Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: j-pee on March 21, 2026, 11:02:49 am
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hi,
I'm not into tubes, and I'm not a great builder... and not an engineer, at all...
I've read stuff though. This is part of it my problem :)
I's been over a year since I purchased a 12ax7, and haven't done anything with it yet...
the idea was easy: build a Butler Tube Driver.
I was vacillating with the Bajaman version with a +/-15V regulators, which need 17.5 volts DC at least...
okay, I'm aware, here 300V is the normal... :)
Either way, I've never built it, because I didn't think I had a cleat shot.
Now, yesterday, listening to a Sheryl Crow concert, and a Guitar solo,
whose sound, again, like so many times so far, took me...
https://youtu.be/bkhfv6vhMkI?si=T5Ih14cNjEfVWBzH&t=164 (https://youtu.be/bkhfv6vhMkI?si=T5Ih14cNjEfVWBzH&t=164)
What I mean is most "obvious" between 2:45 and 3:30...
(before the solo part)
My idea with the whole Butler project was to have a tube preamp stage, that can be ovedriven...
Today I almost got to building it...
But (again) I listened to samples and they sounded "bad",
no overdriven tube sound but like a tube screamer or other... transistor OD
And that's what a cool expert at another tube site said about it...
which I had also read before...
namely, that these "starved plate" preamps distort but it's by far not the same thing as what happens in
an "actual" tube amp...
So, my question is whether one can have a tube overdrive sound using one 12ax7?
What I also gathered is that without a bypass capacitor it is better...
But still, is this viable at all?
at low tension (voltage), like 15 or 30 or 40 or 60 or 120?
(I'd like to avoid HT that can kill a human being, and I live in Europe, meaning 230V from the wall socket)
Or rather,
I would rephrase: what do we actually need to have that overdive-effect?
Or at least half of that :)
I mean, other than a typical high profile amp and everything attached to it :)
People tend to like 1 watt apms for recording...
or attenuators for bigger amps...
this is what I'd like to have :)
Just for fun...
Thanks for any advice! :)
Peter
PS:
I'm aware that there is power tube distortion and preamp tube distortion...
I read the article about "cold clipping"...
I know Rob Robinette's website, and also been there a few times, and I know Marilyn Bancroft's...
(OH.... as I googled for his name so I'd spell it correctly I caught sight of an obituary :( ))
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okay,
I was reading my notes from last year, I realized that
it is the McTube that I might unawares want :)
Luckily I also found a modification advice...
to lower the cathode resistors (to 1K, 820R, for V1, V2), as well as the anode resistors (to 100k both), and also the bypass cap...
I think I will go for it...
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My advice is to look into some of these designs:
https://c2celectronics.com/
They run the tubes at high voltages and are well documented.
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I seem to recall seeing a PDF by TheValveWizard (Merlin Blencowe, not Marilyn Bancroft) that was all about starved-plate 12AX7 usage, but I can't seem to find it, atm. It might have some useful info if you can hunt it down. FWIW, Guyatone had a HV tube pedal, and there's a bunch of derivatives, the Nady TD-1 being one of them. Looking into those might help you along in your journey.
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I seem to recall seeing a PDF by TheValveWizard that was all about starved-plate 12AX7 usage, but I can't seem to find it, atm.
Here it is:
https://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf (https://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf)
And here is another way to create valve overdrive at low voltages:
https://valvewizard.co.uk/sparkgap.html
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I seem to recall seeing a PDF by TheValveWizard that was all about starved-plate 12AX7 usage, but I can't seem to find it, atm.
Here it is:
https://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf (https://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf)
And here is another way to create valve overdrive at low voltages:
https://valvewizard.co.uk/sparkgap.html
Merlin,
Is something going on with your website? From your home page many of the links aren't working. For example, the link in your post https://valvewizard.co.uk/sparkgap.html (https://valvewizard.co.uk/sparkgap.html) if I select it in this post I get to it just fine. If I select it from your home page, it and many others just hang. Very strange...
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Here it is:
https://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf (https://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf)
And here is another way to create valve overdrive at low voltages:
https://valvewizard.co.uk/sparkgap.html
I'd second this, its worth having a go with the low voltages first, if its your first project etc it might a bit safer, I've tried some Valvecaster variants, I think you can get pretty close to the sound your looking for in your example,
What amplifier are you using?