The Nemesis 813 PP Amplifier

[Dave the bass]

I have seen that a lot of times. Without the HT to provide current there is no anode resistance to stop the plate from picking up hum from the heater. At least that’s how I have thought of it in the past.

I used to ponder the heater hum through the speakers with no HT scenario on some of my amp builds (remember when I used to build amps, erks?!). Thinking about it right now, with no HT there's probably no bias to the grid either so the Grid wont be doing it normal job as a control grid either so its ability to 'screen' the Anode/plate wont be there either, hence the heater hum coupling perhaps?

DIET. The heater current is quite large too isn't it, usually in 'amps', that electromagnetic field surrounding the filament/s must be able to couple its way quite quite easily through the close-confines of a Valve.

DIET 2. Another thought, with no HT, there's probably no heater DC lift or offset potential either thats sometimes used to pull the cathode away from the heater insulation to keep the valve happy and within its safe operating limits too.

[izzy wizzy]

All I can say is I must have been incredible lucky with mine.
Now how sensitive are the Tannoys ?
I know things like the 288's with horns ( 109 ) will pick up buzz from the speaker cables, without the amp on. Not with the TQ UB's though , one other reason I like them.
However you must have existing set-ups with same wires that don't buzz, I assume.

Tannoys are about 95dB so not super sensitive but reasonable.

What Nick and DTB said about buzz with only heaters on happens in my line stage as well so not too much of a concern.

Hum has been nulled out so I can't hear it over the buzz hum. It's about 10mV peak and a mix of stuff but it is quite annoying. Forgot to take a pic on the scope.

I shorted the grids to ground. No change so I have to consider the possibility of a parasitic. I have only a 330R stopper on the G2 tied to plate, nothing on G3 tied to fils (not sure if anything should be on that pin) and nothing on G1 going to the transformer. I have some ferrites lying around I could try another night.

Or maybe some snubbing of the choke which is a bit noisy.

A fiddle with the grounding might help.

The noise is quite low and I suspect at normal listening levels it would be fine but I can't help think it is a symptom of something else and to my mind, the output stage should be slient. Am I being unrealistic on an amp like this?

Cheers,
Stephen

[Nick]

Am I being unrealistic on an amp like this?

IMHO, no, less than 10mv should be achievable with a SET, so I would expect a PP will be able to be much quieter,

Looking at the hum is the next step IMHO,

[izzy wizzy]

Am I being unrealistic on an amp like this?

IMHO, no, less than 10mv should be achievable with a SET, so I would expect a PP will be able to be much quieter,
Looking at the hum is the next step IMHO,

Here's what's left after a bit of hum nulling. The null pots in use are very crude at the moment but they work.

I'll try grounding first and then try some ferrites and grid stoppers.

Cheers,
Stephen

[Nick]

Still looks like heater induced at the moment, I would try playing with the hum pots while looking at the scope (if you are not already) I suspect that there will be a optimum point found from moving both pots together.

[Mike H]

+1

When I've done this sort of thing, best I could do was get all the peaks the same height. Wasn't 813's tho.

[izzy wizzy]

I put in the new hum pots. They're very nice! Got hum even as Mike suggested. Other buzzes seem to have pretty much gone. The hum now is very low 4.2mV true RMS so the scope says.

I'm now wondering if I chase this further? My feeling is no for now.

One of the big problems in the past was interaction between the two channels. I don't know how but they did. I think my next move will be to make the other output stage go to test if this "feature" comes back to haunt me. It's so tempting to get the 46s going. No, must resist.

Cheers,
Stephen

[izzy wizzy]

A question for the panel.

I was going to try a "kind of snubber" around the choke input like Morgan Jones suggests and like Lynn Olson has in his amps; to quieten down (is that a proper phrase?) the choke input I believe. I'll need to series caps up to do this to get about 0.22u at ?V. It's the ?V I'm not sure of as it's hard to fiure what that will be in PSUD (unless I'm doing something wrong) I was thinking about 3kV should be right which is 3 series caps of 0.22u 1000V and then maybe parallel up some more of that combo.

So my question is, will I need balancing resistors or can I just slap in that combo of series parallel caps? If ?V is under 2kV, the only 2 caps in series will do.

And another question. Is the ultrapath combo of caps - HT to common fils and common fils to ground something to do with hum rejection or something else like shortening the AC imbalance path in a PP amp? I can't make up my mind.

Cheers,
Stephen

[Nick]

You will need balancing resistors sized to be about a tenth of the cap leakage current. Assuming you mean the cap to ground before and after the choke. Never sure what the cap after the choke is going to do that the cap that follows it won't, unless its just closer.

As for the ultrapath, I have always thought of it as injecting power supply ripple into the cathode in just the right ratio to cancel the ripple on the anode. But you can also look at it as creating a AC current loop around the output transformer and valve that avoids the power supply cap.

