Wow, beautiful ! 

19 hours ago, mwl168 said:

Matched quad of EH 6CA7 for the output CCS and matched quad of SED EL34 for the output gain tubes. I don’t believe the CCS tubes need to be matched to the gain tubes. Maybe Kevin, Birgir or someone can correct me if I am wrong.

Beautiful work Michael!

 

12 VDC is perfectly acceptable for offset (remember Stax in their tube amps specifies +/-15V), but if you look at the schematic, the resistors that set the quiescent point are identical between bottom (output) and top (CCS) tube, so I believe the offset depends on the match between the bottom and top tube. Matched octets! Actually, probably 2 sets of matched quads would be fine.

Thanks Jim! That is very helpful! So I really need matched octets or at least two matched quad, one for each channel to get the lowest offset.

I remember your past comments on filament voltage. What are your thoughts on my CCS tube filament supplies being just slightly over 5.7v? I have been wondering if I should look at options to bring them up.

Switched to matched quads of Svetlana EL34 for the CCS tubes and SED EL34 for the gain tubes and output balance and offset for both channels are all under 3VDC now. 

Filament voltage of 5.7v should be fine. David Berning used 5.7VAC filaments in his EA-230 power amp, which was rated as Stereophile Class B back in the early 1980s. I used it to drive my Quad ESLs for many years.

Thanks Jim!

is good to see more megatrons being born!

I made some modifications to my Megatron. Most of them are under the hood.

• changed the CCS tubes filament supply transformers from 25VA Antek AN 0206 to 50VA AS 0506 (one for each channel). Now the under-load filament voltage is at 6.15v.
• changed the transformer supplying the front end filament and current sink to 50VA Antek 0512 to accommodate the higher filament supply when using E80CC in place of the 12AU7 and drill some holes on the PSU chassis to help with cooling.
• added an LED indicator on the amp chassis which comes on when HV supply is on.

 

Edited April 29, 20188 yr by mwl168

mwl168, you are really making me want to build one of these.  Darn you!  Well done all around and thanks for sharing with us.    

mwl168, you are really making me want to build one of these.  Darn you!  Well done all around and thanks for sharing with us.    

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nice case, where did you get it from?

I put the 4 LED on the amplifier boards upside down so they stick up a little outside the top of the chassiss so when the light up I know the HV is on.

24 minutes ago, eggil said:

nice case, where did you get it from?

I put the 4 LED on the amplifier boards upside down so they stick up a little outside the top of the chassiss so when the light up I know the HV is on.

I did the same thing with the LEDs but felt something looked amiss on the amp chassis when it's powered on. I happen to have an unused pin in the power connector so I wired the DC supply for the CPC1117N delay to it to power the added LED. Easy peasy!  

The chassis is 3809H, I ordered it at AliExpress. Here is a link to one of a few that are on AliExpress.

https://www.aliexpress.com/item/The-new-small-fillet-decode-Amp-amplifier-aluminum-chassis-1Piece/32620866436.html?spm=2114.search0204.3.223.5dfb374eM3n5KD&ws_ab_test=searchweb0_0,searchweb201602_5_10152_10065_10151_10344_10068_10130_5722815_10324_10342_10547_10325_10343_10546_10340_5722915_10548_10341_10545_5722615_10697_10696_10084_10083_10618_10307_5722715_10059_10534_308_100031_10103_441_10624_10623_10622_5722515_10621_10620,searchweb201603_25,ppcSwitch_7_ppcChannel&algo_expid=e682a1ea-5ec6-4d8c-a5f1-2c4ed26fb865-35&algo_pvid=e682a1ea-5ec6-4d8c-a5f1-2c4ed26fb865&priceBeautifyAB

Edited April 28, 20188 yr by mwl168

where is the volume knob?

1 hour ago, eggil said:

where is the volume knob?

On my DAC 

Edited April 29, 20188 yr by mwl168

@mwl168 where are the heatsinks of the grhv boards?  Are they inside the case or does the case do enough cooling...

Beautiful build!

The GRHV is fed 350v AC supplies and regulated to 430VDC rails.The GRHV transistors are bolted to the bottom panel of the chassis and that’s enough for heatsinking. 

How's it sound compared to the Carbon/GG?

You can check this thread at the other place 

If that actually tells us anything, the hybrid / all-tube DIY electrostatic amps seem to change hands a lot less (and quicker) than the solid-state ones.

My megatron is already working.

A couple of details:

1. HV 456Vdc and -452Vdc instead of 300Vdc I have 267Vdc, should I change the 21K resistor (I prefer something in parallel to it)  or can I leave it running with 267Vdc?

2. Offset is between 40 and 29Vdc although the balance is about 5 and 7Vdc. With this offset I have not dared to listen to it. Should I change the output tubes? 

3.Amp runs hot, very hot, very very hot ... no pets friend 

 

you can listen to it. But better to pick pairs of tubes that result in better balance. or trim the cathode resistors, but really better to pick matched tubes.

yes it gets pretty warm. touch a 120 watt fender head sometimes.

As Kevin said the upper tubes (CCS) need to match the lower tubes (output) on each channel to get the lowest DC offset. I was able to get mine to within  3 -4 VDC using better matched tubes.

I paralleled the 21K with another resistor to bring it down to 15K and get the front end B+ to about 290VDC. In my case, the front end tubes draw about 8.5mA total.  

And where are the pics?

 

Thanks !

I'm using other quartets and I've found two that work relatively well (8 EH 6AC7 "Platinum Matched Quad"). Offset is located at -8 and 14Vdc and low to -3 and 8Vdc. Now I feel more code to listen to the amp. I'm listening to it right now, It's totally silent and I like its sound although the sound is a bit "bass" for my taste. In the first and second stages I am using  6189 Sylvania JAN and ECC83 Svetlana W"C" . I'll leave a little time before going through those E80CC... what the hell? I'm going to buy them now ?

About 21K resistor I think that you used a 50K in parallel to 21K . I think that it´s a good idea use parallel resistor here. This resistor is a 3w and runs to around 45º. 

Edit update: 47K parallel to 21K, now I have 302Vdc. 

Some pics ☺️My amplifier is not finished yet.

 

 

I have not been able to order the wiring, finish the LEDs, etc...

 

 

 

 

Megatron and my "Universal HV PSU"

 

 

P.S. yes, I know, the appearance of all my amplifiers are the same. ?

P.S 2º this amp runs HOT like hell

 

 

Edited June 6, 20188 yr by jose
Update

I think that now it´s finished but I want to add a few resistor on the heaters filaments because I have 6.8vAC. Sometimes this happens to me with the Toroidy transformers.

Front end tubes draw about 11,5mA.

I was surprised that this amplifier has very little gain. On the other hand it makes up for it with a very pleasant and musical sound. I´m waiting a pair of E80CC for replacement the 6189.

 

Updates: 

. With the E80CC has more gain. I think it is an improvement over the 6189. I like it much more with the E80CC.

. JJ octett EL34 matched, off set is around 1~2volt. Balance 1~2volt too. It really deserves to invest in a completely matched octet (It's expensive but it makes up)

 

 

Did I say that it runs very hot? 

 

Edited June 29, 20188 yr by jose

What’s your opinion on the Megatron sound signature vs the GG?