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Megatron Electrostatic Headphone Amplifier


kevin gilmore
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  • 5 weeks later...

A brief discussion about Megatron on the Stax T8000 clone thread got me reading the Megtron schematic more closely.

There are total of 8 .22uf coupling caps between stages that are in the signal path. 4 of them are rated 1000V and another 4 rated 630V. These caps will be critical to the sound of the amp.

Any suggestions on which caps to use? 

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Hi,

I joined the just closed GB too for this amp so I thought I should shoot some n00b questions to this thread.

This first one is about tube matching. It is asked on this thread before a while ago but I did not understand the answer given at that time.

So the question is: for matched *pairs* see the following arrangement options:

591ae2fd2fd55_matchedpairs.thumb.png.d07a90b508613a54c057c84f3143cb5c.png

For the small tubes we should obviously match black to black and grey to grey, but what about the el34/6ca7 ones?

  • green-yellow pairs?
  • red-purple pairs?
  • blue pairs?

And if I can obtain matches quads:

591ae323a5145_matchedquads.thumb.png.29dd6b250259812340ded60665898b30.png

Same question..

  • green-yellow quads?
  • red-purple quads?
  • blue-grey quads?
  • left quad and right quad (not color-coded in the picture)

Thanks!

Steef

Edited by steefdebruijn
added quad option
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On 7-12-2013 at 10:23 AM, spritzer said:

I for one don't care if you blow up your amp, that's your problem entirely.  What I do care about is people reading that and thinking this tube is a sub when it clearly isn't suitable in any way.  This amp is supposed to use El34/6CA7 and no other tubes.  Not KT88, KT77, 6550, KT66. KT90, KT120 or KT150. 

Hi spritzer and others,

I am investigating tube options and have a question about the compatibility of the KT77. First of all I am a tube n00b but trying to understand all this.

The megatron is designed for 6CA7 tubes. From what I read these are also called EL34 tubes. So these are the same. If I compare the info on the tube directory (http://www.audiomatica.com/tubes/el34.htm vs http://www.audiomatica.com/tubes/kt77.htm) I see no difference in specs and max ratings. The connections are slightly different but the effect is zero (pins 1(g3) and 8(c) are separate on the EL34 but connected on the board, the KT77 has internal connection between g3 and c on pin 8 with pin 1 not connected to anything).

The measurements differ slightly however (from my eyes). Is this difference in measurements the culprit why the KT77 cannot be used in the Megatron?

Steef

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About the PS...

In the GB we are supplied with the GRHV boards originating from the Carbon. It can be configured for +-450V by changing 2 resistors from 390K to 442K right? Somewhere in this thread it is stated that we need a 360VAC secondary on the trafo. Is this correct?

We need 12VDC filament supply for the little tubes on the GB megatron board. Can we lower the LV supply on the GRHV board to +-12VDC by just changing the 7815 and 7915 to 7812/7912 ones or should we lower the 15VAC too? And, can we just use the -12VDC for the -15VDC connection on the megatron board (I trust trying +12 and -15 from this one supply is a stupid idea)?

If so, I would like to try to keep this in one (large...) box with the following trafos:

Will this do it? Any thoughts or corrections welcome.

Steef

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The GRHV has a very low drop-out voltage so 360vac secondaries should be good enough for the 450VDC regulated output. And yes, changing those two 390K to 442K on GRHV to adjust for 450VDC output.

I am planning to do precisely what you said for the +/-12VDC supplies needed. The 12VDC output needs to supply 1.2A 600mA current for the 4 front end tubes so the 7812 needs to be properly heatsinked. The -12VDC should work for the -15VDC current sink. I am also contemplating using the +12VDC to drive a timer logic for the HV delay. I am hoping someone(s) with technical knowledge can chime in if this is a good idea. 

I plan to use the Antek transformers too although I may use the 50VA ones for the filament and LV supply.

By the way, the CCS EL34 tubes need their filament supply wires very short (can someone help me understand why?) so those are the ones I plan to supply with the 50VA trafo. I will use the 6.3v secondaries on the 400VA/360v trafo for the output EL34.  

If I traced the board correctly, the CCS EL34 tubes are the pairs on the outside edges of the amp board.

Edited by mwl168
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The tubes need 600mA at 12V, not 1.2A. No problems using same supply for delay. 

I mentioned this in PM but be careful using 360vac secondaries... that means around 510V on power supply caps which is just not enough margin for 550VDC rated caps. If you have a high mains voltage like mine then they will see 530V which is right at the limit, meaning they are gonna die relatively soon. Especially if there are spurious voltage spikes in the system. Ideally you want 20% derating for electrolytics and 50% derating for tantalums for absolute long term reliability. If you are using 600V caps then it should be safe but I don't see a lot of options there. 

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Thanks for correcting the filament current, it's 150mA for 12.6v so 600mA total, not 1.2A which is good news.

