Preamp stage help please.

[Stratz]

Thank you very much Justin. Much appreciated.

I just finished a single input 6L6 conversion with 2 gain stages (one 12AX7 for gain and half of the second 12AX7 for the PI leaving 1/2 of a triode unused) and with 5881's, while it doesn't scream dirty it does get a nice bluesy crunch but only at very high volume.

I think another 1/2 gain stage would have been perfect.
Is there any way to use the unused 1/2 of the 12AX7 as a gain stage? I used the AA964 Princeton schematic as a template (no tremolo on my build but with adjustable grid bias) which I've attached a link to below.
Rob.

Princeton Schematic

[Stratz]

If independent volume control isn't critical then you could put a second volume control at any of the other locations and it should be fine. Just a little noisier than necessary and without independent channel control for the higher gain channel.

Independent volume controls would be nice Chuck but either way would be ok as long as I preserve the blackface tone stack sound.
Has the circuit that you've proposed been tested? I really like the idea of the boost level control.
Thanks for the incredibly detailed response.
Rob.

[Justin Thomas]

I'm more of a "tack it together & see" guy, but, if you take out the reverb & tremolo on an AA1164 Princeton Reverb... might need to use a voltage divider between the second and third stages (which in the PR seems to be the 3.3MR/Reverb circuit) and tweak some values. But, it's a starting point.

A lot of the multi-gain-stage uber-distortion amps use the voltage dividers to keep the next stage from getting slammed with such a large signal that the end result is "AC/DC with Volume on 1, Metallica on 5, and Sonic Mayhem on 10!" It happened to that "Thorange" I made, having a gain stage, Volume, Tone, gain stage, gain stage, PI - no attenuation... Completely awesome, completely out of control, almost completely useless for anything less than Ted Nugent. But your results may differ, because of a more complex and lossy Tone stack.

As I said, I'm not entirely sure what I'm doing, but... you won't kill anything trying! Except your hearing. Maybe. And Chuck has probably already tried just about anything I mention, so... I get the impression we tend to bark up the same trees.

Justin

[Chuck H]

Well the circuit I posted has only been tested in my mind... But it works. The level control would sort of be the second volume control as I see it. You'll have the first volume control for the nice channel and the boost level would be the only additional gain control when selecting the naughty channel. I would surely use it on the second gain stage to overdrive the third. The TW circuit has only a single load resistor of 150k between the second and third stage and most TW users report that the useful gain adjustment peaks at one o'clock on the volume. So Justin's on to this one. You may need to attenuate between stage two and three.

[Stratz]

Well the circuit I posted has only been tested in my mind... But it works.

Sounds like a great plan Chuck and I'd really like to use it but first I need to solve another issue.... It never ends.

I replaced all of the old wax style coupling caps last night and I wanted to test it and see how it sounded with 2 gain stages or possibly 2 1/2 gain stages because I still cant figure out what type of phase inverter it has so one triode of the last 12AX7 (V3) may be used for gain. I'm not sure.

I'm running only input (1 meg grid leak and a 33k grid stopper) from V1 (12ax7) to V3 (12ax7) to the output stage. The 6AV6 (V2) is totally disconnected except for the heaters.

I get no audio unless I touch my DC volts probe to grid A (pin 7) of V3. Then the audio kicks in but the volume fluctuates very slowly. If I knew more about the Princeton style tremolo I might be able to figure out the problem.

I bumped up the preamp plate feed voltage so they now read higher than shown on the schematic which is attached in my first post.
Voltages off the top of my head are as follows.
V1 (12ax7) approx 190v on both plates. 1.4v on the cathodes.

V3 12ax7 (either PI or PI and 1/2 gain stage) pin 1=170v. Pin 7 = 310v <--- Should match pin 1?
V3 cathodes measure pin 3 1.5v. Pin 3 1.1v

V4 & V5 (6L6) = 440v plates/320v screens. Screens seem a bit low.
20v drop across the cathode bias resistor.

I need to figure out what type of phase inverter this amp has so I'll know whether or not both triodes of V3 are properly biased.

Thanks all.
Rob.
.

[Stratz]

Here are a couple of pictures for whatever it's worth. Very difficult to see whats going on with point to point.

The tone stack is from a Princeton AA964. Some components are jumpered together and tacked in.
One of the jumpers is going from the input to V1 grid (pin 7) to the middle lug of the volume control. The other jumper is V1 plate (pin 6) to V3 grid (pin 7) The mid range resistor (bass pot left lug to ground)is tacked in temporarily as I may add a mid range control.
EDIT. I forgot to mention. I measured all of the filter caps with my ESR meter and while one is border line they're all within spec.
Thanks guys.
Rob.

