Valve amplifier service

[Nick]

If you feel like a trip to Halifax some time, we could have a play with it.

[simon]

Thanks. If I run out of options and inspiration I might just do that.

[Mike H]

May be my imagination, but seems like the 'scope trace is blurry, like it has HF in it? Could be RF?

[Nick]

I think its a result of over exposure of the phosphor.

[simon]

No you're correct Mike. I used a low pass filter too to block anything above 20k (IIRC) thanks to a tip from Nick previously. So I'm not sure what's going on - at greater signal strengths I get a decent sine wave or whatever. But down low it's just mush.

[simon]

Okay, so putting the probe on either 5881 grid doesn't affect the hum.

I lifted a leg of the 1M resistor and it measured okay but I replaced them both anyway. No affect on hum.

I sprayed some contact cleaner on the pins and inserted in the socket a few times, but this didn't affect the hum either.

Ho hum.

[Nick]

Okay, so putting the probe on either 5881 grid doesn't affect the hum.

Ok, good, so what do you see on the scope when connected to the grids?

It sounds like pin 7 of the ecc83 needs a grid stopper, but that's very unusual for a 83.

[Mike H]

That's a possibility - say 1k - that half of the '83 is in common grid mode which is a favourite config for wide-band applications (i.e. radio).

I'm assuming it isn't interference from outside?

Or, it may be acting as an oscillator. Where is C5 located relative to V2 and R6 (137k) ? I'm thinking of HF feedback anode to cathode.

[Mike H]

11 days later, any updates?

[simon]

No I've been having a bit of a break from it. Not felt so good again, but will get back to it shortly.

[Mike H]

Okely-dokely.

Probably a good idea, come back to it later with fresh eyes.

I typially do this after a bunch of soldering stuff up. Wait until the next day before checking for mistakes before turning it on.

[simon]

Well, poor old Dave will be wishing he'd found someone who really knows what they're doing. And not a poorly sicky. I've not been up to much for a few weeks but I've had a look under the hood again this afternoon.

A quick recap - there's hum on the right channel which wasn't there till recently. The amp has had quite a bit of upgrading of some quite expensive parts at some point but the circuit looks to be the same.
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I've replaced all the caps in the amp, film and lytics, but this didn't make any difference.

I've replaced R7 but this didn't make any difference either.

Putting the meter probe on V2B grid silenced the hum, so today I added gridstoppers on the grids. I didn't have any 1k carbon as Mike suggested so used 820R. But this didn't make any difference either.

I'm beginning to wonder if maybe it's the socket???

[simon]

This said, I was a little surprised when I turned it on again today as I couldn't detect any hum. A little perplexed I had a gentle poke with the meter probe and found that by gently moving R7, the 1M resistor soldered on the socket pins, across the socket, I could create all sorts of horrible noise. I could also gently coax R7 and make it stop. Both ends of R7 seemed to do this.

Not being a sound solution with longevity I fitted the grid stoppers and the behaviour was back to before - hum which disappeared when the probe was applied, and I couldn't provoke the noise any more that I could do freely earlier in the day.

So maybe it's the socket? Dunno.

[simon]

I'm assuming it isn't interference from outside?

Or, it may be acting as an oscillator. Where is C5 located relative to V2 and R6 (137k) ? I'm thinking of HF feedback anode to cathode.

No, I don't think it's external interference. But could be wrong of course!

C5 is now one if the orange drops to the middle right in the photo below. R6 is hiding under the yellow LCR 220nF cap, and the valve is bottom left.

The grid stoppers and R7 are also visible bottom left.
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20230824_182608.jpg (195.87 KiB) Viewed 1409 times

[Nick]

Worth a new r7 I would think.