ST12 - 6AK5 - SELA T12 rip-off - SIMPLEST TUBE MICROPHONE IN THE WORLD, FULL PRO

[ricardo]

I think the theoretical pros & cons are quite clear.

quite clear but not for you
you can make it work with virtually ALL tubes as long as its inter-electrodes specs allow operation
you  will have more chances with higher inter-electrodes capacitance , leakage, etc  ...

Now that us theoretical nerds have trashed it all out,  8) lets hear from those who have tried it in practice.

Which tube?
Which circuit?
What did you find?

“In theory, theory and practice are the same. In practice, they are not.”

 

[granger.frederic]

if you have time to waste , i haven't... sadly...

i've said everything that i have to say about this schematic : it works but i don't like it (instability, dummy variables..)

i'm an electronic tech, that's my job, my father was also ...i've learned the basements from the tubes era to the solid state devices  era

each time i want to create ,repair, or built an electronic device , i'm used to theoretically think about it before , mostly because my wife asked me to not fire our house 

by the way, each time i do something, i try to think about it before ...not always with success but that's another story...

why would i deeply test a schematic that i don't like ?

i'm  not masochist , are you ?

however i don't see why any  preamp triode or sharp cutoff pentode wouldn't work in this case.
if it could work very  fine  ? is it reliable ?
i doubt but that's just an opinion

 

[lampas]

Build it, and at this time I don't like it at all.... I use GE 5654W, mechanical parts and capsule from Recording Tools MC200 and
Haufe T14/1 for output. I think that T14/1 have too high ratio for that circut as the gain is too low for my taste ... I'll try with
CM 2480 and BV8 .... Other than that the tubes I did try were all very noisy ... I did order bunch of NOS ones and I will see...

Photos will follow ...

 

[ln76d]

For some time i had to stop all my projects, so i didn't test 10:1 but definately i will back to it.
If you will have troubles with the tubes i have some stock of great military 5654W french made. All NOS. I tested few in two circuits and all were great. With 6:1 it sounds fine.  CM 2480  have similar ratio to T14/1 but BV8 should give you bigger difference.

 

[lampas]

Photos will follow ...

 

[lampas]

Another

 

[lampas]

If you will have troubles with the tubes i have some stock of great military 5654W french made.

Hi,

I did use GE 5654 and they are all really noisy ... I did try with Chinese BV 08 copy, but It appear that it is fake. Ratio is higher than
6.5:1 ... But real BV8 is too big for MC200 housing ... Yes I am interested in your 5654, especially as you test them in this circuit and I didn't gave up on this schematic ... Please send me your email in my private box.

 

[ln76d]

Sent you my email on PM. Write me email i will give you some more details

Nice build!

If you will had some problems with that circuit you can always try to make ptp my S16 circuit. It can be used without any problem with 6ak5/5654 tube and 10:1 transformer (probably also even higher), but need higher B+ voltage. Otherwise i would also recommend to make some mods on ST12.
Important thing! PSU for ST12 need to be very well filtered.

 

[alexc]

I did my diy mic with the with ge 5654-as-triode and  cinemag 'nico' 6.45:1 (I think!)

in a cheap 47 style donor mic and china psu 

Was mightily impressed with the sound .. even with a cheap capsule and some very hasty point-point wiring.  Using some good caps and gohm resistors. And duck tape!

Been in my room ever since

NEXT round of mod is a beezneez capsule.

ps  - I liked the  nico+5654  combo so much, I use it in a couple of d.i. boxes for guitars and bass too. Super minimal and very satisfying.

It's a mighty tube with probably the lowest microphonics I've come across in an  high-ish gain app.

 

[ln76d]

Could you put pics here?

Did you tried it without 1G resistors?

Fully agree about the tube -  it's really great
I have in plans design two diferent microphones circuits with it and probably i will build  third - freely based  on u47 circuit.
5840 according to datasheet is miniaturized version of 6AK5, but anyway when compared  5654 with 5840 in the same circuit, 5654 sounds much better.
I also tested it in my S16 microphone, with 10:1 ratio transformer and it was sounding amazing.
http://groupdiy.com/index.php?topic=62423.msg791063#msg791063

 

[alexc]

Here's the outside pic ....  an apex 480 ldc fet was the donor.

It is a long body, fat type, with shock mount on a mounting ring at the lower binder. It has some switches as well on the front.

psu + cable were  bog standard with fixed polatising voltage - I rejigged it for my needs. changed the mic body binder to accomodate.

