If it makes any difference to you guys, wouldn't it be cheaper to just use an existing preamp from your rig as a front end. I figure the impedance from the output of the preamp would be 600 ohms. Don't know if this jibes well with Bluebird's mod to put a 10K:10K as the input transformer, but something tells me it would work.
Regarding the interstage transformer, my shoddy research suggests that you replace the plate follower resistor with an interstage transformer. How that applies to the unit in question (PRR Vari Mu) is what befuddles me.
Regarding the input and output gain, I'm happy with just the concept of a -18db pad on the input before the transformer and one on the output after the transformer. What might be worth looking into is installing a volume pot on the pathway to the primary of the interstage transformer so you can mess around with how hot that transformer gets hit.
The line amp is a good idea, would you put it before the 600:600 output transformer? Sounds like a sexy place to put it. I'd almost be game to suggest a much more arcane solution to address this concern. ;D
What if we created a crude open aka insert loop with a 150ohm Rshunt? I'm thinking that if there's no dangerous voltage traveling along the signal path after the tubes and the interstage transformer, that we could simply create a gap in the circuit whereas the signal leaves the system and then can be routed back in. If the signal would be stepped down after the interstage transformer to 600 ohms, then we could attach the secondary of the interstage transformers to a pair of XLR outputs aka the "insert loop outs" with a 150 ohms Rshunt inline. Then what the end user could do is take the signal (at 150 ohms/mic level) and feed it into a preamp of their choice to bump up the level and then take the output of that preamp (at 600 ohms) and feed it back into the compressor at the "insert loop ins." Since the output transformer is 600:600 ohms then it should be rather compatible.
Unless there's some issues with dangerous voltages, I'd say that this would cost us maybe 7 bucks in parts? If someone doesn't want to use the preamp method for making up gain after the interstage transformer, then if we put the Rshunt on a switch that can bypass it at the insert loop out, then the end user can just patch the insert loop with two XLR to 1/4" cables and the impedance will be at 600 ohms going into the output transformer.
For more suggestive madness.
Seeing as how Bluebird suggested the interstage transformer as 15K:600 ohm, this would actually allow us to use the mighty Cinemag CM-9600T aka the Ampex 351 block transformer clone. These are the transformers used in the JCF 8 channel D/A with all those tubes and 8 of the Cinemag CM-9600Ts.
They are rather pricey I think just under 90 a piece, but two of them combined with some crazy tube action going on around them along with a way to really feed signal to that output transformer makes for some sick compression!
Right now I've got my scheme like this for parts.
Transformers:
Edcor XSM 10K:10K (2) Input transformers
Cinemag CM-9600T 15K:600 (2) Interstage transformers
Cinemag CMOQ2S 600:600 (2) Steel core Output transformers.
EH 12AU7 (2)
3RU case to accommodate the Cinemag CM-9600Ts.
Dinesh PCB and I will probably go for the biggest knobs that will fit on the face of the case!
Everything else like the power transformer is pretty wrote and I'll probably go for decent Xichon caps etc but not go crazy with manufacturers, just what makes the most economic sense.
If you really want to go even nuttier, I'm certain that some OEP 10K:10K or 600:600 on the inputs would be pretty interesting since they have a colored sound that some folks prefer in the 1176, while others like the Cinemags.
The Edcors seem to be the order of the day though and since the ones on the input are really high headroom, I figure they make the most sense, otherwise I would go for something with more nickel to get a clearer input out of these things. However if the insert loop holds water then whatever preamp you insert in the loop offers more flavor to the mix too!
Design wise, Im leaning towards having a 5"X5" square cut in the middle of the face of the case so that you can see the tubes. I found a place that sells these really cool 6"X6" brass mesh tiles for 8 bucks plus shipping and I think mounting this on the inside of the case where the hole is would be a nice play off of the older guitar amps and tube hifi gear where you could see the tubes lit up when the unit is on, it would probably help with ventilation immensely. As far as metering, I kind of don't see the need. This is a unit that you'll use by ear anyways.
I know that PRR will always suggest the cheaper options first and he is right. My only reason for even thinking of using that Cinemag CM-9600T is because I have been searching for the right project to use it in. It just seems this project is perfect for it!
Hope this adds some fodder for brainstorming and discussion!
Bluebird, where you at??
Peace
Illumination
