The Nemesis 813 PP Amplifier

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IslandPink
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#46 Re: The Nemesis 813 PP Amplifier

Post by IslandPink »

Sounds like a methodical grind through this is required.
If you ground the Tx secondaries to the 813 grids, is it still there ? If not then it's coming from earlier in the chain.
Good effort so far, anyway.
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#47 Re: The Nemesis 813 PP Amplifier

Post by pre65 »

Somewhere on here are details of a way to heat 813 using HF AC.

The author was Alex Kitic and it was for a single ended 813 amp.

This is a link to his web site.

http://rh-amps.blogspot.com/
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#48 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

IslandPink wrote: Sun Nov 11, 2018 10:31 pm Sounds like a methodical grind through this is required.
If you ground the Tx secondaries to the 813 grids, is it still there ? If not then it's coming from earlier in the chain.
Good effort so far, anyway.
Grind being the operative word :angry5:

Do you mean just ground the gids? They are grounded through the secondary and the input is grounded as the 46s aren't in circuit.

So far all I've got is IT-813-OPT with the input going into the IT primary which is shorted out for testing. Makes no diff even if it's open.

I'm glad it isn't induction from the fils tx into the OPT.

Cheers,

Stephen
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#49 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

I've found 2 fil transformers so could have one per valve and have seperate hum pots on each. I did this on one of the previous builds and I remember it nulling hum but that one buzzed as well so I never pursued it further. Another PP813 builder used this method to great success.

Cheers,

Stephen
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#50 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

I have a question for the panel.

Is it possible the hum could be an issue with cathode bias but maybe less so if the fils were grounded and I used fixed bias?

I ask as it is something I tried in previous versions but not methodically. I have the transformers to do it but don't want the detour unless there's promise in it.

Cheers,

Stephen
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Nick
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#51 Re: The Nemesis 813 PP Amplifier

Post by Nick »

izzy wizzy wrote: Mon Nov 12, 2018 6:35 pm I have a question for the panel.

Is it possible the hum could be an issue with cathode bias but maybe less so if the fils were grounded and I used fixed bias?

I ask as it is something I tried in previous versions but not methodically. I have the transformers to do it but don't want the detour unless there's promise in it.

Cheers,

Stephen
I cant think of a reason why it would be, but without knowing what the hum is like its hard to say. Its possible for example for it to be singing away at 500kHz, and the only effect you notice is the hum riding on that signal. I know I am singing the same song, but scopes are really really useful.
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#52 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

Nick wrote: Mon Nov 12, 2018 8:59 pm I cant think of a reason why it would be, but without knowing what the hum is like its hard to say. Its possible for example for it to be singing away at 500kHz, and the only effect you notice is the hum riding on that signal. I know I am singing the same song, but scopes are really really useful.
I wish I had a decent scope but I only have a simple USB one. Anyway, maybe the pic will show you something.

hum1.jpg

Another builder of these has seperate fil tx per 813 with humpot on ea and has more sensitive speakers than me and he says its quiet. That's my next move. What would be an OK disappation for a 5W WW pot?

Cheers,

Stephen
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#53 Re: The Nemesis 813 PP Amplifier

Post by IslandPink »

I tend to think about 40% for most things vs. 'rated' dissipation.
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#54 Re: The Nemesis 813 PP Amplifier

Post by pre65 »

Are the filament connections "in phase" ?

There is some info here on PP filament heating.

http://www.bartola.co.uk/valves/categor ... l-designs/
Last edited by pre65 on Mon Nov 12, 2018 9:57 pm, edited 1 time in total.
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#55 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

IslandPink wrote: Mon Nov 12, 2018 9:47 pm I tend to think about 40% for most things vs. 'rated' dissipation.
Good. We're on the same wavelength :)
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#56 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

pre65 wrote: Mon Nov 12, 2018 9:49 pm Are the filament connections "in phase" ?

There is some info here on PP filament heating.

http://www.bartola.co.uk/valves/categor ... l-designs/
Thanks for thelink. I'll check that out. The in phase thing occurred to me last night and is the next thing to try before two fil txs. Would be great if that's the issue.

Cheers,

Stephen
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#57 Re: The Nemesis 813 PP Amplifier

Post by Nick »

So, nice and clean (ish) 50Hz. Swapping one of the filament connections is worth trying. You can increase the effective power rating of the pot by adding resistors to either end so it only covers a smaller area around the middle.

That does look like heater supply hum to me.
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#58 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

pre65 wrote: Mon Nov 12, 2018 9:49 pm Are the filament connections "in phase" ?

There is some info here on PP filament heating.

http://www.bartola.co.uk/valves/categor ... l-designs/
Nick wrote: Mon Nov 12, 2018 11:11 pm So, nice and clean (ish) 50Hz. Swapping one of the filament connections is worth trying. You can increase the effective power rating of the pot by adding resistors to either end so it only covers a smaller area around the middle.

That does look like heater supply hum to me.
I reversed one of the 813 fil and hum went down from 30mV to 22mV. I then put the hum pot back in, lowered the resistors I had on it to give it wider range and got the hum down to 15mV :D

So my next move will be seperate fil tx and hum pot for each 813 which I have the transformers for.

Thanks everyone.

Cheers,
Stephen
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#59 Re: The Nemesis 813 PP Amplifier

Post by Nick »

izzy wizzy wrote: Mon Nov 12, 2018 11:28 pm
pre65 wrote: Mon Nov 12, 2018 9:49 pm Are the filament connections "in phase" ?

There is some info here on PP filament heating.

http://www.bartola.co.uk/valves/categor ... l-designs/
Nick wrote: Mon Nov 12, 2018 11:11 pm So, nice and clean (ish) 50Hz. Swapping one of the filament connections is worth trying. You can increase the effective power rating of the pot by adding resistors to either end so it only covers a smaller area around the middle.

That does look like heater supply hum to me.
I reversed one of the 813 fil and hum went down from 30mV to 22mV. I then put the hum pot back in, lowered the resistors I had on it to give it wider range and got the hum down to 15mV :D

So my next move will be seperate fil tx and hum pot for each 813 which I have the transformers for.

Thanks everyone.

Cheers,
Stephen
Is the 15mv the same sort of hum or is it another sort appearing below the filament hum (could happen to make life harder).
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#60 Re: The Nemesis 813 PP Amplifier

Post by Nick »

Nick wrote: Tue Nov 13, 2018 8:54 am
izzy wizzy wrote: Mon Nov 12, 2018 11:28 pm
pre65 wrote: Mon Nov 12, 2018 9:49 pm Are the filament connections "in phase" ?

There is some info here on PP filament heating.

http://www.bartola.co.uk/valves/categor ... l-designs/
Nick wrote: Mon Nov 12, 2018 11:11 pm So, nice and clean (ish) 50Hz. Swapping one of the filament connections is worth trying. You can increase the effective power rating of the pot by adding resistors to either end so it only covers a smaller area around the middle.

That does look like heater supply hum to me.
I reversed one of the 813 fil and hum went down from 30mV to 22mV. I then put the hum pot back in, lowered the resistors I had on it to give it wider range and got the hum down to 15mV :D

So my next move will be seperate fil tx and hum pot for each 813 which I have the transformers for.

Thanks everyone.

Cheers,
Stephen
Is the 15mv the same sort of hum or is it another sort appearing below the filament hum (could happen to make life harder).
Have you got a shared cathode circuit. You could add a 50R unbipassed resistor below each cathode before they join. Then try a separate hum pot but the same transformer.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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