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Megatron Electrostatic Headphone Amplifier


kevin gilmore
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On 4/3/2020 at 7:09 AM, simmconn said:

I'm not sure if I understand your reply. In the Megatron the first stage uses 12AU7, and the second stage uses 12AX7. What do you mean by "provide 24db gain to drive pair 12au7s"?

Sorry,the first stage 12au7s to drive 12ax7.

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  • 9 months later...

My apologies if this has been asked before....

As normal my first steps in a build is to get the circuit diagram and reverse engineer the gerbers to check them against each other.... looking at the circuit diagram pdf the decoupling caps have been omitted... fair enough, however

1. the circuit diagram shows a single 150K resistor from the led D3 to ground but the reverse engineer of the gerbers has two 150K resistors in series to ground....

2. the gerbers have two 330K resistors, one each from the 300V line to anodes 1 and 2 of the 21ax7. These resistors are not on the circuit diagram and are used to drop the 300V psu line to down to about 250V for the 12ax7 anode voltage...

here is my reverse engineer with components in pink missing from the pdf:

 

NOTE this circuit diagram has not been checkedmegatron.thumb.JPG.dcb8de58197c9aec077e9290d54c3d82.JPG

Edited by jamesmking
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  • 3 weeks later...

Yesterday, after three years of regular use (from 2018 to now) I measured some voltages for check how my Megatron aged.

It is very nice to see that the voltages are still similar to that I did as a reference once finished it. The EL34´s glass have gotten a bit darker with use but otherwise my "baby" is perfect.

As an anecdotal detail, my preferred valve for the first stage is still E180CC.

 

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  • 1 month later...

well my amp boards for the megaslow build finally arrived today. I have decided to use a golden reference HV psu for the +-450V and a second golden reference hv for the +300V. The psus are finished other than I don't have and cant source the pass mosfets.... *sigh*

One question. since I will be using a 300V psu do I need to populate the 3*100V series zener string connected from the 300V line down to ground? I assume the zener string is there to stop the 300V line rising above 300V with respect to ground if the 300V line is derived from the +450V line via a dropper resistor.. or is there another reason why the string is there?

 

Edited by jamesmking
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My megaslow build.

Thank you JoaMat for supplying the 3 mosfets I needed for the golden reference psu.

The megatron is workingish. This build was a little perplexing. I used groove tube 12ax7, electroharmonix 12au7 (gold pin) and groove tube el34s.

_MG_6205.jpg

_MG_6204.thumb.jpg.ce5eee1bd02887af3f1af711cc9170f3.jpg

 

I implemented the 12ax7 increased current modification form the outset.

I found that I could only get about 220Vrms output before clipping. The clipping was not symmetrical and it was the bottom of the wave on the +O that was clipping only. I started to probe with a scope and found that the voltage drop at the 330K anode resistors on the 12ax7 was only about 19V on the +O side and about 35V on the -O side. Everything else looked fine. I scoped the output of the 12au7 when the amp was fine - no clipping when the output of the amp clipped. I checked all the psu voltages - no problems. It was the output of the 12ax7 was asymmetrically clipping.

I took the valves out and measured them and the 12ax7 was only managing about 0.5mA output on one plate and about 0.8mA on the other. The avo manual said I should be expecting about 1.2mA. I was not sure if the amp had damaged the valve or if the valve was weak. So I checked the amp and removed the extra current modification and tried with a stronger 12ax7. Now both the +O and -O clip at approximately the same point but the clipping is still asymmetrical. I'm getting about 35V to 40V across the 330K anode resistors on the 12ax7. The bottom of both outputs clips before the top, and I am getting about 440Vrms output before clipping.

I got in an email conversation with watford valves and the proprietor said that almost all new production 12ax7s are only capable of about 0.8mA and that only new old stock could do 1.2mA. So the high current mod might not be an option if you are running new production 12ax7s.

