Amp should be 100 watts but low on power, why?

[SoundmasterG]

The amp started life as a Sovtek MIG 100U, and I kept the power transformer because it had the correct voltages and was running a quad of EL34's, so I figured it should keep up. I modified it into a clone of a VOX AC100, which was a single channel amp. I added a second channel with higher gain in addition to the stock Vox single channel. I'm using the same phase inverter as VOX used, a 12AU7 seesaw variant of the paraphase. To mix the channels, I added another triode (half a 12AU7) in parallel with the first triode in the phase inverter. They share a plate resistor and cathode resistor, and the second triode in the phase inverter has it's own plate resistor for necessity, and it's own cathode resistor for better performance. The VOX circuit had both halves of the phase inverter sharing the cathode resistor and only used a single 12AU7 instead of the mexer setup like I'm using.

I seem to have a problem in the phase inverter area as I only get around 23v AC signal out of the first half of the phase inverter, and this is with a 55v signal coming in to the phase inverter out of the high gain preamp. The bias is -35v in fixed bias. Cathode bias has around 29v on the cathodes. I pulled the output tubes and get about the same signal swing without them in there, so I don't think the output stage is affecting things. I built the stock Vox circuit with the single triode, and a Fender LTP in an outboard chassis and subbed it one at a time into my amp in place of my phase inverter circuit, and I get around the same performance...which is not enough. The supply voltage for the phase inverter is around 410v in cathode bias, and about 430v in fixed bias, so there should be plenty of signal swing. The signal looks good on the scope too. I've checked and verified all of my wiring several times, and checked parts values and everything appears correct. The amp is perfectly stable with low hum and good tone, no oscillations that I can hear or see on the scope....yet I still have low power.....


As a Sovtek, the power transformer was running a quad of EL34's at 485v, and three 12AX7's. In my amp as an modified AC100, I have an EF86 and 12AX7 in one channel, and a couple 12DW7's in the clean channel that ends up with the same arrangement of a 12AU7 triode as the input and a 12AX7 gain stage and cathode follower as in the Vox. I also have a 12AU7 phase inverter, with the other 12AU7 half of the 12DW7 as the mixer triode for the first half of the phase inverter, and a quad of JJ KT77's. With the extra tubes over the Sovtek, the filaments are still at 6.3v, so no issue there.....and the B+ drops to 466v in my circuit instead of the 485 in the Sovtek.

Here are the measurements on the amp.

IDLE - FIXED BIAS

Plate = 466v
Screen = 450v

DIMED - FIXED BIAS

Plate = 422v
Screen = 371v

IDLE - CATHODE BIAS

Plate = 435v
Screen = 420v

DIMED - CATHODE BIAS

Plate = 423v
Screen = 373v

I measured output power and I get about 37.4 watts clean in fixed bias and 44.5 watts clean in cathode bias. That seems backwards to me! Dimed I get 61 watts fixed bias and 54.3 watts cathode bias. This is with a quad of JJ KT77's and a primary impedance on the output transformer of 1750 ohms. I tried a quad of OS Mullard EL34's and the power output was about the same. Each power tube is around 40ma of idle cathode current. I'm not sure how to measure the plate current at the moment.

I don't think I get enough drive out of the phase inverter. When measured clean in fixed bias, I get about 23v AC at the plate of the first triode in the phase inverter, and 17v out of the second triode. I know they aren't balanced all that well right now but I'm not worried about that as much as the low power. I don't think it gets back to 0v on the power tube grids. I looked at it on the scope, and I switched the scope from monitoring AC to DC and it was below the 0v line. Is that how you check? Does anyone have any ideas of what kind of things I should check or where I should go from here?

Thanks for any help you can provide!

Greg

[Leevi]

Have you tried to use 12AX7 in PI instead of 12AU7?
Is there any difference in power if you measure the power separately with one power tube pair at a time?
/Leevi

[PRR]

> To mix the channels, I added another triode (half a 12AU7) in parallel with the first triode in the phase inverter.

That trick gives low output. It is fine in small-signal stages, but not as the last stage before the Power bottles.

Can you sketch the exact schematic for the driver with values and voltages?

[SoundmasterG]

> To mix the channels, I added another triode (half a 12AU7) in parallel with the first triode in the phase inverter.

That trick gives low output. It is fine in small-signal stages, but not as the last stage before the Power bottles.

Can you sketch the exact schematic for the driver with values and voltages?

