reducing hum

[mrjcfreak]

I managed to pick up a wonderous beast, a 1960s radio containing all your mechanical capacitors etc. and 10 tubes!

My project is to use the pickup socket on it to make it a straight guitar amp, which works fine, but I've quite a big hum, especially with extension speakers...

Is it caused by old valves or dud capacitors? I don't think so; it sounds like a normal 50Hz hum, running independent of signal through the front. Can I rectify it easily by changing old valves or caps?

And does anyone know of a product which is solely a 50Hz filter? because it would be damn useful in so many applications, and would save everyone lots of headaches.

[astouffer]

Replace all the filter capacitors and that will cure your hum problems. I won't even attempt to power up any old radio or amp thats been sitting for years. If the filter caps aren't bad or shorted already, they soon will be.

It would probably also be a good idea to replace all the wax coupling caps while you are at it. They tend to absorb moisture slowly and leak. Leaky caps can blow up or cause tube failure.


Adam

[anatech]

I agree with Adam here. I normally run a "new find" on a variac, incrementing the voltage bit by bit while watching currents and leakage voltages. That's to see if there are any expensive problems (or terminal faults). Then, change the coupling caps & whatever else. Note: don't increase the value of the coupling caps any great degree. Sometimes a rolloff is intentional to keep the output transformer from saturating.
Also note that a consumer amp may not handle your application too well. Your average power will be much higher than the design goal on the radio.
-Chris

[honsten]

try replacing all the valves with a set of new matched ones (same serial numbers) and for extra hum -bucking,try replacing any rusty,old steel connections with nice clean gold plated ones.

good luck

[mrjcfreak]

I'm starting to realise the best way to preserve my amp is to replace most of it!

Thang is, my guitar is so hot, the thing overdrives when the master is anywhere above 3/10.

I don't think it's going to be abused that much, it's going to sit in my bedroom looking cool, I might get myself an arial and use it for it's intended purpose (Medium wave radio) as I don't have a stereo in my room atm, and I might stick guitars into the back of it for recording or a little special something.

I found it at a fair, they couldn't sell it as it was un tested electrical, so I 'safely disposed of it' and my intention was to conduct a-level physics tests on it. I have decided to use a new valve and experiment circuit for that though, so it's a convenient little procurement.

Caps are gonna have to be replaced though, they look pretty old and my house is too damp.

[SY]

Basic diagnostic question: is the hum present when nothing is plugged into the input?

If so, take everyone's suggestions and replace the filter caps and any selenium rectifier stacks.

If not, your issue is grounding.