I did some frequency response measurements and found the -3db point was only at about 23-24Khz... which seems rather low.

Could someone tell me what output voltage rms they are getting before clipping and the -3db point for verification.

thanks in advance James

Edited by jamesmking
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  • 3 months later...

Hi All!

I have had the fortune of having owned one of Tran's (Thuan, Thuy etc...) Megatron builds, shipped from Vietnam, for the past two years... I bought it from the same Head-fi account that was selling T2 boards and kits since 2005: kytuphicanh (now using tranhieu on head-fi and fls.audio on ebay).  

It was a fairly temperamental build at the outset, eating through the power supply filtering caps... until I eventually upgraded them to Kemet ALC10 550V (from Nichicon 450V) and added a couple of MS-22 thermistors to slow the startup.

However, the amp recently began eating the 5A inlet fuse, and I suspect that either the primary or secondary filament transformer has given up the ghost.   

Would someone in North America be interested in taking a look, without obligation?  If you can't fix it, you can simply bin it.  It seems like a waste to bin it without at least giving it a second chance.  It was a fine sounding amplifier when it was working.  I would be happy to UPS the amp to you.  I have some very nice tubes you may be interested in for your trouble, I'm under the same handle on Canuck Audio Mart.

I have a DIY T2 coming from GeorgeP, so I'm not in a terrible rush or anything like that.

Thank you!

1.jpg

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You know, not many things upset me too much, which is why I suppose I've put up with this amp for 2 years; however, the IMMEDIATE and IDENTICAL reply I received from Thuan BOTH here on head-case (thuytn) AND on head-fi (tranhieu) to the above post does get under my skin:

I've just seen your post on HC. Could you please remove my name from that post? It could heavily affect my future sales. Hopefully you could understand my situation.

Cheers
Thuan

Wow, this fellow has guts.  Hence, I will provide some helpful, clarifying details:

1. Thuan's amp was dead the week it arrived.  It had eaten through it's first set of 6 under-sized 220uf 450V Nichicon power filtering caps, and blown the soldered fuses between the GZ34 rectifiers and the caps.  The inrush current was virtually uncontrolled on startup.

2. Thuan provided ZERO support, and was completely unresponsive.

3. Thuan was using a different account for amp/T2 board and transistor sales on head-fi (kytuphicanh), which was scrubbed by head-fi moderators, who added the warning: Caution: Incomplete Sales  Why?

Good question. The next fortunate head-fi member to buy one of Thuan's Stax Megatrons was paramesh; Paramesh contacted me directly regarding my Megatron build, and we basically had the same experience: amp dead within a week, zero support or accountability.  I have all the PM's.  Hence, the head-fi moderators deleted Thuan's account, kytuphicanh.  No worries, Thuan has several other accounts, (including thuytn, tranhieu, fls.audio on SBAF and tee8tee4388 on reddit, among many others, no doubt)

4. Thuan's Megatron contains no delayed-start or soft-start circuit of any kind, not even a thermistor for crying out loud.  The power filtering caps blew repeatedly.

5. For additional clarity, please see pictures attached of Thuan's caps upon arrival, and within a WEEK of use.  Notice any tops blown right off?  This happened 4-5 times before a thermistor was installed, and then the power transformer died...

6. The main power transformer was described as (pictures of his winding rig are on SBAF): "Custom-wound Kawasaki 0.23mm Z9 core transformers".  In reality, the mains power transformer is under-sized and is now dead.

7. Anyone on head-case who would like to further evaluate the quality of this fls.audio Felitsa Audio amp build is welcome to receive it at my expense via UPS Express, worldwide.  It is a large, single-chassis build, and it is quite dead, yet again.

Cheers, indeed!

