I was able to scam a trimpot in there without removing the board. Glued it down to keep it stable, and set the bias to about 65% plate dissipation. Also added a 100pF WIMA film cap across the feedback resistor (R27) to help tame some of the icepick. Both of these helped quite a bit.

I also removed the wiring from the useless ground polarity switch and converted it to a tonestack bypass switch. Gives about a 10 dB signal boost and a flat frequency response.

I think a lower-gain tube in the V1 position is going to have the best impact.

Nate Dort wrote: Fri May 03, 2024 7:03 pm I was able to scam a trimpot in there without removing the board. Glued it down to keep it stable, and set the bias to about 65% plate dissipation. Also added a 100pF WIMA film cap across the feedback resistor (R27) to help tame some of the icepick. Both of these helped quite a bit.

I also removed the wiring from the useless ground polarity switch and converted it to a tonestack bypass switch. Gives about a 10 dB signal boost and a flat frequency response.

I think a lower-gain tube in the V1 position is going to have the best impact.

You could also increase the plate to plate cap in the PI, C11 in the schematic (but I'm sure you know this).

benadrian wrote: Fri May 03, 2024 3:22 pm Has anyone made a 5F6A preamp into a cathode biased 6V6 power section in a 1x12" combo?

I have come close-ish. This is a plexi circuit with two 6V6 in a 1x12 Ampeg reissue cab, but its fixed bias. My guitar player uses it and it sounds great.

Dr Tony Balls wrote: Sat May 04, 2024 6:33 am

benadrian wrote: Fri May 03, 2024 3:22 pm Has anyone made a 5F6A preamp into a cathode biased 6V6 power section in a 1x12" combo?

I have come close-ish. This is a plexi circuit with two 6V6 in a 1x12 Ampeg reissue cab, but its fixed bias. My guitar player uses it and it sounds great.

That sounds fantastic!

Okay, amp gurus.

With great irony the only amp purchased new in my arsenal is my most problematic.

I might have griped before on her about my Dr. Z, Jetta. For being a pricey thing it's been a nuisance. It sounds magnificent so I'm committed to getting it in shape.

It has long held a tendency to get microphonic. Since it's cathode biased I overlooked my neanderthal fears and popped in some new Soviet 7591s (the relatively unique tube at the heart of this circuit). After what I thought was a bad batch I got a new pair sounding nice. The thing became my favorite recording amp.

Then it seemed to randomly become microphonic again. I had a tech friend tell me Dr. Z is known for creating amps that eat up tubes. I also found that sometimes when it was misbehaving I could gently wiggle or push in the power tubes and it sometimes temporarily settled down.

I start to wonder if none of the tubes were a problem, but instead it is the sockets. Last night at band practice it started actually growling at me, a jiggle got it to behave for 20 minutes and it started growling again.

Thoughts?

Tube sockets can be retensioned. I'd probably start there.

Not that this 100% applies to your amp:

After having tube problems with certain amps I am now 100% for going w/ NOS or good condition vintage tubes in problem spots as long as they aren't crazy money. It can actually save money over time.

I had an amp that would eat JJ EL84s once a quarter, i put some old Soviet military equivalents in as an experiment and it totally fixed it. The tubes were more expensive than the JJ’s upfront but I havent spent a dime on those tubes since.

losthighway wrote: Mon May 06, 2024 12:42 pm Okay, amp gurus.

With great irony the only amp purchased new in my arsenal is my most problematic.

I might have griped before on her about my Dr. Z, Jetta. For being a pricey thing it's been a nuisance. It sounds magnificent so I'm committed to getting it in shape.

It has long held a tendency to get microphonic. Since it's cathode biased I overlooked my neanderthal fears and popped in some new Soviet 7591s (the relatively unique tube at the heart of this circuit). After what I thought was a bad batch I got a new pair sounding nice. The thing became my favorite recording amp.

Then it seemed to randomly become microphonic again. I had a tech friend tell me Dr. Z is known for creating amps that eat up tubes. I also found that sometimes when it was misbehaving I could gently wiggle or push in the power tubes and it sometimes temporarily settled down.

I start to wonder if none of the tubes were a problem, but instead it is the sockets. Last night at band practice it started actually growling at me, a jiggle got it to behave for 20 minutes and it started growling again.

Thoughts?

I found a schematic here, which I can't verify.
https://el34world.com/Forum/index.php?topic=26563.0

The amp looks incredibly simple, which is a good thing all around. I don't have much experience with 7591 tubes, except maybe converting a V4 to use 6L6 tubes instead

When it makes noise, does the noise happen when no signal is present? Either way, here's some stuff to do.

Cleaning and re-tensioning the tube sockets would be a good first step. You can use rubbing alcohol or deoxit, apply it to the tube pins, and then when still wet insert and remove the tubes a few times. Re-tensioning is a little more involved. Pull the amp from the chassis, drain the caps, and then use a small pick or jeweler's screwdriver to bend the contact inside the tube socket so that they will more firmly grip the tube pins.

The amp has an effects loop. This is a perfect opportunity to decouple the preamp from the power amp. You can plug the guitar right into the power amp in, or go guitar ->boost/distortion pedal/power amp in. Try playing it like that and seeing if problems persist. If so, then the problems are in the phase inverter or power amp section. if not, then they are in the preamp section. Signs point to power amp.

If you're concerned about power tube life being short from being run too hot, you could always increase the value of the cathode resistor. The schematic says 10k, but you can go to 15k, 20k, etc. As long as you only go higher, nothing bad will happen... other than the possibility of the power amp sounding kinda meh if you go too high.

Good luck!

benadrian wrote: Mon May 06, 2024 1:39 pm
When it makes noise, does the noise happen when no signal is present? Either way, here's some stuff to do.

Great, useful, detailed advice. Not sure if I'm prepared to drain the caps. I might have to employ a tech if it goes there.

But yes, the amp makes noise without signal. At points with the guitar plugged in, no playing, it has approached a feedback type ring. I love the effects loop strategy because it's easy and logical, although I agree signs point to power section. It would be fun if this was all just a bad 12ax7 though!

Since I’m back on the forum, I feel like I might as well ask something I got a question on a guitar I recently swapped pickups on.

I recently swapped pickups on one of my guitars to a set of GFS Surf 90s (their version of a DeArmond single-coil). They sound good and preeeetty close to the Jumbo MFDs on my G&L ASAT Special (currently my number one), but the MFDs still sound a touch brighter.

The pots in the guitar are 250k for both volume and tone.

Two questions here:

1. Would switching to 330k pots get me just *slightly* brighter on the Surf 90s? If it’s gonna be too trebly or not have an appreciable effect I’d prefer to not do the swap.

2. If I do decide to swap, is it necessary to only do the tone pot or should I swap the volume pot as well?

This may be a lot of words to end up with “you won’t know until you try” but I figured to ask in case someone’s had experience here. (Of course, I played a gig last night and didn’t feel too much of a difference switching between the guitars, so there ya go…)