High Voltage Hijinks

Asdrael said:
Hah ok. But there is no software like PCB softwares that make it fool proof by saying "hey you should have a connection here". Damn, needing brain cells makes everything complicated.
Click to expand...

What year do think this is? 2025?!

You will forget stuff, but the process is much more forgiving in those situations. I trace out my layout with the schematic (of course). For each pin on a tube socket there should be a connection in the layout (with exceptions like dc coupled cathode followers, etc). For each pot lug.. etc.

Use your Mentat powers!!

It's pretty fast really. I did the two channels of the Soldano X88R in 80 mins or there abouts. I just put something on youtube like a podcast and trance it out. Most of the guitar amp stuff is pretty simple and linear.
 
Asdrael said:
Yeah, I guess I just wouldn't know where to start. Probably wise to try to build "block by block" and not be too fussy about lengths
Click to expand...
Guess it depends mostly if using tried&done circuit design or planning a frankenamp.

I’d go setting chassis dimensions roughly as area for layout, then go first for PT/OT placement, then setting external connections (jacks, sockets, switches) and after that start planning where to add circuit blocks (aka stages) and connections most efficiently.
 
JohnnyCreepy said:
Guess it depends mostly if using tried&done circuit design or planning a frankenamp.

I’d go setting chassis dimensions roughly as area for layout, then go first for PT/OT placement, then setting external connections (jacks, sockets, switches) and after that start planning where to add circuit blocks (aka stages) and connections most efficiently.
Click to expand...
Sounds feasible. It would be for an idea mixing different blocks (clean from X, dirty from Y, Pi and power from Z) so nothing ground breaking but would require a bit of thinking. I suppose you forgo relays for actual switches then?
 
Hey my PCBs just arrived

IMG_1036.jpeg

I will have to do a test fit before I go soldering but so far it looks fine. I am not that happy with the socket distance I picked for the power tubes as I somehow messed it up and I will have only 2cm between tubes but it should be fine temperature wise still. As it is the same distance as between preamp tube it should lower heat transfer between components. Even my home made Fasel footprint is correct zomg.

Going to ponder reprinting the power board and do a test fit while I wait for the transformers to show up.
 
Exciting!!

I owned a Mesa Mark V 35 head and it had a little PC style fan in it that I replaced with a very quiet Noctua fan. A little 40mm would work.
 
Asdrael said:
Hah ok. But there is no software like PCB softwares that make it fool proof by saying "hey you should have a connection here". Damn, needing brain cells makes everything complicated.
Click to expand...
I use my PCB software (Altium) to design turret boards for exactly that reason. I just keep in mind that when I make the board, everything in as a "trace" is going to be a wire. I made a separate parts library for the components I would typically use on turret board builds so I can get the spacing of everything right.

For simple stuff that I'm not as worried about I'll knock something out in CorelDraw.
 
Today had 10 mins in garage to do some amp work. Added 10 Kohm cement resistor across OT secondaries for protection as there was this one occasion I tried to raise guitar volume and amp was silent because speaker cord was off. 🤡

Also measured plate and anode voltage without power tubes. Plate Grid -60.19 VDC (bias pot maxed) and anode 541.5 VDC. Tomorrow gonna measure again with tubes in and read voltage drop over fresh 10 ohm cathode resistors and set bias like an advanced amauteur. :cool:
 
Last edited:
JohnnyCreepy said:
Also measured plate and anode voltage without power tubes. Plate -60.19 VDC (bias pot maxed) and anode 541.5 VDC. Tomorrow gonna measure again with tubes in and read voltage drop over fresh 10 ohm cathode resistors and set bias like an advanced amauteur. :cool:
Click to expand...
Plate is another name for anode, and -60.19V is a very unusual voltage to have on a plate. I'm assuming you mean cathode?
 
JohnnyCreepy said:
Today had 10 mins in garage to do some amp work. Added 10 Kohm cement resistor across OT secondaries for protection as there was this one occasion I tried to raise guitar volume and amp was silent because speaker cord was off. 🤡
Click to expand...
Hey didn’t you ask me a few posts earlier if I had even ever seen a schematic ? 😁

Regarding the voltages and the bias, and given some tubes production is pretty unstable, I have put a bias switch to change the range of the bias setting. Just a resistor in or out. Hopefully will allow me to use everything.

Are you testing with light bulb and/or variance at first? I tend to follow rob robinette start up check list but don’t have a variac.
 
Model T turret boards:
Photo Oct 16 2025, 9 04 47 PM.jpg

I don't want to brag but I'm getting pretty proficient in this 80 year old technology!

I did some chassis drilling today too, mostly just the front. I think I've solved all the important stuff, I just need to do a sanity check on placement with the turret board locations and then it's all execution left.
 
Asdrael said:
Are you testing with light bulb and/or variance at first?
Click to expand...
The parts for light bulb current limiter have been waiting to be assembled almost for a year now…

So far doing first startup without pre and/or poweramp tubes in and measuring pin voltages on tube socket has been enough precaution. I do double or triple check parts changed and compare orientation and connections to the schematic.
 
JohnnyCreepy said:
The parts for light bulb current limiter have been waiting to be assembled almost for a year now…

So far doing first startup without pre and/or poweramp tubes in and measuring pin voltages on tube socket has been enough precaution. I do double or triple check parts changed and compare orientation and connections to the schematic.
Click to expand...

Take your ADHD pills and put it together already ;)

It doesn't have to be pretty:
Photo Oct 21 2025, 8 41 42 AM.jpg

I have a cheap variac too but the limiter is way more useful and important.
 
Last edited:
Asdrael said:
Love it, looking clean!

I might be dumb but what are those jack pot looking things tht you have (4 in a row) above the multimeter plugs? Individual sunken bias trims? That looks ave, I need that for my next build...
Click to expand...

Thanks!

Yes, just a pot with an internal slot instead of a shaft. CTS 10K, you should be able to source them since it’s a part fender uses. The upc is 609722158343
 
Last edited:
Hal Harvey said:
Take your ADHD pills and put it together already ;)
Click to expand...
:D THERES JUST TOO MANY THINGS TO DO AND KIDS TO FEED. PRIORITIES MAN PRIORITIES!

Btw, all didn’t go well this time. Had some problems during a test play. Clean sound was amazing, but when driven with square wave not so. Overall volume was also reduced and rising preamp gain caused outputs partial/total cutoff and crackling note decay.

Schematic here, red marks most recent changes. It’s missing 1Kohm resistor between OT speaker out leads.

IMG_7334.jpeg

Haven’t dived into troubleshooting yet, but I’ll gather some thoughts here:
  • Bias was set hotter than before. Now over 60%, before it was way over under 40%, maybe even under 10%. Hard to say, I cannot recall pot setting. Could it be my output transformer is with low turns ratio and prefers super cold class B biasing? Or do non-isolated divided two sided bias circuit cross talk when bias set warm?
  • Oscillation? Preamp grid resistors aren’t soldered near socket, but in input jacks with shielded wire leading to tube.
  • Cutoff happening with gain. V2 and PI are missing input grid resistors. Blocking distortion somewhere before big bottles?
  • Raising treble made note decay worse, bridging inputs brought more acceptable sound and cured note decay. HF Oscillation?
  • Are my 2x15 speakers causing all this? Should test another cab before diving too deep.
  • Maybe PI plate resistor balancing was bad choice? Is snubber cap necessary? Does MerlinB talk shit which I believed? :unsure:
I really need an oscilloscope. Alixpress messed my order and never received one I ordered in june. Got money back, but didn’t place a new order… I’ll start with bias first.
 
You must log in or register to reply here.
Back
Top