GM70 PP

What people are working on at the moment
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izzy wizzy
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#91 Re: GM70 PP

Post by izzy wizzy »

RhythMick wrote: Thu Oct 31, 2019 1:10 pm Slightly confused. They are the small boards but with the 3 legged transistors mounted on the wrong side? How are the heat sinks mounted?
No big boards left and not rated for 20V fils so slightly uprated components and some bodging to use a bigger heatsink all fed from SMPS which is supplying fils at present.

How I mount the heatsink hasn't been figured out yet 🤔

But if you mean how are boards mounted to heatsink, then nuts and bolts sandwich board and transistor /regulator through on board hestsink holes thru to hestsink. See the side on pic.
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#92 Re: GM70 PP

Post by RhythMick »

What I was trying to understand (perhaps I'm being dumb) was that if you mount from below then you need to turn the transistors around so the legs are in the right positions for the board, which expects the transistors mounted from above. So presumably you do that and, what, bend the transistors away from the centre, which would leave the base exposed to sit on the heatsink. Just a problem getting the screwhead in I guess ?

I don't need to understand, it just struck me I didn't.
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izzy wizzy
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#93 Re: GM70 PP

Post by izzy wizzy »

Yes exactly that. You can see the legs twisted and them sitting at an angle. Luckily they then line up with the original heatsink mounting holes. The nuts can then sandwich them to hestsink. Luckily I have some very small spanners to get in there.
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#94 Re: GM70 PP

Post by RhythMick »

I see the pic from above on page 6, is there a side pic elsewhere ?

IGNORE - me being dumb again.

As long as it works for you, just interested.
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#95 Re: GM70 PP

Post by izzy wizzy »

Maybe this helps.

IMG_20191031_173702455_crop_24.jpg
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#96 Re: GM70 PP

Post by RhythMick »

It does thanks
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#97 Re: GM70 PP

Post by izzy wizzy »

Got the new modified Lehane VCCS boards in feeding the GM70s in one channel so able to compare channels. Was surprised intially how close they sounded until after a while, realised there's more clarity and silence between notes than the pure SMPS channel. Overall more engaging and there's a kinda fog over everything with the SMPS only channel. However the SMPS only channel is still good, just not up the refinement of the VCCS channel. The listening revealed more music and subtlety in the VCCS channel so can't wait now to do the other one hopefully in the coming week.

Cheers,
Stephen
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#98 Re: GM70 PP

Post by izzy wizzy »

All the valves are lit by Lehane VCCS supplies and I really like what they do.

So now to try something related to this decoupling thread http://www.audio-talk.co.uk/phpBB3/vie ... f=5&t=5263

Plan is to use what I have lying around in the shed. Most of it is way too big but it's there not doing anything. tx @ 300V, rectifiers RG1-250 MVs and some silicon in a hybrid bridge, 15H, 220u, 10H, 220u or maybe 20u not sure yet which should give me the 235V or so that I'm after.

Question is, not sure if the 15H choke will handle choke input. It's 200mA rated and I'll be pulling 60-70mA. If it doesn't, what happens apart from a lot of noise? I'm guessing it saturates and then not work as well? What would I look for in this regard? Don't have experience in this area as only used parts rated for the job before.

If so, another guess, can I pull it apart and increase the gap to help things out?

Cheers,
Stephen
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#99 Re: GM70 PP

Post by RhythMick »

Not sure I'm following the concern, 60mA well under 200mA rating?
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#100 Re: GM70 PP

Post by Nick »

RhythMick wrote: Tue Nov 12, 2019 6:30 pm Not sure I'm following the concern, 60mA well under 200mA rating?
Yes, but choke input is much harder on a choke than cap input.
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#101 Re: GM70 PP

Post by RhythMick »

I have only used choke input filters personally, I have less experience with cap input.

I follow Jac's advice (Lundahl dealer) and choose the choke which gives me the maximum inductance which is rated for the current. For example my 26 PSU draws about 11mA, 6 for the valve and 5 for the bleed resistor. I use an LL1667-15mA as input choke giving 270H.

For 60mA I might be looking at something like the LL2743-70mA.
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#102 Re: GM70 PP

Post by IslandPink »

I assume because the choke isn't shielded from the steep dV/dt slopes from the rectifiers, the choke sees greater current swings, and hence the max flux it has to cope with must rise. Hence bigger core required. Probably also higher insulation rating, if that requires any change, though I suspect most are well over-specced.
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#103 Re: GM70 PP

Post by izzy wizzy »

As Nick says, the choke in a choke input filter gets a hard time. According to PSUD, the choke sees 130V RMS, 420VP-P across it. I thought if it was derated a bit, that might help. I need it to be choke input in order to use MVs. The Lundahl chokes can usually take a large AC swing. I don't know about the one I have even though it is wound more like a plate choke.

I thought a larger air gap on the choke might help. May have to ask elsewhere. Don't want to spend money if I don't have to.

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Stephen
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#104 Re: GM70 PP

Post by Nick »

Yes, remember the following cap is a short circuit at AC, so the choke has the full AC voltage across it and current through it. MJ covers it well.
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#105 Re: GM70 PP

Post by RhythMick »

Harshest conditions would be at startup, maybe consider a slow start circuit? I've honestly never worried about the choke. Even if it briefly saturated at startup it just makes it a heavy resistor but it will quickly return to normal? Am I missing something?
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