Need suggestions for 50's tube amp debug

[vortex]

I picked up a 50's McEachern (Licenced under patents of Northern Electric, Western Electric) tube amp on ebay. I've never heard of the make before but was impressed with the hefty power transformer and apparent build quality. The tube compliment is 2-6N7 pre and 2-6V6 power. The rectifier is a bit of a mystery, but at the moment, it is reputed to be a 5U4 according to the seller.
I had a peek under the hood and there is one replaced cap, the rest are original but appear in OK condition for their age. I know they should be replaced and if I keep the amp they will be. There is a choke.
I am getting quite a bit of hum and intermittant crackling on the amp and it is low on power as well. I tried replacing the original metal 6N7's with some glass 6N7's I had on hand and noticed that they did not light up.
The seller told me that he had just had the amp serviced and that the rectifier had been replaced. He also said that he had tested the amp and that it was working very well with no hum etc. I'll give him the benefit of the doubt for now.
I've held off trying a 5Y3 in the rectifier position as I don't know what the repercussions might be. It occurs to me that it might be a better choice than the "unmarked" 5U4.
I thought I would toss this out here at Ampage and see if anyone might have some suggestions or cautions. Much appreciated. Cheers.

[Mars Amp Repair]

Votex,
I would think if the pre-amp tubes won't even light up, then I'm surprized you're getting any guitar out of it all. With some tubes it can be difficult to see the filiament glowing. As far as trying the 5y3...don't see any issues there...if memory serves, the 5y3 is about the weakest of that rectifier family, so no issues with over voltage..wouldn't be anyway for a short test.
other than that, you'd need to start measuring voltages & troubleshooting.

[stokes]

The 5Y3 is going to give you less power than the 5U4.It sounds like it just needs the normal maintenance that would be expected in any amp that old.Before putting new tubes in change those caps,all the electrolytics in the amp,that is a very likely source of the hum and low power.And caps that old tend to burst,I would make this a first priority.Clean and retension all the tube sockets,dirty sockets can cause the crackling.Clean out the pots with some contact cleaner.These are basic musts to do before you do anything else.I cant stress the filter caps strong enough.Without an up to par power supply you will not be able to find any problems there may be.

[John_H]

As the others said "replace those capacitors". Check out this webpage. http://www.geofex.com/ampdbug/ampdebug.htm

OBTW With switching to the glass preamp tubes You may need to shield one, or both of them.