low voltage "high gain" amp

razorrick1293 · 4th April 2011, 10:40 PM

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?

db! · 4th April 2011, 10:47 PM

You won't get much gain out of a 12AU7 at 30V; you'll need around 200-300v if you want "high gain"...

razorrick1293 · 4th April 2011, 10:49 PM

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?

Matt.B.H. · 4th April 2011, 11:03 PM

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.

razorrick1293 · 4th April 2011, 11:06 PM

+/- 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

Matt.B.H. · 4th April 2011, 11:10 PM

OK so I guess you dont know if the output stage is push pull or single ended?

razorrick1293 · 4th April 2011, 11:11 PM

the output is a TDA2030, or something similar, just a simple power opamp output

Matt.B.H. · 4th April 2011, 11:13 PM

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.

Matt.B.H. · 4th April 2011, 11:17 PM

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.

razorrick1293 · 4th April 2011, 11:17 PM

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

4th April 2011, 10:40 PM	#1
razorrick1293 diyAudio Member Join Date: Nov 2010 Location: England Hertfordshire	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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4th April 2011, 10:47 PM	#2
db! diyAudio Member Join Date: Nov 2009 Location: Ontario	You won't get much gain out of a 12AU7 at 30V; you'll need around 200-300v if you want "high gain"...

4th April 2011, 10:49 PM	#3
razorrick1293 diyAudio Member Join Date: Nov 2010 Location: England Hertfordshire	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?

4th April 2011, 11:03 PM	#4
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	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.

4th April 2011, 11:06 PM	#5
razorrick1293 diyAudio Member Join Date: Nov 2010 Location: England Hertfordshire	+/- 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

4th April 2011, 11:10 PM	#6
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	OK so I guess you dont know if the output stage is push pull or single ended?

4th April 2011, 11:11 PM	#7
razorrick1293 diyAudio Member Join Date: Nov 2010 Location: England Hertfordshire	the output is a TDA2030, or something similar, just a simple power opamp output

4th April 2011, 11:13 PM	#8
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	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.

4th April 2011, 11:17 PM	#9
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	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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