 

After.jpg

Before.png

Inside.jpg

Outside.jpg

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Seeing that Felitsa audio is still alive and well on Ebay (fls.audio on eBay), below, you can find Paramesh's full PM, outlining his Stax "Support" experience with Thuan and fls.audio :

 

my name is paramesh .. i see that you have a megatron ..got from tuan
just wanted to ask you hows the amp doing ...?
i am asking you cuase i have the megatron from him as well and it has an issue and i am bit stuck ..and not able to reach him ..
any leads a can help

oh my goodness !! sounds very similar saga !!

thank you so much for replying sir !! greatful ..
i been feeling badly stuck and alone with the amp
after the guy turning his back on me after shelling out a lot of money ..
atleast your message gives me some hope ...
but i am glad atleast your amp is fine now !!

i got the megatron from him after i saw his feedback which was great from lot of people
till i got the amp ..he also send a lot of messages and images of testing the amp etc etc..

i got to know your megatron is the one before mine ..and i asked him whats the diff and he told me mines a SS rectifier..or something
and rest is same . same tube stages of 2 QUADS of EL34 and a pair of 12au7 and 12ax7 and all internals ..

i even got the amp picked up by a shipping company through air cargo so its handled well

it arrived ..i plug the amp .runs for some time ,,say a track or two ( i was might impressed )
i bump up the volume ,... say halfway stage .. tragedy strikes ..
minor flames erupt from under one the EL 34 ..i unplug the amp..
i check with him ...he says he's checked all and sent and suggest some issue in transit ..
i even opened amp on his request and sent him images ..no solution

he asks me to try the amp again .. it works but with half gain ..50% Of what it is ...
say from 9 am on volume knob .. now i am listening at 3 pm .. which is strange !! but all FR is ok..
something gone wrong in the Pre amp stage is what i thought ..

i been asked by him to check with a local repair chap !!! with a bad hand drawn schematic he sends me ..

i get a local electronic guy check all internals .. he shows me a busted Zener diode in the circuit of one of the channels . everytime we replace the diode and again it burns

then i carry the amp to another city to a respectable someone who worked on tube amps .. and he checked and said one of the EL34 has busted ...which is true,,,
so after replacing the EL34 .. the zene diode stops blowing ..but the gain is still an issue ..

the person checks all the stages of amp paths and says no faults ..
but the issue of HALF GAIN is not sorted out ...

I wrote hundreds of mails and messages only to be told ... i must find someone and debug the issue !!!!
( so did he send me a faulty amp ?!...fully aware of the issue ... i dont know )

its been over 3 months now .. and i have heard only 3 tracks ( sounded great ).. but now its lying aside
hes stopped comminucating with me and is not available on headfi .. hes been black listed ...by someone ..

do you see any probablities .. what could be the reason in my amp ..
for sure i did not follow any of the steps as you mentioned in point 1 of your write up .. swicthed on off in intervals ..
point 2 is what evidently same issue experienced by me ..

can you guide me to someone who can help me on this or even consult on chargeable basis ..to help fix this ...
i will be really greatful ! i am stuck

thank you so much

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Hello to all

I can briefly tell my experience with Thuan.

I bought last year a Megatron, but according to the photos i believe it is a different construction (don´t know if mine it´s an upgrade or it's a previous construction).

All aspects with Thuan have been very positive, concerning my Energiser (quality and reliability), all upgrades that i asked for, many doubts that i've been having since that time till present (not only regarding to the Megatron but also in general).

My Energiser is running 2 Quads of EL34 power Tubes, a pair of GZ34 rectifier tubes, pair of 12ax7 and pair ot 12au7 pre-amp tubes. The sound is excellent and kept improving during some months; now it is stable and working at full potencial (i believe so). The only issue i had was due to a electrical power peak that smoked the fuse - i bought a new one (now i opted for an audiophile grade fuse) and it's been behaving perfectly so far.

I don´t question any of the above statements or experiences, but in my case (fortunatelly) everything worked fine.

I know that he is an enthusiast and is making all its gear on the spare time, apart from its main job. That's why i also have been more tolerant (dispite the fact that this is an expensive gear). Nevertheless, all the support i needed was given, faster or slower, but i was answerd.

