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pgraham 26th September 2009 07:00 AM

plug-in input transformers in tube PA?
I have a Bell Sound 5630 tube PA amp I am restoring. It has 3 XLR mic inputs, which are each wired through an octal socket on the chassis before going to the first preamp stage. The sockets are marked (on the chassis and the schematic) as "Low impedance input transformer socket". They currently have plugged in what is called on the schematic "Input transformer socket shorting plug". This straps pins 2-8 and 4-7. The schematic shows that the XLR jack signal conductor 1 goes to socket pins 2 and 5; signal conductor 2 goes to socket pins 1 and 4; socket pins 6 and 7 are wired to chassis ground; and socket pin 8 is the input to the first preamp stage. So currently one XLR conductor goes straight to the preamp and the other to ground.

My question is basically whether I ought to be using an input transformer of some sort for best sound to actually use this amp as a PA with modern mics and low-z XLR cables (only intending short runs - 20 foot max); and if so, whether the relatively commonly available vintage Altec octal plug input transformers would work; and if so, which one would be best?

There's no indication of suggested input transformers to use in the (limited) tech sheet I have for the amp. Were these transformers generally pin-compatible between manufacturers? from what I've seen, I think the pinouts on the Altec would work. For instance, the 15335A shown here:
if plugged in to my sockets, would run XLR conductor 2 to one end of the primary (pin 1), the other end being ground (pin 6), and connect the preamp to one end of the secondary (pin 8), the other going to ground (pin 7). Is that how these transformers should be wired? Or are the 2 signal conductors supposed to go to the 2 connections for the primary? I could at any rate rewire the socket easily enough.

My understanding is that low-z mic inputs are expecting to see a high impedance at the receiving end; I thought the Ampex 15095A might be the appropriate choice? That has the primary at 600 or 150 (depending on how you connect it) and the secondary at 15k. Should the low-z to hi-z interface be within the input transformer itself, with the primary at low-Z matching the mic and cable and the secondary at hi-Z; or should the primary of the transformer be the hi-z load for the mic and cable? As you can tell, I am confused. If I do want a low-impedance primary, would 600 or 150 be most appropriate? Would this amp be expecting or able to work with something like 15k on the secondary (preamp in)?

I mainly want to make sure I'm not trashing my signal response at the input stage!

m6tt 26th September 2009 08:04 AM

They're supposed to be step-up to grid impedance, I believe. Many were rebranded to suit manufacturer. I encountered these for Low-Z inputs (200-ohm balanced) to 50k grid or so. Really depends on the transformer quality. For what it's worth, I just dug out a schematic that shows one of these with pin 7 (50k winding start) to grid, pin 8 (50k winding stop) to ground, XLR pin 1 to ground, pin 3 to low-Z winding start, pin 2 to low-Z winding start, pin 1 to ground. For what it's worth, this particular PA schematic had a grid leak biased input stage with a fairly high plate load.

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