Gates (?) mic pre

[rove]

I have a mic pre that I bought off of ebay that I have been trying to get working properly. It is supposedly a Gates, though it has no writing whatsoever on it. The input transformer is a UTC A-11 and the output Transformer is unbranded marked AO 3040. 4 tubes: 6SJ7; 6J7; 6SN7; 6X5.
The problem is a high noise level, at first I thought it was hum, but it seems more like just some low frequency noise. Otherwise it sounds pretty good, looks like some of the caps have been replaced, but not all. Is this the type of thing that could be caused by some aging caps?
am I explaining myself? thanks for any ideas.

 

[NewYorkDave]

The filter caps may have been replaced at some point, but very likely they're due to be replaced again. Judging by the tube complement, your preamp is probably 50 years old or older.

 

[rove]

thanks for the reply. It has 2 30/30 MFD can caps and one Mallory "type FP" 40 400 can cap.
for sound comparison I am using another Gates pre, the M-5287, which, incidentally I would love to get any more info on. they don't share any of the same tubes, but it hasn't had any caps replaced. The M-5287 is quite quiet in comparison to the other pre.
Which caps would likely be the filter caps (forgive the uninformed question)?
thanks.

 

[NewYorkDave]

The can caps are the filter caps.

Are you using it with an external power supply? Look at how the heaters are connected. They need to be referenced to ground (or a fixed DC voltage) at some point, and this is often done back at the power supply. If they're just floating, you'll get hum.

 

[rove]

It has a built in power supply, Triad R-3A power transformer.
the heater wires are connected to the Green/Green wires of the PT which are 6.3V secondary (I think). The unit has a 2-prong power cord.

 

[rove]

any advice on what exactly to use to replace the vintage can caps?
thanks.

 

[soundguy]

rove, the cans probably have 2 30uf caps and a 40uf and 400uf cap. I wouldnt waste a bunch of time trying to find cap cans, just get the values you need and mount them inside the chasis if they will fit. You can try to mount them externally and put a tray cover over them, like some fender amps did, or shrink tube the leads and just leave them (probably not the best idea though). NYdave came up with a neat solution if you look in the altec thread, just cram a bunch of caps into a small bud box and hang it off the back. Many modern caps are small, you can likely fit them inside the chasis in lots of cases with little ado.

dave

 

[rove]

that sounds good, I was thinking I would just use new caps. However, without a schematic, how can I test what the connections are on the capacitors? I think there should be plenty of room in the chassis, or else I'll work out space for them.
thanks

 

[rmburrow]

If you are sure the can cap is bad, carefully remove the cap from the phenolic mounting so it doesn't break.  Try to recover the terminal disk, and then remove everything else inside the can (wear gloves) and dispose of the old innards.  Clean the inside of the can once everything has been removed.  Wire replacement capacitors inside the can (ground side closest to the bottom). Use shrink tubing to insulate leads.  Typically, one can get all of the replacement caps inside the can with space to spare.  It may be necessary to use some inert filler of some type to keep the caps mechanically stable.

This way, the "vintage" look is preserved and no extra "stuff" is paralleled across the dead can cap inside the chassis.