Snail Phono Pre

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little eddy
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#226 Re: Snail Phono Pre

Post by little eddy »

So just had an extended listening session and sugar, one channel dropped in volume.

Felt the top and it was quite warm. Switched off, allowed to cool, and all was fine again..

Dissipation is higher to me now running higher currents and perhaps the original 6 off 1" square vents are inadequate for the necessary airflow.

So I've ordered a 400x200mm piece of 1.5mm alu sheet with 10mm perforations. But the question is how to keep the enclosure neat (with no proud bolt heads if I can get away with it), yet ensuring the sheet will not come loose.

Is glue an option in which case what types are stable up to normal enclosure temperatures?

Or I'll try to bolt using countersunk screws.

Any other ideas? Could swap the valves around to see if one of them is the culprit, but think resolving the enclosure temperature should be my primary aim.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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IslandPink
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#227 Re: Snail Phono Pre

Post by IslandPink »

Did one channel reduce in volume then remain stable ?
If so - if you have a cathode by-pass cap on a valve, and it goes, this would be the effect.
"Once you find out ... the Circumstances ; then you can go out"
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Nick
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#228 Re: Snail Phono Pre

Post by Nick »

But it would be unlikely to recover. You need to see if the gain drops again and when it does measure the voltages looking for a change.
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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IslandPink
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#229 Re: Snail Phono Pre

Post by IslandPink »

Yes, data needed. Maybe it's a dry joint that comes open when it's hot then tightens up again.
"Once you find out ... the Circumstances ; then you can go out"
little eddy
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#230 Re: Snail Phono Pre

Post by little eddy »

Yes but no but!.

Measuring no problem, but that means taking both the top and bottom off the enclosure, so can't allow it to heat up again (if that is what's causing the issue)!

Must admit that one of the Tele ECC83 I'm using hums. You can't hear it when playing music so thought might as well use that one since emissions were good since can't really sell. But perhaps a weak link, and susceptible to higher than normal temperatures? With tiodes in parallel, maybe one's stopping conducting because there's still a reasonable sound level out of the faulty channel.

While I'm at it might swap the valves over and if the problem reoccurs and moves channel then know it's a valve.

Unclear as to the temp rating of epoxy, some recommend as low as 50C. Should be OK with the much improved ventilation.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#231 Re: Snail Phono Pre

Post by little eddy »

little eddy wrote: Fri Nov 01, 2019 5:06 pm While I was at it had taken a couple of pics of the SUTs. Does anyone recognise who they might be made by?
I have talked to the original builder, and he has suggested that the SUTs fitted were made by a now closed company called Goring. Has anyone heard of them and have any understanding of their likely quality?
SUTs top small.jpg
SUTs bottom small.jpg
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#232 Re: Snail Phono Pre

Post by little eddy »

So my challenges to get the most out of this amp continue.

When scoping the installed SUTs in my phono, the input is out of phase:
20-05-11 SUT trace small.jpg
When looking at the underside and markings, guess it was easy for the original builder to get them the wrong way around - thought he would have tested it!

Just going to swap the secondary wires over:
Underside of SUT small.jpg
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#233 Re: Snail Phono Pre

Post by little eddy »

Did some playing today with a 1k square wave and the Goring SUTs. 1st was to find an optimum Zobel network - 160pF and 4k7 gave me this:
20-05-12 Image #4 SUT, 47k load, no series R & 160pF+4k7 Zobel small.jpg
But then the ECC83 and grid stoppers are in themselves a HP filter so what if I were just to add resistance to the grid stopper? So with an additional 2k2 in each channel, got this:
20-05-12 Image #4 SUT, 47k load and 2k2 series R small.jpg
Which of the two approaches might be 'technically' better? I personally like the simplicity of just adding to the grid stopper resistance rather than additional capacitors, (if either are needed at all)?

Mike.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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Nick
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#234 Re: Snail Phono Pre

Post by Nick »

Assuming you mean grid stopper, then I would keep it as low as possible. TBH an ecc83 won't need one. If you mean grid leak then that’s a different question.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
little eddy
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#235 Re: Snail Phono Pre

Post by little eddy »

I have 100R on each triode grid.

Just by connecting up the ECC83s, the ringing/overshoot is already much reduced:
20-05-12 Image #3 SUT, 47k load and ECC83s only small.jpg
I'm thinking that adding further components is probably an unnecessary over-complication. But then we go on about the accuracy of the RIAA so perhaps getting this as flat as possible is a pre-requisite for a Vinyl front end because any inaccuracies are passed down the line?
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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