Because I suspected my circuit as being a possible problem, I disconnected it and built the stock AC100 seesaw paraphase, and a Fender LTP in a seperate chassis and temporarily subbed those in, one at a time. I had the same low power problems, and my circuit actually seemed to have more gain than the stock Vox one that it is based off of. I wouldn't think that circuit would give half the power out of the amp as it should, but you're the expert PRR. I can post a copy of the schematic later this week. I just started a temp job and am working a ton of hours at the moment.

I did try a 12AX7 in the circuit, and changed my other tube to a 12AX7 instead of a 12DW7 so I'd have an AX triode as both halves of the first section in the inverter, and I had a little more gain, but not much.

I'll see what I can add later this week, and thanks for the help.

Greg

[PRR]

Screen voltage drops a lot for a "100W" amp.

On a recent project I had a similar 80V sag from idle to ROAR but in that case my goal was modest power with gentle failure (burn a resistor instead of a transformer). There coulda been 35W, but with the oversize screen resistor I actually got 20W, which was ample for jazz. However you are obviously aiming much higher. Maybe your screens sag too much? I never studied a large stage-worthy EL34 amp.

[The_Gaz]

I suspected something fishy with the screens too over at the AX84 forum when Greg tried his luck there - In a recent build I had mistakenly used 10k screen resistors on a pair of KT77s instead of 1k, and it brought my 45 watt amp to a mere 10 watts. Greg said all seemed well there... I really hope you can solve the problem, and I could offer something more!

[SoundmasterG]

Hmmm, thats interesting about the screen sagging too much. Since I've never seen a stock AC100 and haven't messed with 100 watt EL34 type amps before, I have no idea if the screen sag is normal or not. I would think it might be for an EL34 since they are a true pentode, but I'd think a beam tube like a KT77 wouldn't be as susceptible to sag on the screen.....I used an LCR 50uf/50uf can in parallel that I had for the screen cap. I had it on the shelf for awhile and I reformed it on my Tel-Ohmike and it seemed to be ok, but I suppose it may not have been. Could the electrolytic cap I used for the screen be a possible cause for the saggy screen? If so that isn't that hard to change out once I pull the board out of the way. I would have to look at the amp but I could possibly bypass it with a single 100uf cap to see if there is a difference. I am currently bypassing it with a small film cap too. I appreciate the help guys!

Greg

[PRR]

> wouldn't be as susceptible to sag on the screen.....

Sure it would. Maximum current (beam-tetrode or pentode, different patents for the same thing) is roughly proportional to G2 voltage. And 3,500/pair loading is a low impedance load, needs HIGH current, which means high G2 voltage. You could plot it, but from other amps I know these tubes will need G2 very near to plate voltage to pull 3500 well.

> Could the electrolytic cap I used for the screen be a possible cause for the saggy screen?

No. Or unlikely.

What is between your 430V plate supply and your 370V G2 point? Reisistor? Choke?

An unexpected possibility is that your OT is mis-tapped (misconnected) and not really 1750 but some higher value. That would give low power with high G2 current.

[SoundmasterG]

I just meant that I would think the true pentode (EL34) would have the possibility of having more max screen current than the beam tetrode (KT77). That isn't based on me looking at the datasheet and reading the numbers or anything, just from me having heard that true pentodes have more excessive screen current issues and need a larger screen resistor when compared to beam tetrodes, but maybe that is people comparing EL34's to 6L6's or something.

The 430v plate is cathode biased.....its more like 466v when in fixed bias, and the screen likewise is higher in fixed bias of course as I noted above. I've got a 19 H 100ma Heyboer choke in between the plate supply and screen supply, and there are individual 1k 5 watt screen grid resistors at each screen grid on the socket. I guess I could bypass those on the bench to see if there is much of a power difference. I was looking at some of the original Vox AC100 fixed bias and VOX AC80/100 cathode biased amps and they used a 470 ohm screen grid resistor on the cathode biased version and nothing on the fixed bias one. Even so, the cathode biased version is supposed to make around 80 watts and I'm at half that. Its an easy check to bypass the local screen grid resistors so I'll try that in a little bit. I'm also working on fixing my schematic to my latest design in the amp and I hope to upload it later tonight. I believe I have a couple things wrong on it with the location of my HT fuse and the channel switching isn't working right, but I'm not worried about that right now. It might just be that I have something miswired in the amp, but at this point I'm not sure where to look, and I still suspect my PI is not working correctly or giving enough swing, but am not sure where to begin on there either.

