12 v tube preamp

[flester]

I've seen a few references to building tube preamps e.g. https://www.instructables.com/id/The-ValveLiTzer-Low-voltage-Tube-Booster/
https://www.instructables.com/id/The-Valve-Caster-20-Tube-Boost-and-Overdrive/
Any comments on whether this is worth doing e.g. will it really sound any different to  solid state version?

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[willpirkle]

The tube-purists will hate the starved-plate designs but I’ve done this myself by altering a tube-screamer. It was noce and spongy distorted, but a little noisy (I used mini co-ax cable for critical runs). I was a tube-purist also until I got a Radial Tri-Mode ToneBone for $5 off a broken hardware table at Sam Ash. It only needed a 15V regulator to fix, and was yet another starved plate design. I liked how it sounded so I tried it with the tube screamer as a side-project.

[gfr]

http://www.valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf

Some theory, and some ideas for improvements.

[willpirkle]

Great article, man! The Butler patents are worth checking out too.

[flester]

I ordered some 12*U7 and sockets and am going to give this a try.
I see that there are some hybrid amps e.g. the Orange Tiny Terror which has a 12 AX7 preamp and uses a 15v power supply. Is that likely to be a starved plate or is the voltage stepped up internally? Is this a realistic option for DIY?

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[phatt]

hi Flester,
AFAIK, Tiny Terror is all valve running on high voltage.
I have no doubt that running Valves on low voltages will work but I doubt you will do much better than SS pedals.
The big thing with Valve rigs is the High voltage and the way in which the supply is delivered to each stage.
I think the term is  High Impedance supply, these sag as power is increased giving the classic compression feel. There are of course many other things that make all valve amps do the magic but just altering the values of the drop resistors in the supply chain changes the way the whole amp responds to how you play.
I doubt that will happen if running an AX7 at 12Volts.
Phil.

[flester]

Sorry I meant Orange Micro Terror which has a solid state power amp. Just curious about how they run the 12AX7 preamp using a 15 v power supply.

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[g1]

Appears to be running on low voltage:

https://gitlab.com/jofirel/uterror-amp/-/archive/master/uterror-amp-master.zip

[Enzo]

You can get a tube to do SOMETHING at 15v, but don't expect it to work like a 12AX7 in a real circuit at real voltages.

[willpirkle]

This would be considered starved plate and the B+ supply is +15V and not internally boosted, but the design does not include biasing so the first half 12ax7 only amplifies the bottom (-) half of the waveform, which is supplied via the boost amp. The second half 12ax7 reamplifies the flipped, attenuated signal (100K/4.7K resistor divider). I ran a SPICE simulation this morning and attached the results. The lowboost.png file is the output of the two tube halves (top is 1st half, bottom is the 2nd tube output) with a small input signal (low boost knob setting), hiboost.png is the same thing with a massively boosted input, and combogain.png is a combination plot. The harmonic distortion was 38% for the most distorted output.

I am not a fan of this design and I agree with Enzo - it does *something* --- but there is no duty cycle modulation you expect out of a true high-gain cascaded tube preamp when you overdrive the input, and it's going to add noise.

Have you looked at the classic PAIA Stack in a Box (SIAB)? it used a switching charge multiplier circuit to get the plate voltage up from 15V to almost 50V or so, getting it out of starved-plate region. I never built it, but it does seem to have a following of sorts.

[flester]

Yes I have seen the Paia version in Craig Andertons project book. Might be an option if the kits are still available. Building it from the  schematic is probably beyond my skill level.

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[willpirkle]

PAIA used to sell just the PCB for their project kits. If you can find one of those, could be do-able! And I’d guess someone is probably cloning the PCB by now.

[willpirkle]

Here’s the screamertube I built many years ago that might split the difference and give you something to experiment with that isn’t overly complicated. Note that there is a small error concerning the heaters - they should be wired in series, not parallel (causes excess heat). The circuit was changed, but not the blog.

http://screamertube.blogspot.com

Disclaimer: tube purists will hate it, but it was a fun project to put together!