Hot Cat 30 out of higher voltage EL34 amp

[sluckey]

Swap the tubes to determine if the tube or the circuit is to blame.

[bcmatt]

AAAAAHHHHHHHH! TOO MUCH FUN!

So, yes, one of the Cathode Bypass Caps had a cold solder joint. I had them buried under the Cathode Resisters on the terminal strip and I originally didn't give myself enough space to solder it properly. I wanted the Resisters on top to be able to swap them.

Gain channel finally works, and it really comes alive and is really fun once I get close to Halfway up on the VVR. The clean channel always sounds good, but for stage volume you kind of want it sort of below there. So there is probably a happy medium somewhere around 11 o'clock on the voltage so that you don't have to run the clean channel too spanky clean. Because there are so many triodes in the gain channel, I find myself having to crank the Level and Master controls more to equal the volume of the clean channel as the voltage comes down. So, there is a lot of re-messing with all the controls on the gain channel every time the VVR is adjusted very much. Too low of voltage (maybe below 25%?) and the gain channel will not be very nice at all (just fizzy and lifeless sounding).
If just playing a single channel, you can get nice tones at any volume, it's just when I want to switch with the ABY pedal and get them balanced with each other that you sort of want it to be a basic minimum volume and voltage up at least 40%.

So ya, this amp basically has to be played with the VVR running in that 40-60% range where you know the MOSFET is needing to Dissipate the most heat.

I'll be continuing to play a bit with this thing. I anticipate the the clean channel could use a 3-position bright switch just to counteract that the Brilliance control is shared with the Gain channel. I also feel like the clean channel has maybe a bit too much bass (but that might be because I'm playing it through a 4x12 right now).

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I think I will set the VVR to have a maximum available voltage setting that allows reasonable preamp voltages, then dial in the cathode bias resistors for that voltage.
I should probably just limit the minimum VVR voltage as well so that the gain channel doesn't ever sound too bad.

[bcmatt]

Swap the tubes to determine if the tube or the circuit is to blame.

Yes, good call. So that's what I did and the problem persisted. So then I found that solder joint on the cathode bypass cap.
I gave the Express back its tubes and put in some slightly more mismatched old Phillips that will stay in there.

[bcmatt]

I was quite worried about this amp running with the VVR at full voltage, but it might actually be OK. I took Voltage measurements everywhere with the VVR turned up all the way:

Sorta Hot Cat Full Voltages.png

It would seem that the Preamp voltages aren't crazy high compared to the Old Hot Cat Schematic. I'm reading mostly about 10V higher on all the plates. The furthest would be V2A being about 23 Volts higher than the schematic at 248V being fed from B+5. I could try to bring it down a bit with a 27K resistor instead of the 22K.
Maybe V3A is most concerning since it's at basically the max rated voltage for 12ax7s at 329V... but apparently that's part of the design I guess.

Anyways, with these 470 Ohm Cathode Resistors on the EL34s, I'm calculating about 92% dissipation at full VVR voltage. It is higher voltage than the design but is it still considered reasonable?
Or is 421V plate Voltage just too high if I'm wanting to cathode bias? (393V plate to cathode)

Any thoughts?

Realistically, I expect to run this with the VVR down almost all the time and so the Power tube plate Voltages will be down around 150-250V almost all the time. That calculates at a tube dissipation between 12-30%.

[bcmatt]

Oh, another question. I didn't have any 1n5 caps, so I was either paralleling a couple together or in the case of the clean channel, I just settled for a 2n2 cap off the clean input triode's plate that feeds its volume pot. Would that be allowing extra bass? If I dropped that to a 1n cap, would it cut a bit of low end?

[maxkracht]

If I dropped that to a 1n cap, would it cut a bit of low end?

yes

[bcmatt]

If I dropped that to a 1n cap, would it cut a bit of low end?

yes

OK, thanks. I actually will try the stock value by using a 2n and 5n in series. I added that this morning but haven't had a chance to try it yet. I have a front panel hole next to that volume pot that may eventually be used as either a bright switch or a bass switch to flip between a couple values if needed. I'll try this stock version through a couple speaker cabs and see what might need be needed.