I thought about the OT being mistapped, and thought it was at first, but I was testing it wrong. I still had the primary hooked up when I tested it initially. After I disconnected the primary and ran a small AC voltage into it, I did get a ratio that worked out to be right around 1750 ohms, and thats what Heyboer said it should be too. I tried it mismatched on the output and do get more power of course, but still not where it should be.

I post again later with the schematic hopefully and the results of my bypassing screen resistor tests. Thanks a LOT for your help PRR!

Greg

[PRR]

> true pentodes have more excessive screen current
 
Yes, and this can lead to melt-down; OTOH if screen is too low you can't make big plate current swing which means low output which is your complaint.

1K per screen seems high for a HIGH power EL34 amp. 6L6/KT66/KT77 (all similar) might do OK with 1K, but we more often see 470.

If you go 1K per screen, you might want 10uFd at each screen..... ah, but on a quad-bottle amp that's just too-too cluttered.

Short the screen resistors and hit a chord or two. That will tell you if it makes a big difference.

> testing it wrong. I still had the primary hooked up

If connected to some cold plates, that should not affect the measurement. Unless you have plate-ground diodes or some other stuff loading the primary. A cold tube's plate is just a little bit of metal in a hard vacuum, next best thing to "nothing". `

[SoundmasterG]

I had to think about what else I did wrong when I was testing the OT. I think I was measuring it though the load instead of direct to the secondary leads......its been a little while since I did it and I had forgot what I did eariler.......but when I did it the right way it was 1750 ohms. When I did it the wrong way through the load, it was more like 4650 ohms.

I just tried to bypass the screen grid resistors a little while ago and had something smoke. I am not sure what happened yet....either a cathode wire was touching against a plate turret (it is partially melted there) or the 50uf/50uf can cap had an issue. I'm not totally comfortable with that particular can...even though I reformed it....so I ordered a new F & T to stick in it's place just to be sure. I think the cathode wire I must have partially melted when I was soldering something else because it was shorting out to the plate turret....sooo I'm going to clean that up and pull the board anyway because I want to make sure everything is correct underneath it, (wires from turret to turret on bottom of board) and replace the can too once it comes in. So I'll be delayed a bit in testing the screen numbers when the resistors are bypassed, but when I get to it I'll post results. I may try some 470 ohmers in there too like Vox did.

Working on the schematic right now and should be able to post it in a couple hours. Thanks again!

Greg

[SoundmasterG]

Ok, I have a copy of the schematic....but the file is too large to upload. It is a pdf, or a couple dwg's...but way too large to upload here. I can email it if you would like me to PRR...if so send me your email in a PM. Otherwise I'm not sure how to get it sized smaller to upload here. Its a work in progress at this point but I believe I fixed a couple issues I had to reflect the current wiring. Right now the channel switching works backwards from how I would like it to work so it is disconnected. I plan to add power scaling after the amp is working correctly and tweaked for sound. As you will see once you see it, my phase inverter setup is basically like the Vox AC100, but I have that 2nd triode (V4A) in parallel with the first one (V5A) to give a mixing function. When both channels are used there is slightly more gain than when just one input is being used....but it isn't excessive. It also reduces hum from what I've read elsewhere, and this amp is very quiet so maybe that contributes....I was wondering if I move the coupling caps to the power amp (C28 and C29) out of the local feedback loop if there would be any improvement in the phase inverter function?

Comments and/or critiques are appreciated once I'm able to get it to you.

Greg

[PRR]

I have to run off to a lecture. I've condensed the schematic to the driver section so it will post. Maybe someone here knows if this plate-mixer has enough ooomph to flog the EL34s.

[HotBluePlates]

I'm using the same phase inverter as VOX used, a 12AU7 seesaw variant of the paraphase.

Nah, Vox is using a plain old paraphase. For a real seesaw, go to www.pmillett.com, and under the books section, download High Fideilty Circuit Design by Norman Crowhurst (I'm assuming your internet is better than mine, which is slower than dial-up). In the Drivers and Phase Inverters chapter, you'll see a real see-saw, which should work well for you.

But the Vox circuit worked, and the power really was 100w. So I think you got too clever with the 2 preamps and 2 tubes to mix them. Have you tried disconnecting 1 of the 12AU& sections fed by the preamps and doubling the plate and cathode resistors? Try it, and see if the power comes back. It'd be faster to test than calculate...

[SoundmasterG]

I'm using the same phase inverter as VOX used, a 12AU7 seesaw variant of the paraphase.