I just wanted to share my experience and hoping that experience was only a bad luck.

C7EEE598-6BDB-4275-9511-9F11F43D2FDCJPG.jpg

IMG_0046JPG.jpg

IMG_0077.jpg

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59 minutes ago, orientexpedite said:

You know, not many things upset me too much, which is why I suppose I've put up with this amp for 2 years; however, the IMMEDIATE and IDENTICAL reply I received from Thuan BOTH here on head-case (thuytn) AND on head-fi (tranhieu) to the above post does get under my skin:

I've just seen your post on HC. Could you please remove my name from that post? It could heavily affect my future sales. Hopefully you could understand my situation.

Cheers
Thuan

Wow, this fellow has guts.  Hence, I will provide some helpful, clarifying details:

1. Thuan's amp was dead the week it arrived.  It had eaten through it's first set of 6 under-sized 220uf 450V Nichicon power filtering caps, and blown the soldered fuses between the GZ34 rectifiers and the caps.  The inrush current was virtually uncontrolled on startup.

2. Thuan provided ZERO support, and was completely unresponsive.

3. Thuan was using a different account for amp/T2 board and transistor sales on head-fi (kytuphicanh), which was scrubbed by head-fi moderators, who added the warning: Caution: Incomplete Sales  Why?

Good question. The next fortunate head-fi member to buy one of Thuan's Stax Megatrons was paramesh; Paramesh contacted me directly regarding my Megatron build, and we basically had the same experience: amp dead within a week, zero support or accountability.  I have all the PM's.  Hence, the head-fi moderators deleted Thuan's account, kytuphicanh.  No worries, Thuan has several other accounts, (including thuytn, tranhieu, fls.audio on SBAF and tee8tee4388 on reddit, among many others, no doubt)

4. Thuan's Megatron contains no delayed-start or soft-start circuit of any kind, not even a thermistor for crying out loud.  The power filtering caps blew repeatedly.

5. For additional clarity, please see pictures attached of Thuan's caps upon arrival, and within a WEEK of use.  Notice any tops blown right off?  This happened 4-5 times before a thermistor was installed, and the power transformer died...

6. The main power transformer was described as (pictures of his winding rig are on SBAF): "Custom-wound Kawasaki 0.23mm Z9 core transformers".  In reality, the mains power transformer is under-sized and is now dead.

7. Anyone on head-case who would like to further evaluate the quality of this fls.audio Felitsa Audio amp build is welcome to receive it at my expense via UPS Express, worldwide.  It is a large, single-chassis build, and it is quite dead, yet again.

Cheers, indeed!

 

 

 

 

I truly feel sorry for you. I feel its totally unfair that someone should take the designs here and butcher them and then offer no actually useful support or help. At the very least they should supply proper electronic schematics for the power supplies and amp boards. That would help in debugging and finding out what is actually going wrong (probably multiple implementation issues).

I see nothing here that remotely resembles a megatron build from the gerbers released on these forums. The golden reference HV is a low noise will respected power supply use in many amps here. Why did the creator of this amp not use it? this amp looks like its had lots of changes for changes sake rather than for any good reason.

 

 

 

Edited by jamesmking
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I'm not sure what those things are but they are sure as shit not Megatron's.  Looks more like a SRX with some version of the Megatron output stage... This is why Kevin and I try to expose these fuckers as they are literally running away with peoples money. 

On a similar note, I have another amp from JR Audio here now and well... stay put for another horror story.  That thing lasted for a few days before the PSU blew up... a sloppy clone of one of my old PSU's BTW. 

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I agree: there are multiple implementation issues.  The power supply schematic likely existed on a napkin at one point, but Thuan doesn't have it.  In two years, I've never seen a copy, in spite of repeated requests.  Upsizing the caps to 550V 220uf slowed the issue down, but didn't eliminate it.  In two years, when the amp worked, it was excellent, which was usually for bouts of moderate length, until the mains transformer finally gave out.  However, any excellence could likely be attributed to the vintage tubes stuck in the amp by myself.

