Type 1 Trimax valve pre

Hi guys,

I have a rack containing a single 'Type 1' mic preamp (40dB), two 'Type 2' line amplifiers and a power supply. There schematics are here:

http://www.retrovox.com.au/Images/type1spec.gif
http://www.retrovox.com.au/Images/type1cct.gif
http://www.retrovox.com.au/Images/type1cct_1.gif

Now, the mic pre is fixed at 40dB of gain. What would you guys suggest I do to make it a little more usable? I was thinking of a simple pad network??

Thanks a bunch,

Chris

Replace R1 with a pot or switched attenuator.

Samuel

Maybe you can use a swicth at R11?

Try this:

Remove R11 (lift it in one side). This removes all feedback.

Replace R6 with a 1M log potentiometer.

Connect R7 and R8 to the pot's viper.

You're now running the circuit no-feedback, with control of gain down to none.

If you loose top end, try lifting C1.

Might sound good - and if it does, this would be the easiest way of implementing variable gain.

If it dosen't sound good, there is some other - less simple - ways. Try this first.

Jakob E.

[quote author="gyraf"]Try this:

Remove R11 (lift it in one side). This removes all feedback.

Replace R6 with a 1M log potentiometer.

Connect R7 and R8 to the pot's viper.

You're now running the circuit no-feedback, with control of gain down to none.

If you loose top end, try lifting C1.

Might sound good - and if it does, this would be the easiest way of implementing variable gain.

If it dosen't sound good, there is some other - less simple - ways. Try this first.

Jakob E.[/quote]

Thanks Jakob, you're a legend.

I'll give it a whirl and let you know how it goes.

Cheers

Chris

With that pentode running open-loop when feedback is removed--as well as the 12AT7, which is not the most linear triode to say the least--distortion is going to be pretty high. Then again, like Jakob says, you might like the way it sounds. If not, then pads or variable feedback are the next things to try.

If you reduce the value of R11, increase the value of C4 by a proportional amount. The circuit has loads of open-loop gain, so you could even increase the value of R11 somewhat if you wanted more gain, and still maintain a good feedback margin.

Chris,

How'd you go with these mods in the end?

I have the same amps, but just running as standard for now.  I know Barney Loveland has a couple, too, and he was maybe looking at adding a gain mod to his at some stage.  Not sure if he ever decided to explore that or not.

Cheers,
Evan.

Just doing my 6 yearly check-in here .. .
  !woah!

Chris - did you ever get in and mod the TYPE 1 amp(s)?

Myles and I have some down here in Melb and were just looking at the in the past days..

I think Rob Squier and others (eg. Barney Loveland) have looked at them and just added pad(s) either before or after the module (along with maybe phantom power / phase switch, such as offered all-in-one on the JLM 'Go-Between' module, for example)
but I'd def be keen to hear if any of the other aforementioned mod's added nice characteristics too.

Cheers!
Evan
(Melb)