The Nemesis 813 PP Amplifier

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IslandPink
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#31 Re: The Nemesis 813 PP Amplifier

Post by IslandPink »

Looks like it means business.
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izzy wizzy
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#32 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

Drilled some more holes in my swiss cheese PCB that holds the audio stuff so mounting the 46s, the interstage and choke.

audbase1.jpg

audtop1.jpg


Cheers,

Stephen
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#33 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

IslandPink wrote: Mon Nov 05, 2018 11:20 pm Looks like it means business.
I used to hate turning things on with this high a voltage but now I have a variac. Couldn't use it when I had hollow state rectifiers. I have a test load but it can't run for long as it isn't heat sinked well enough. Still scary enough.

Cheers,

Stephen
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#34 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

It's nearly time to turn on the output stage. 813s cooking for a bit to get them nice and warm after a long rest. A first for me will be ramping the HT up on a variac.


audbase2.jpg

813ampbuild1.jpg


Fingers crossed it works.

Cheers,

Stephen
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#35 Re: The Nemesis 813 PP Amplifier

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Have the fire brigade gone yet ? :D
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#36 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

Blue smoke stayed in on first power up. Fire brigade stood down for now.

It worked but not exactly how I would like. I lowered the bias for about 75mA per 813 as it was a bit high at 100mA earlier. Both sides were within 2mA. On the little test speaker, there was hum measured at 26mV but it isn't very sensitive nor does it go very low.

Brought it in and hooked it up to the Tannoys. It is too audible on them.

So the question is what to do? Is it induced? Is it something else? What frequency is it?

Encouragingly, I can hear music albiet at a low level but it sounds decent enough for a first go. Will that mean I only need the 46 stage to make it go loud enough?

Things to ponder ....

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Stephen
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#37 Re: The Nemesis 813 PP Amplifier

Post by IslandPink »

Hmmm, not ideal.
OK let me chuck out some ideas, not all fully thought-through.
As for frequency , you can double-check with a signal generator fed into the amp - if you have one ?
Output valves you've checked for current - however if you swap 813's, does it change at all ?
Output transformer secondary, does it have same inductance and/or same DCR on both sides ?
You haven't ultrapath-ed the output stage, like the LynnO stuff - was there a reason ?

My only ref for this sort of amp was the Aurora I built with 300B's, it was really quiet from the word go.
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#38 Re: The Nemesis 813 PP Amplifier

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IslandPink wrote: Sun Nov 11, 2018 5:25 pm Hmmm, not ideal.
OK let me chuck out some ideas, not all fully thought-through.
As for frequency , you can double-check with a signal generator fed into the amp - if you have one ?
Output valves you've checked for current - however if you swap 813's, does it change at all ?
Output transformer secondary, does it have same inductance and/or same DCR on both sides ?
You haven't ultrapath-ed the output stage, like the LynnO stuff - was there a reason ?

My only ref for this sort of amp was the Aurora I built with 300B's, it was really quiet from the word go.
Thanks for the suggestions.

I checked the output with no input and it is 50Hz 80mV p-p which is roughly in line with the 26mV I measured in the shed on the meter.

I didn't ultrapath the output but I have tried to bypass the cathode R with a 220u and not a bit of difference to the hum level I could hear or measure.

I'll let it cool down and swap and measure things. I can't measure inductance.

I wonder how well balanced the filamant transformer is and whether that is an issue? I could also try moving it as it is relatively near the opt. Bloody hard to do that as I wired it in single core and the leads aren't that long.

I have to resisit the temptation to wire up the 46s to see if there's enough gain.

Cheers,

Stephen
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#39 Re: The Nemesis 813 PP Amplifier

Post by Nick »

50Hz may just be a earthing issue or the heaters, I would expect 100Hz from the power supply or the heaters (as well).
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#40 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

Nick wrote: Sun Nov 11, 2018 7:16 pm 50Hz may just be a earthing issue or the heaters, I would expect 100Hz from the power supply or the heaters (as well).
Well that's what I thought. It's earthed, the PCB is earthed and that did make a difference. Earthing the OPT did nothing. I measured the filamant tx and it is the same both sides to the limit I can see on my Fluke multimeter. The OPT resistance is the same both sides, again to the Flukes limit.

Not sure why I would see 100Hz from the filaments as they are AC. I thought that would appear as 50Hz?

Anyhoo, It's back in the shed and I've moved the filamant tx down the other end on croc leads but will have to wait until after dinner to test.

Cheers,

Stephen
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#41 Re: The Nemesis 813 PP Amplifier

Post by Nick »

Not sure why I would see 100Hz from the filaments as they are AC. I thought that would appear as 50Hz?
Depends on if a induced voltage (50Hz), or a heating effect (100Hz)

Just having a equal voltage on the transformer may not be ideal as you dont know what the resistance of the heater means WRT the way the voltage is distributed in the valve. I would try a hum pot as a simple first thing to try (instead of the TX centre tap).
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#42 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

Well I suppose it would happen again. I'm stumped. It doesn't look like induction with the filamant tx up one end and the OPT down the other. I can only guess it's the AC fils. 300B 2.5V, 813 10V. Maybe even in PP, it doesn't work. The only way I'll know is to go DC fils. Ian built my first deisgn and the only real difference was he used DC fils; seperate linear PSUs for each valve. I never wanted to do that but maybe I have to. I'll have to have a rummage in the shed to see how I can do this.

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Stephen
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#43 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

Nick wrote: Sun Nov 11, 2018 9:42 pm
Not sure why I would see 100Hz from the filaments as they are AC. I thought that would appear as 50Hz?
Depends on if a induced voltage (50Hz), or a heating effect (100Hz)

Just having a equal voltage on the transformer may not be ideal as you dont know what the resistance of the heater means WRT the way the voltage is distributed in the valve. I would try a hum pot as a simple first thing to try (instead of the TX centre tap).
OK thanks. I'll try a hum pot but it will have to be one for both rather than individual valves.

Cheers,

Stephen
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#44 Re: The Nemesis 813 PP Amplifier

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izzy wizzy wrote: Sun Nov 11, 2018 9:43 pm I can only guess it's the AC fils. 300B 2.5V, 813 10V.
Stephen
300B, 5V
It is surprising, as Nick says hum pot for now.
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#45 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

IslandPink wrote: Sun Nov 11, 2018 9:56 pm
izzy wizzy wrote: Sun Nov 11, 2018 9:43 pm I can only guess it's the AC fils. 300B 2.5V, 813 10V.
Stephen
300B, 5V
It is surprising, as Nick says hum pot for now.
Whoops. Yes of course. Wouldn't have thought there would be much diff between 5V and 10V.

Anyhoo, tried hum pot. Didn't do a thing. I had 33R each side of a 50R pot but there was no change. I suppose I could get rid of the 33R each end but I feel I'm clutching at straws and may be a bit much for the pot. It's 5W. 2W would be dissapation across the 10V then there's the bias about another 0.5W or so for 2.5W on a 5W device. Maybe for a short time it would be OK.

I'll sleep on it but I can't believe it won't work as is which is the frustrating thing and hence the name of this thread.

Cheers,

Stephen
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