Valve amplifier service

[Nick]

Yep, hard to say for certain. Is that with no input? Looks to be a 100Hz trace there, assuming the X axis is 2ms/div. 100Hz says power supply to me. The glitch, might me 50Hz rectifier switching, but hard to know.

Is that in the "its humming" state? if so I agree there is no clear sign on RF.

What did the 5881 grid's look like? Preferably both at once with the two channels.

[Mike H]

I find it hard to imaging a ecc83 needing grid stoppers TBH.

Ditto!

What I can't quite get my head around is what's changed.

Very good question. I went back and reread the thread. In summary, started humming, tho not a huge lot. Valves changed to new ones, V1 & V2 = ECC83. No change, On advice, restore V2 to (new) 7025. (Is equiv low-noise ECC83 and original spec for the amp.) No change. Now humming right channel only?

Post #17 said: "What I did notice was the small amount of hum stopped when I put the meter probe on the left channel grid of V2B."

That shrieks RF oscillation to me, and RF oscillation can cause hum.

Ditto, hence my earlier post on the subject.

I wonder if it's an earthing problem? Does the chassis form some part of the ground circuit? (I usually try to avoid doing this.) Are there tag washers to it that could be loose or 'dirty', so bad connections? Something has deteriorated over time?

The 'spike' is interesting (to me anyway), I have seen somethng like this before, years ago, and it was a type of audio sinewave oscillator that used a R-C-R-C-R-C network to introduce a phase delay at the required frequency (one of the R's is a pot for manual control) and thus positive feedback. To this day I don't know what caused the spike, so not helpful; in hindsight I'm guessing the valve (an EF86 or EF36) was 'clipping', as the spike occured on one peak only.

[simon]

Sorry it's taken me a little while to reply chaps.

Yep, hard to say for certain. Is that with no input?

The inputs are terminated to earth with 100R (as suggested when I was looking at the C3g phono)

Looks to be a 100Hz trace there, assuming the X axis is 2ms/div. 100Hz says power supply to me. The glitch, might me 50Hz rectifier switching, but hard to know.

Yes 2ms/div. The rectifier diodes have been replaced with HR4120 which are hyperfast with soft recovery, but presumably still have a switching spike?

Is that in the "its humming" state? if so I agree there is no clear sign on RF.

Yes, all the photos in the previous post are with hum, except for the very last

What did the 5881 grid's look like? Preferably both at once with the two channels.

Very, very difficult to get a photo as both traces were dancing all over, up and down, and sometimes as if the timebase had been doubled (say 4ms/div), it's quite odd really. But they are both the same, with the spike at the bottom of the wave. This is what the 5881 grid looks like:

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[simon]

What I can't quite get my head around is what's changed.

Very good question. I went back and reread the thread. In summary, started humming, tho not a huge lot. Valves changed to new ones, V1 & V2 = ECC83. No change, On advice, restore V2 to (new) 7025. (Is equiv low-noise ECC83 and original spec for the amp.) No change. Now humming right channel only?

Yes, just the right channel as far as I can tell. And thanks for rereading the thread!

I wonder if it's an earthing problem? Does the chassis form some part of the ground circuit? (I usually try to avoid doing this.) Are there tag washers to it that could be loose or 'dirty', so bad connections? Something has deteriorated over time?

As far as I can tell the chassis must be grounded at the input phono tags. The two phono tags have a thick piece of wire soldered between them and this seems to be the star earth and everything looks to attach here.

Now, my best guess is these tags must also make contact with the chassis, I can't see anywhere else where it could be grounded, and the manual doesn't make any specific mention of chassis contact that I've found. I don't think there's any earth lift resistor, so the chassis is at the same potential as signal earth I think.

I checked continuity and the chassis is grounded, but I can't tell if this is where it actually is in contact with the chassis - these are the input phonos and there's no clue that the paint has been removed from the chassis.

20230903_184127.jpg (209.29 KiB) Viewed 8057 times

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But if it was a dodgy/dirty earth connection I presume it would affect both channels as the star earth is common?

[simon]

Running out of ideas I went over all the humming channel's solder joints I could and rewet them, but it made no difference.

However, listening carefully there is a little hum on the other channel, but it's much less than the other channel which masks it.

The hum on both channels goes when the meter probe is touched on V2B grid on both channels. The meter doesn't have to be switched on BTW, but does need to have the neg probe grounded.

[Nick]

Can you check that if its playing music that putting the probe on that pin doesn't stop the amplifier from playing music. Worth checking before going further.

[Mike H]

I wonder if it's an earthing problem? Does the chassis form some part of the ground circuit? (I usually try to avoid doing this.) Are there tag washers to it that could be loose or 'dirty', so bad connections? Something has deteriorated over time?

As far as I can tell the chassis must be grounded at the input phono tags. The two phono tags have a thick piece of wire soldered between them and this seems to be the star earth and everything looks to attach here.

Now, my best guess is these tags must also make contact with the chassis, I can't see anywhere else where it could be grounded, and the manual doesn't make any specific mention of chassis contact that I've found. I don't think there's any earth lift resistor, so the chassis is at the same potential as signal earth I think.

OK, so what I would do (from experience!) is have the input phono sockets insulated with nylon shoulder washers, and an earth lift resistor from there to the chassis, which would be mains earthed ('cause you've got mains coming in). But yes for star wiring from the phono sockets grounds.

Doesn't alter the fact is was 'OK' before ...

[Mike H]

I checked continuity and the chassis is grounded, but I can't tell if this is where it actually is in contact with the chassis - these are the input phonos and there's no clue that the paint has been removed from the chassis.

Ah, another thought, the paint insulated them before, now it doesn't.

[simon]

Can you check that if its playing music that putting the probe on that pin doesn't stop the amplifier from playing music. Worth checking before going further.

That's a good point that hadn't occurred to me. Will check it out.

[simon]

OK, so what I would do (from experience!) is have the input phono sockets insulated with nylon shoulder washers, and an earth lift resistor from there to the chassis, which would be mains earthed ('cause you've got mains coming in). But yes for star wiring from the phono sockets grounds. ]

Me too. But as you say, it was working before. TBH it's pretty tight access around the phonos with a lot if wiring in the way, and I'm reluctant to do any modification here.

[simon]

I checked continuity and the chassis is grounded, but I can't tell if this is where it actually is in contact with the chassis - these are the input phonos and there's no clue that the paint has been removed from the chassis.

Ah, another thought, the paint insulated them before, now it doesn't.

But if it insulated them before I'm not sure there was any connection of the chassis to ground. I think it would be surprising if there was no chassis ground on a kit?

[simon]

Can you check that if its playing music that putting the probe on that pin doesn't stop the amplifier from playing music. Worth checking before going further.

Yep, putting the probe on (either channel) stops the music coming out of the speaker. Does this take me a step forward or a step back? Or am I still lost in the same place?

[Nick]

You are in a different place, but not sure just what that place is. However 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.

[simon]

There's a lot to like about SE amps.......

[Nick]

But ironically, if the problem is the first stage, and that's a single ended stage

I think its likely that the effect of the probe on the high impedance point in the phase splitter is to force the valve to cut off and prevent any signal from getting through. I assume that the result of the probe on the high impedance point is caused by the input impedance of the scope. Might be interesting adding a blocking capacitance to the point you are probing and then connecting the probe to the other side of that.