Curious schematic

[EFK]

Can some of you more learned individuals take a look at this and offer some basic explanation as to (1) why all the crazy capacitors placed willy-nilly everywhere and (2) why such large values in dropping resistors as well as what type of PI is this? I know my drawing is unprofessional, but I'm fairly confident that I've got it right. My first impression - and granted I don't think I really understand all this - is that there is a lot of signal attenuation going on, so I wonder, Why?

[BiBi]

These Valco amps are really difficult to follow because of the draping of wires going every which way.
From my limited understanding: the "crazy capacitors" on the plate resistors are there to quell oscillation because the designer used such high values of plate resistors-- to get more gain and less harmonic distortion (!) from that individual stage. That might also explain the amount of attenuation going on.
As for the PI it is a paraphase type.
Hope that helps-- there are some Valco schematics out and about. Alot of them I have seen have a similar style. I've only heard one of the amps in person, but it had a really unique character.

[octal]

Are you sure you got that PI correct? The cathodes are really directly connected to ground? I don't claim to be an expert on grid-leak bias, but usually you'll see higher grid resistor values on a tube set up this way.

The tone control appears to use the "Steve" miller capacitance of the PI tube to achieve treble roll off with the series resistance the tone pot adds. That's different!

I agree that the various extra capacitors are used for tone shaping & decoupling to prevent oscillation.

Nathan

[EFK]

Thanks guys. I did make a mistake in the PI and fixed it but haven't had a chance to upload the correction - the PI cathodes are tied to ground via a 2.2K resistor. Completely wanky amp to work on, and while it isn't very loud for a 6l6 amp, it's got a very unique tone. Crank the volumes and it's dead-on for the lemon song off LZII, and that's w/ a SG... I'd love to try it w/ a Tele!!! hahahahaha Some of the inputs have a faint 'cocked wah' sound, and the treble inputs sound like they're plugged into an old rangemaster.

I'd love to get a little more volume out of it, but I don;t think that can be done w/o changing the tone.

PS: here's the updated preamp schematic

[Merlinb]

My first impression - and granted I don't think I really understand all this - is that there is a lot of signal attenuation going on, so I wonder, Why?

There is certainly a lot of treble attenuation, but there isn't much broad-band attenuation. Most of those "dropping resistors" don't really drop anything but treble, they simply add to the grid-stopper resistance.
The PI is a simple paraphase with shared cathode:
The Valve Wizard -Paraphase

[EFK]

Hi Merlin - after Bibi mentioned above that it was a paraphase PI, I went to your site as I remembered seeing the detailed explanation of the paraphase therein. Very helpful in breaking it down - thank you! What I meant by using the term dropping resistors was an incorrect reference to the plate resistors being so large, and/or there being two of them in series (100K and 270K) as on V1. You are referring to the grid resistors either directly inline as on V2 grid or those being used as voltage dividers (at least, that's how it looks) on V1 as being used to cut treble, or am I misinterpreting?

I would love to get a little more volume out of this amp; it's one of those amps that sounds so great (to my ear) that I want more! I wish it could at least hit the volume level of my deluxe, but my guess is that there is just not enough juice being generated in the preamp for the power section to work with?

[Merlinb]

I wish it could at least hit the volume level of my deluxe, but my guess is that there is just not enough juice being generated in the preamp for the power section to work with?

That seems unlikely- there looks to be enough gain in the high-input channel to drive the power valves to full output (they're not 6550s or something are they?). I suppose you could bypass that 2.2k cathode resistor...

What I meant by using the term dropping resistors was an incorrect reference to the plate resistors being so large, and/or there being two of them in series (100K and 270K) as on V1

Believe it or not, those 100k/0.05uF combinations are actually PSU filters, not load resistors! By using very large PSU dropping resistors they can use the really small 50nF smoothing caps instead of electros.

[EFK]

Here's the power amp. I'm using two JAN 6l6wgb and I changed out the bias resistor from 250 ohm (which had them dissipating 19 watts at idle by calculation) to a 200 ohm resistor (which raises that to about 22W at idle). I also added a 30uf bypass cap on a switch but frankly it doesn't seem to do very much. I'm now getting 382 DC on the plates, and that's at 123VAC input. I am still using the 5V4 rectifier, I think it could take a GZ34 with no problems as I've got new electrolytics in there but it would probably change the feel of the amp a bit.

I think I'll probably just leave it alone. I thought about adding a bypass cab to V2 but haven't done so as I don;t really want to change the character of the amp too much - I love fiddling with old amps, but there's no use in trying (too hard... ) to make them something they are not. Maybe something like a minimal 1 or 2 uf would boost volume a bit without changing the frequency response too much?

Is this weird tone control sucking any volume off somehow? Also wondering why there are two different value "mix" resistors - 220K into the tone off channel 1, but 470K into the tone off channel 2? That doesn't seem the usual thing to do.