GM70 SET

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simon
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#76

Post by simon »

You may be right Andrew, have to say I've never really heard a benefit whenever I've tried it.

Do you mean a specific choke for choke input, or the first choke in the PS Mike? The PS is 2 6AX4 dampers - 10H - 80uF - 10H - 160uF. Both chokes are the big 400mA Danglebury jobs. There's about 240mA draw so I would have thought the first choke would be okay. But something's upsetting it. A bit of ringing perhaps?
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Mike H
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#77

Post by Mike H »

Oh really - well when I've had vibrating like that it was an input choke, and the value was too small.

Hmmm!
 
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#78

Post by Mike H »

Well I don't know I've just done a spice sim of it and even if taken down to 2 Henries it still works properly. So I dunno.

Mind you it's power is peaking at 60W 200 times per second, that might have something to do with it :shock:

Can you make some flexi rubber mounts for it :lol:
 
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#79

Post by Mike H »

PS: I guessed the secondary Voltage as 450 - 0 - 450, and max. current 500 mA :?:
 
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#80

Post by simon »

I think it's more like 700 - 0 - 700 and 350mA IIRC.

The DAC buffer choke input choke vibrates the same and I found that altering the current through the shunt reg altered the degree of vibration in the choke. Perhaps the AC component is just too much in addition to the DC? Looks okay in PSUD though. Perhaps I'll try paralleling the two 220nF snubbers to give 110nF and see if that has any effect.
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andrew Ivimey
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#81

Post by andrew Ivimey »

snubbers ..... dear oh dear oh dear.
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#82

Post by Mike H »

simon wrote:I think it's more like 700 - 0 - 700 and 350mA IIRC.
Image

The DAC buffer choke input choke vibrates the same
Image

and I found that altering the current through the shunt reg altered the degree of vibration in the choke. Perhaps the AC component is just too much in addition to the DC? Looks okay in PSUD though. Perhaps I'll try paralleling the two 220nF snubbers to give 110nF and see if that has any effect.
OK so more DC makes it vibrate more, so probs just the power then. It is quite a bit.

I was once using a small choke that vibrated but worked, that was a low value like 2 - 3H, but it was then tuned with 220nF (I think) which raised its resonant frequency and so reduced the buzz, nevertheless it was flexibly mounted, else screwed straight onto an ally box lid the whole panel would buzz enough to drive you mad. :D
 
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#83

Post by simon »

Parallelling the two 220nF snubber caps as a 110nF first "tuning" cap made no difference. There's a little vibration in the first motor run cap too, though none in the mains trafo.

Playing with the driver operating point. I'm not convinced Nicholas Chua's published circuit is that great. But what do I know?
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#84

Post by pre65 »

simon wrote:
Playing with the driver operating point. I'm not convinced Nicholas Chua's published circuit is that great. But what do I know?
My version (the one DTB raves about) was the basic circuit with about 600v HT
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#85

Post by simon »

It's a shame he didn't put some voltages on as they would have been useful to cross reference. The power supply shows a 30k dropper to the driver, but if each valve draws 20mA say that's a drop across the resistor of 1200V! 3k might be more like it perhaps.
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#86

Post by pre65 »

On GK71 I'm running the driver (12GN7 / 12HG7 as a triode) at 35ma with a CCS on anode and 100R cathode resistor.
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Mike H
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#87

Post by Mike H »

Vibrating capacitors? Summink a bit odd going on?
 
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#88

Post by pre65 »

Mike H wrote:Vibrating capacitors? Summink a bit odd going on?
Did they come from Ann Summers ? :wink: :lol:
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#89

Post by simon »

Mike H wrote:Vibrating capacitors? Summink a bit odd going on?
Perhaps. But if there is I don't know what.
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#90

Post by simon »

simon wrote:Parallelling the two 220nF snubber caps as a 110nF first "tuning" cap made no difference.
Errr, 440nF rather. I added another 1uF to see if a first cap might ease the vibration but it didn't seem to have any effect. I was reluctant to go much further as I'm a bit concerned about the current rating of the mains transformer as the power supply tends to cap input.

Anyway, I dragged the amp downstairs earlier this week to see how it really sounds. Not bad, not bad at all - it's definitely a bit of a party animal :-D. They get hold of the speakers in a way that I don't think any of my other amps have done, even Madness. So there's plenty of bass grunt, but there's also the lovely brightly lit treble that thoriated filaments seem to give.

It might not be the most nuanced and subtle amp, though I'm still evaluating this. There's a little too much buzzy hum for comfort though, which I'm putting down to layout, but I don't think I can be bothered to chase it down. I'll just rebuild them in a proper box.

Upstairs I wondered if the gain was a little low, but I measured it and the choke loaded 12GN7 has a gain of 39. Downstairs on the far more efficient horns they're plenty loud :-).

So the question for those that might have some experience is should I up the HT? I'm getting ~520V as it is, so ~475V across the GM70. It would be relatively easy to go up to 750ishV (new mains transformer) but beyond that means a lot of new components etc. And much higher risk of corona effect. I've read a few posts on DIYAudio that say that as nice as the GM70 is at 500V it really shines over 1000V. So is it really worth chasing that?
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