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kevin gilmore

Megatron Electrostatic Headphone Amplifier

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Nothing on my end.  The board is fully populated save for the tube sockets which should be here soon.  I'll probably finish the BATE before I ever get to the Megatron though...

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With the group buy of boards going on, I've been putting together a BOM for the Megatron.  I will post a link once it's more complete.  I have a few questions.

 

  • What tolerance and wattage is recommended for the resistors?
  • Are 680K Ohm or 690K Ohm resistors used?  I'm looking at the jpg of the amp board and that number is hard to read for me.  I think it's 680K after looking at Spritzer's populated board but I wanted to make sure.
  • There is one capacitor (in the corner) labeled as .1uF / 4630V.  I presume this is a .1uF / 630V cap?
  • There are two caps labeled as .47 / 50V.  Is this .47F or .47uF or something else?

Thanks!  :)

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Mouser

Resistor/Qty    

220R/8   71-CMF60221R00FHEK

500R/2   71-CMF60499R00FHEK

2K2/4   71-CMF602K2100FHEK

3K3/4   71-CMF603K3000FHBF

5K1/4   71-CMF605K1100FHEK

10K/6   71-CMF6010K000FHEK

100K/4   71-CMF60100K00FHEK

150K/4   71-CMF60150K00FHEK

250K/4   71-CMF60249K00FKEB70

330K/4   71-CMF60332K00BEBF

470K/4   71-CMF60470K00FHEK

680K/4   273-680K-RC

1M/4   71-CMF601M0000FHEK        

1K5/8   71-CPF3-B-1.5K

21K/1                    

 

Capacitor      

0.1uF/630V/9   505-MKP4-0.1/630/5

0.22uF/630V/4   505-M10.22/630/5

0.22uF/1000V/4   505-M10.22/1KV/10

220uF/100V/4   667-EEU-FC2A221

0.47uF/50V/2   80-C330C474J5R                

 

Transistor      

2SA1968/4    

2SC1815/2                    

 

Zener      

12V/4   78-BZX85B12-TAP

100V/3   78-BZX85B100-TAP

LED/4   859-LTL-4213                

 

Tube      

12AU7/2    

12AX7/2   

6CA7/8    

 

I've done that before, will anyone check?

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Mouser

Resistor/Qty    

220R/8   71-CMF60221R00FHEK

500R/2   71-CMF60499R00FHEK

2K2/4   71-CMF602K2100FHEK

3K3/4   71-CMF603K3000FHBF

5K1/4   71-CMF605K1100FHEK

10K/6   71-CMF6010K000FHEK

100K/4   71-CMF60100K00FHEK

150K/4   71-CMF60150K00FHEK

250K/4   71-CMF60249K00FKEB70

330K/4   71-CMF60332K00BEBF

470K/4   71-CMF60470K00FHEK

680K/4   273-680K-RC

1M/4   71-CMF601M0000FHEK        

1K5/8   71-CPF3-B-1.5K

21K/1                    

 

Capacitor      

0.1uF/630V/9   505-MKP4-0.1/630/5

0.22uF/630V/4   505-M10.22/630/5

0.22uF/1000V/4   505-M10.22/1KV/10

220uF/100V/4   667-EEU-FC2A221

0.47uF/50V/2   80-C330C474J5R                

 

Transistor      

2SA1968/4    

2SC1815/2                    

 

Zener      

12V/4   78-BZX85B12-TAP

100V/3   78-BZX85B100-TAP

LED/4   859-LTL-4213                

 

Tube      

12AU7/2    

12AX7/2   

6CA7/8    

 

I've done that before, will anyone check?

 

That list is what I have so far.  I have the .1uF caps separated into 4x 450V and 5x 630V but all 630V caps will work also.  Tube sockets, a pot, and terminal blocks are all that remain.

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That looks ok and the tube sockets are the CMC teflon units (or the ceramic ones would probably work) and the terminal blocks are just the standard 0.2" pitch. 

 

Here is how far mine has gone.

 

EUPCcJCh.jpg

 

Almost took off my hand with that bloody drillpress so this is on hold while I make a few KGSSHV's. 

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That list is what I have so far.  I have the .1uF caps separated into 4x 450V and 5x 630V but all 630V caps will work also.  Tube sockets, a pot, and terminal blocks are all that remain.

 

Thanks for working on the BOM chiguy and palchiu... but if i'm not wrong the 5.1k resistors should be rated for 1W?

Edited by RiStaR

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They need to be high voltage rated, power isn't a problem.  1/2W is fine as most are rated at 500V. 

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They need to be high voltage rated, power isn't a problem.  1/2W is fine as most are rated at 500V. 

 

Thanks for clarification :)

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is there a blessing-endowed list of transformers to grab for this amplifier? i need to make another order with sumr and this seems like a worthwhile addition to the order if a kgsshv type transformer can work in this setting

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Using the BH transformer requirements would be a good idea with this one.  I will be using one of my spare BH transformers so only +/-400V but for 450V use a ca. 400VAC transformer. 

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thanks spritzer, I'll go ahead and use the specs for the 450v kgsshv transformer then. i was also glancing at the pcb layout and i noticed that a 21k resistor now taps off the positive hv rail- is there any advantage now to using a separate supply for +300v?

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It is better but you can also just use the dropping resistor.  I'm going that route but this is team overkill anyway so an extra PSU would be welcomed. 

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It has been mentioned previously that the megatron draws more current than the KGSShv psu can supply. The concern perhaps is that if we tap it for the 300v, the draw on the current shortage will be further added.  

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The front end draws very little power so that isn't a problem.  It's also easy to increase the current limiting of the KGSSHV supply. 

 

Now people are going to do the same on the KGSS... again. Hennyo?

 

Now that would make Kevin truly furious...  :)  Did you ever see the inside pictures of that amp?  So he didn't want to spring for a +/-15V supply but the amp was filled with Z-foil resistors...  :palm:

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Can anyone comment on the feasibility of buying NOS EL34's as substitutes? I know it's an old topic so I've been digging through the internet... all I can find is "they're interchangeable but one is a pentode, the other is a beam tetrode", European EL34's are America 6CA7. It's all shaping up to be a serious mindfuck

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EL34 = 6CA7.  The former was developed by Mullard/Philips in Europe and the latter is just different enough to not fall under the former patent and thus avoiding the royalties manufacturers would have to pay.  It was made only in the US by Sylvania/GE though Sylvania would end up being Philips in the end...  Electrically they are almost identical and can be exchanged at will. 

 

As for NOS, some are better... some are not. 

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