Nah, Vox is using a plain old paraphase. For a real seesaw, go to www.pmillett.com, and under the books section, download High Fideilty Circuit Design by Norman Crowhurst (I'm assuming your internet is better than mine, which is slower than dial-up). In the Drivers and Phase Inverters chapter, you'll see a real see-saw, which should work well for you.

But the Vox circuit worked, and the power really was 100w. So I think you got too clever with the 2 preamps and 2 tubes to mix them. Have you tried disconnecting 1 of the 12AU& sections fed by the preamps and doubling the plate and cathode resistors? Try it, and see if the power comes back. It'd be faster to test than calculate...

Thank you for condensing that down to post it PRR! I couldn't figure out how to go from a large pdf to a smaller file. If you get time later, I'd love to hear your thoughts.


HBP,

If you look at Merlin's site on the floating paraphase (seesaw) version of the phase inverter, the arrangement matches that which Vox used so I believe it is a seesaw paraphase in the AC100. Regardless, if you read the first post, you can see that I built an outboard chassis with the stock Vox phase inverter, and also the Fender LTP, and I disconnected what I needed to in my amp to sub those in one at a time, and got about the same swing as with what I get now, though slightly less. I also tried a 12AX7 in place of the stock 12AU7 and there was a slight difference but not enough to get the 100 watts this amp should make. I've actually got a copy of that book by Crowhurst and I looked in there also. I did try disconnecting the mixer and doubling the plate and cathode resistors back up and I had a little less gain than what I have with the mixer setup. I am basically using what Kevin O' Connor calls a "Tweed Mixer" though I'm using it in the phase inverter. Merlin also talks about that mixer circuit in his book, though again, not in a phase inverter. I figured since the first half of the seesaw is a standard gain stage that it should work fine, and it does do the mixing part well.

As I was telling PRR in my email when I sent the schematic, I had another problem with the amp when I tried to bypass the screen grid resistors and test for sag. A cathode wire was shorted to a plate turret and a component started smoking. Of course I shut it off quickly, but I believe the tubes are toast, though I have to test them still. There may be other parts in the amp that are suspect, and I will have to test before I can safely fire it up again. I will be replacing the reformed LCR can cap with a new one, just to be sure, and when I do that, I have to pull the board. So when I do that, I will look underneath it to make sure all my wires are correctly connected. I'm not happy with my layout anyway, due to parts being too close together and what not. So I may redesign the board to be one long board instead of two different ones. (I had to allow room for the Z mount OT, but I figured out a way to mount it differently to allow room for a full length board.)

So I'm delayed a bit, but after I replace the cap and check for other suspect parts, and make sure my connections are correct, I will probably still have the low power and lack of swing problem, so I'll still be looking for help. :)

Greg

[PRR]

> if this plate-mixer has enough ooomph to flog the EL34s

No.

> coming out of the first half of the phase inverter I only get around 25v AC or so. On the scope it doesn't drive to 0v either, so I've wondered if the phase inverter is my problem. Maybe I have something wired wrong or maybe the mixer circuit that I used is a problem, etc.

Yes. See simulation below. With two plates parallel, but only one grid driven, there is a strict limit how far "up" you can swing. When I drive (my) TU4 off, TU5 still pulls current, so the plates node does not rise much. Additionally the common cathode resistor diverts *more* current to TU5, though separate resistors or a common cap makes little difference. Neither does changing the 100K plate load.

The sim shows 11V peak up swing initially. After a while, the asymetric wave and coupling cap drifts toward 19V peak upswing. But we are needing 30V-40V.

The plate mixer "may" be good for very small signals and very low budgets. And maybe not, considering how rarely it is seen.

Why mix this way? The gain is also low. If you omit (my) TU5, and use two 470K resistors to mix your two paths onto the grid of TU4, the gain is the same but output is MUCH larger.

I also note that the VOX-original 47K loads got changed to 100K and 220K.

> I did wire up the stock VOX AC100 seesaw/paraphase inverter... actually got less swing out of it. I also did the same with a Fender LTP...
> bypassing the 1k screen grid resistors  ....I had a component smoke. ..., I think a cathode wire had shorted to a plate turret.
> my tubes are now suspect as when they are in circuit, the B+ is dropping to 400v or so and two of the tubes start to glow a bunch
> I pulled the power tubes (JJ KT77's) to test them in my B & K 747B tube tester and when I tried it with the first tube, the needle went backwards,

There's multiple other gremlins on your bench.

