Valve amplifier service

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Paul Barker
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#16 Re: Valve amplifier service

Post by Paul Barker »

Whereas the 7025 is a plug in replacement for ECC83 it is a low noise version. So you should go back to 7025 in position one for a start. So looks like you have a Mark 2.

This doesnt mean the hum will go, but Im a boiler repair engineer. You fix what you find, and await the rest of what you need to fix, as it comes forward for attention. Doesnt mean you were wrong to fix what you found so far.

Address the question of induced noise from power leeds running near interconnects and so on. Cross leeds at right angles if not avoidable.

More advanced is whether power supply capacitors are bulging at the top where they are designed to break first when over heating. Hum youre not used to might be caused by capacitors losing capacitance incrementally until they spew their guts and short the power supply to ground a little at first. Physical signs easy to read.
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Greg
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#17 Re: Valve amplifier service

Post by Greg »

As Paul says, looks like you have the Mk2 push pull kit. Obtaining a pair of 7025’s for V1 would be the intended preference, but I doubt that will correct your hum problem. If you PM me your email address, I’ll send you the build instructions with circuit diagrams pdf for your benefit and that of whoever helps you out fixing it.
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#18 Re: Valve amplifier service

Post by simon »

Do you have a multimeter and can check some voltages? Are you comfortable doing so? If not maybe bring it round to mine and I could have a poke around it?
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#19 Re: Valve amplifier service

Post by Tony Moore »

As Paul said, I think I'd suspect that LCR PSU cap since it is common to both channels. Might be on its way out.
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#20 Re: Valve amplifier service

Post by simon »

Thanks for the "second opinion" Tony, Bostod's bringing the amp around on Tuesday so I will certainly check the cap out.
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#21 Re: Valve amplifier service

Post by simon »

Dave brought the amp round this morning and I had a look. Plugging it in to some old speakers there wasn't obvious loud hum, but this might be because the speakers probably aren't very efficient.

It took me a while to work out what I was looking at as it's fairly densely stuffed, but checking some voltages nothing particularly jumped out at me as a problem.
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What did strike me as odd was that HT was bang on but the current through the 5881s was 65mA rather than 56mA yet the anode voltage was 25V higher than indicated on the circuit which I wouldn't expect.

The voltages on V2 were a bit higher than expected but I don't have any experience of a long tail pair to know whether this is an issue.

What I did notice was the small amount of hum stopped when I put the meter probe on the left channel grid of V2B.

I didn't take any components out of circuit to measure as it will be a bit of a job to do so. The 50uF+50uF LCR cap looks to have a date code of 1993 so is a definite contender, though the HT checks out.

There are a few other lytics, most of which are Blackgates (remember them?!) and along with the Audio Note PIO coupling caps look to be an upgrade at some point so it's more difficult to age them.

I wonder if the hum might be some kind if inter-kit hookup issue (there's no ground lift resistor) but does anyone have any thoughts or suggestions that could be tried next?
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#22 Re: Valve amplifier service

Post by Nick »

I would check the voltages on the 470k grid resistors. If the PIO were leaking that would show as DC on that and higher valve current than expected. The higher anode voltage is probably down to the output transformer having a lower DCR than the prototype.
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#23 Re: Valve amplifier service

Post by Mike H »

Image
I had exactly the same thought, if C5 & C6 have gone leaky, that could explain it. You could try disconnecting them then testing the Voltages again?
 
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#24 Re: Valve amplifier service

Post by simon »

Thanks both. I've been mulling this over, not very successfully :lol:.

Dave took the amp back with him and I wish I'd checked some more voltages but it wasn't too easy to navigate around the amp. The big AN PIOs were retrofitted above the tag boards so it was all a bit tricky to get at.

It's up to Dave what he wants to do next I guess - live with it, find someone who really knows what they're doing :-D, or perhaps bring it back and leave it with me and I'll work my way through some things. But it might take me a little while.

What are your thoughts Dave? And I really won't be offended if you want to try someone else.
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#25 Re: Valve amplifier service

Post by bostod »

Thank you so much with all the help so far. I tried Simons advice and took out the PS audio Trio p-200 preamp and replaced it with a NVA passive preamp to test any additional ground issues going on.
I was surprised that the noticeable hum changed into a much quieter buzz, if that makes sense. The noise is still equal in both channels even with the new 7025 valves in the p1 position on both channels.
I'm not sure what this means. The sound is very different with the passive pre, seems to have lost alot of the soundstage and air. Everything has closed into a more centre position but with tighter bass.
I would love to get to the bottom of the issue and get the amp working back to close to what it should be.
I would have no problem with Simon keeping hold of the valve amp for a while. Luckily I have a very good reserve amp.
Let me know your thoughts
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#26 Re: Valve amplifier service

Post by simon »

I'm happy to have another look at it Dave, spend some time delving deeper. Drop it round when you're able.

I'd probably try it in my system too to see what happens - my speakers are a lot more efficient than those old cheap Tannoys I used for testing and would give me an idea of what the hum's like. (I still wonder if an earth lift resistor might be needed - removing the pre amp might have confirmed this perhaps.)
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#27 Re: Valve amplifier service

Post by simon »

It's taken me rather a long time to have a look at this again, haven't really felt up to it. The updated circuit below has the voltages I took earlier ringed in red.

I couldn't get a steady reading on g1 of any of the 5881s - the voltage was bouncing around between c150mV and c300mV, occasionally going negative. Not a lot but I would have expected it to be closer to 0V or at least not bouncing around. This does perhaps explain the hum though maybe?

C4 is also a AN PIO - perhaps explains why cathode voltage of V2B is a bit high?
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Edited - 150mV to 300mV, not 0.15mV to 0.3mV
Last edited by simon on Wed Jun 14, 2023 2:58 pm, edited 2 times in total.
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#28 Re: Valve amplifier service

Post by simon »

I did plug the amp in to my speakers to see what happened. There wasn't a lot of hum but plugging the amp in after the active crossover, volume pot and op amp gain stage there was motorboating which surprised me a little. Plugging my phone into the amp and there was no motorboating.

Any thoughts on what's going on? Replacing the AN PIOs seems the obvious course of action, but not a cheap one.
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Nick
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#29 Re: Valve amplifier service

Post by Nick »

If the cap to ground was leaking, in that position I would expect lower cathode voltage not higher.

If I understand the initial problem it's hum. Combine that with getting it to motorboat I would think that there may be insufficient decoupling in the power rails. Especially between the output stage and input stage.
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#30 Re: Valve amplifier service

Post by Nick »

Maybe check C3 and 7 and consider larger values.
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