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Megatron Electrostatic Headphone Amplifier


kevin gilmore
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Probably be a while for that to happen - typically takes 3-4 weeks for stuff to reach me and I haven't ordered the delay, yet. I'm also not sure I want to finalize the layout until I know what I'm getting for the tube delay.

 

The components inside my PS are going to be:

  • KGSSHV PS
  • Soft-start circuit
  • HV Time-delay circuit / relay
  • LT1083-based LV PS set at 6.3v (though I'm contemplating dropping this down to 5.5v and using it for the delay relay as well?)
  • HV trans
  • 9V trans
Edited by RiStaR
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The local Electronics shop where I "invest" a fair amount of my money just loaned me this (http://www.ston.com.tw/spec/H3Y-2.pdf) to try out.

I intend to use that in front of the HV transformer. Is there value in putting the soft-start circuit after this? Or should i leave the soft-start just for the filaments. Open to suggestions since it's my first tube amp and there's just so much contradicting info about HT delays ><

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Well scratch the hum issues, but I am getting music out. SumR transformer hums when powering the board, and I'm getting hum issues from what I assume is the input filament transformer. I'm getting 270 on the rail, which increases to 307 with only one channel populated. 

 

My first impressions are that there is noticeably more treble detail and slightly less bass impact than the kgsshv. I find the presentation more even across the spectrum as a result, but it also makes me aware of my sensitivity to sibilance on certain recordings- something I didn't experience as much with the kgsshv. I also agree with ristar's comments, there's more of a sense of "space" and "separation", but that could also be related to the frequency response. All in all sounds great so far but I need to fix the hum issues before I make a real assessment. 

 

On the aesthetic side, this thing looks fucking insane with the filaments going everywhere.

 

Some specs (I know I've mentioned these before, but collecting it in one place might be useful for others):

normal PSU, 2n3904s ripped out

20k to drop to 300V

 

SumR transformers for HV and output filaments

antek transformer on input

 

wima between 12ax7 and 6ca7

mundorf mkp between 12au7 and 12ax7

telefunken 12au7

toshiba 12ax7a

national electronics 6ca7

 

I'm considering that Maida 21st century regulator for the 300V line, but it seems like serious overkill.

I also have an extra bridge left over from the PSU, the input stage heaters could also work with just rectified DC right? Maybe a future revision could have an optional spot for this.

 

EDIT: ground your XLR's kids, it works perfectly. 

Edited by nopants
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Congrats 'pantsless' :)

 

What do you mean by 270 and 307 on the rail? - this is your 300v line?

 

I guess we should list down the tubes for the sound impressions - I'm using a matched octet of 'cheapish' EH 6ca7s because of a great contact from Victor. The 12ax7 and au7s are also EH.

 

While I only listened to it for 4-5 hours, I do think the KGSSHV has just as much detail and separation. Which headphone were you using for the comparison? While I enjoyed both the 007mk1 and SR-202 with the Megatron, I do think the SR-202 got more out of that pairing - I didn't think I got much more out of the SR-202 from the KGSSHV versus the SRM-252 and I thought it was just because the SR-202 doesn't improve much with better amplification. I guess I'm wrong on that front.

 

Agree on bass impact being better on the KGSSHV versus the Megatron, but it also seems that the Megatron does that more naturally? Waiting on parts to increase the height of my casing so I will update on this when I get more time with both. End of it all, it looks like I may be keeping both amps afterall. I just need to think of an excuse to justify that to the "finance minister" :P

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Per kevin's suggestion I ran the board with only one channel's tubes populated- that made the 300V rail sit at 307, both channels populated read as 270. 

 

I dropped extra on the rebranded RFT's because I wasn't really planning to roll tubes- that would straight up drive me insane. I did end up impulse buying JJ's 12ax7 though since they were fairly cheap, and there are differences... but I have to resist rolling for the moment. Life finances are taking their toll, luckily I had all these parts sitting around before all that bullshit.

 

I only have the mk1 at the moment- there's a lot of detail in both amplifiers but the presentation is different enough between them that it makes you question which is "more neutral". The only thing I readily perceive listening to the Megatron is a lack of any particular emphasis on the three main frequency bands, which leads me to believe it's more even/neutral overall. Right now I'm just enjoying the fact that there is sound and it's fairly glorious though.

 

This all being said, I have zero desire to case the thing up now that the damn thing works. Maybe I'll just cover it in cardboard when I'm not using it

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  • 2 weeks later...

Have finished my PS unit, but only just realised Mouser didn't pack one item for my connector so I can't actually use them. It's been about 2 months since that order, hopefully Mouser does good on it. Still waiting on some wood so I can finish my amp chassis.

 

In any case, to test the HV delay I connected directly to the terminal blocks using wire. The good news is it works properly and I spent another 8 hours on it. The HV comes on about 66 secs after power on. The issue I had previously with the DC offset going to 450v momentarily doesn't happen anymore - so the delay did the trick there :)

 

hfu6VDp.jpg

 

Further impressions, the bass on the Megatron is perfectly fine - it may even be tighter than my KGSSHV. There is more volume in the bass with the KGSSHV so the impact is stronger there, but the Megatron with the big bottles has enough bass to please. On the upsides: There's definitely more clarity with the Megatron - I picked up on stuff I didn't notice with the KGSSHV. Going back to the KGSSHV, I can hear them, but the Megatron just made it more obvious. You just tap more to the music with the Megatron and things just seem more 3D. I changed my 12ax7 from EH to Tungsol and now there's less reverb - not negative just different. Where the Megatron really is enjoyable are with tracks with strings (Kotaro never sounded better) and drums with cymbals. 

