The Nemesis 813 PP Amplifier

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izzy wizzy
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The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 5:39 pm

I've been trying to build an 813 PP amplifier since about 2003 on and off. It has been the most disheartening journey in DIY audio as I still don't have an amplifier that works properly. It is my nemesis. I've lost count of how many versions of this thing I've built. They started off as variations of Lynn Olson's Aurora with the front end of Kevin Carters amplifiers K&K Audio. It morphed into many versions of E-Linear types based on ideas from Douglas Piccard. I've had it as triode, pure pentode, ultra linear. I've had cascaded triode drivers, cascodes, pentodes and so on. Some have sounded fantastic, some not so great. When they have worked and sounded great, I know I'm on the right track.

However, there has always been one big problem. Noise. Hum or buzz of some kind that means it's a non starter. Some have oscillated as a pair and sounded like a Theremin. The audio circuits have only one thing in common; the output transformer and 813s. The power supplies have the same power transformer and MV rectifiers but all else different.

As the original idea was on my web page, someone got in touch and asked if he could build it which was a bit of a worry as mine didn't work properly. Ian built a pair of beautiful amplifiers and they worked superbly. Kinda annoying but also a relief. Meanwhile ....

I've tried trouble shooting and seeking advice but to no avail so I'm going to try this; a show and tell build and maybe when it doesn't work yet again, somebody might be able to help given all the information will be here in one place. OTOH, maybe it will work this time.

I'm returning to the original idea as I have virtually all the parts. It's a derivative of Lynn Olson's Karna but instead of 300B finals, it will use 813s. Instead of 45 drivers it will use 46s. Instead of 5687 input, it will use 12B4. It will also borrow ideas from Kevin Carter again as the two amplifier designs share ideas.

It will be my typical "breadboard in a box" construction that has been successful for phono and line amplifiers. I hope this thread produces a result I can use that isn't geological in its timeline. Fingers crossed.

Cheers,

Stephen

izzy wizzy
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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 5:47 pm

This is a rescued board from the last failed amplifier. I can add a few more holes to mount the 46s and the other holes will do for the others. The signal transformers will go underneath.

813layout1.jpg

Cressy Snr
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Re: The Nemesis 813 PP Amplifier

Post by Cressy Snr » Sun Nov 04, 2018 6:06 pm

Look forward to following this Stephen.
At last, after all these years, a boatanchor build to follow. I think the last one was Simon's "Madness" amplifier , and that's a looong time ago now. :)

izzy wizzy
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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 6:10 pm

Here's the sketch of the audio circuit for starters.

813schemo1.jpg

The front end might be stacked as shown or it might be cascaded with the first stage RC decoupled from the 2nd. Not decided yet. The cascade version was one of the nicer sounding versions.

No version has ever used the 46s and this is a risk especially as I intially propose to AC heat them using the now surplus 866 transformers. Hopefully being PP, I'll get away with it.

The PSU will be full wave, choke input but this time, will use silicon for rectifiers instead of MVs to remove that as a variable. HT will be about 950 to 1000V. In the end, I'd like a seperate PSU for the front end but for now, I will use RC dropper off the HT as I have loads of 50W resistors for the job.

I have some 6D22S for rectifiers but no transformer to power them. It's not a project that gives me confidence to throw more money at just yet. I also have 866 MVs which I really want to use. I love the sound of MVs (smaller ones) in my phono and if I can use them there, why not here? It should be simple but ....

Cheers,

Stephen

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IslandPink
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Re: The Nemesis 813 PP Amplifier

Post by IslandPink » Sun Nov 04, 2018 6:21 pm

I'm going to raise an unwelcome subject from the start : why 813's ? ( I assume this is partly because you have some ? ) .
Only I heard 813 in SE compared to 211 in the same amp ( with suitable mods ) in a short space of time, by Paul Barker, years ago, and it was very clear the 211's were better. The 813's don't look that linear in triode mode on paper, and it isn't 2nd-harmonic that would cancel in PP - when we did the listening in SE it sounded like higher-order stuff and just a general 'crudity' of the sound .
If I was doing an amp like this I'd go the extra 10W per filament and use GK-71 in triode mode which are very linear and I can confirm sound excellent. They were about £12 per valve when I bought a couple.
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Re: The Nemesis 813 PP Amplifier

Post by Nick » Sun Nov 04, 2018 6:29 pm

Yes, I must admit I was having similarity unhelpful thoughts about using GM70's
Resistance isn't futile it's V / I.

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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 6:52 pm

IslandPink wrote:
Sun Nov 04, 2018 6:21 pm
I'm going to raise an unwelcome subject from the start : why 813's ? ( I assume this is partly because you have some ? ) .
Only I heard 813 in SE compared to 211 in the same amp ( with suitable mods ) in a short space of time, by Paul Barker, years ago, and it was very clear the 211's were better. The 813's don't look that linear in triode mode on paper, and it isn't 2nd-harmonic that would cancel in PP - when we did the listening in SE it sounded like higher-order stuff and just a general 'crudity' of the sound .
If I was doing an amp like this I'd go the extra 10W per filament and use GK-71 in triode mode which are very linear and I can confirm sound excellent. They were about £12 per valve when I bought a couple.
There are always better valves out there and I bet true great triodes like the 211, 845 and so on would be a better bet. However like you surmised, I have the parts so why not use them. My ultimate aim was to use it as a pentode not a triode but in theory, getting it to go as a triode should be easier. And I have nothing to compare it to luckily other than the amp it has to beat; the EAR509s. When it has made a noise, it handily makes the 509s sound like a transistor radio. Who knows, maybe GK71s will end up in there.

