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Megatron Electrostatic Headphone Amplifier


kevin gilmore
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68VA would be the rating with free air circulation. In a can with another transformer stacked on top would seriously knock the maximum dissipation way back.

I dunno what winding wire SumR use, but it might not have been really high temperature insulation anyway. Some use solderable wire for ease of connection. I had a Weller soldering iron transformer self destruct as a result of that. High temperature winding wire can cope with 180C.

All in all I'm not too surprised it cooked.

In a T2 the transformers are in the open chassis with ventilation slots in the top panel.

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compared to the wire sizes on the T2 transformer, these new transformers have wire sizes at least 1 size smaller on all the secondary windings. if i had another t2 transformer which i don't i would put both in the can and test for that. on the high voltage transformers which i have not lit up yet, the 5v windings are definitely 2 wire sizes too small.

i don't want to pick on richard because everything else in the past was perfect, but these transformers lasted less than 1 hour. i did finally get a quote from tigertoroids.uk and they say they can definitely do it, and have seen all the pictures. with a outside diameter of 95mm and 49mm high. virtually identical in size to the sumr transformer.

Edited by kevin gilmore
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Years ago I had problem with high temperature when stacking two transformer. It was so high so upper transformer started to “boil”. I bought some BERGQUIST GAP PAD VO Soft, 3mm thick, from Mouser. One piece between lower trafo and 3 mm aluminum bottom plate, one between trafos and finally one between top trafo and top aluminum plate bent and bolted to heat sink. This way the temperature was reduced considerable.

Determine inner temperature of a transformer is not easily done. One way is the measure resistance of primary winding when cold and when hot. Put those figures in this formula,  T(hot) - T(cold) = [[R(hot)/R(cold)]-1] x 235  and voila..  you have calculated the temperature raise of the transformer.

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Tore one of the dead transformers apart. At least a little bit. There was no wiring error.

3 amp winding is .0485 inch diameter which is 17 gauge and good for 2.9 amps (2 x el34)

1.5 amp winding is .0295 inch diameter which is 21 gauge and good for 1.2 amps (1 x el34)

American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength (powerstream.com)

Other charts do less current. Some charts do higher current.

So at a minimum, right on the absolute edge.

Can’t get deep enough to measure the primary winding which might be more important.

and the bergquist stuff is non-stock, lead time 26 weeks :(

Some of the toroid companies sell covers for their transformers. I would think that as a blackbody radiator, they would help the cooling, not hurt it. But next time i will be testing without the covers on.

With the transformers the amp is tipping the scales at 75 lbs. Making it really hard to work on.

Edited by kevin gilmore
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  • 3 weeks later...

Here is one of my mini T2. Left channel is changed to this…

BZ8A0418.thumb.JPG.40d01ee7dab41d1d35b6ddbcb1f537c8.JPG

1717684726_BZ8A0417(2).thumb.JPG.091e6e1aab79f34c5c7508021fa91eba.JPG

A Megatron something. Upper EL34 changed to T2 style CCS with 3 x 2SC4686 and 01N100D driving two LEDs. Current 28mA, cathode resistance 1390 ohm (the 500R resistors and a trimmer). The big black capacitors are Mundorf MCap Supreme silver/gold/oil, expensive but they look nice.


2116574748_BZ8A0415(3).thumb.jpg.0894a4ee426f0998316949b8a68c8128.jpg
My kitchen at lunch time.

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A right channel is finished now. Here is an aerial picture.1319785627_BZ8A0423(2).thumb.jpg.0395ed27e6d16f0bb64dc9c3aa711665.jpg

Output current sources differ between channels. Left, as said in a few posts earlier, has T2 style CCS while right channel only has 01N100D/DN2540 CCS - a “Carbon style” CCS.  The boards are ready for both CCS. 

The boards have same size and tube positions as mini T2. I run this on +400/-460V PSU, same PSU as I use for mini T2.

Edited by JoaMat
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Just made a third board (right side) of “Megatron something”. Output CCS are T2 style - 3 x 2sc4686 and 2sa1968. All ten LEDs are on top side of board. I’ve to compensate for the missing tubes.

BZ8A0427.thumb.jpg.42f10d4a6906230a589fcfee270305fa.jpg

 Picture is shoot in weak light, ISO 12800 and 1/6sec.

Edited by JoaMat
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JoaMat,

question on umbilicals (or better the connections though both) - are those absolutely identical for both channels?
Probably shouldn't, as Safety Earth (that must connect both Amp/PSU cases to Earth in the PSU case and to circuit/star ground as well) should be carried through one only, right, in order to avoid loops.

What about circuit ground, as lately in the Stax connector both left and right channel ground connect together, so if ground is carried through both Umbilicals, this would introduce a loop as well? 

 

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I’m still working with “Megatron something”. Here is another version.

Snapshot_21-11-29_14-39-53.thumb.jpg.92ccfd2a66257792a749d0d60338fcda.jpg
Similar to earlier, but no trimmers. Room for 3W cathode resistors and opto servos (the only servo you need).

I built an original Megatron back in 2013, but it’s disassembled long time ago and that was kind of a challenge, top tubes with separated floating filaments. I must say this is a bit easier and sonically it’s all right.

I can’t call this Megatron, can I? I’m thinking of Hybritron – a combination of Hybrid (tubes and solid states) and Megatron. Or maybe I just call it my little ampy. Don't want to be sued by Bourns.

 

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