 

I have some 12bh7s coming soon, so hopefully I can get everything else up before they arrive.

 

Now I can't wait to finish this thing and listen to it more.

 

Edit: oh the temperature on the valves vary a fair bit depending on tube... some are at 105c while some are at 130c. My transformer is running at 41c and really quiet. The amp's top surface approaches 42c. The sinks on the KGSSHV PS is about 61-66c still. The amp is definitely hot.

Edited by RiStaR
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hfu6VDp.jpg

 

 On the upsides: There's definitely more clarity with the Megatron - I picked up on stuff I didn't notice with the KGSSHV. Going back to the KGSSHV, I can hear them, but the Megatron just made it more obvious. You just tap more to the music with the Megatron and things just seem more 3D.

 

Awesome build and congrats!

 

And I do agree there's something extra special about the sound of a good tube amp over sand  :jbl:

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What position are you going to put the 12bh7 in?

May have to change the resistors in te cathode bias a bit.

The megaton has lower distortion at higher levels than the

Electra or the Wes. In fact it's lower distortion and a much wider

Frequency response than some of the solid state amps out there.

Edited by kevin gilmore
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What position are you going to put the 12bh7 in?

May have to change the resistors in te cathode bias a bit.

 

 

I intend to use them in place of the 12AU7s. Ok, from reading it seemed I just had to make sure I could supply the heater current (which I have gobs of with the LT1083). So do let me know what I need to swap around to test them :P [While I got a tube amp up and running, I'm still pretty much clueless on tubes apart from looking at pinouts/gain and the bh7s looked just like a straight drop-in replacement]. I know Birgir talks about the ECC99 a fair bit as well. Let me know what you'd recommend?

 

 

Awesome build and congrats!

 

And I do agree there's something extra special about the sound of a good tube amp over sand   :jbl:

 

 

Cheers Jon :) Almost there!

 

Congratulations Cal. Where did you get the PSU case from? I may be headed that way as well.

 

http://www.ebay.com/itm/SL9155A-10-DIY-FULL-Aluminum-Audio-chassis-Audio-Amplifier-Chassis-Enclosure-/261050882766?pt=US_Pro_Audio_Parts_Accessories&hash=item3cc7d85ace

 

It's in champagne... tried stripping it but didn't go too well... so ended up spray painting it with Metallic paint - it's darker than the other chassis, but it's good enough for me.

 

Edit: Link didn't show for some reason.

Edited by RiStaR
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I intend to use them in place of the 12AU7s. Ok, from reading it seemed I just had to make sure I could supply the heater current (which I have gobs of with the LT1083). So do let me know what I need to swap around to test them :P [While I got a tube amp up and running, I'm still pretty much clueless on tubes apart from looking at pinouts/gain and the bh7s looked just like a straight drop-in replacement]. I know Birgir talks about the ECC99 a fair bit as well. Let me know what you'd recommend?

 

The pinout is the same but electrically they are very different.  We would only endorse changing tube type if it was actually better in any way, though we would probably have used said type in the first place.  Also rolling in different tubes without recalculating the operating point and swapping out the resistors as needed is a terrible idea. 

 

So basically, just because something fits doesn't mean there is any reason to put it there.  Stick with the stock tubes. 

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  • 1 month later...

My father in law  was going to take this radio to the goodwill.
It is a GM 150A (I didn't know General Motors made radios in the 1930's).
I am thinking on using it as the case for my Megatron build.
It needs a lot of work, refinishing the wood and gutting everything out.
The amp/tubes would be where the turntable is, and could close the lid when not in use.
The power supply and Stax jack will go where the radio dials are.

It is quite heavy, and would be a real challenge to take it to meets :)

post-2299-0-06394100-1385853741_thumb.jp

post-2299-0-43339200-1385853210_thumb.jp

post-2299-0-82899000-1385853258_thumb.jp

post-2299-0-29118900-1385853305_thumb.jp

post-2299-0-29372700-1385853450_thumb.jp

post-2299-0-31922000-1385853510_thumb.jp

post-2299-0-18123600-1385853552_thumb.jp

Edited by eggil
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one chicken on each of the 4 x 6c33's

 

That sounds interesting - just googled the tubes and they look nice too :D

 

On my megatron I think i found my tubes that work best for me... TS 12ax7; sylvania 12bh7 (it just sounded better than all the au7s i tried); Russian 6P3S-E (they also put out less heat than the EH 6ca7s). Bass is as much as the KGSSHV and the decay sounds perfect for me... now i'm wondering if I should take a gamble and a large box of unmatched ones or pay the premium for the matched quads for backups.

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Yes, 6c33 is just awesome in what Andy is doing.  Using 6P3S-E in a 900V amp is just untold amounts of stupid.  Just because some idiot on the net claims this tube is similar to the EL34 doesn't make it so.  It's a lower spec version of the 6L6 and is not compatible. 

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