I also agree that 550VDC rated caps would not be enough for pre-reg position using 360vac secondaries where they will see well over 500VDC. Mouser has 600VDC rated electrolytic caps but they only go up to 220uf.  

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Just to be sure: 25va 15v is plenty for the LV (regarding the filament current of 600mA)?

And: if the two 4A 6VAC secondaries from the AS-4T360 - 400VA 360V are enough for the center tubes (equals 2A per tube), 25va 6v (2.1A per tube) should also be enough for the outer tubes? Or is it better to give the trafo for these outer tubes more headroom?

Regarding the caps: so we should look for better caps then presuming we cannot lower the HV trafo secondary and/or the +-VDC to the Megatron board...

I hope these are overkill (wo'nt fit to board also) with needing four of them... :-)  http://nl.rs-online.com/web/p/aluminium-capacitors/8707188/

Maybe easier (and cheaper) to lower the PSU to +-400VDC (using 320VAC secondaries) and use normally available 680uF/550V caps? Or is that simply not enough for this amp...

Edited by steefdebruijn
added 400VDC question
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I typically over-spec my transformers plus the Antek 50VA ones are not much bigger and pricier than the 25VA ones. But, yes, the 25VA ones should work too just won't provide as much headroom.

The post-reg capacitors only see the desired regulated voltage so 550VDC rated caps are sufficient for those positions.

 

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38 minutes ago, mwl168 said:

The post-reg capacitors only see the desired regulated voltage so 550VDC rated caps are sufficient for those positions.

I think this n00b is beginning to understand. So one of the large caps (the one connected to the rectifiers) should be a large 600V+ one and for the other one a standard 550V will do.

So only two expensive caps then for the PS (trying to avoid expensive audio-grade caps on the Megatron board) :-)

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4 hours ago, mypasswordis said:

Just use the 200VA 350V Antek: http://www.antekinc.com/as-2t350-200va-350v-transformer/

Solves all your problems :) Smaller, lighter, costs less, lower voltage so you can use 550V caps

I got damned short memory. Just checked some PM with congo5 who provided some really good information for the Megatron.

The HV current draw on the positive rail is about 122mA for the front end and output stage together. The current draw for the negative rail is about 110mA. mypasswordis's recommendation of the 200VA 350V Antek is spot on - the 350vac secondaries are rate at 290mA. This is the transformer I will use for my build.

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5 hours ago, JoaMat said:

Antek transformer is a good alternative but you might consider a custom made transformer. If you live in Europe – Toroidy is IMO a good alternative.

Thanks! Asked for a quote on separate 350vac 300ma and four 6.3vac 3a trafos. All with two windings and specs per winding. Will parallel the secondaries on two of them for the center tubes.

What secondary is best suitable for the LV if we use 12vdc output? I would guess 12vac would be enough but maybe it does not provide enough headroom. Also no other mods necessary to the LV than using 78(79)12 instead of the *15 ones? (I think remembering from GRLV discussion that also zeners had to be lowered but maybe that is not applicable here)

thanks for all your input and feedback!

Steef

On sockets. Will these do? http://www.jakeband.it/sockets.html 

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I’m not sure that 3A per winding is optimal. You might end up with a voltage higher than 6.3 V if load is just one and a half ampere, but my knowledge on this subject is poor. Perhaps someone with better knowledge can tell?

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4 hours ago, JoaMat said:

I’m not sure that 3A per winding is optimal. You might end up with a voltage higher than 6.3 V if load is just one and a half ampere, but my knowledge on this subject is poor. Perhaps someone with better knowledge can tell?

Thanks again,

I rephrased my quote. Will not receive answer before next week with High End Munich going on this weekend.

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On 16-5-2017 at 0:00 AM, mwl168 said:

There are total of 8 .22uf coupling caps between stages that are in the signal path. 4 of them are rated 1000V and another 4 rated 630V. These caps will be critical to the sound of the amp.

Any suggestions on which caps to use? 

No suggestion (because no experience) but will investigate if these can somehow be fitted:

https://www.intertechnik.com/Shop/Cross-over-parts/Capacitors/Audyn-True-Copper-Cap/_-ATC022630_1768,en,6559,53292

https://www.intertechnik.com/Shop/Cross-over-parts/Capacitors/Audyn-Cap-True-Silver/_-ATS0221000_1768,en,6226,123941

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Hi,

I have a quote on the toroidy transformers. They advise 50VA 6,3VAC (2 windings) trafo with a lower than normal off-load voltage (normal 6,67V, for this application 6,4-6,5V).

Are there more people interested?

For the LV I think I will settle for the 50VA 15VAC version; if I understand the calculations correctly that will give me 5W dissipation on the LM7812 (and LM7912) with an input voltage in the middle of its optimal operating range of 16-22V.

Steef

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