[bcmatt]

I got a bit of a chance to try the amp out yesterday again after dialing in the proper value cap 5n + 2n in series to equal about 1.5n and it does sound much more balanced. I should note that I also had brought it to a new location where I had my open-back 2x12 with Creambacks. I do think it is the cap though. I'll bring it home today and test it through some other cabs:

Mesa Roadster 4x12 with Creambacks
4x12 with V30s and ET65s
2x12 with WGS Liberator and Celestion V30
I'll also try it through my Badcat Unleash into an IR box.

I do have to say I am really liking this amp a lot so far and am quite excited about it. I still don't know about an actual band setting because I always seem to gravitate to my Liverpool being my favorite. But I think this has real potential. I really think this cathode-biased EL34 clean tone is actually quite pleasing and takes pedals well. Its easy to dial in subtle different degrees of "pretty clean" (I think thanks to no NFB) But, it also can be cranked up to very full saturation. (My impression so far is that it is not quite as aggressive as the Liverpool and Express, but definitely more than the Songwriter/ My guess is maybe it is similar to the Rocket or Z-Wreck gain-wise. So I think it may end up being the ideal amp for use with a pedalboard. But then there is the Gain channel which is a whole different animal with a whole lot of control. It's definitely more exciting than just using different gain pedals in front of the clean channel. That channel maybe needs an FX loop though.
I've tried an ABY pedal with the pedalboard feeding the clean channel. I was hoping I could use the Y setting to keep a bit of cleaner wet sound behind the Gain channel, but haven't been able to dial out hum in that setting. A and B are pretty quiet when the pedalboard is in the A, (and Y with no pedalboard is good too) but the pedals in there for the Y setting is too noisy so far... not sure why.
I'll try the wet effects after the amp by using the Unleash and IR box, then both channels will get them.
It also might be fine with just a single shorter delay pedal in front of the gain channel instead and that might be enough for when that channel gets chosen. I know it seems like I'm getting into the weeds with this, but I just think that is what makes this amp so great. The two channels are so distinctly different, but they both sound REALLY good... you want to be able to use them both in a set as rhythm and lead tones.

Meanwhile, I'm trying to decide on what kind of faceplate I want to make:
1. Carved leather
2. Spray-painted tin
3. Wood with burnt lettering
4. Plexi-glass that I spray-paint the back of and scratch in lettering

Out of the choices I'm considering, the wood option is the only one I've tried before for a faceplate
I did try the leather thing for my tele pickguard, so that might be fun to carve in a ferocious cat of some kind and turn it black with Vinagroon again.

20250411_071548.jpg

On this pickguard, I made the mistake of cutting out the shape before doing the carving. The carving majorly stretches the leather so it should be done first.

[bcmatt]

I found some leather locally. It wasn't cheap so now I'm invested in making this work:

a4066746-beb2-4a9d-a34c-f27867e1e890.jpg

[bcmatt]

I'll need to make a small hardwood badge just to offset all the black.

20250412_134942.jpg

[Colossal]

Nice work, Matt. I have enjoyed this thread.

[bcmatt]

I'd like to put some white paint or something into the lettering to make the controls more readable in dim scenarios. I'll do some tests...
Then maybe I should seal the leather with some resolene or something.

20250413_154619.jpg

[bcmatt]

I tried wiping in acrylic paint with my thumb then wiping it off with a clean finger but it sorta leaves a residue on the surface. Maybe it's all about timing... Being fast with a damp finger to wipe away.
Anyone have ideas or done something like this before? I'm going to test some acrylic paint fine tip pens.

20250413_171059.jpg

[bcmatt]

So, these both these 2 channels have an odd number of gain stages... but then they feed opposite sides of the phase inverter, so I guess that explains why it seems the two channels are out of phase with each other. I can flip this phase of one of the channels with my Radial Bigshot ABY pedal, but I'm curious the motivation behind that. I haven't noticed channels feeding opposite sides of the PI before. Is that a thing done fairly often?

Sorta Hot Cat Full Voltages.png

[Helmholtz]

"Gain" channel has 4 gain stages (last stage is a non-inverting cathode follower), so both channels are out-of-phase and mix in-phase at the PI.
A classic example of using the LTPI as channel mixer is the Vox AC30.