That's the good pic 

 

[alexc]

Inside, is the nico traffo at the bottom, crappy perspex mounting ring at top.

has a big *** 3.2 uF decent quality coupling cap and a ge 5654.

I did use a 1gigohm resistor for capsule bias of +63.5V and another 1gigohm for tube grid leak bias.

A very simple triode circuit with 15K plate load resistor on +125V supply, +1.2V bias on 300R for 4mA thru the cathode. (I like higher current biasing on my mic amps)

I estimated the rp at 5K3//15K  -> 3K8 and the nico is 6.45:1 stepdown for approx 25K reflected Zload.

When I tested the circuit sans capsule, the BW was ridiculously wide at both ends, noise was super low  .....  mostly due to the low gain of the circuit, which is possible because of the low step down ratio of the nico traffo.

My tube supply was bog standard zener string with a couple stage pi filter after that. Capsule supply was similar. Heater supply was a regulator chip.

Here's the inside 

 

[gyraf]

So what is the consensus - is grid leak bias an acceptable way to headamp a condenser?

Jakob E.

 

[ln76d]

Probably yes

First tests when i started this topic didn't showed anything wrong with the circuit.
Another thing is  that popular years ago "royer mod" circuit also is made same way.
Both tubes are also similar since 5840 is miniturised 6AK5

 

[ricardo]

So what is the consensus - is grid leak bias an acceptable way to headamp a condenser?

There are theoretical caveats but some makers and some DIYs have had good results.

When there is a discrepancy between theory & practice, the sensible thing to do is to gather more info. ie who has had success, with what etc and who has had bad results, with what etc.

Then theory can be refined to reflect 'real life'.

Alas, some people who have had success are refusing to give details cos their 'real life' experience doesn't support their theories.  ???

 

[micaddict]

Interesting thread this.

Both in the narrow and the broader sense.

 

[ln76d]

Ok, another one out from the list of unfinished projects.
I had problems to find all the stuff needed to build PSU for it (still have many things packed in many boxes ).
For PSU i used parts which i had, maybe it's not so best design, but works great, very silent.
Both sections are regulated, Heater with 7805 and B+ with 62V Zener (35.5V direct on plate).
In the final build i used NT1a edge terminated capsule and 6.3:1 Funkwerk Leipzig transformer.
I also had to remove internal mesh from headbasket - it's definately better.

Few words about high impedance resistors  from grid to ground
I tested 100M, 1G and direct capsule to grid connection in A/B comparison.
Between direct capsule connection and 100M there was probably difference in low end, probably because if it even was it was very subtle. To be honest, i'm not sure it wasn't only illusion.
Between 1G and direct connection there wasn't difference at all.
Microphone is very quiet, no problem with noise, distortion, hum or hiss.

Output transformer - i'm too lazy to test other transformer in this mike
There's probabllity that output impedance is too high, because with NT1a capsule it sounds really smooth.
Overall microphone have pretty strong output and sound incredibly pleasant.
I'm keeping it as it is - i'm fully satisfied.

Pretty good capsule for it could be 32mm K67 (definately most of chinese microphones could be really good donor for this project - MXL2006 could be good candidate - pretty ok headbaset and look as also 32mm K67).
If it would be too bright, then there's an option to use cap from plate to ground.
In my build audible difference starts from 390pF, but i don't have any need to use it.

Again - worth to build

 

[lampas]

Even I built one and I was not happy with the result of it, I did not gave up on that schematic... This weekend I'll try to build it again...
I have two Gefell M7 capsules mounted  and in the final stage I will put this capsule in the Banzai M49 housings. As the capsules are with dual diaphragm, the question is, how to make remote polar settings ? Similar like on late Oliver Archut U47 schematic with remote polar pattern ? This mean 120 V on the back capsule ? Does capsule on Sela T12 "see" 60V on the front side of the capsule ?

Peace.

 

[Gus]

threads from 2006 and 2008

reply 25 is a guess at the circuit in the thread I would use >=4:1, dotted line is microphone and power supply separation.

https://groupdiy.com/index.php?topic=29901.msg362898#msg362898

Another thread https://groupdiy.com/index.php?topic=19965.msg233789#msg233789

 

[lampas]

Great Gus,

thank you very much... I finally build it again, this time with B5887.1 transformer from Spencerleehorton...
First impression is good, I'll make a few audio samples in the next days and will post them here.