[Justin Thomas]

I need to figure out what type of phase inverter this amp has so I'll know whether or not both triodes of V3 are properly bias ed.
.

It's a Paraphase. Not exactly sure which specific "kind," because there are several. But, it's clearly not an LTPI or Cathodyne. If it were a Cathodyne, one of the power tubes would be fed by a plate, the other by a cathode. If you follow the grids of the 6L6s BACKWARDS from their .033 coupling caps, you see each cap fed by a plate. R24, 25, 28 are a series of voltage dividers which take the amplified signal from the plate of V3A and take it down to the same or similar l level as the signal feeding the grid of V3A. Then that triode (V3B) amplifies it to the same level as what's coming from the plate of V3A, but out of phase with V3A. The main trick with these is balancing the two...

Hope that helps! Funny enough, it's about the only PI I feel I have even a remote grasp on!

Justin

[Stratz]

Thank you Justin. If I change it to a cathodyne will I be able to use one triode for a gain stage?
Phase inverters are a weak point for me so I'll need some hand holding if I can use the entire tube.

[Justin Thomas]

Yup, just like a Princeton.

I guess I personally don't think of the "triode before the Cathodyne" as a "gain stage." I consider it as a part of the PI. I see its job as "drive the PI enough so the PI can drive the power tubes suitably." I guess it's technically a gain stage. It looks exactly like the others, so...

I would be curious, because the Paraphase uses a gain stage to drive one power tube, and another out-of-phase gain stage to drive the other. So basically, there's a gain stage before each power tube. The Cathodyne has a gain of 0.9 or something, so it needs a single gain stage before it to get enough drive. So it seems it may be kind of equivalent to me - a gain stage driving each power tube.

Where the difference comes in is how the two halves are balanced, how the circuit distorts under regular & overdrive conditions, and other such phenomena. Probably no more "gain" in one over the other, but the CHARACTER of the sound is where it counts.

The LTPI can also provide gain in and of itself, it's just how that one works. Personally, I enjoy amps with Cathodynes - there's a certain crispness right at the edges of overdrive. I've never played a Paraphase amp, other than Ampegs, and they were meant to never distort anyway, so I don't use them to judge.

Justin

[Stratz]

I absolutely understand your point and I'd be thrilled to leave the LTP right as it is but I believe my volume swell problem is in the PI. Unfortunately I am much less familiar (read "not a clue") with the LTP as I am with the cathodyne.

If you look at this schematic you'll see that the cathodyne only requires one triode of a 12AX7. Even though the other triode is being used for the tremolo circuit you can get away with only 1/2 a 12AX7 and just leave the other 1/2 un-wired..

I'm very new to building amplifiers. I started with a Mojo Deluxe Reverb kit last summer. Built a 5e3 Tweed kit and an 18 Watt Marshall from a kit and I really didnt learn much about amps at all because it's pretty much "paint by numbers" when you follow a layout. So I started to convert old PA's and Hi-Fi's to guitar amps and not only did I learn about 1000X more than any kit build I also taught myself how to read a schematic.

I've done a bunch of conversions and they sound really, really great. I did a 6L6 conversion and I made a LTP PI and I had nothing but problems so I guess I'm a bit intimidated by the LTP. The other problem is that I still dont know how to use the left over triode from a cathodyne PI for a gain stage so I'm equally stuck with the cathodyne and LTP if you get my drift,

I'm hoping someone will chime in and explain how to use the left over triode from a cathodyne as a gain stage. It's a shame to leave 1/2 a tube on the table so to speak.
Rob.

[Enzo]

You get no audio passing unless you put a meter probe on a grid? That sounds to me like that grid has nio return resistor. Powr off, what resistance to ground from that pin 7?

[Stratz]

You get no audio passing unless you put a meter probe on a grid? That sounds to me like that grid has nio return resistor. Powr off, what resistance to ground from that pin 7?

I get exactly 0 ohms sir.
Sounds like you're on the right track Enzo I had ran a jumper (just testing) from a .1 cap off the V3 pin 7 grid to the plate of the previous gain stage. I dont usually have to use grid resistors on Fender style preamp's so the grids must be seeing resistance from somewhere down the line.

What do I need to do to correct this?
Thanks!

[Chuck H]

Are you sure you got zero and not infinity? It should be pretty plain to see if that pin 7 is wired directly to ground.?.

[Enzo]

You can't let grids float.

[Stratz]

Are you sure you got zero and not infinity? It should be pretty plain to see if that pin 7 is wired directly to ground.?.

Sorry Chuck. I should have wrote infinity or OL. I had no continuity.

I hooked up my decade box and a 33kR seemed the best match between noisy and lower output. The tube is also properly biased now.
I can continue on with the modified preamp section now that I know the PI and output stage is wired properly.
Thanks
Rob.