Closeness of O/P valve, mains and O/P transformer

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vinylspinner
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#16

Post by vinylspinner »

Hi Greg,

Would it be possible to mount them under the turntable shelf, out of view at normal level, not sure what length leads you could get away with, it would be simple to fit a baffle on the underside and mount the tx behind it.

Failing that give the amp and speakers to me, I will kick out wifey and take over our large room, you would be welcome to visit and listen any time.

regards

Nigel
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#17

Post by Andrew »

Greg, what about just making a new top plate? Is that viable? If it were, that might be preferable to a complete new chassis.....

here's hoping...

Andrew
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#18

Post by James »

Hi Greg.
The only explanation I can think of is that the magnetic flux leakage from the output tx air gap ( due to the dc in the opt tx)is coupling into the mains tx due to proximity pushing it towards saturation.
I know you have tried different orientations but have you tried ensuring the air gap is furthest from the mains tx. Look for a small cardboard shim between the E and I laminations. Also ensure the laminations are at 90 degrees to those in the mains tx.

James.
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#19

Post by James »

Failing that,

You could try some copper tape as a flux damper band.

Ensure loop continuity with a small solder joint in the end of the tape.

James.
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Nick
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#20

Post by Nick »

Also make suer the gap is tight. Loosen the bolts (holding the lams together) and power up, then tighten again (under power) to allow the gap to close as much as possible.
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#21

Post by Andrew »

James wrote:Hi Greg.
The only explanation I can think of is that the magnetic flux leakage from the output tx air gap ( due to the dc in the opt tx)is coupling into the mains tx due to proximity pushing it towards saturation.
I know you have tried different orientations but have you tried ensuring the air gap is furthest from the mains tx. Look for a small cardboard shim between the E and I laminations. Also ensure the laminations are at 90 degrees to those in the mains tx.

James.
Good idea James, worth trying, it may not be what is causing the problem for Greg but it can't hurt to try it out - in fact I wish I had thought of that !!
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Greg
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#22

Post by Greg »

James wrote:Hi Greg.
The only explanation I can think of is that the magnetic flux leakage from the output tx air gap ( due to the dc in the opt tx)is coupling into the mains tx due to proximity pushing it towards saturation.
I know you have tried different orientations but have you tried ensuring the air gap is furthest from the mains tx. Look for a small cardboard shim between the E and I laminations. Also ensure the laminations are at 90 degrees to those in the mains tx.

James.
Aha! Thank you James. I had them with the lams at 90 degrees to the mains Tx but the right O/P Tx had the air gap furthest away and the left had it adjacent to the mains Tx. I rotated this Tx through 180 degrees to distance the air gap and the problem has gone away. Very many thanks for everyones advice.

I've now decided not to bother with drop through mounting and will surface mount in this orientation. Just need to get the covers chromed now. I've found a place in Yates that will do them, but minimum order is £50 :cry:

Greg
Last edited by Greg on Thu Aug 01, 2013 1:03 pm, edited 1 time in total.
JamesD
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#23

Post by JamesD »

I suspect this is setting up as a resonant transformer, where the extra iron is coupling with the magnetic field from the mains transformer causing a high Q resonance at (or close to) the hum frequency.

If this is correct James suggestion of the copper band as a flux dumper is just what is required.

Good Luck! I understand just how frustrating this must be!

James
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Greg
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#24

Post by Greg »

Thanks, James (D). It looks like 'tother James' orientation suggestion has worked :D
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IslandPink
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#25

Post by IslandPink »

Great news.
This is a thread worth saving for reference !
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Greg
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#26

Post by Greg »

IslandPink wrote:Great news.
This is a thread worth saving for reference !
I spoke too soon about the hum being cured. Re-orientating with the air gaps farthest away initially seemed to cure the problem but after the amp had been on for a while, the hum in the mains Tx built up again, albeit not so bad as before. I read up in Morgan Jones and he comments on the problem of flux leaking into steel chassis and he recommends a Paxolin 1-2mm gasket as solution between mains Tx and chassis. I have a thin cardboard one at the moment. I have some rubber sheet I used for the 300B amp so I'm going to make a set of three and see if that improves things further. I have already temporarily slipped some spacers between the O/P Tx's and chassis and that has brought a further improvement, so I think I am on the right lines.

I don't want to give up on this conversion because musically I have the best sound I've ever had. I'll have a go at tightening the lams as Nick has suggested and have started exploring the copper tape suggestion, albeit copper tape is expensive stuff and could easily cost me around £40 :cry:

I continue to very much appreciate all the kind help and advice I have been receiving.

Greg
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#27

Post by James »

Greg.

See PM.

James.
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#28

Post by James »

Greg,

I do you have insulating top hats installed on the mains TX core bolts?

Circulating currents could reduce the saturation margin making the mains TX more susceptible to this sort of issue.

James.
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#29

Post by James »

Greg,

Flux band should look something like this, a more representative picture than the last one.

James.
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Greg
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#30

Post by Greg »

James wrote:Greg,

I do you have insulating top hats installed on the mains TX core bolts?

Circulating currents could reduce the saturation margin making the mains TX more susceptible to this sort of issue.

James.
Hi James,

I've replied to your PM.

I didn't use top hat washers on the mains Tx on the original build, so no they are not fitted. I will look and see what stock on these I have but expect it to be low so may need to find some more.

Regards,

Greg
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