Seeking more clean power from this design

[rawnster]

Mike, I would want to get the voltages back to where they were, or per schematic, and attempt to fix the issue. Have you tried a 5751 in V1?

[Atmospheric]

I don't have a 5751, but that would be interesting to try.

Mike, I would want to get the voltages back to where they were, or per schematic, and attempt to fix the issue. Have you tried a 5751 in V1?

[Tone Lover]

Also try a 12ay7 there I find I like the drive and sound with one of those if a 5751 isnt quiet enough.
Thanks Bill

[Atmospheric]

I decided to take rawnster's suggestion about trying to get the voltages back to spec on the original Fender schematic.

I reinstalled the original 27K/4K5 dropping resistors. I decided to be more thorough with my measurements this time (measure plate voltage at all 4 plates).

It turns out that I think the 12AT7 is a bad tube. Installing it loaded both V2 plate voltages substantially (see attachment).

I reinstalled the 12AX7 in V2 and noticed something really strange. The V2b plate voltage is REALLY high and the 56K plate resistor is only dropping 6V! (C+ is 345V; plate voltage on other side of the 56K resistor is 339V).

I verified that the 56K resistor does measure correctly (~57K).

So it looks like what happened when I installed the (bad) 12AT7 is that it loaded down the PI so much that it provided much less signal into the power stage, which sounded better at full volume. But this amp sure doesn't produce much volume. It's just whisper quiet until about 6.5 on the volume control.

That just doesn't seem right. I dunno, maybe the Princeton I owned was just exceptional. But I know Mike Campbell mic's up non-verb Princetons on tour. I just cannot imagine that an amp producing as little volume as mine could be useful in that situation.

I'm perplexed.

[ToneMerc]

I decided to take rawnster's suggestion about trying to get the voltages back to spec on the original Fender schematic.

I reinstalled the original 27K/4K5 dropping resistors. I decided to be more thorough with my measurements this time (measure plate voltage at all 4 plates).

It turns out that I think the 12AT7 is a bad tube. Installing it loaded both V2 plate voltages substantially (see attachment).

I reinstalled the 12AX7 in V2 and noticed something really strange. The V2b plate voltage is REALLY high and the 56K plate resistor is only dropping 6V! (C+ is 345V; plate voltage on other side of the 56K resistor is 339V).

I verified that the 56K resistor does measure correctly (~57K).

So it looks like what happened when I installed the (bad) 12AT7

A 12AT7 draws about 3-4 times as much current a 12AX7.

TM

[Atmospheric]

OK, so if I understand you correctly, the lower measured plate voltages are just due to a larger IR drop across the plate resistors? That probably explains why Fender schematics with 12AT7 PIs run the plate voltages on the pins at ~315V, instead of 260V for a 12AX7.

Thanks.

A 12AT7 draws about 3-4 times as much current a 12AX7.

TM

[Firestorm]

Check the other 56K (the cathode resistor). Make sure you've got a good ground. Also check the 1K. With only a 6V drop, it seems like something is way out of spec or not properly connected.

[oldhousescott]

Check the other 56K (the cathode resistor). Make sure you've got a good ground. Also check the 1K. With only a 6V drop, it seems like something is way out of spec or not properly connected.

Yes, you should have about 300v on the plate and about 50v on the cathode. As measured, you're only flowing about .1mA. Somethings not right in the cathodyne.

[rdjones]

Aside from all the other good suggestions I have to point out a mistake in both versions of the original schematics.

Do Not pull B+ from pin 2 on a 5AR4 (GZ34).
As a matter of convention B+ should always be taken from pin 8.
Hopefully you've already corrected this oversight but I haven't caught it mentioned in the thread.

rd

[Atmospheric]

Thanks rd. I will look into that. I was just removing the standby yesterday. I *think* B+ is connected to pin 2.

Can you elaborate on why this is bad? I realize "it seems to be working" isn't always a good rationale for anything.

Aside from all the other good suggestions I have to point out a mistake in both versions of the original schematics.

Do Not pull B+ from pin 2 on a 5AR4 (GZ34).
As a matter of convention B+ should always be taken from pin 8.
Hopefully you've already corrected this oversight but I haven't caught it mentioned in the thread.

rd

[Atmospheric]

First off, thanks again for the great help on this one. I couldn't have done it without you. I've uploaded an updated schematic.

What I did:
o Eliminated V1b gain stage.
o Removed 470K grid stopper from V2b-7 to ground. That snapped the V2 voltages right back in spec.
o Removed standby.
o Tweaked first dropping resistor. Voltages are now ~220V on all plate resistors and ~270V on the PI plate.
o Verified that B+ is indeed coming off pin 8, not pin 2.
o Tweaked tone stack by changing treble cap back to orig 250pf and bass cap to .022uf.
o Added a mid control. I'm really liking this. The stock 6.8K setting is great with pedals; cranking the mids up works great if I go straight into the amp.

The amp now responds musically over the entire volume control range and has noticeably more oomph and muscle.

-mlt

[Firestorm]

Removed 470K grid stopper from V2b-7 to ground. That snapped the V2 voltages right back in spec.

Glad you got it sorted out. But for anyone looking on, that 470K isn't supposed to go to ground; it's just a grid stop. Grounding it would throw off the bias of the stage.

Since you've eliminated the extra gain stage, you probably won't risk overdriving the cathodyne anyway, so it should be fine without Merlin's large grid stopper.

[ChrisM]

Looks good, how's it sound? Does it function well as a bass amp?

Essentially a modified Fender, nice! You might consider grid stoppers on the output tubes though (1K-10K).

[statorvane]

Excellent.

BTW, what are you going to do with V1b? How about using it for a tremolo?

[Jana]

I know what I would do with that extra stage from V1b, but you will all think I am crazy!

Okay, so you already know that.

Very simple--make the first stage a cathode follower. Drive the tone stack off the cathode follower. All you need is 1 more 100K resistor to do it.