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Megatron Electrostatic Headphone Amplifier


kevin gilmore
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Here is my solution for filament transformer… 625549642_BZ8A0437(2).thumb.jpg.91dde704b7aa0babf65741a0738fa582.jpg

 …mounted on bottom case plate, belonged to a now retired Blue Hawaii. I also found a piece of Bergquist Gap Pad for the transformer to rest on.
IMG_0444.thumb.jpg.8a20b6a217b6d060859c80705c08cb74.jpg

It seems that cathode resistor voltage is much lower for Emission Labs tube than for EL34, so I decreased the negative high voltage from 460 V to 400 V.

 The lowered voltage called for change of Cathode resistor and trimmer, now 402R resistor and 100R trimmer. A few things had to be removed before component changes.

BZ8A0438.thumb.JPG.4f38fa607f25b49b3ffb0b02c6602909.JPG

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This Emission Labs amplifier wasn’t planned, it just happened along the way. Although it seems to work, I have no idea how to best use the direct heated triodes in an electrostatic headphone amplifier. 

Here are maximum conditions stated in datasheet from Emission Labs with note 6.

 1942021920_InkedEMLdatasheet.thumb.jpg.9f2fa673675aba624fa23afa6f03e544.jpg
Note6) Maximum conditions do not apply simultaniously. This is not a working point.

I think I’m within most of the maximum limits, but what how about Power Output in Class A: 8W and Grid resistor: max 470K? 

Is there anything to consider in my case? Now I’m running at 400V/27mA roughly 11W and grid resistors are 680K as in the original Megatron. Any advice and comments are appreciated.

Edited by JoaMat
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  • 3 weeks later...
On 11/26/2021 at 5:29 PM, audiostar said:

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? 

 

Sorry for a really slow response.

Yes, the umbilicals are identical.

Some background – my first electrostatic build was a KGSShv with a PSU with on board heat sinks in its own house. Second build was the DIY T2 and I liked the chassis design. Thereafter I have only built things (almost) with off board heat sinks. With T2 I built an original T2 PSU in a Modushop case. Later I built another T2 PSU with my own designed PCBs. That are the only PSUs I’ve built, along with the first KGSShc supply. But I’ve built a lot of Kevin designed amplifiers, and all have been power by those two T2 PSUs. I found the T2 PSU with its two identical umbilicals quite convenient. With the good quality Amphenol connectors it’s easy to connect/disconnect as wanted when I want to try/test one channel at the time.

What about ground loops? I’ve read/heard about it, but so far, I haven’t, or I don’t know I have had ground loops. For me it’s more important I always have a safety ground connected. Therefor I’ll continue to use two identical umbilicals …

P.S.
I’ve had some issues with mini T2 that was solved by removing one 580V bias wire, so I probably have had a loop. problem. But safety ground wires will stay!

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On 1/7/2022 at 2:15 AM, JoaMat said:

Sorry for a really slow response.

Yes, the umbilicals are identical.

Some background – my first electrostatic build was a KGSShv with a PSU with on board heat sinks in its own house. Second build was the DIY T2 and I liked the chassis design. Thereafter I have only built things (almost) with off board heat sinks. With T2 I built an original T2 PSU in a Modushop case. Later I built another T2 PSU with my own designed PCBs. That are the only PSUs I’ve built, along with the first KGSShc supply. But I’ve built a lot of Kevin designed amplifiers, and all have been power by those two T2 PSUs. I found the T2 PSU with its two identical umbilicals quite convenient. With the good quality Amphenol connectors it’s easy to connect/disconnect as wanted when I want to try/test one channel at the time.

What about ground loops? I’ve read/heard about it, but so far, I haven’t, or I don’t know I have had ground loops. For me it’s more important I always have a safety ground connected. Therefor I’ll continue to use two identical umbilicals …

P.S.
I’ve had some issues with mini T2 that was solved by removing one 580V bias wire, so I probably have had a loop. problem. But safety ground wires will stay!

Thanks, JoeMat and a Happy New Year as well! 

So, you carry circuit ground through both umbilicals which you connect separately to both channels in the amp's chassis).
Safety Earth, you carry through both umbilicals as well and connect both to a single point in the amp's chassis?
Bias line you split in the PSU chassis and carry through both umbilicals as well, but connect in the amp's chassis only one of them to the actual Bias output?

In the power supply chassis you tie ground of all PSUs together and connect through a circuit breaker (or not, or simple 10R, etc) to the chassis itself & Safety Earth coming off 1) the IEC mains connector and 2) both umbilicals off the amp's chassis.

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