Valve amplifier service

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Mike H
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#136 Re: Valve amplifier service

Post by Mike H »

What are those crimp connectors for?
 
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#137 Re: Valve amplifier service

Post by simon »

Mains connection to the on/off switch on the front. No, I wouldn't have had it that close either.
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#138 Re: Valve amplifier service

Post by Nick »

As I said yesterday:
I would look at what seems to be an input coax and I assume first stage next to what looks like mains connections.
Maybe worth moving those mains carrying wires away for a test and see if the hum reduces. I assume that the humming side is the one near the switch?
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#139 Re: Valve amplifier service

Post by simon »

Both sides hum but yes the closest channel hums more. Not sure what would have changed to start the hum but definitely worth moving the AC away to see what happens as it should be easy enough to do.
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#140 Re: Valve amplifier service

Post by Nick »

Not sure what would have changed to start the hum
Its always hummed while you have had it I believe?

This is where reliable measurements would have been helpful to know what it was then against what it is now. Maybe its worth a recap of the problem(s) and things done and results?
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#141 Re: Valve amplifier service

Post by simon »

I very quickly shorted the input end of R2 to the phono earth but this created more hum, an earth loop I imagine. Then I had to clear everything away again as I needed my desk for work :-(. I've been through the thread and tried to summarise...

Post 7 – circuit and parts appear to be based on the upgrades discussed in the second attachment

Post 13 – Dave replaced all the valves but this made no difference to the hum. On switch on amp is quiet, but after 15 secs or so the hum starts, becoming constant after a couple of seconds.

Post 14 – valves are Mk 2. Upgrade parts look to be standard WAD upgrade components. Considering their age the AN PIOs are good candidates for being suspect.

Post 15 – V1 has ECC83 rather than the low noise version 7025

Post 16 – try changing V1 to 7025. Also look at power supply caps for signs of bulging.

Post 21 – first look inside the amp. Voltages close, HT fine, some others a little off. Current through the 5881s was 65mA rather than 56mA but the anode voltage was 25V higher than indicated on the circuit. Voltages on V2 (LTP) were a bit higher than expected. The 50uF+50uF LCR cap looks to have a date code of 1993 so is a definite contender, though the HT checks out.

Post 22 – try checking voltages on 470k grid resistors (R11&R12)

Post 25 – Dave 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. The noticeable hum changed into a much quieter buzz. The noise is still equal in both channels even with the new 7025 valves in the V1 position on both channels.

Post 28 – I plugged 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. Plugging my phone into the amp there was no motorboating.

Post 29 – “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.”

Post 30 – Maybe check C3 and 7 and consider larger values

Post 32 – Putting the meter probe on pin 7/grid of V2B on one channel the hum stopped. (Music also.) The LCR 50uF+50uF reservoir cap has what looks like a date stamp of 1993.

Posts 43 to 45 – caps changed, no difference. All original caps measured okay out of circuit.

Post 49 – heater supply at 0V, heater/anode breakdown? Or something simple like crud on the valve socket that is allowing heater AC to get to one or more grids?

Post 54 – rewetted joints around V2. Can't see any heater lift - all valves are wired in series and tied to ground. But looking at the circuit and voltages again, the upper triode's cathode is at c120V so well within the 180V cathode to heater max. for an ECC83.

Post 66 – Putting the scope probe on either 5881 grid doesn't affect the hum. Lifted a leg of the 1M resistor (R7) and it measured okay but replaced them both anyway. No change in hum. Sprayed some contact cleaner on the pins and inserted in the socket a few times, but this didn't affect the hum either.

Post 72 – added gridstoppers on the grids of V2. I didn't have any 1k carbon as Mike suggested so used 820R. But this didn't make any difference either.

Post 73 – when amp was turned on after c7 weeks break there was no hum. Poking around the components on V2 pins with DMM probe (R7 1M) invoked all sorts of noise. Gentle coaxing of R7 stopped it again. 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.

Post 86 – RF oscillation, which can cause hum. I doubt if the phase splitter is oscillating, but the cathode follower in that SRPP is a definite possibility. Agreed, ECC83 is usually too lazy to oscillate, but usually isn't the same as never. A 10k grid stopper on the upper grid of that input SRPP might cure the problem. Cathode followers sometimes need cathode stoppers too. A 1k in series with the cathode's lead to the phase splitter is another possibility. A third possibility is that the power supply isn't adequately decoupled at RF to that stage. A 100nF to chassis might work.

Post 90 – inconclusive scope work! There is an odd spike on the waveform.

Post 92 – could be an earthing problem?

Post 94 – thought it was humming on the right channel only (actually the left but the amp is upside down so inverted… It appears the chassis is earthed via the input phonos, and there doesn’t appear to be an earth lift resistor.

Post 95 – rewet all the accessible solder joints on the humming channel. There is hum on the other channel too, but much lower. Touching the meter probe on V2B grid stops the hum, same on both channels.

Post 102 – applying the meter probe to V2B stops music as well as hum

Post 103 – It may well mean that the hum is not related in any way to the phase splitter. First stage looks likely I would say now.

Post 129 – Disconnected the existing s1-l -> s2-l and s1-r -> s2-r so its s1-l -> s2-r and s1-r -> s2-l and swapped the feedback over, hum changes channels suggesting V1, the SRPP, is the problem.

Post 132 – consider the coax

Post 134 – plugging the phone in to the amp now causes all sorts of noises and hum – it didn’t when I originally tested the amp
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#142 Re: Valve amplifier service

Post by Nick »

I think a lot of that list was misdirection because of the effect of the scope (we must check that over as it may be doing more harm than good). So if I understand, the amp hums like before, and sometimes gets very noisy.

Is it equally noisy in both channels?

