The Nemesis 813 PP Amplifier

[IslandPink]

I applaud -
(I) no interstage from st.1 to st. 2
(ii) 46's as drivers

[izzy wizzy]

I applaud -
(I) no interstage from st.1 to st. 2
(ii) 46's as drivers

I'm interested to hear the 46. These are the only directly heated valves I've played with but still no pure triodes. As an experiment along the way, I'd like to try the thing as just 46 - 813. Previously it went damn loud without the line stage I have now. Maybe the input transformer could be a slight step up to help a bit. Be brilliant if I didn't need stage 1.

Cheers,

Stephen

[Nick]

Either a simple bead on the wire (useful for heaters in ID valves as well)

https://www.rapidonline.com/rvfm-k5b-rh ... ad-23-0730

or larger ones with a few turns around

https://www.rapidonline.com/w-rth-elekt ... te-54-8636

[izzy wizzy]

Either a simple bead on the wire (useful for heaters in ID valves as well)

https://www.rapidonline.com/rvfm-k5b-rh ... ad-23-0730

or larger ones with a few turns around

https://www.rapidonline.com/w-rth-elekt ... te-54-8636

Thanks Nick. Should I use the small single lead through on G1, G2, G3 and do away with the stoppers. The larger one with a few turns on the filaments?

Cheers,

Stephen

[pre65]

Is this of any interest Stephen ?

http://mackiefamily.unospace.net/audioc ... 13main.htm

[izzy wizzy]

Is this of any interest Stephen ?

http://mackiefamily.unospace.net/audioc ... 13main.htm

That's what got me started on 813s. I know Paul and heard that amp a long while back. Must have made an impession

Cheers,

Stephen

[Nick]

Either a simple bead on the wire (useful for heaters in ID valves as well)

https://www.rapidonline.com/rvfm-k5b-rh ... ad-23-0730

or larger ones with a few turns around

https://www.rapidonline.com/w-rth-elekt ... te-54-8636

Thanks Nick. Should I use the small single lead through on G1, G2, G3 and do away with the stoppers. The larger one with a few turns on the filaments?

Cheers,

Stephen

I would start with small ones on all the wires (they are available with different internal dia) and add the multi turns as needed. You can also make a common mode for the heaters by winding both wires around a core. Like many things there is no single answer, but they are useful things to have in the box when needed. Some will need grid stoppers as well, but you need to find the instability and address the frequency its oscillating at.

[Mike H]

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.

The Thing that covered half the floor at Owston?

[simon]

You referring to me or the amp Mike?

[Ali Tait]

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.

I recall you mentioning that with the bits for the static amp?

[Mike H]

You referring to me or the amp Mike?

Just a thought - the circuit looks very like what I was first trying to do with my EL84 p-p amp, circa 2016 (?)

Viz, p-p interstage transformers or centre-tapped choke, but in any case some kind of balanced p-p input / driver stages, using two EF86.

I first simulated in LTspice. Just as well! I could do the p-p input / driver stages and the p-p output stage separately, and by themselves they worked beautifully in simulation.

But soon as I plugged them together it oscillated like buggery and the sim totally crapped out. Absolutely hopeless. Didn't matter what I did. Finally it went to bog-standard cooking pentode + concertina phase splitter, just to get something that worked. No p-p input / drivers, no interstage transformers or chokes (CT or otherwise). That works fine. There was still a bit of a problem with low frequency instability, such that I had to reduce the input pentodes' gain to stop it. This also showed up in simulation, but only if I extended the sim time to several seconds, as it took that long to start up. Oops.

Anyway it's a lovely amplifier and works very well, altho not at the moment cos the mains on-off switch has disintegrated.

I think the problem with the original idea, p-p input with TX's or chokes, was too much open-loop gain.

[Mike H]

PS: do you have an oscilloscope? Your hum / noise could be an audible manifestation of radio frequency oscillations and suchlike.

[izzy wizzy]

PS: do you have an oscilloscope? Your hum / noise could be an audible manifestation of radio frequency oscillations and suchlike.

I kinda have a scope; 2 in fact. A basic USB scope and a Tek that's sat around for ages and doesn't seem to go very well. The main issue is the scale of the voltages which renders fault finding very difficult.

Anyhoo, I'm trying to come at this from a safer angle this time given the problems of the past. Ian built pretty much my first design http://izzy-wizzy.com/audio/powamp813.html (bottom of page) and his worked. I've seen some other PP813s and they've worked. With some more attention to detail, I'm hoping I can make this work.

Plan is to make output stage and hook it up as is to see if it works and is silent. Then move 1 stage back, the 46s and see if that can be made to work. If I'm lucky, that's all I need do. If not, I may need the 12B4 stage. The PSU will be simpler than the past and so I'm hoping I won't need the scope.

I hate fault finding so much, I tend to pull something apart and build another except this time, I may be at the end of the road for that approach.

Cheers,

Stephen

[Mike H]

Yep sounds like a plan.

[izzy wizzy]

Wired up a power supply. Decided I had enough diodes for 5 per leg so why not with these delicate things. I like the external link cable so will use only one RG214 coax for HT. It's got 3kV rating and the N series connector is rated above 1kV RMS. Bit late tonight to power something like this up.

Cheers,

Stephen