This started out as an amp to knock the stuffing out of my Acoustat electrostatics. It's way overkill for the Tannoys where a much more sane amplifier would do. But hey, where's the fun in that :roll:

Cheers,

Stephen

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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 7:14 pm

Nick wrote:
Sun Nov 04, 2018 6:29 pm
Yes, I must admit I was having similarity unhelpful thoughts about using GM70's
Did you mean I should/could use them instead for a better result? If so, I guess that is an option further down the road.

Cheers,

Stephen

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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 9:01 pm

This is the power supply. In the past I've put the power transformer and choke in one chassis and the caps in the other. They were linked with the HT lead connectors which were RG213 coax and N series RF connectors; one lead for +ve one for 0V. In hindsight, I'm not sure that's a great idea. The mains enters the PSU chassis and then gets linked to the top chassis for the heater/filament transformers.


813psu1.jpg

Cheers,

Stephen

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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 9:02 pm

This time I might put the caps in with the iron and rather than an external HT connections, go with banana plugs suitably isolated and make it an internal connection. The audio chassis sits above the PSU chassis. Chassis being a loose term. It's actually a sheet of 18mm ply with threaded rod to support the top layer of 18mm ply. Of course it has castors and needs to be in 2 pieces so I can lift it. The down side of an internal connection is I may not be able to flip it upside down to work on it like below without some further thoughts on that.

813elinshed.jpg

Cheers,

Stephen

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Nick
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Re: The Nemesis 813 PP Amplifier

Post by Nick » Sun Nov 04, 2018 9:44 pm

izzy wizzy wrote:
Sun Nov 04, 2018 7:14 pm
Nick wrote:
Sun Nov 04, 2018 6:29 pm
Yes, I must admit I was having similarity unhelpful thoughts about using GM70's
Did you mean I should/could use them instead for a better result? If so, I guess that is an option further down the road.

Cheers,

Stephen
It all looks pleasantly similar to the 211 I made years ago. You have to love MV rectifiers :-)

I meant that GM70 should be a simple replacement for the 813's and the true triode nature may make them less prone to RF oscillation. I also mean that I must one day build the PP gm70 amp I have the audio transformers for.
Resistance isn't futile it's V / I.

izzy wizzy
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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 9:52 pm

Nick wrote:
Sun Nov 04, 2018 9:44 pm
It all looks pleasantly similar to the 211 I made years ago. You have to love MV rectifiers :-)

I meant that GM70 should be a simple replacement for the 813's and the true triode nature may make them less prone to RF oscillation. I also mean that I must one day build the PP gm70 amp I have the audio transformers for.
Well it's that RF oscillation thing I want to avoid. In the past, I don't know what has been oscillating but I've tried just about everything but obviously not everything, to suppress it at least on the 813s. Suppressor coils on the plate, high value CC grid/screen stoppers.

I think this time, if I build in stages and test each one in turn, I hope to be able to identifiy the problems as they arise.

Do you have any suggestions to avoid the 813s oscillating? For starters I'll have 10k CC on the grid and 1k CC on the screen.

Cheers,

Stephen

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Nick
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Re: The Nemesis 813 PP Amplifier

Post by Nick » Sun Nov 04, 2018 10:11 pm

izzy wizzy wrote:
Sun Nov 04, 2018 9:52 pm
Nick wrote:
Sun Nov 04, 2018 9:44 pm
It all looks pleasantly similar to the 211 I made years ago. You have to love MV rectifiers :-)

I meant that GM70 should be a simple replacement for the 813's and the true triode nature may make them less prone to RF oscillation. I also mean that I must one day build the PP gm70 amp I have the audio transformers for.
Well it's that RF oscillation thing I want to avoid. In the past, I don't know what has been oscillating but I've tried just about everything but obviously not everything, to suppress it at least on the 813s. Suppressor coils on the plate, high value CC grid/screen stoppers.

I think this time, if I build in stages and test each one in turn, I hope to be able to identifiy the problems as they arise.

Do you have any suggestions to avoid the 813s oscillating? For starters I'll have 10k CC on the grid and 1k CC on the screen.

Cheers,

Stephen
Ferrite’s can be helpful. The problem that grid stoppers can cause is that while they add low pass filter, they also reduce the control the outside the valve world has on the grids, so if they are going to oscillate because of coupling inside the valve the stoppers can make matters worst. Adding inductance can add the low pass at RF while still keeping the LF inductances low. That may be badly described, but I think that sometimes the trick is getting energy out of the valve rather than stopping it getting in.
Resistance isn't futile it's V / I.

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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 10:20 pm

Had a rummage in the shed and came across 6 x 220u 400V caps which I can use in the HT on the amp side.

813schemo2.jpg

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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Nov 04, 2018 10:28 pm

Nick wrote:
Sun Nov 04, 2018 10:11 pm
Ferrite’s can be helpful. The problem that grid stoppers can cause is that while they add low pass filter, they also reduce the control the outside the valve world has on the grids, so if they are going to oscillate because of coupling inside the valve the stoppers can make matters worst. Adding inductance can add the low pass at RF while still keeping the LF inductances low. That may be badly described, but I think that sometimes the trick is getting energy out of the valve rather than stopping it getting in.
Thanks. That sounds useful if I understand right. The one thing I've not tried on the grids is ferrites. Any suggestions on that? I'm open to try just direct connection to the IT and may start there again to see what happens at the beginning.

Cheers,

Stephen

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