[SoundmasterG]

Thank you for taking the time to look at that PRR. Maybe I did get too cute with trying to mix this thing. I had thought about utilizing that extra triode as a gain stage and just using the Fender method of mixing, but I didn't want to lose any possible gain since the Vox circuit was already pretty clean. It isn't a huge thing to change that though if I'm going to build a new board anyway.

Why mix this way? The gain is also low. If you omit (my) TU5, and use two 470K resistors to mix your two paths onto the grid of TU4, the gain is the same but output is MUCH larger.

I guess it is a sign that I don't know what I'm doing here...haha. I'm mostly self taught so there are lots of gaps in my knowledge. It is an interesting circuit even if it doesn't seem to work as intended.

I also note that the VOX-original 47K loads got changed to 100K and 220K.

The actual circuit progression was that Vox made the 80/100 version of the AC100 which was cathode biased and about 80 watts output. This is the version the Beatles used. It used 220 plate resistors for the PI and a common 1k5 cathode resistor. This was the circuit I was emulating. There aren't any good schematics of that amp that I could find online, though the Jim Elyea book on Vox has a copy of it. (Excellent book btw) Later, Vox made the fixed bias version of the AC100 and then a later AC100/2 version that was also fixed bias. Both of them changed a few things, but notably they changed the plate load resistors for the PI to 47k. My amp has fixed/cathode bias switching so I had to pick one version to go with and I chose the cathode bias version for the phase inverter area and preamp on the clean, Vox channel. So I sent you the later fixed bias version of the AC100 because that was the only one I had to send since I don't have a scanner, and it is close to what I am doing in my amp.

There's multiple other gremlins on your bench.

Yes I would agree with that statement at the moment. Too many things breaking at once for my liking!


Thanks again! I'll post more once some things are sorted.

Greg

[HotBluePlates]

I guess it is a sign that I don't know what I'm doing here...haha. I'm mostly self taught so there are lots of gaps in my knowledge.

Me, too! Which is why it's nice to have PRR correct me when I goof. I learn a lot be being told how wrong I am! 

Actually, I don't think this point would have been noticed except that there's a thread going now about the Seymour Duncan Covertible amp, which has a funky master volume circuit. It uses a parallel triode with no drive except for variable d.c. to the grid to drag down a signal-carrying triode, thereby reducing its output. The circuit exists as a solution for how to control a master volume of an amp remotely without noise issues involved in running signal-carrying wires long distances and outside the chassis.

I saw what you were trying to do, but since your stated issue seemed to be low output from the phase inverter, and that was the only deviation from stock, it seemed reasonable that there would be unforseen interaction.

Hopefully your tubes are good. A short across the socket from plate to cathode would be external to the tube, and might not cause damage unless the cathode had way too much current pulled through it. But this is a fixed bias amp, so more likely the current was pulled through the chassis metal.

[SoundmasterG]

I guess it is a sign that I don't know what I'm doing here...haha. I'm mostly self taught so there are lots of gaps in my knowledge.

Me, too! Which is why it's nice to have PRR correct me when I goof. I learn a lot be being told how wrong I am! 

Actually, I don't think this point would have been noticed except that there's a thread going now about the Seymour Duncan Covertible amp, which has a funky master volume circuit. It uses a parallel triode with no drive except for variable d.c. to the grid to drag down a signal-carrying triode, thereby reducing its output. The circuit exists as a solution for how to control a master volume of an amp remotely without noise issues involved in running signal-carrying wires long distances and outside the chassis.

I saw what you were trying to do, but since your stated issue seemed to be low output from the phase inverter, and that was the only deviation from stock, it seemed reasonable that there would be unforseen interaction.

Hopefully your tubes are good. A short across the socket from plate to cathode would be external to the tube, and might not cause damage unless the cathode had way too much current pulled through it. But this is a fixed bias amp, so more likely the current was pulled through the chassis metal.

My amp is actually fixed bias or cathode biased, depending on where the switch is, and I don't recall where the switch was when I was testing and had the problem. I know that a component was smoking, but I'm not sure which one yet. When I tried to test the tubes, I was only on the first one and it blew the fuse on my B & K 747B tube tester. So I got a new fuse and will try to test the others here in a little bit. The fact that the first one was making the needle on the meter move backwards doesn't bode well for the other tubes in that quad. If they are bad I'll just get some others. Eurotubes is just across town from me so its not a big deal...just some money. I'll probably end up redesigning the amp to utilize that triode stage elsewhere and mix the channels with resistors. If I wire it right, I can add that triode stage to the Vox channel after the tone stack and use either a 12DW7, a 12AX7, or a 12AY7 and get some choice in the tone and gain that I want, and still be able to get the stock AC100 sound, so its good. It'll just take awhile to redesign and build, but I'm learning a lot so its a good project.

