Valve amplifier service

[Nick]

I would look at the earthing arrangement of the amp, I think the phase splitter is a white elephant.

[simon]

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

Aye, but I'd have got there quicker

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.

IIRC putting the probe on the ground side of the 220nF cap from V2B grid to ground also stopped the hum but I'll check later.

[simon]

I would look at the earthing arrangement of the amp, I think the phase splitter is a white elephant.

Erk! Don't much fancy getting in to that. Horrid! But maybe that's the next job, unless there's something simpler to try in V1 first?

[Nick]

I would add a isolation .1uf cap to the signal from the first stage to the phase splitter, then using that check on the scope if there is a hum signal. Then work from there to try and find what may or may not cause that to vary.

[simon]

IIRC putting the probe on the ground side of the 220nF cap from V2B grid to ground also stopped the hum but I'll check later.

Nope, don't know what I was thinking, unsurprisingly putting the probe on the earth side of the 220nF cap doesn't stop the hum or music.

[simon]

I would add a isolation .1uf cap to the signal from the first stage to the phase splitter, then using that check on the scope if there is a hum signal. Then work from there to try and find what may or may not cause that to vary.

Not sure I've understood correctly - add a 0.1uF cap to the scope probe, then use the other end of the cap to probe around?

[Nick]

Yep, I was generally unsure about your scope and it seeing 200v or so. What with it not wanting to switch to AC input. If its ok with it, just use the probe.

[Mike H]

Wanted to post something but thought might just confuse the issue. Waiting for the above.

[simon]

I think it does "see" AC signals, I was just a bit confused that channel 2 was referred to on screen as V DC. But I'll try the cap and see what happens.

Yesterday was hospital day so it might be a few days before I feel like probing further.

[Nick]

I think it does "see" AC signals

I was thinking more about protecting the scope input stage from DC. Then turning the sensitivity up so you can see hum/noise.

[simon]

Another 2 weeks slips by... Adding the cap didn't seem to make any difference.

There doesn't look to be any hum before V2. As far as I can tell anyway - the scope produces that low level "mush" I get. As soon as I put a probe on the top side of C4 (or either side of the gridstopper I put on V2B grid) the hum stops. Tricky to scope the hum when applying the probe causes it to stop humming!

Incidentally, applying the probe to V2A grid increases the hum.

[Nick]

Adding the cap didn't seem to make any difference.

I didn't suggest the cap would make a difference, I was just trying to protect your scope when probing around 300v with the input set to DC.

As soon as I put a probe on the top side of C4 (or either side of the gridstopper I put on V2B grid) the hum stops.

Yes, we also know it stops the music as well, so there is no information to be found by doing that.

You are stuck as you have a scope that doesn't reliably show you anything other than "mush"

Incidentally, applying the probe to V2A grid increases the hum.

Try a .1uf cap between the first grid in the phase splitter and ground to shunt any signal from the first stage to ground.

Or even better (just thought), as its all in the same amp, cross connect the two input stages with the two phase splitters and see if the hum follows the input stage or the phase splitter.

[simon]

That's a good idea. Just tried it. Horrible loud feedback through the speakers - had to switch off ASAP.

Just to be clear, I clipleaded from V1 top triode cathode to the other channel's V2A grid.

[Nick]

You will need to disconnect the existing s1-l -> s2-l and s1-r -> s2-r so its s1-l -> s2-r and s1-r -> s2-l And also swap the feedback over as well, I forgot about that.

[simon]

I'm sure I'm being dumb but I haven't followed this. What's s1 and s2?