GM70 PP

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RhythMick
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#31 Re: GM70 PP

Post by RhythMick »

You'd have been pulling an extra half amp through the DC boards as well remember, assuming you run 1 board per valve. An extra amp if they are in parallel.
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izzy wizzy
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#32 Re: GM70 PP

Post by izzy wizzy »

RhythMick wrote: Sun Jul 14, 2019 10:23 pm Surely you're going to try both and tell us which sounds best...?

:o :) :lol:
I might try the different sides of grounding the fils but you persuaded me against the resistors ;)

And yes there would be more current which would have meant a different pre reg. It may still have to happen.

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Stephen
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Nick
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#33 Re: GM70 PP

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simon wrote: Sun Jul 14, 2019 1:02 pm Nuts! You'd convinced me with the space charge scenario! :)

I can't think why a different part of the DC supply might sound different though?
Well, if its regulated, one end will be the emitter of a transistor, the other the ground or a sense resistor for the supply. Remember Mark found that he could hear the cap in the integrator.

It may also be a case that connecting the resistor may alter the conditions the DC supply sees.
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RhythMick
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#34 Re: GM70 PP

Post by RhythMick »

izzy wizzy wrote: Mon Jul 15, 2019 7:55 am
I might try the different sides of grounding the fils...
I'd be interested if there's any difference, assuming the bias is kept constant. I only tried the connection to the positive.
RhythMick
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#35 Re: GM70 PP

Post by RhythMick »

Nick wrote: Mon Jul 15, 2019 8:51 am Well, if its regulated, one end will be the emitter of a transistor, the other the ground or a sense resistor for the supply. Remember Mark found that he could hear the cap in the integrator.

It may also be a case that connecting the resistor may alter the conditions the DC supply sees.
Both interesting points. I always thought of a valve heater in terms of simple resistance but as well as stray capacitances if its coiled it will have inductance. Placing a significant resistance in parallel with that could have all sorts of effects.

The more I think about it the more I favour simplicity.
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Nick
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#36 Re: GM70 PP

Post by Nick »

Nothing is a simple resistance. Certainly not a heated wire. But remember the part that matters is not the hefty heater power, its the tiny fragile audio signal that has to coexist in the same circuit.
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#37 Re: GM70 PP

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Nick wrote: Sat Jul 13, 2019 4:59 pm I have argued that a centre tap with DC will help reduce any residual noise. But I think it practice its not worth doing and may be worst as you are adding extra resistors in the circuit.
I could be late to the party here, but I would agree with Nick..(didn't a politician once say that?) ;)
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izzy wizzy
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#38 Re: GM70 PP

Post by izzy wizzy »

Audio circuit plan. Replace 813s with Gm70 is the only thing needed to do here.

IMG_20190728_163113429_compress45.jpg

Replace 813 fil supplies with Gm70 fil supplies only thing to do.

IMG_20190728_163213076_compress57_crop_46.jpg
RhythMick
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#39 Re: GM70 PP

Post by RhythMick »

Just a thought Steve, but with my transformer coupled all DHT PP amp I started with shared cathodes and found a big improvement from switching to separate (bypassed) cathodes. Maybe something to try one day, if you do I'd be interested in the result.
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#40 Re: GM70 PP

Post by simon »

Looking at the HT PS again Stephen, it kind of looks like you're using connectors to connect the trafo and first choke to the rest of the filter. Have I interpreted that correctly?

2120V CT is an interesting voltage!
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#41 Re: GM70 PP

Post by RhythMick »

I would also be worried about those SMPS directly connected to the TX that serves the GM70 bias. Aren't they really noisy?
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Nick
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#42 Re: GM70 PP

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RhythMick wrote: Tue Jul 30, 2019 12:56 pm I would also be worried about those SMPS directly connected to the TX that serves the GM70 bias. Aren't they really noisy?
You could add some filtering between the SMPS and line voltage, but again, as I said, it depends on how good the supplies are made and how conforming they are. I found this at random, but it is a good discussion about conducted emissions.

https://courseware.ee.calpoly.edu/~darakaki/Paper2.pdf
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izzy wizzy
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#43 Re: GM70 PP

Post by izzy wizzy »

simon wrote: Tue Jul 30, 2019 12:32 pm Looking at the HT PS again Stephen, it kind of looks like you're using connectors to connect the trafo and first choke to the rest of the filter. Have I interpreted that correctly?

2120V CT is an interesting voltage!
You got that right. I use 1000V rated 2mm banana plugs and sockets. So I can separate the top and bottom to work on.

It's a custom transformer to get me the HT I thought I wanted 15 years ago. Don't ask me now how I arrived at that but it does give me the 980V or thereabouts which seems a good number. I think psud had something to do with it.

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Stephen
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izzy wizzy
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#44 Re: GM70 PP

Post by izzy wizzy »

RhythMick wrote: Tue Jul 30, 2019 12:56 pm I would also be worried about those SMPS directly connected to the TX that serves the GM70 bias. Aren't they really noisy?
Dunno. They might be. The 813 amp has them serving both valves. I used the same ones as another manufacturer and that's about as far as I looked at things. Something to fiddle with at some point :D

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Stephen
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#45 Re: GM70 PP

Post by izzy wizzy »

One 813 amp is now a GM70 amp. I left out the CM chokes so it would be a fairer comparison. After a few records, I felt the GM70s are considerably better which I guess will come as no surprise round here. The 813 amp sounds cruder, a bit rough round the edges and generally the GM70 amp is a bit more involving. So back out to the man cave to convert the other one.

Cheers,
Stephen
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