Power supply buzz

[Brouilly]

I have recently rebuilt my SE 6DT5 into an old valve radio I bought on ebay, for my sisters birthday. In a late night beer fueled rush to get it finished in time, I rushed the wiring and there was a hurrendous buzz in the signal path a few seconds after turn on. I put this down to me being a fool, and bought her chocolate instead.

So over the last week I have rebuilt it to a much better standard, but the buzz remanes there.

Originally I thought that it was the new transformers I am using, mentioned here...

http://www.diyaudio.com/forums/tubes-valves/176798-small-size-output-transformers.html

... cheap I know, but an upgrade from the hallogen light heaters I was using as OTs before! Ill fork out for decent parts when Im more experienced.

So the buzzing comes after a few seconds of turning on, but when touching a probe to the primary on the OT is cuts out and gets louder, and makes all sorts of horrible noises. Can anyone suggest what this might be?

Ive checked all my wiring, again, checked the diodes and the p/s caps, moved the transformers around, and have now reached the limits of my knowledge.

I was concerned that the transformers might not be up to the job, but from the other thread that doesnt seem the case.

I did spend all night searching the threads, but nothing really hit the mark.

Thanks for any help in advance.... still learning

Charlie

 

[chrish]

Star Grounding

 

[Brouilly]

Thanks for your help, but this was the first thing I checked

Charlie

 

[Brouilly]

interesting read though chrish, thanks

 

[korey]

Is it possible for you to post a pic? It sounds like a power supply filter capacitor issue, but don't want to guess. Maybe a fresh pair of eyes can see something that you may have missed.

 

[Brouilly]

I just got in and fired it up again, and after a while the buzz slowed down to a clicking.

To an inexperienced ear it sounds very much like a low frequency oscillation.

There isnt much room inside the radio, and i tried to use exterior holes, but this placed the mains switch close to the speaker terminals. The change in 'buzz' was me moving the switch away from the terminals.

So I guess im looking at mains interference of some kind?

 

[Brouilly]

ok, and touching the mains plug results in this oscillation to speed up again.

im certainly stumped

 

[Brouilly]

ok.....

i hadnt realised that the volume pot was floating. Having seen my mistake, I thought to myself that this couldnt be the reason for all this horrible noise....

... how wrong I was. There is still a bit of hum, so im going to go back over everything.

But at least there is hope!!

Thanks to everyone for putting up with me

Charlie

 

[Sch3mat1c]

RFI? Try grid stoppers?

Contrary to popular belief, chassis grounding, done responsibly, has much better results. Particularly at RF, where you can't tolerate the huge ground return inductances that star grounding inevitably gives you. Radios and TVs are all chassis grounded, and with good reason: they need excellent RF performance!

Tim

 

[Arnulf]

I wouldn't want my AF amplifier to have excellent RF performance 🙂 Quite the contrary, I want it to suck at RF amplification.

 

[20to20]

Lot a folks mistakenly (understandably) use the term "RF" in place of "EMI." RF is normally in the frequency range above 30Khz. Electromagnetic interference is what we normally fight with grounding/shielding in amps. Of course we don't want someone's CB to be able drill into our amps either.

 

[DF96]

For "excellent RF performance" read "stable RF performance" i.e. it doesn't oscillate. A valve doesn't know it is only supposed to amplify audio in an 'audio amp' so if it sees the appropriate impedances in the right places it will quite happily oscillate - it thinks you were building an RF oscillator.

However, don't forget that for a radio or TV receiver hum and distortion are less important than stability and sensitivity so the correct grounding technique for a TV IF stage at 36MHz might not be appropriate for an audio amp at 20-20kHz, and vice versa.

 

[Brouilly]

The hum is not there when the pot is fully open or fully closed, could anyone comment on that?

Just a faulty pot? Spose it gives a good case for stepped attenuators....

 

[DF96]

The hum is being injected into the pot slider, or whatever it connects to (a valve grid?). Is the cable screened? Is the screen grounded at the right place? (Somewhere near the valve cathode circuit)

 

[Brouilly]

I havnt used screened cable from the pot to the valve circuitry, just from the RCA to the pot. So screened cable, grounded at one end near the valve?

thanks

 

[M Gregg]

Charlie,
If the pot is not earthed you will get the buzz in the middle of the track. Because it is not being held down by signal or earth.

 

[Brouilly]

Thanks M Gregg, so from you other post, you suggest grounding the pot to star ground...?

Is the norm to use screened cable from pot to valve circuity?

Charlie

 

[M Gregg]

Charlie,

Screened cable from the pot with screen earthed at one end and an earth cable from the thread of the pot to ground.
Any signal cable that has low signal will pick up if it is not screened. Take care not to short the screen to any power connections.

Regards
M. Gregg

 

[Brouilly]

Thanks,

Ill give it a go and let you know

Charlie

 

[M Gregg]

Keep any AC away from signal path. Twist heater cables and keep away from inputs to tubes. Keep input capacitors lead between valve and cap short.

Just for fun

M. Gregg