Try removing the mains wiring from the area of the input and see if that makes any difference.

Cant help think that it may be worth considering a earth lift, but maybe worth checking out further before trying that.

Have you tried a pair of shorting RCA plugs (shorted with 100R resistor or similar, a dead short to ground can make things worst sometimes.

"Post 94: It appears the chassis is earthed via the input phonos, and there doesn’t appear to be an earth lift resistor."

Thats not ideal, I would normally expect the chassis to be earthed solidly to the earth on the IEC, then the phono's isolated from the chassis, and then signal earth taken to chassis earth via small value resistor with a bipass cap, or bridge rect (to work as back to back diodes, so any voltage above 2*Vf is dropped to earth, but other than that the inout ground is isolated from chassis ground), again with bipass cap.

"Post 25 – Dave 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. The noticeable hum changed into a much quieter buzz."

Maybe the gain of the trio is too high, and or ground problems.

"Post 134 – plugging the phone in to the amp now causes all sorts of noises and hum – it didn’t when I originally tested the amp"

Maybe same as above, or its just the grounding problems.
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#143 Re: Valve amplifier service

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Nick wrote: Mon Jan 08, 2024 9:27 pm I think a lot of that list was misdirection because of the effect of the scope (we must check that over as it may be doing more harm than good). So if I understand, the amp hums like before, and sometimes gets very noisy.

Is it equally noisy in both channels?
Now, it's noisier on one channel than the other. I have a suspicion that it might have been noisier overall when I first started, but as I'm unable to properly measure the noise it's difficult to be sure. Just as you've said before.

The noise with the phone is interesting - it didn't do it before but it was pretty horrid the last time I tried it.

And yes, I need to understand what's going on with the scope or if it's just me.
Have you tried a pair of shorting RCA plugs (shorted with 100R resistor or similar, a dead short to ground can make things worst sometimes.
Yes, the shorting plugs have 100R to ground.
Try removing the mains wiring from the area of the input and see if that makes any difference.

Cant help think that it may be worth considering a earth lift, but maybe worth checking out further before trying that.

...

"Post 94: It appears the chassis is earthed via the input phonos, and there doesn’t appear to be an earth lift resistor."

Thats not ideal, I would normally expect the chassis to be earthed solidly to the earth on the IEC, then the phono's isolated from the chassis, and then signal earth taken to chassis earth via small value resistor with a bipass cap, or bridge rect (to work as back to back diodes, so any voltage above 2*Vf is dropped to earth, but other than that the inout ground is isolated from chassis ground), again with bipass cap.
I really don't like the look of the grounding, both the star earth and chassis connection (whatever that actually is). I can play with the mains AC routing to the front mounted switch, but much more would need a major rewire. I'm not sure I want to do that, but it needs a discussion with Dave.

In fact I've been coming to the opinion for a while now that I think Dave should plug the amp in back at his to check how it sounds now, whether the hum/noise is better/worse/same. I'd quite like to hear it too to get a better handle on the issue in his system. Are you reading Dave? Thoughts?
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#144 Re: Valve amplifier service

Post by Mike H »

Does it have an earth lift resistor, or if not, can one be added without too much work? (Resistor that separates mains earth and chassis from amplifier 0VE or ground rail.)

Just occurred to me, in my experience I've got RF oscillation for usually 2 reasons, 1. it's a radio valve, ergo wide bandwidth, so it just wants to oscillate (just try and stop it - in some cases, just a length of wire is a 'coil' and off it goes); 2. a power valve, i.e. an o/p valve, because of high current and low impedances, relatively speaking. I note the 5881 current and anode Voltages have increased over what they should be? This might be a symptom of something, or, not.

Right now I'd like to isolate the o/p stage, disconnect C5 & C6, see if that changes anything.

I agree ECC83 etc. shouldn't be a problem as it's designed for audio.
 
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#145 Re: Valve amplifier service

Post by simon »

Mike H wrote: Tue Jan 09, 2024 9:19 pm Does it have an earth lift resistor, or if not, can one be added without too much work?
Not that I can tell. The problem is I don't know for sure where the chassis connection is. My presumption is it's the input phonos. If it is I'd need to replace the phonos with isolated ones, removing the ruck of star earth connections soldered to a short piece of wire stretching between the two phonos, and then create a new connection to chassis.
Mike H wrote: Tue Jan 09, 2024 9:19 pm Right now I'd like to isolate the o/p stage, disconnect C5 & C6, see if that changes anything.
You know, I'm not sure if I ever actually did this. I think I must have done but have no recollection of it.
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#146 Re: Valve amplifier service

Post by Greg »

Looking at the original build instructions I sent you, you’ll see all grounds go to the bridged input RCA sockets ground tabs, as does the earth wire from the IEC socket. I would have thought making a close comparison between the amp you have the the instructions should indicate if it has been built accordingly. Not all early WAD kits, including this one, had an earth lift resistor network, although builders sometimes modified to include them.
Last edited by Greg on Thu Jan 11, 2024 1:15 am, edited 2 times in total.
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#147 Re: Valve amplifier service

Post by Greg »

Wiring Up, Pg 7, Item 6.
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#148 Re: Valve amplifier service

Post by simon »

Yes Greg, it does look like the instructions describe, that's what made me think the chassis is grounded in this way. And why it's not an easy fix.

Here's the photo of the phonos again, yuck!
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#149 Re: Valve amplifier service

Post by Ant »

They look like they are already insulated to me, so is the earth floating inside the amp?
Or is there a wire to the back of the iec socket?
Also appears to be a bit of a busbar cut off so where did that go originally?
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#150 Re: Valve amplifier service

Post by Mike H »

A modification too far without consulting the owner :shock:
 
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