mwl168

interest probe - Megatron and GRHV PCB GB

50 posts in this topic

Posted (edited)

Someone, who will remain unnamed here, got me interested in building a Megatron:angry: 

I got quotes back from the fab houses. If we can get 10 sets we can get the cost per board down to just under $40 USD (plus shipping from me to the participant). The PCB is 228mm x 228mm and will be 2mm thick with 4oz copper.

I did some research and the only obsolete/hard-to-get part is the 4 2SA1968 which Kevin confirmed can be replace with KSA1156 mounted backwards. The board also uses 2 2SC1815 or MPSW06. I have a stack of the 2SC1815 so I am happy to supply them to the GB participants that need them (no cost). 

Please respond here if you are interested. 

Thanks!

Edited by mwl168

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Posted (edited)

Rendering of latest board:

 

 

megatron.jpg

Edited by mwl168

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I thought the ksa1156 only has breakdown of 400V? Do you mean the 2sa1486 or stn9360?

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1 hour ago, mwl168 said:

Someone, who will remain unnamed here, got me interested in building a Megatron:angry: 

 

He is a bad man!!!! :ph34r: 

 

I want 2 please, ;)

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15 minutes ago, mypasswordis said:

I thought the ksa1156 only has breakdown of 400V? Do you mean the 2sa1486 or stn9360?

I checked with Kevin and in this position it only runs at 300V so KSA1156 should work. STN9360 should work too but will need to be mounted vertically and observe the pinout difference.

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2 boards for me please!!

Enviado desde mi Z Pro mediante Tapatalk

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1 board for me please :D, 

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1 board for me too. The pcb buying/ group buys here are starting to feel more like subscriptions... :D

Whats in the "set"? 

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Posted (edited)

3 hours ago, gwrskien said:

1 board for me too. The pcb buying/ group buys here are starting to feel more like subscriptions... :D

Whats in the "set"? 

I started this interest probe with only the Megatron amp board in mind but I am open to include the PSU board as well. 

Kevin layout a "universal PSU" for Megatron longtime ago. I am not sure if that's the one people like to use.

For me personally I have been thinking if I can use the GRHV with some revision to power the Megatron. We need +/-450VDC, +300VDC (can be derived from the +450VDC supply with a drop resistor), -15VDC plus 3 sets of filament supplies - one set for the front end (12AU7 and 12AX7) which Birgir recommended using DC supply, one set for the 4 output EL34 and another set for the CCS EL34 which needs to be very close to the tubes.

What are people's thoughts?   

Edited by mwl168

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I was hoping there was a dedicated psu board for this amp that had all the different voltages on board.. :) but after reading a little in the Megatron tread, i think it is the setup you describes that people use.

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The positive rail GRHV already has the little +-15V supply built in so you can just use that, and similarly a little 7812 or similar regulator with heatsink could be used for the DC heaters. IIRC the universal PSU is basically just a KGSSHV PS with an extra C- and 7815/7915 PS so it would be better to go with the GRHV

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i used ac filaments everywhere without any hum issues, might help cut down on what you have to route

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Posted (edited)

no on the 3 terminal regulators.

each output tube is 1.5 amps so that is 4 x 1.5 amp windings and 1 x 6 amp winding

all the input tubes are in parallel, so 12V at 600ma

 

Edited by kevin gilmore

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Looking at the latest Megatron Gerber file I downloaded and used in my 2nd post here, the front end 12AU7/12AX7 also uses 6.3V filament. I do remember reading on the Megatron thread that 12.6V filament can be used but will involve modification of the board.

3 hours ago, kevin gilmore said:

no on the 3 terminal regulators.

each output tube is 1.5 amps so that is 4 x 1.5 amp windings and 1 x 6 amp winding

all the input tubes are in parallel, so 12V at 600ma

 

Hi Kevin: Can you confirm if my observations below (from the Gerber) are correct?

1. The CCS EL34 tubes are the four EL34 on the outside edge of the board, each of these EL34 needs its separate filament winding and the filament supply wires need to be very short so the transformers pretty much need to sit right next to or below the board.

2. The output EL34 tubes are the ones in the center, two for each channel and can share one filament winding for each channel.

3. All the EL34 filament supplies need to be floated.   

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1 ) yes

2) yes

3) the 4 tubes in the middle can either float or tie to -vcc

yes the front end tubes are 6.3v, so 1.2 amps

 

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Thanks Kevin!

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the conversion to 12V filament for the small tubes is easy and I should be able to get that done tomorrow, too many other things got in the way today.

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Posted (edited)

12v filament version posted

pretty sure the original was done the way it was to accomodate the 6dj8 tubes for those that wanted to do that

edit: the filaments of the output tubes are tied to -450v on the board

Edited by kevin gilmore
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Awesome, this version would be the way to go :D

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Posted (edited)

Yes. We will be using the updated board for the GB.

Updated board image courtesy of congo5. 

Megatron12v.jpg

Edited by mwl168

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1 board needed for balanced stereo right?  The set is just the on board with no psu?  Perhaps I should build this instead of the BH

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Yes, one amp board for both channels for Megatron. 

The PS is a bit of a dilemma. The old Megatron PS is based on the KGSSHV PS, quite big and not as accurate and low noise as the GRHV. 

I have been exchanging PMs with a few knowledgeable headcasers in the last few days. With Kevin's recent modification to run 12.6VDC for front end filament supply (Thanks Kevin!) this is what I plan to do personally:

I will run my Megatron with a GRHV PS. I will use 7812 and 7912 regulators on the LV section on the GRHV board. The 7912 will provide the -12VDC for the front end current sink and the 7812 will provide 12VDC for the front end filament supply. I will mount the 7812 (which need to drop about 3VDC and pass 600mA) under the PCB and bolted to the chassis below to provide proper heatsinking.  

I will use a drop resistor to get the 300VDC from the 450VDC supply.

I can include the GRHV PS board in the GB if people are interested. 

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14 hours ago, mwl168 said:

I will run my Megatron with a GRHV PS. I will use 7812 and 7912 regulators on the LV section on the GRHV board. The 7912 will provide the -12VDC for the front end current sink and the 7812 will provide 12VDC for the front end filament supply. I will mount the 7812 (which need to drop about 3VDC and pass 600mA) under the PCB and bolted to the chassis below to provide proper heatsinking.  

I will use a drop resistor to get the 300VDC from the 450VDC supply.

 

I think that it´s a good idea. simple and easy to do.

I need more GRHV boards if it´s possible.

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Are you thinking a two-box build? The GRHV will need external heatsinks for the 10M90S and the SiC MOSFET parts, which complicates a single-chassis layout, given the size of the Megatron board, or calls for a huge box.

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