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kevin gilmore

Megatron Electrostatic Headphone Amplifier

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I must have measured things wrongly late last night. Balance is around 3v but I'm getting -120v offset. Any ideas Spritzer?

I think it may be my 1968s (the ones I'm not certain about). The led besides it turns on and off while the tubes warm up. Will swap those out and see.

Edited by RiStaR

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Thanks for inputs Spritzer... Changed the 1968s and I'm still at -100v for offset. Will continue tomorrow ;p

Edit: ok just measured the voltage from the plus line... It has dropped to 150v... Probably the reason why. I'm using a 22k resistor to drop the 450v line to 300v... Will check again to see what's causing the problems... Looks to be my PS... May need to remove my 2222A and have no current limiter?

Edited by RiStaR

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That would certainly account for the offset.  Bypassing the current limiter might work but it could also end badly... 

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Took out the 2222a... Offset is down to 5v ;)

Stupidly... Right channel balance keeps climbing up... It's at 8 now...

I'll do some research on an even higher substitute than the 2222a (I did put it the right way around)... Dang.. I thought 3x the capacity of the 3904s would be enough

Edited by RiStaR

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If the power supply voltages are right and the offsets and balances are in the 10 volt

Range you are done.

Go and listen to the music. Leave the 15 ohm resistors alone for now, let me know

The voltage across them.

Leave the current limiters off for now. Will have modification soon.

Too many projects reaching completion at the same time

Edited by kevin gilmore

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Thanks guys for all the help! Especially Kevin for the recent Squarewave and now this ;)

For the record I did listen to music ;) it seems smoother compared to my kgsshv. It also seems a tad bigger as i get a sense more reverb in some of the tracks, too.

I'm also using 10ohm resistors instead of 15. Instead of 1.5k on the amp boards I'm using 1k. Thats possibly why the current pull is so high. The 2.5inch heatsinks on the kgsshv ps are between a very toasty 61-66c. My 400VA transformer hits up to 40c, while the EL34s are at 99c.

It's stabilized quite well. Only thing is on power-on the offset momentarily goes to almost 450v before settling at those voltages. Want to see if I can do something about that because I want to leave a headphone plugged in.

Edit: sorry missed your request Kevin. Voltage across my 10R resistor is around 0.77v and still rising - just really, really slowly.

 

Spent 4 hours listening to it :) Now to fit the amp inside the case:

 

r6Tbjxl.jpg

Edited by RiStaR

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Great, well done.

 

Like your case – is it your own design? could it be bought somewhere?

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needs t2 style power on delay for the high voltage. Or separate switch, one for filaments, one for high voltage.

 

150ma on the power supplies.  about right with 1k resistors.

 

room heater for sure. 135 watts of high voltage plus about 85 watts of tube filament power.

 

no front end tube distortion, no limited output frequency response due to limited size center tapped inductor core etc...

plenty of gain, low distortion.

 

yeah baby.

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How much time delay would be recommended? Would 10 seconds be enough?

 

Just to clarify, we change the 2k in the PSU to 1k?

 

Do we still leave out the 2N3904?

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60 seconds to let the tubes fully warm up same as the T2. you really want to do this at these current and voltage levels

especially with expensive tubes.

 

Ristar used 1k cathode  resistors on the amp board instead of 1.5k resistors.  The 10 big ones.

 

Ristar changed the 15 ohm resistors on the power supply to 10 ohm an he left off the 2n3904.

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Joamat, I bought it. I'll pm you the link tomorrow ;) that said I underestimated the size of my transformer... Going to try to mod the case by adding maybe some wood trim (?) to the base.

Cheers Kerry!

Got it Kevin, I'll look into getting a time delay.. Right now I'm just using a soft-start circuit. I'm not sure why but the DC offset of my right channel goes up to 450v momentarily but the left is fine... Maybe the time-delay will help there?

I'm going to miss it (took it all apart) while I try to get everything housed - weekend just isn't long enough ;p

Time for my rest... Been a long fruitful day - thanks to all for the support when I ran into issues! I'm also almost caving in and getting the rk27.... Having a pain in the butt moment trying to mount my own rotary attenuator. ><

Edited by RiStaR

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If you don't mind I'd like the link for the case as well. That's just too gorgeous.

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If you don't mind I'd like the link for the case as well. That's just too gorgeous.

 

I believe that's a "3607T" case. They've been popping up on the various Chinese exporters lately (eBay has some, aliexpress, etc.).

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I believe that's a "3607T" case. They've been popping up on the various Chinese exporters lately (eBay has some, aliexpress, etc.).

Yep, this is it. I bought mine from aliexpress with the option to customize - you just don't get the accessories then. (http://www.aliexpress.com/item/Aluminum-Case-of-HIFI-Tube-Amplifier-Sandblasting-Silver-Process-HIFI-DIY-Cutting-Holes-on-Upper-Cover/1302491615.html)

Kevin I only have eight 1k cathode resistors. I hope I didn't miss any? Also, thanks for the schematic!

Edited by RiStaR

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Thanks Kevin; just making sure cause I saw you mentioning 10 big ones earlier in the thread.

Spritzer: I've measured only in Celsius... All in this post () this is all running outside a chassis and in an air con room set at 25c. It's definitely a heater ;p

Sorry for the late responses.. Busy with chassis work at the moment. Almost finished the one for the square wave. Will complete that by next weekend. I'm trying to find some way to increase the height of my megatron amp chassis to fit my filament transformers under it... So will take longer with this - same with the time delay circuit.

For others who are still building, here is a list of things I used that are different from the BOM. Hopefully it helps some folks?

1k cathode resistors instead of 1.5k

10R instead of 15R for PSU

I tried 2222As instead of 3904s but I still run into problems here so they are removed (waiting for Kevin's proposed workaround while I get everything else in order)

1.7v LEDs instead of the 2.1v (may have remembered this value wrong, but it wasn't the 1.7v ones used in the KGSSHV)

330uF instead of 220uF capacitors - I wanted better caps but the shop only had this value closest to it. From what I read it shouldn't make a big difference; maybe just a tad more bass?

Edited by RiStaR

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60 seconds to let the tubes fully warm up same as the T2. you really want to do this at these current and voltage levels

especially with expensive tubes.

 

Ristar used 1k cathode  resistors on the amp board instead of 1.5k resistors.  The 10 big ones.

 

Ristar changed the 15 ohm resistors on the power supply to 10 ohm an he left off the 2n3904.

 

We should still be fine with the default values populated on the amplifier and supply right? My wallet is pleading with me to not place any more orders from Mouser, fucking taxes and shipping

 

I hope ristar's can post some guts pictures once he's done placing components in the power supply chassis.

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