The Nemesis 813 PP Amplifier

[Mike H]

Yeah it thinks the cold fil's are a short-circuit. You could instead try series resistors, to get it going, then switch out after.

[IslandPink]

Damn ! - that just reminded me I had that problem with a Dell one I used initially for the GK-71 - mind you, that was specced too close to the limit. I got a 20V 5A afterwards and that one started OK. So it is surprising this 20A one is not happy.
Can you put a beefy 0.1R resistor in each connection to the valves, and run it at 12V ?

[izzy wizzy]

Damn ! - that just reminded me I had that problem with a Dell one I used initially for the GK-71 - mind you, that was specced too close to the limit. I got a 20V 5A afterwards and that one started OK. So it is surprising this 20A one is not happy.
Can you put a beefy 0.1R resistor in each connection to the valves, and run it at 12V ?

I don't have resistors but I do have a relay so I'll time delay one of them to get it going. It's possible maybe that in the future with the CM choke in there, that might be enough. But for now, more SS stuff in my amp

Cheers,
Stephen

[izzy wizzy]

Digging in the shed produced the 555 timer PCB the relay used to be attached to so now reunited, they switch on the 2nd filament after a wee while. And it all goes but ...

It isn't completely quiet but a lot better. I haven't put the scope on it yet to see what's coming out of the SMPS. Maybe I was a bit naive thinking it would be fine however it's good enough to go forward with 46s to see if I can introduce more hum/buzz further up the chain.

Cheers,
Stephen

[IslandPink]

Ah good , so what are you hearing now - in term of noise or anything wrong in the sound ?

[izzy wizzy]

Ah good , so what are you hearing now - in term of noise or anything wrong in the sound ?

I'm not into comparing channels for sound just yet as I only got an output stage going. It would be hard for me to say if there's something wrong with the sound in one channel.

The buzz it produces is very low which I'm sure the scope would show if only I could be arsed to hook it up coz if I do and see it, then I have to do something about it. I could live with it as I don't listen to the speakers softly preferring headphones.

Off to the shed to wire up 46s.

Cheers,
Stephen

[izzy wizzy]

Finally got some time to wire up the 46s in one channel and it goes! Woohoo!

I don't think the 46s add to the hum/buzz even AC heated which is great but then I was fairly keen to see if it went loud enough. One channel is marginal but two going together might be enough. If it isn't then I can wire the input tx as a step up.

I've never heard an all DH amplifier and although I only played a few tracks, it does show promise and is headed in the direction I like.

Hope I don't have to wait another week to get the 2nd channel done.

Now something else has cropped up which is strange and maybe the panel can help. As I change volume on the AVC, I can hear click click click audibly like there's DC on the switch or some such. The line stage OPT has a Sowter TVC wired as an autoformer on the output which then feeds the power amp input tx. Any ideas?

Cheeers,
Stephen

[Nick]

I have had that effect when there is some RF going on. May not be in your case, but thats when I have seen it. less likely I would think with a 46.

Do you have a resistor from the input to ground, it may be the switch allowing the input to float between positions.

[izzy wizzy]

I have had that effect when there is some RF going on. May not be in your case, but thats when I have seen it. less likely I would think with a 46.

Do you have a resistor from the input to ground, it may be the switch allowing the input to float between positions.

My first inkling was RF but was hoping for some other reason. I have no resistors to ground anywhere on the transformers so that's easy to try. It's quite possible they are floating up a bit.

Cheers,
Stephen

[Nick]

If they are break before make switches, at that point the grid will be floating.

[Mike H]

Could put high Megohms resistors between grids and ground?

[izzy wizzy]

Thanks for the suggestions.

The lure of getting the second channel going was far stronger than solving problems. Having two channels might be loud enough but as it 9:45pm fire up and people were in bed, I couldn't get much of a go. Full volume is loud but there's nothing in hand and a bit more might be useful.

It's hard to get a real comparison to what I've had before as I've moved the speakers to another part of the room and I've not heard them in a year so it's all a bit new but it does things I really like so hopefully on the right track.

Next move is to try and chase down the hum/buzz issues and the clicks on the volume. It is a make before break switch.

Cheers,
Stephen

[Mike H]

Watching.

[izzy wizzy]

Not much time tonight however I did stop the volume clicks by removing the earth on the screen of the interconnects

This will require more investigation as I thought they were a screened twisted pair, earthed at one end. Previously when just driving the output stage, the earth was needed. Now not so apprarently. But maybe there's more to this than just what I can remember. The interconnects are long and have transformers at both ends all floating. They could be a balanced connection I think but are rca terminated at each end with the screeen wired to an extended wire which I can croc clip to things; only at one end.

I think I need to update the schematic and so can zero in on where the grid resistors should go and maybe get help as to why this odd stuff is happening.

Cheers,
Stephen

[izzy wizzy]

This is starting to get exciting. The wierdness/volume clicking earlier has gone away. Might have had something to do with me mis reading the pin out for the input transformer. For some reason Lundahl show it looking down on the tx rather than the pin side. Although it's kinda symmetrical, something odd was going on. Anyway, I wired it up as a 1:4 or 1:2+2 stepup and now I have enough gain to go bloody loud which is about right so have a bit of gain to spare. The IT should be 1671 not 1674.

Still no trouble shooting as I had time for a good listen tonight. I'm really liking what I hear. Makes me want to put on more and more records which is the right idea. It does what I want in terms of bottomless power plus it's way more subtle than anything I've had before but then again, that's not saying much. I've only had one amp for 35 years. I was worried that having a year of headphones, that the speakers wouldn't live up to that experience from my memory previously but with this amp, they do. The thing I like most is the difference between recordings is very large which usually means good things. It's way better than previous versions which were big and beefy but no finesse. I'm guessing that's the effect of the 46s.

My daughter was in the room while this was going on and we agreed the SMPS channel was a bit sharper than the AC channel. Might be time to get the scope out and have a look at the SMPS to see what's needed to make it quiet and a little less sharp in the top. The hum/buzz at present isn't stopping me enjoying listening to them although between records, it's clearly audible. Had to laugh when she came in. "What's that burning smell?" Never a good question but in this case, there are a lot of hot things. The cross over box got a bit close to the 813s the other day; balancing on the output transformer and it is a bit blackened now; that wasn't the burning smell today tho

Cheers,
Stephen