V72a Gain control

I've got a pair of V72a that I want to put to work. Whats the best way to add an external gain control? The only way I can see doing it is to add an external stepped attenuator at the input and not use a real gain control.
Any Ideas from the group?

RonL

A stepped and floating 600-ohm output attenuator would probably work, too. Combined with the input attenuator it would give flexibility to drive the thing accordingly. Granted, these are slap-on simple solutions and I presume you'd like an interstage gain control.

External attenuator at the input should work great. Maybe you could add a 40dB attenuator at the input and a 20dB attenuator at the output and have more "sound control"...

I'm thinking a semi bridged "T" at the output with about a -20dB range and a -20dB pad on the input. This should cover most of the bases.



RonL

In that case, head for http://www.groupdiy.com/index.php?topic=6711

there's a 6-step bridged-T attenuator with -30dB range, and the -20dB pad hidden in those pages. I don't know wether the V72a has a true 600-ohm output though.

A while back Joe Malone recomended a veriable pad before the pre in order to keep the dynamic range of the pre in tact.

I am currently working on setting up my V72 set up with a simple -20dB pad at the input and a variable 0 - -20dB pad (2 dB steps) on the output, I will post some info when I have it tested / running.

Just think like this... With the pad before the pre, you loose signal-noise ratio. With the pad at the output you loose headroom.

Good!
We can compare notes!
I'm still working on sheet metal.
I'll post some photos when I'm done.


RonL

great :thumb: There must also be a possibility to adjust gain within the unit, as gunter wagner does, for example.

Another thing I am really interested in is adding a direct input for recording bass guitar, I know it must be possible to connect a high impedance instrument directly to the first stage tube after the input trafo in a V72, but don't know the details of the "modification", maybe there is nothing more to it :roll:

side note: interesting to note that the V72 is a different animal compared to the V72a, a friend compared them both in the studio and said they sound quite different.

To adjust gain you would need to play with the feedback path and compensation capacitor.