Seeking more clean power from this design

[Firestorm]

Here's a little more background on what I intended this amp to be. I previously owned a BF Princeton (non-verb) that I was forced to sell. I had always regretted doing that. So I basically wanted THAT particular amp but without trem. Just a minimalist signal path. I always dug the sweetness of that old Princeton, especially compared to DRs.

Take the wiper from the volume pot and go directly to pin 2 on V2a (totally bypassing everything related to V1b).

A lot of people prefer the tiny additional gain that you get from the reverb mix stage of a Princeton Reverb (that's what V1b is doing), but if you're pining for your non-reverb Princeton, Jana's suggestion is spot on.

You may still have an issue with the cathodyne (but maybe not). Distortion artifacts from this kind of PI are typically really ugly, so you should know easily if it's a problem. Just add a 470K right on the grid of V2b (after the 1M) to clear it up.

[Atmospheric]

Changed the dropping resistors to 18K and a jumper wire. Voltages are:
V1/V2 plate=247V
B+ to V1/V2=371V

I got a tiny bit more volume out of the amp, but also kind of a nastiness on the top. I suspect that 247V is probably too high.

Next, I turned the bias down to 18mV. That helped a fair bit. Monkeyed with the bias and found that I liked it around 20mV.

I did feel like playing with the bias helped me hear what the power tubes were contributing to this. The only way I can describe it is that it sounds like the power tubes are doing what I want them to, but that the input to the power tubes has a distorted fuzziness to the signal.

Next, I jumpered around V1b (wired volume control wiper to V2-2). I liked that a lot. Much more "honest" and uncluttered to my ears. Overall the amp had a bit less volume, but I was able to get further around on the volume control before it started falling apart.

It sure doesn't seem like I am getting much volume out of a pair of 6V6s running at these plate voltages. The amp is quieter than some Champs I've played.

Speaker is a 20w Weber alnico Jensen clone.

The amp feels a tiny bit overfiltered to me. AA964s have a total of 80uf filtering (1x40, 2x20, no choke). This design has 100uf (1x40, 2x20, and a choke).

Could something as simple as the input network to V1-2 be a problem? The AA964 is a 1M+68K divider. This build is a 1M+32K divider. I suppose my build should develop a bit more signal into V1-2.

[Firestorm]

Next, I jumpered around V1b (wired volume control wiper to V2-2).

You also disconnected the existing connection at V2-2, right?

[Atmospheric]

Yup.

Next, I jumpered around V1b (wired volume control wiper to V2-2).

You also disconnected the existing connection at V2-2, right?

[Atmospheric]

I decided to swap in a 12AT7 in the PI.

Instant major improvement. Bass tightened up dramatically. Amp is now clean through the entire volume control.

I see from bassman schematics that 12AT7s can take up to 315V on the plate. So I may monkey with the dropping resistors trying to find the optimum voltages for both the preamp and PI.

I still think there's other things I can do. I really appreciate the links to the cathodyne stuff. I'll be looking into that. Maybe a few tweaks to the tone stack.

rawnster and I swapped out the Fender 250pf for 110pf in the tone stack. I forget, does 110pf give me more or less treble than the 250pf?

[Firestorm]

A little less. Also affects mids and upper mids.

[Jana]

You might also want to try a 1uf bypass cap on the first stage instead of the 25uf.

[Atmospheric]

What would that do?

You might also want to try a 1uf bypass cap on the first stage instead of the 25uf.

[greekie]

With the voltages running as high as they are (505V across the first filter cap on Standby), I'm reluctant to go the SS recto route. I'd probably to have to get a new PT. In which case, I might as well just build another amp (which I may do at some point anyway). Even if I build another amp, I would still want to get this one functioning a bit better.

Just a side note - change the standby-switching so it mutes the amp instead of removing B+. Such an arrangement as yours is very bad for the rectifier tube as it sees a good bit of inrush current, and as you mention yourself, your capacitors go overspec on the voltage.

My point is: why take any of these risks?

Which speaker are you using? This plays a big part as well!

Also, perhaps change the type of phase inverter as well as the tubes to 6L6, so they get more juice.

YM2C

[Jana]

The 25uf cap is bypassing a full range of frequencies--including the lower bass that tends to flub and fart. A 1uf will roll off higher in the frequency range--somewhere in the mids depending on the cathode resistor value. Remember, the gain is increased for any frequencies that are bypassed with the cathode cap. So, changing the cap to 1uf will decrease the gain on the low frequencies.

[M Fowler]

Use a solid state rectifier plugin and change the V1a cathode bypass cap to 1uf for less gain. Use all 12AX7 tubes and swap 6V6 for 6L6.

Mark

You might also want to try a 1uf bypass cap on the first stage instead of the 25uf.

Jana is correct

[Atmospheric]

I'm pretty darned happy with the amp after changing to the 12AT7. Early breakup, too much or farty bass doesn't seem to be a problem now. Originally, the power stage coupling caps were .1uf like the AA964. But rawnster and I changed them to .022uf to try to tame the bass a bit. However, now I wonder if I shouldn't change them back to .1uf to see what that sounds like.

With the voltages running as high as they are (505V across the first filter cap on Standby), I'm reluctant to go the SS recto route. I'd probably to have to get a new PT. In which case, I might as well just build another amp (which I may do at some point anyway). Even if I build another amp, I would still want to get this one functioning a bit better.

Just a side note - change the standby-switching so it mutes the amp instead of removing B+. Such an arrangement as yours is very bad for the rectifier tube as it sees a good bit of inrush current, and as you mention yourself, your capacitors go overspec on the voltage.

My point is: why take any of these risks?

Which speaker are you using? This plays a big part as well!

Also, perhaps change the type of phase inverter as well as the tubes to 6L6, so they get more juice.

YM2C

First, I don't really use the Standby - I keep it set On for the very reasons you mention. With a GZ34, the amp takes a good 40 seconds to fully warm up. AA964s don't have Standbys. This was just something extra we decided to add. You're right, we'd probably be better off without it.

Again, with the 12AT7, I think the amp (and speaker) does exactly what I want it to. I think it's now a matter of refining things.

As previously mentioned, I have tried 6L6s and I don't like how they sound as much as 6V6s. Also, I want to retain the cathodyne PI for now believing that it contributes a certain sweetness (when it's not misbehaving).

I really appreciate the help guys. Thanks again.

[wicker]

You could try to add local feedback in 2nd stage to reduce overall gain and make amp smoother. Just my 0.02$

[rawnster]

The amp has a 470k grid stopper on the cathodyne, per Merlin's recommendation. Mike (atmospheric) mentioned that he liked it a bit better afterwards. Slightly less grind.

[rawnster]

1uf bypass cap on the first stage is a great idea. Should do that for sure. Easy mod.