GT66 tube mic voltage requirements

[gct1]

Hi
I have a GT66 without a PSW1 power supply that I picked up in a car boot sale. I have a Tbone SCT800 psu that says 6.3v and 200v. I can buy another tbone psu if that would work or the tbone retro 2 psu if that would work. Can anyone give me the voltage requirements for the gt66 or the output from the tbone retro 2?
Cheers

 

[LevinGuitar]

The B+ should be 120v

I have the schematic of the AM62 / GT67 mic, it's a multipattern version of the AM61 / GT66, hope it will help

All schematics were aviable on aspenassociates web not so long ago but not anymore.

Edit: Is the grid2 used as plate and the plate non connected?

 

[Khron]

What're the odds those 200v are unregulated and measured with no mic connected?

 

[mad.ax]

Here's the AM61 psu schemo if that helps...

Axel

 

[gct1]

Thanks for the info. I need some time to digest.

 

[robovox]

Same PSM1 also supplies Sterling Audio ST66/ST69 (based on GT66/GT67). STPSM1SV and STPSM1BK (in silver or black) are listed at musiciansfriend.com for $70--with a 15% ROCKTOBER discount code available.

 

[gct1]

Thanks for the info.

My GT66 has a FET and a pot in the centre unlike the GT67. What's the pot for?

Looking at the mic it should run with just a B+ at 120v as there is no B- marked on the CB? The heaters I guess will already be wired for 6.3 v between A+ and A-?

 

[Khron]

B- will be the overall circuit ground / 0v.

 

[gct1]

I checked my Tbone SCT800 voltages and measured 252V and 8.57V (not 200V and 6.3V as stated on the CB) So I need to drop the voltage to 120V - about half - to use it with the GT66. What's the simplest way to do this or should I look for another PSU?

 

[soapfoot]

I checked my Tbone SCT800 voltages and measured 252V and 8.57V (not 200V and 6.3V as stated on the CB) So I need to drop the voltage to 120V - about half - to use it with the GT66. What's the simplest way to do this or should I look for another PSU?

Did you measure those voltages without a mic connected?

If so, those are the "unloaded" voltages and the voltages under load are likely to be different.

 

[gct1]

It was a loaded measurement. I checked again after 5 min and 30 min and the reuults were similar.
8.5V on the heater is a bit high with the centre earthed no? Specs say 6.3V and the life of the valve depends on how close you get.

 

[gct1]

So does anyone know the B+ for the TBone SCT2000 or Retro Tube II PSUs as they are readily available in Europe.

 

[mad.ax]

8V5 on heater is definitely wrong. Usually those PSUs use a regulator like 7806 for the heater. If that regulator dies and shorts, you end up with unregulated voltage, which might be those 8V5...

On the HV side, you'll most likely find a few RC, and a pair of zeners.
Upping the value of the Resistors, and replacing the zeners with a couple of 60V, will set your B+ to 120V, just like in the AM61.

Axel

 

[mad.ax]

Edit: Looking at the schematic of your mic, there's a 10Ω 3W resistor in serie between A+ and the 5840 heater. Since the 5840 draws 150mA that means there will be a voltage drop of 1,5V.
Dunno how close the AM61 and GT66 are, because 6V (output of the AM61 psu) minus 1,5V would result into serious underheating... 4,5V!

Idealy you should mod your PSU and replace the fixed regulator by an adjustable one, then tune the heater voltage for the best noise/performance compromise...

Axel

 

[gct1]

I'm confusing you. The mic being tested is the one I had with a PSU TBone SCT800 with a 12AX7B that I swapped for a ECC83. This circuit is below and has no 10R 3W resistor. I should probably add one on pin2 ?

 

[gct1]

And this is the SCT800 PSU

 

[mad.ax]

OK. Your PSU picture confirms the heater voltage is regulated by a 7806. 8,5V at the output of said 7806 is NOT normal.
Measure how many Volts you get on pin 1 (input), then on pin 3 (output)

 

[LevinGuitar]

Edit: Looking at the schematic of your mic, there's a 10Ω 3W resistor in serie between A+ and the 5840 heater. Since the 5840 draws 150mA that means there will be a voltage drop of 1,5V.
Dunno how close the AM61 and GT66 are, because 6V (output of the AM61 psu) minus 1,5V would result into serious underheating... 4,5V!

Idealy you should mod your PSU and replace the fixed regulator by an adjustable one, then tune the heater voltage for the best noise/performance compromise...

Axel

Haven't noticed that resistor

 

[gct1]

Pin 1-2 16.8-16.3V
Pin 2-3 7.8 - 8.6V
I took a few measurements no mic attached

 

[imbuzzed]

So does anyone know the B+ for the TBone SCT2000 or Retro Tube II PSUs as they are readily available in Europe.

I've got them both. Both are zener regulated for the B+, so you could easily adjust them by swapping the zeners. The SCT2000 seemed to be originally set to 170V B+.

The retro tube II I'm not sure about the voltage, as I installed a new board in the enclosure. But the circuit of the original seemed quite similar to the SCT 2000.