This Thuan fellow's immediate and selfish PM response today, after two years of providing no support for an amp he asked 5.5K USD plus shipping for, has raised my ire.  Curious and telling, too, is the nearly simultaneous appearance of an apparent shill, as above. 

I actually have no hard feelings, overall - this community has been fantastic to me, I've rolled more vintage tubes than I ever thought possible, and I've been the recipient of a great deal of kindness and outright generosity from tube collectors, fellow enthusiasts, and so on.  Everything from NOS pinched waist Philips E180CC's, to several quads of iconic gold pin USA 7308's and Holland 6922 from the early 60's, were practically gifted to me by a kind-hearted Canadian collector for my T2 build; this community has been stellar.  A fellow with two Shangri-La SR's sent me one pair from Korea for roughly half the Canadian cost of a new pair.  GeorgeP is a kind-hearted and generous builder, as you well know.  It's too bad that I wasn't aware of George and these threads a couple of years ago.

I'm sending a hi-res picture of the innards to Birgir, who has also been very helpful over the years.  If he or anyone else wants, I am happy to get Thuan's unique "Megatron" masterpiece to them. Thanks for being so kind as to reply.

Thanks for all of your help and suggestions.

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2 hours ago, orientexpedite said:

No worries, Thuan has several other accounts, (including thuytn, tranhieu, fls.audio on SBAF and tee8tee4388 on reddit, among many others, no doubt)

I've seen this person's builds on SBAF and they look spectacular. Extremely unfortunate that they seem to have some issues.

Oh, and I found your for sale post on Canuck Audio Mart, which has bigger/nicer pictures. I was tempted to put my hand up and give this a shot, but there is *no way* I could find the time and energy to work out that rat's nest of wires!

(But also, at the same time, so much thought and detail has been put into some aspects. I mean, the ventilation around the power tube bases is really neat, but then to undersize the power caps? SMH)

Edited by Beefy
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I’ve received several helpful replies

You are very welcome to have a look without cost or obligation, and bin it, if that’s all it’s really good for.  I don’t have high hopes for it, and George’s T2 is already near completion.  It would be good to shed light on what this builder is actually doing.

I get 80% off on UPS domestically, and it should be at your door next-day, within province.

UPDATE: A very reputable member, local to me, has kindly agreed to take a look, and shed some light on this build...  Thank you, all!

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5 hours ago, orientexpedite said:

UPDATE: A very reputable member, local to me, has kindly agreed to take a look, and shed some light on this build...  Thank you, all!

Great to hear, I was semi-tempted because I like fixing broken things, but I don't really use estats these days so I'd have trouble reliably testing.

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5 hours ago, orientexpedite said:

UPDATE: A very reputable member, local to me, has kindly agreed to take a look, and shed some light on this build...  Thank you, all!

Outstanding, pleased to hear it!

Please do report back with whatever issues/solutions are found, it may help out others who have similar amps.

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Well, a couple of nice outcomes have arisen from the ire induced by the self-serving PM of thuytn , so far:

First, upon reporting the head-fi PM (word-for-word identical to the one he simultaneously sent here), Head-fi moderators immediately flagged his current account https://www.head-fi.org/members/tranhieu.83205/ "Caution: Incomplete Sales".  The account hasn't been wiped like the first account (with the T2 board and transistors), yet, since it doesn't seem he's actually sold anything via the current head-fi account.  But there's always fls.audio on Ebay with a couple of auctions about to close, now... and I suspect there is already more than one unfortunate victim like Paramesh out there.

Second, Thuan's "Megatron" is ready to go out the door to a member here who is more than capable of providing a comprehensive assessment.

With gratitude, my thanks!

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