Valve amplifier service

[Nick]

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?

I think what may look like an insulated washer is just the solder tag.

[simon]

Yes I think it's just a solder tag. There's no obvious connection to the chassis other than where hardware is mounted.

[Ant]

Is the only ground connection to the outside world just the outer of the phono sockets on this example of the amp?
I ask because the fabulous exploding cole phono stage was wired in such a way that there was no connection from the phono stage to the box with the mains tx in it via the connwcting cable, and the iec was on the back of that box. thus the earthing scheme relied on it being connected to something else downstream. That was partly why i asked nick to reconfigure the grounding scheme when i asked him to mend it.
The comment that the phone sent it wappy brought that up from the foggy memory, there would have been no ground via the source in that case

[simon]

As far as I can tell yes. It's how it's described in the build manual, and how it appears in reality. Can't say I like how it's implemented, but AFAIK it's been fine for many years like this until recently.

[Ant]

Fair enough, if it is built to spec then thats how its supposed to be. I was just trying to wrap my head around how it was wired in the first instance

[simon]

I'm just back from popping round to Dave's with his amp for a sanity check.

The good news is that Dave thinks the hum is much better than it was. But it's still audible, quite clearly on the left channel, but your head has to be next to the right speaker to hear it over the left channel.

Dave said the amp had always had some hum, which he found tolerable, but had got worse to the extent it wasn't tolerable. I didn't know this (or had forgotten) and is interesting I think.

The hum now is inaudible when playing music but not between tracks. I wouldn't be able to live with it like this as it's quite annoying.

Dave used a Transcenent preamp to control volume and interestingly there was very little increase in the hum with gain.

I think this reflects the situation - Dave will hopefully put me right if not.

So, I think a couple of things need checking: moving the 240V AC wiring away from the front of the amp (by the left channel pre stage); and disconnect the coupling caps to the output stage.

But most of all a trip to Dr. Nick's I think!

[Nick]

But most of all a trip to Dr. Nick's I think!

Always welcome (though check before if weekend as I have a bunch of weekends planned for sprints this year).

[Mike H]

A sanity check is very important.

[simon]

Indeed.

I just wish it was as easy in the rest of my life

I'll drop you a line Nick.

[Mike H]

I was wondering if it's noisy mains, being more noisy than normal. Whatever the normal is.

[bostod]

When Simon brought the amplifier round for a check, I was pleasantly surprised. As Simon said there is still some low level hum that is noticeable between tracks. I also agree that the hum is quieter. When the problem was at its worst the noise was more like a buzz or drone that increased with volume. That has now completely vanished, leaving the low level hum. Not very technical I know but that was my best way at describing the problem. The low level hum has always been present since I bought the amp. Any improvement to that would be a bonus

[Mike H]

Sorry I've lost track. Did Simon change any components? (@ Simon )

[Mike H]

It was interesting to revisit this after a fair break. The hum is greater on the right channel than the left. Connected to 18" Fane drivers which will be around 97/98dB the right channel is low but audible. Back in the day, many of our amps hummed like this and we were quite pleased with them . The left channel needed my head to be quite close to the speaker to hear the hum over the PC fan. So overall I would say the hum isn't bad. But it is there and needs fettling.

I am reminded of this..... re valve hi-fi... from 'Popular Science 1962 - Hi-Fi Systems That Last A Lifetime - Google Books'
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[simon]

Not since I changed some of the caps and a couple of resistors some time ago Mike.

I wanted to have a look at the mains wiring to the on/off switch at the front this weekend but didn't manage to.

[Mike H]

OK thanks.