MK7 - tube mic project

[okgb]

is there a pic of your finished mic ?
i'd love to see how you mount the switch

 

[JW]

Arggggg.

I think I was literally sleeping when the Black Market post went up. No chance. Damn.

Anyway, Max, will the new design transformers be cheaper maybe since they aren't the vintage laminations?
Keep us posted as to how those are coming along.

 

[bobschwenkler]

Wow, that was fast. Not that that's surprising...

 

[e.oelberg]

I was at the kindergarden picking my kids up 

 

[ioaudio]

just to clarify: there will be a second, identical batch, with the same laminations used for the transformer - maybe two weeks or ten days to go.

some pics can be found on page 5&6 of this thread.
okgb, which switch? the pattern selector in the psu?
i will add more information/pics after shipping etc.

-max

 

[e.oelberg]

it's fine,
great that there was so much reaction !


nicholas

 

[okgb]

yeah that answers it  , pattern switch in the power supply
didn't think that far !

 

[ioaudio]

i updated the first post with some instructions.

 

[pH]

Nice pictoral Max.

I'm wondering about the differences between using a PF86 and an EF86 in your design. The former is quite scarce here in the US but it looks pretty affordable on ebay.de. Do you prefer the sound of one over the other?

 

[ioaudio]

no, i had good sounding pf´s and good sounding ef´s.
if you already have ef86 use them. if you have to buy the tubes, it will be cheaper to go with pf86 - you can buy ~5 for the price of 1 ef86 and find a suitable, low noise tube within the batch.

 

[seavote]

did i misunderstand the first entry in the thread. the pf86 heater is run at 6.3Vs? ???


just read it again.  can you tell us more about why this is so in this circuit?

 

[gevermil]

The reality of actually getting one these seems slim , dose AMI have an equivalent transformer for the available pcbs  ?
Thanks ,
Gary

 

[plumsolly]

2 x 100 Megaohm 0,25W  [247-7806]
1 x 68 Megaohm 0,25W  [247-7834]

Hey Max and everyone - these two rs #s are backward - should be:
2 x 100 Megaohm 0,25W  [247-7834]
1 x 68 Megaohm 0,25W  [247-7806]

also im going to try to put together a parts list with North American sources
cheers, Ben

 

[ioaudio]

thanks for the correction, ben - a usa parts list for the uncommon things would be great.

gary, i´m already working on a successor transformer.

seavote, the heater rail is used for the heater AND for biasing. for example the pf86: the 6,3v go enter the mic, 0,7V drop at the 2,2R resistor (additional filter together with the 0,22F caps) = 5,6V.  than the tube heater drops about 4,3V = 1,3V before the 3,9R* resisitor.
the cathode is connected at the heater at this point, so the cathode is lifted to 1,3V while the grid resistor is grounded to 0V.
it´s called fixed or "stiffened" bias (stiffened - there is still a resistor in the cathode, but its very small)
G7 and many other circuits work self-biasing, using a resistor between the cathode and ground (would be about 2,2k in this circuit) and with the heaters seperated from the audio.

-max

 

[plumsolly]

Hi - I'm putting together the parts list and had a couple of questions before I posted it:

For the 68 Meg resistor I was only able to find 4w metal oxide (I only checked Mouser and Digikey) will that be ok? Gregory(drip) says in his la2a manual to "Avoid metal oxide resistors for anything but the power distribution
resistors , (the resistors that ‘feed’ off of the main power rail)."

for the tubular film caps I was able to find polyester film caps for all the values, but I was also able to find polypropylene for some values. I saw that the rs number you(Max) gave is for polyester. any thoughts?

Also, I was thinking about the mxl 960 as the donor mic: http://www.mxlmics.com/condenser_mic/900_Series/MXL960/mxl960.htm
I really like the way it looks, it comes with Mogami cable!, it's readily available in North America, and it costs only $200 usd.
I haven't been able to find a schematic, I think it might have a 200vdc b+. I was going to call the company and try to get a power supply schematic. we'll see. what was the b+ of the t.bone SCT-700 before the modifications?

I think that's it for now. I'll post a complete parts list in the next couple of days.

thanks, Ben

 

 

[imo]

I found the 68m metal films at Mouser. Here is the part number. They seem to be well in stock:

660-GS1/4LC686J
Ian

 

[plumsolly]

I found the 68m metal films at Mouser. Here is the part number. They seem to be well in stock:

660-GS1/4LC686J
Ian

awsome. i dont know why i couldn't find em
-Ben

 

[bradb]

thanks for all the info everyone is giving

 

[ioaudio]

excellent.
the 1µF is a filter cap, polyester is fine here. the 0,33 - 068µF is the coupling cap, here you can take a polyprop (or paper/oil or...)
dont forget that size matters - and check footprints. i think it´s better to wait for the pcbs to arrive before ordering the parts.

i had some very productive days so expect the next batch on the black market within the next days.

-max

 

[okgb]

If anyone knows for sure
can you reference other mic brands that are the same size ?
Particularly here in North America

i need to find a donor as well , tia