an AO-43 kicking around
S.Luckey
Did you do anything else to the circuit besides:
- adding a preamp tube
-fender pre circuit
- and adjust the power supply resistors


Thanks

[sluckey]

Everything I did is shown on the schematic.

Oh, I did one more thing... I scrapped  Pee Wee and used the iron to build a Magnetone M-2.

[Toxophilite]

Right!
I wonder if I should do all the Peewee mods first before building the Magnetone??