onailime

Ampex 8 channel tube mixer mod... please help !
« on: January 25, 2008, 12:39:45 PM »
Hi,

So, I have two Ampex / Sigma 3671 4X1 tube mixers (also sometimes named 3761 and 3716 !). They each have five UTC HA-100X transformers (4 ins, 1 out reverse) and four 5789 tubes. They are meant to be microphone level in, microphone level out, so I have to pad the inputs about 30 dbs to avoid tube saturation, and then add gain with a mic pre. But that's ok, they sound GREAT !

schematic:

 http://twin-x.com/groupdiy/displayimage.php?pos=-1934

pic :

http://twin-x.com/groupdiy/displayimage.php?pos=-1935

(on mine, inputs and outputs are 600 ohms-strapped, and there is no resistor network after the output transformers)

Of course, I want to turn them into an 8-input mixer. For now, I have done a simple mod which turns them into a 8X1 mono mixer. To do this, I have simply taken the summing bus of one 3671 and added it to the other summing bus.

So... I think by doing this, I have changed the summing bus impedance (divided by two) and lowered the output level 6dbs, too. What my ears tell me is that it still sounds great.

The next step is obviously two things : add panpots to get an 8X2 mixer and/OR add ONE aux send and keep it mono. I have read numerous posts on this forum about panpot design. I have found the NYD panpot values/schematic. I am aware that there are a lot of impedance issues in my case...

so my questions...

1) Suppose I ONLY want to add one pre-fader aux send. Should I replace the 100k "fader" (in fact, a pot) with a 200k, and add another 200k in parallel for the aux bus, so that the tube still sees about  100k load? Or am I completely wrong? Is there another way to do this?

2) Suppose I ONLY want to add panpots to each channel. If I use NYD panpot coefficients, I will either have to

     a) lower level pot, pan pot, and summing R values, so i can have a decent (not too high) mix bus impedance. But then, each tube will see a lower impedance than first intended. (bad?)

OR

     b) keep 100k level pot and find myself with 500k summing resistors. (even worse?)

so, which one of these two options do you think is best (or least bad)? Are there values that you recommend (for level pot R, panpot double R, summing R) for a good compromise? Or is it impossible to make it sound good without buffer stages in my case (which I will not do, i want to keep it simple).

3) Is it decently possible (?) to have BOTH a pre-fader aux send AND a pan-pot on each channel? (of course then I would have to add one summing bus and one output transformer...).

Any mod suggestion VERY welcome ! I admit I am a bit lost... Thanks for reading !

Onailime.


onailime

Ampex 8 channel tube mixer mod... please help !
« Reply #1 on: January 28, 2008, 08:39:36 AM »
... so I guess I will just try different pot values and see what sounds best. thanks.


 

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