Convert Transistor EQ to Tubes?

[tbryanh]

I want to convert the attached EQ from transistors to tubes.

Any ideas on how to do it?

[Alf]

Here is the eq from the Acoustic GT100 tube amp using a 12ax7.

Alf

[loudthud]

Some rambling thoughts on the subject:

The Mesa circuit came from an article in Popular Electronics, some time in the late 60's. It breaks down to an emitter follower and an opamp. A cathode follower would be fairly easy, but the opamp poses some challenges. A simple diff pair and a cathode follower might have enough gain to function as an opamp. That would be a gamble. You would really like to scale the impedance of the Mesa circuit up by a factor of 10 to make it tube frendly.

One possibility is using a power amplifier as the opamp. KOC hints at this following the way Hiwatt does a presence control where the pot goes between the two inputs of the phase inverter.

There are a couple of other circuits I looked at. The Fender Studio Bass and the Vox V125. Both are similar to the Acoustic but use pots with a center tap that is grounded. Worth looking at as possible alternatives.

The hard part is obtaining suitable inductors at a reasonable price. The Acoustic circuit doesn't give values on the inductors, just part numbers. I've heard people say they obtained inductors from Mesa. Most other places they are real expensive. Then there is the synthetic inductor approach. That takes one cathode follower and a couple of other parts per inductor. That would be a good place to consider a Mosfet or high voltage opamp.

[Alex R]

The V125 has a tendency to be noisy, the inductors pick it up. I've wrestled with a few. Maybe it's too early in the preamp so the noise gets amplified too much. Just a side issue.

[Alex R]

I mean the V125 EQ. But it makes for a noisy amp. Mains hum pickup. Nice amp otherwise. I think it has a single FET on the input.

[tbryanh]

Hi Alf, I was wondering if you could repost the schematic. It seems to be missing 1/2 the circuit.

Thanks

[tbryanh]

Is there a link to the Acoustic GT100 schematic. I am not finding much.

Thanks

[tbryanh]

Fender Studio Bass

That is a nice looking tube EQ. the frequencies are for the bass range, but that can surely be tweaked for guitar. Awsome.