Audio hum from amplifier

[Station X]

That's definitely a diode. The power resistor connected in series with it suggests it's not original.

Is it wired to the reservoir capacitor?

[paulsherwin]

It looks like a 1970s replacement. Half wave rectifying the power isn't a great idea if you're trying to minimise hum, especially with a 120V supply, but we don't know the circumstances of the repair or what was there before.

You said in #1 that you've changed the smoothing electrolytics. Can you double check your work there?

[Guscarguy]

I put in a oscilloscope to the speaker terminal and I get noise immediately and not much else afterwards once the tubes warm up. Same as I don't see any difference when I power down the amp. To me I'm thinking that it may be AC mains noise instead of amplifier noise. What are your thoughts?

[Station X]

That points to mains hum on the HT supply. The semi-conductor HT rectifier will start conducting as soon as the player is switched on, but the amplifier won't start working until the valves have warmed up.

Are the new smoothing capacitors of the correct value and connected the right way round?

[Herald1360]

A hot chassis set with half wave rectification would have mostly 60Hz hum from poorly smoothed HT though there could be enough second harmonic present to dominate the sound from a small loudspeaker with limited LF response.

[bigfathairyvika]

Any LED lighting switched on in room when testing?
Or even fluorescent lamps?

[Diabolical Artificer]

Some pictures of what your seeing on the scope would help. Andy.

[John10b]

Check the ripple on your ht, using either your scope or dvm.
Then take some pictures/ readings so we can determine what is occurring on your ht supply,
Do you have any other amp or radio, so you can carry out some tests and compare with your amp?
When I listened to your amp I did think it was possibly external interference, so as suggested try switching off any other electrical item near you.
Also if you have a small transistor radio try and use this to see if it picks up any interference, hope this helps.
John

[Guscarguy]

I keep seeing HT in the comments. What are you referring to? Checked all connections of work that I've done and the capacitors replaced are in the same location as the original ones are.

[John10b]

Sorry for any confusion, HT means High Tension(Voltage).
John

[Station X]

They call it B+ in the USA. Perhaps it's the same in Canada?

[paulsherwin]

Yes, Canadians generally use US electronics terminology rather than British. HT = B+, anode = plate, valve = tube, all the usual stuff.

[John10b]

Well you live and learn.
John

[Guscarguy]

Thank you for the clarification. As of now the changer is disconnected from the amp to eliminate the possibility of that being the problem. Maybe the motor needs to be connected to eliminate the hum perhaps being that's an Ac motor? Somebody mentioned to me the motor acts like a transformer for the amplifier.Tried it in different rooms with no fluorescent light no other devices. Same result. I purchased this product to help my situation.

https://www.amazon.ca/gp/aw/d/B07835...b_b_prod_image

[Guscarguy]

The product came today. Didn't help the situation at all.

[Gabe001]

I remember reading a thread where the problem with hum was solved by the addition of a pi (RC) filter to add additional mains smoothing. Could be particularly relevent here as the mains is half wave rectified with a lot of ripple. If should be 60hz but with harmonics you can't be sure

Found it: https://www.vintage-radio.net/forum/...&postcount=162

I'd also try a 220ohms 5w resistor followed by 33uf capacitor as an additional layer of smoothing, as suggested by lawrence. I cant see a schematic but this would generally go between your reservoir capacitor and output transformer. You can work it out by looking at where Donald fit his and placing it in the analogous place in your set.

[Edward Huggins]

Originally Posted by Guscarguy

Thank you for the clarification. As of now the changer is disconnected from the amp to eliminate the possibility of that being the problem. Maybe the motor needs to be connected to eliminate the hum perhaps being that's an Ac motor? Somebody mentioned to me the motor acts like a transformer for the amplifier.Tried it in different rooms with no fluorescent light no other devices. Same result. I purchased this product to help my situation.

https://www.amazon.ca/gp/aw/d/B07835...b_b_prod_image

I think it may be well worth reconnecting the motor especially if its windings provide the heater/filament voltage.

[Radio Wrangler]

Originally Posted by Guscarguy

The product came today. Didn't help the situation at all.

There is a Youtube channel, "Big Clive" where an experienced engineer on the Isle of Man likes opening up that sort of thing to show what's inside and talks about what they can and can't do. He has a rather dry sense of humour. Most of these things are just snake oil. A few have basic suppression components in them, almost none of them make any difference. Some of the things he opens are actively dangerous. It's a good channel for opening your eyes.

David

[Guscarguy]

It's funny you brought up filters as I mentioned that product didn't work. Soon afterwards I ordered a line filter. We'll see if that makes any difference. Much cheaper than the other one for sure. This gentleman has the same problem as me has 3 videos uploaded, in the end no results found. Motor was connected.
https://youtu.be/vKvTLpYYzs4?si=oOA3Xfrr1PV6QX91

[John10b]

Just took a look at the video.
As I understand it he did conclude the problem was mains borne, he demonstrated this by removing the mains plug momentarily and interference stopped.
As I understand it you have tried the amp in a different room, however, if you do have mains borne interference this will occur in any room.
You tried a mains filter with no success and you are trying another one, as I understand it. Let us know how you get on.
John