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frank2 3rd January 2013 12:43 AM

12ax7 power transformer
Hi, i want to build a dumbleator, an external buffer for fx loop. It is a tube buffer. My question is: they want 135V from a power transformer that convert 115V to 135V that will be rectified. Ain't that a very low voltage for a 12ax7? If not, why not using directly the 120V from the wall, it is not that far from 135V? I would just use a 6.3V transformer for the heater. The problem is that i can't find any 135V transformer.... I have found a 170V transformer at a good price, i don't know if it is way to much...

What do you think?

Thank you


SY 3rd January 2013 03:01 AM

Using the mains power directly is an invitation to death. Don't!

Yes, 135V is low, but you're after effects, right? Don't sweat small voltage differences- a 120 or 150V transformer will work fine. Here's one that will give you 140V:

Antek - AN-05T160

DUG 3rd January 2013 03:03 AM

1 Attachment(s)
Common problem.

Common solution.

Isolation is important for safety.

Be careful...Isolation does not mean the secondary at 115Vac cannot harm you.

DUG 3rd January 2013 03:05 AM


Originally Posted by SY (

That is a nice transformer.


frank2 3rd January 2013 04:44 AM

okay, i didn't thaught about it that way (safety).
That is a nice transformer, i have used Antek transformer in the past, for my linkwitz labs pluto's speaker, they are very well made, great stuff. I am not sure it will fit in the enclosure i was planning to use, but i will try to see if it is possible.
Thank you for your help!


benb 3rd January 2013 05:13 PM

That Antek IS a nice transformer, but looks like overkill for running a single 12AX7. I'd use two small 6V or 12V filament transformers (a 1A secondary rating should be enough) as Doug shows in #3. Together they're smaller and probably weigh a third of what that Antek weighs, and will fit in your enclosure.

SY 3rd January 2013 06:01 PM

Yeah, but it's cheap and the right voltage. In "Valve Amplifiers," Morgan Jones suggested rescuing small transformers out of bathroom strip lights. Those are typically smaller (20VA) but give a 1:1 voltage ratio and excellent isolation.

frank2 4th January 2013 01:38 AM

Yeah it seem very much cheaper to run a 12ax7 for a buffer with two 6V transformer... i will go see what is available at digikey...

JMFahey 4th January 2013 11:22 AM

Yet, if that 170V transformer is cheap, available, small, cute, etc. , you *can* use it, lowering voltage is easy.
And anyway, I think the Dumbleator used the lowest voltage he could , for unknown personal reasons of the highest mystic value (did I say $$$$:D?).
Of course, you may have to use some slightly higher voltage caps.
Please post the schematic here :)

Stormrider 4th January 2013 11:39 AM

This may work: Single Preamp Tube Project Power Transformer High voltage, though.

Another option is to look for a Stancor PS-8415 which was used in the Alembic F2B preamp with two 12AX7's. They can still be found on ebay occasionally.

EDIT: I haven't dealt with them, but it looks like Leeds Radio in NY still has the PS-8415 for $22.

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