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Old 4th April 2011, 11:40 PM   #1
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Default low voltage "high gain" amp

hello there, ive found an old SS amp that ive been wanting to turn into a hybrid, but i was hoping to not spend too much on power transformers, and im using an SS PA so no need for an OPT.

basically, i want to rip out the preamp and make a typical 3 gain stage with cathode follower design, i was thinking of using 12AU7's as iv used them for low voltage, and i was hoping to run them on about 30v maybe less, has anyone had any experience in this or is this going to be a trial and error thing?
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Old 4th April 2011, 11:47 PM   #2
db! is offline db!  Canada
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You won't get much gain out of a 12AU7 at 30V; you'll need around 200-300v if you want "high gain"...
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Old 4th April 2011, 11:49 PM   #3
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well i know that, but i was thinking at that low voltage, surely not too much gain would be needed to get a similar effet of a 12ax7 at 400v?
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Old 5th April 2011, 12:03 AM   #4
Matt BH is offline Matt BH  United Kingdom
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Do you only have 30V? or + and - 30V?

60V is not so bad, try google for low voltage designs.

You could also voltage double your supply.

Google is your friend, try looking at space charge tubes and the low voltage later battery radio tubes.
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Old 5th April 2011, 12:06 AM   #5
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+/- 30v is very possible, im not sure about using negative voltage on the valves though, do i just replace ground points with V- points? and i have looked at some designs, but none of them get you very far in terms of gain, and ive got against voltage doubling, ive used them before and didnt really liked the way it turned out
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Old 5th April 2011, 12:10 AM   #6
Matt BH is offline Matt BH  United Kingdom
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OK so I guess you dont know if the output stage is push pull or single ended?
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Old 5th April 2011, 12:11 AM   #7
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the output is a TDA2030, or something similar, just a simple power opamp output
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Old 5th April 2011, 12:13 AM   #8
Matt BH is offline Matt BH  United Kingdom
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sorry, didnt quite get your meaning.

you should with a voltage doubler and associated smoothing be able to build a valve front end for it.
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Old 5th April 2011, 12:17 AM   #9
Matt BH is offline Matt BH  United Kingdom
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Where do you plan to get filiament power from?

I would think it would be easier to buy a small tranny, maybe suitable for a pre amp to do it.

I use little toroids for this. you can use two back to back or any other combination you can dream of.
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Old 5th April 2011, 12:17 AM   #10
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yes, its an old solid state amp i have, i have a 30-0-30v transformer and dont really want the hastle of getting an output transformer for it, and finding space on the chasis for it. im pretty happy with an SS power amp seeing as i never really drive it that hard
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