[izzy wizzy]

You will need balancing resistors sized to be about a tenth of the cap leakage current. Assuming you mean the cap to ground before and after the choke. Never sure what the cap after the choke is going to do that the cap that follows it won't, unless its just closer.

As for the ultrapath, I have always thought of it as injecting power supply ripple into the cathode in just the right ratio to cancel the ripple on the anode. But you can also look at it as creating a AC current loop around the output transformer and valve that avoids the power supply cap.

I do mean before and after the choke. Much like you, I don't see why after but hey, this is for HF stuff so maybe best to site right on the choke. 1/10 of cap leakage current. That's going to be tricky. In fact balancing resitors of any kind might not be available in my parts boxes. Voltage rating being the main issue and then quantitiy of them. Any ideas what voltage rating should I be going for before the choke? I thought Vp would do so about 1500V for a 1040VAC input.

Re the ultrpath, again, I thought that in a SET but in a PP stage? The only thing going on at the cathode is an imbalance signal and in theory it's a common mode rejection point I thought. Could it be an imbalance distortion cancellation point in PP? I think I have the caps so could be worth a try later.

Off to go make the 2nd channel now.

Cheers,
Stephen

[IslandPink]

I would definitely try the ultrapath in the output stage, I'm wondering if it might have been the key to my 300B PP amp being the quietest thing I've ever built. I was looking for a more detailed discussion from Lynn ( than on the Nutshell HiFi site ) somewhere about this but failed due to lack of time, but Nick's comment about PS noise cancelling sounds good. I only used AC heaters, no humbuckers and a main PS that was cLC, with mine. The GE oil/poly caps are very good in that location.
I was lucky enough to be present at an afternoon-coffee discussion at ETF2005 where Lynn, Gary Pimm, Steve Bench and Allen Wright were discussing the pros/cons of this arrangement vs. the CCS+shared cathode option that Allen used, it was fascinating, about how the imbalance current would be handled in the two cases. Wish I'd taken a tape recorder !

[Mike H]

A question for the panel.

I was going to try a "kind of snubber" around the choke input like Morgan Jones suggests and like Lynn Olson has in his amps; to quieten down (is that a proper phrase?) the choke input I believe.

By "quieten down" I think you mean audible buzz? Yes the cap and resistor make the choke into a tuned circuit, you can tweak the values to alter what the impedance of the choke is at 50 Hz, as it were.

Been a while since I done it tho so rusty on the details. But IIRC I could certainly make a small value choke, like 3 Henries or something, work like a bigger value one, and buzz (vibrate) a lot less.

Balancing resistors - I interpret as, in parallel across each snubber cap? (x2 in series) A few megohms should do, but must be exact same value tho.

HTH

[izzy wizzy]

By "quieten down" I think you mean audible buzz? Yes the cap and resistor make the choke into a tuned circuit, you can tweak the values to alter what the impedance of the choke is at 50 Hz, as it were.

Been a while since I done it tho so rusty on the details. But IIRC I could certainly make a small value choke, like 3 Henries or something, work like a bigger value one, and buzz (vibrate) a lot less.

Balancing resistors - I interpret as, in parallel across each snubber cap? (x2 in series) A few megohms should do, but must be exact same value tho.

HTH

I thought the snubber thing might quieten the buzz a bit but also I've read on Diyaudio that it can smooth the input to the choke a bit; remove some of the harshness of diode switching that can cause oscillations in the amp. That might be a bigger value than a snubber. I dunno. OTOH, I only have 2 screws holding the brute down to the 18mm ply so maybe 4 might help. It's fairly low noise but I can hear it. The iron in this amp is huge. I don't know why but Majestic seems to make their iron goods that way.

I am going to have to chase down the remaining hum. It is not much but is annoying at very low listening levels however I virtually never listen at those levels as I'd use headphones instead but I can't resist having a listen on the speakers in mono at the moment

Second channel is very nearly done.

Cheers,
Stephen

[Mike H]

I believe I have simulated this in LTspice, but as I say was a while ago, but sometimes it shows that what people think is what happens, isn't necessarily what actually happens. If you see what I mean.

" it can smooth the input to the choke a bit; remove some of the harshness of diode switching."

Yes because it's like putting a bigger value choke in. Putting a capacitance across it makes it into a tuned circuit, and if the resonant frequency is at or near 100 Hz (50 Hz -> full wave / bridge rectifier), then it will present a larger impedance to the input from the rectifier. So yes it will do both those things.

It's a kind of filter known as a resonant trap circuit.

[Nick]

Not sure its the type of snubber thats being discussed here Mike (or it wasn't). MJ started talking about a RC network across the choke as you describe, though with a C value that would put the resonance much higher than 100Hz. But he pointed out that the RC provides a path to HF past the choke so suggested a different network, a cap from the input of the choke to 0v and from the output of the choke to 0v, thus not providing a path for HF to bipass the choke.