Greg

[SoundmasterG]

Well the tubes tested out just fine after I replaced the fuse on the tester. Very strange how it was reacting before!

I replaced the LCR can that I reformed for the screens with a new F & T so there is no question about that. When I had the board up I checked my wiring and it was all correct. I changed to 470 ohm screen resistors instead of 1k. I should be ready to test the amp again tomorrow, and I'll check for output again with this setup. If the mixer setup on the phase inverter is the problem, then in order to keep the channels in phase I could probably use the unused triode as a parallel stage for the Vox channel's input. This will give just a bit more gain then what it has now and still keep it in phase with my higher gain channel. I don't think the tone would change significantly to where I wouldn't be able to get the stock Vox sound somewhere in the control's range. I can mix with resistors like PRR suggested. The only other option is to use that unused triode as a gain stage, which puts channel 1 out of phase with channel 2....not a big concern I guess since the two channels as they consist now are very different in volume levels since the EF86 channel has so much more gain.  I had originally intended to be able to use the channels together but that doesn't really work the way the amp is right now with very large gain differences between the channels.

As far as the part that was smoking....I haven't figured that out yet, though I think it may have been one of the 100 ohm plate resistors on the power tubes. I'm not sure why they were used, but I figured I would add them to emulate Vox's circuit performance. I plan to check each one and make sure it is still specing out ok before firing the amp up again on the limiter. I also plan to disconnect my phase inverter circuit with the mixer and sub in the stock Vox one and Fender's LTP that I build in my outboard chassis for more testing. If my mixer circuit is the low power problem then I should be able to get full power with either of the others.

Greg

[SoundmasterG]

Well I was able to test the stock Vox AC100 seesaw paraphase inverter that I built into an outboard chassis in my amp today. Previously when I had tried it, I had not disconnected everything in my amp that I should have from the other phase inverter setup, so my swing was still low. This time I was getting better signal swing and more power so the mixer setup in the phase inverter is in fact my problem for the low power. Thank you PRR for alerting me to that being the problem. I will be redesigning the amp to mix via a couple mix resistors before the phase inverter. Is there any reason to use 470k's as you mentioned instead of say 270k or something? The noise will be less with the smaller values right? I am guessing that channel interaction will be less with the larger values?

I am thinking at this point that I will utilize the unused half of a 12AU7 on the input of the clean channel along with the stock 12AU7 stage, making that a parallel stage. This should bump up the gain a tad to make up for the loss from the mixing resistors, while still keeping most of the character of the stock Vox AC100 channel I hope. The other option is to use it as just a regular gain stage, but this would put the clean channel out of phase with my high gain channel. I suppose it isn't a big deal if I don't use the channels together but I did want that option.

So now I have to do some redesigning and rebuilding.

Incidentally, the smoking component that I mentioned previously was one of the 100 ohm plate resistors on the power tubes. I used too small of a value.

Greg

[PRR]

> I had not disconnected everything in my amp that I should have from the other phase inverter setup, so my swing was still low.

Sorry that I did not see that mentioned. That makes a difference.

> Is there any reason to use 470k's as you mentioned instead of say 270k or something? The noise will be less with the smaller values right? I am guessing that channel interaction will be less with the larger values?

Thin-air wild guess. No, the noise does not matter this far up the gain/level path. Interaction is probably not a big deal. With 12AX7 the frequency response may matter: 470K/2= 235K against the 110pFd grid is barely 6KHz, but a lot of amps don't get much more. With the low-gain 12AU7 you will be well over 15KHz.

It's a point to play with, invest a half buck of solder and resistors, AFTER you get the full POWER you were expecting.

[SoundmasterG]

Thank you PRR for the help, much appreciated!

A redesign is in process. I plan to make it work in my amp the way my board is now, and then after the circuit works, redesign the board to be better suited to the circuit. Heyboer is sending me a mounting kit to mount the Z mount output transformer as a top mount, which will save me a lot of space and allow a full chassis length board instead of two boards like I had before. That will greatly simplify the layout and be better for good layout rules. I pretty much have most of the circuit redesign figured out already...just a couple tweaks still. I'll post more once I have things in process again.

Greg