Ampeg SVT preamp question

[okabass]

Can anyone tell me what the supply voltage to the pre amp should be for the svt? I'm going by the shematic on schematics heaven. My plan is to build the pre amp first, then if finances/time allow, put it together with a 100-150 w power amp.
Also does anyone know where in the uk I could get one of the tone circuit inductors?
Cheers Just

HI
The old schematic says 300 V. But it varies. Many SVT preamps get more, like up to 350 V. If you get 300...350 V you are ok.
You can make the toroid inductor (I've made a few) or you can buy eight 100 mH inductors (Farnell?) and make it that way. You'll get the values from the Ampeg V9 pre amp schematic.
Here's mine SVT-clone rig. I've used SVT-type pre over 10 years. It's the best pre I ever had. Period.
http://www.okabass.suntuubi.com/?cat=1

[Gingertube]

It was suggested above that the inductor values can be read from the V9 preamp schematic.
CAUTION: The V9 Preamp schematic at Schematic Heaven shows the 3 inductors as:
100uH, 300uH and 800uH - Thats WRONG, these values are out by X1000
They are in fact:
100mH, 300mH, 800mH
and the Ampeg part called up in the SVT Preamp Schematic is:
A 800mH inductor with taps at 300mH and 100mH.

You can do the math yourself:
2 pi F = 1/sqrt (L x C)

Examples
That 0.15uF and 300mH puts the centre frequency at 750 Hz
That 0.033uF + 100mH puts the centre frequency at 2.7 kHz
The leg witgh the 800mH makes use of the 0.68uF coupling capacitors to set the centre frequency somewhere below 500Hz (around 300Hz I think).

CAUTION: I saw a Ampeg Spare Parts Price List from a few years ago. That confirmed the values should be milli Henries (mH) NOT micro Henries (uH) BUT the tapped 800mH inductor was listed as US$135 whereas separate 100mH, 300mH and 800mH inductors were listed as US$4 each.

A redesign using opamp gyrators in place of the inductors seems like a good idea.

Sowter(in the UK) makes a range of EQ Tapped Inductors mainly for RIAA Phono equalisation BUT might have something suitable - priced at around 35 UK Pounds. The redesign looks better all the time.

Cheers,
Ian

[okabass]

Hi

Well. I suggested to look the values from "the Ampeg V9 pre amp schematic"
Yes. Those are the values in the _real_ Ampeg V9 schem. There seems to be some redrawings, that has incorrect values.

The correct mid frequencies can be read from the service manual or the amps front panel: you don't have to guess them. The correct values (according the manual) are 220, 800 and 3000 Hz (Some says 3,2 kHz).

" A redesign using opamp gyrators in place of the inductors seems like a good idea. "

I have not tried gyrators, but I don't see any sense to change that legendary SVT circuit. Hi voltage tubes and lo voltage cheap IC:s are no good combination. IMHO the original LCR mid is the best sounding mid control for bass.

By the way. The μ (mu or micro) seems to show correctly; i.e. you don't have to use u.

[Scott Anderson]

Here's my version of a simple B25/SVT amp.

http://home.triad.rr.com/scottvicki/150BassAmpProject.htm

Scott

[okabass]

Here's my version of a simple B25/SVT amp.

http://home.triad.rr.com/scottvicki/150BassAmpProject.htm

Scott

HI

Nice work!

[dhuebert]

Nice work. A lot of it looks verrry familiar. It's nice to see someone doing things the same way as me more or less, makes me feel like maybe I'm not too far astray in my thinking.

Don