Transformer for GT67 psu? (psm1)

[britton]

Hi guys

So I have this groove toobes gt67 siting around without a psu so i was going to build a new one.  What would you recommend for the power transformer?  I've searched for info on this mic and read this post but it seems to have more to do with the older ones:

http://www.groupdiy.com/index.php?topic=30295.20

Anyway, here's the schematic of the mic and psu courtesy of fender i think?

BTW I've found so much useful information on this forum in the past and I just want to say thanks in advance!

 

[britton]

and the mic

 

[pH]

The 6205 tube is the equivalent of the 5840, which is used in both Royer mods. Take a look at those schematics for PSU ideas.

 

[britton]

wow yeah that makes since.  the funny thing is that i'm currently ordering parts for a few royer mod mics (2 603's and the 2001).  i will mention that i'm somewhat new to tube microphone design (good at following instructions and not too great at breaking things down). 

So if I designed made one more like the royer psu and was getting similar voltages would the gt's circuit jive with that or would i need to change a few things around there?

 

[hitchhiker]

That GT67 may be a later name of the 6TM. I think it's supply was the PSM2. It would power 2 mics. If it was then the voltage was 120v and  whatever heater voltage 5840/6205 wants.

 

 

[britton]

Hey thanks guys.

I know that the 2001 royer psu runs B+ at around 100v (close to 120?).  The heater should be the same in both designs. 

So if I made a cable that connected the A- and B- (pins 6 and 1) and grounded to the shielding on the psu side (to go from 7 pins to 5) then I could just use one of the power supplies that I'm already planning on building, correct?

 

[mad.ax]

100 & 120 V are close in the sense that they'll both shock you if you touch them!
The tube will probably run fine with both, but the capsule polarization voltage will be affected, and that's where 20V difference can be important.

A google search for 'gt67 voltage' showed this GS thread as the first result:
http://www.gearslutz.com/board/geekslutz-forum/485116-groove-tubes-gt67-pinout-psu-specs.html
In it, there's a schemo for the PS-1A psu which Fender says is the correct supply for the GT67.
On this schemo, there's an 1N5380 in the HV section. This is a 120V Zener diode, so I guess it's safe to assume that B+ should be 120V for the GT67.

Axel

 

[britton]

mad.ax.

that post on gs was made by me and that schem is actually for an old gt mic (i posted the wrong one).  i didn't really get any replies over there which is why i came here.  i'm not really sure if that old psu would be a good one for the gt67 or not.

but thanks (maybe i'll go update that thread)

don't listen to everything you find on google.

 

[mad.ax]

I see you added the psm1 schematic on your GS thread. Good thing, as it may help to up the level of good information one can find on Google!

Looking closer to the PSM1 schemo, there's something strange... The last HV filtering cap, C23, is labeled : 220µF/100V
This can mean two things, either the B+ is much lower than 100V, either it's a mistake.
I tend to favor the second solution, because C21 & 22 are 400V, and C6 on the GT67 schemo is a 160V part...

Anyways, my point is that you need to find the correct polarization voltage for the GT67 capsule in order to determine B+.
If you look at the GT67 schemo, R3 & R4 forms a voltage divider which brings half of B+ to the backplate.
With 120V B+, that would give 60V of polarization voltage, which is what most LD capsules expect... But, there's still a possibility that the GT67 capsule,
being 3µ thick, wants a lower voltage... For instance, it may be optimized to work with a 48V polarization voltage (would be useful for phantom powered mics).
In that case, applying 60V would stick the diaphragm to the backplate.

You should try to find someone who owns a working GT67 and PSM1, and could measure the B+...

Axel