low voltage "high gain" amp

[razorrick1293]

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!]

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

 

[razorrick1293]

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 BH]

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]

+/- 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 BH]

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

 

[razorrick1293]

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

 

[Matt BH]

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 BH]

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]

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

 

[razorrick1293]

filament is coming from a separate small toroid.

 

[Matt BH]

If you have two of the small toroids you plan to use for filiaments then stick one backwards to get an isolated 240V or so.

Be careful.

 

[razorrick1293]

hmm, ok, about the - voltage though, would i just replace the ground points with ground? (other than the input and speaker ground)

 

[Matt BH]

dont worry about the - supply to your op-amp style output stage.

Ground is ground, certainly do not connect the negative rail for the output to ground(speaker ground is ground, although this should be a nice short connection to the smoothing caps) I mean the around about -30V rail.

You say you are using a TDAxxxx type thing. It wont need any gain in front of it.

 

[razorrick1293]

sorry i worded that wierdly, so would i put +v on the plate resistors and -v on the cathode resistors and grid leak resistors?

 

[Matt BH]

That was not clear at all.

Do not connect your negative rail to ground.

 

[Matt BH]

You do have I guess, you have not given any direct measured DC voltages. 60V to play with. with capacitive coupling you can use this 60V or so for your input valve stage.

You will or may as well only use one stage.

Experiment, find out.

 

[razorrick1293]

of course i wouldnt be conncting negative to ground...

 

[razorrick1293]

i guess il just have to see

 

[Matt BH]

cool we are on the same track then.

For your TDAxxxx you really dont need any gain unless you want tone controls and all that, so look at most line stage designs and play.