GT66 tube mic voltage requirements

[gct1]

Thanks for all the info. I have ordered a Retro ll so I can paly around with the kit to see what else I need to buy for the GT66 as I will need a cable and shock mount too. I will keep you posted.
Cheers

 

[gct1]

So while I'm waiting for my new toys:

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

If I just change the 2 zener diodes for 60V is this enough to do the job? What do I need to do with the resistors?

 

[mad.ax]

Those resistors need to drop the B+ to a bit more than the zeners value so the zeners don't overheat. Say 140V for 120V zener regulation.
Their exact value depends on the voltage after the bridge, and the current that the mic draws. Once you get those two data, you can determine the voltage drop you want for those resistors (Vdrop =Vbridge - 140). Then the value of the resistors is (Vdrop/I)/3 (if you have 3 resistors...

Axel

 

[gct1]

So for the SCT800 PSU I get 48+48+48 across the 30k resistors (near 150V)​

The 2 zeners are 100v but read 131+128v​

There are 250v at the mic and heaters (measured on valve) 8.67v​

I would have expected a higher than 250v across the resistors not lower?​

There is also a 1M resistor after the zeners.​

Any feedback?​

 

[mad.ax]

Which microphone was connected for those tests? The STC800? And does this mic works well?

I ask because nothing makes sense! The 7806 should deliver 6V DC, and you get 8.67! The two 100V zener should deliver 200V DC and you get 250! Looks like everything's fried...

Axel

 

[gct1]

Sorry voltmeter battery low.
98v 96v zeners
36.5v across each resistor, total 106v
6.5v heater
B+192v
297v after the bridge
So from the equation my SCT800 is 1.2mA
To change to 120v I need 2 x 60v zeners and 3 x 47k resistors?

 

[mad.ax]

Ha! with those measurements, it looks much more normal

1,2mA is the current drawn by the STC800... What makes you think the GT66 will need the exact same?

I would plan for a bit more, say 3 X 39K. If the mic draws less, the Zeners will limit the voltage to 120 anyways... But if you see your 120V drop you will need to adjust.

Axel

 

[gct1]

Many thanks.

 

[gct1]

I checked 12AX7 is 1.2mA and GT6205 7.5mA plate current.
So 3 x 6800R should work?

 

[mad.ax]

You checked what? Maximum plate current on a datasheet?
Doesn't tell you how the tube is wired or biased....

Axel

 

[gct1]

Ok, so I will buy a selection of resistors Thanks

 

[gct1]

I started by rewiring the cable connector at the mic end. As I’m using a tbone psu for a sct800 to power the gt66 the heater is just +6.5v and 0v common. The gt66 has a separate cable +a and -a that results in 6.3v. Can I just connect -a to 0v in the cable connecter at the mic end?

 

[gct1]

I figured it out. Thanks a lot to all who contributed.
I added a heatsink to the voltage regulator as it was around 70 degrees.
I went for 40K resistors because my 1w resistors looked smaller than the originals hence 30k plus 2x20k and 2 x 62v zeners gave me 125v and 6.5v heater.
Compared to the Retro II the GT66 is smoother on guitar, not so bright. I prefered the GT66. I will compare on vocals in a few weeks when I have a project. I even got the GT66 into the SCT800 cradle by removing the washer which gives me a choice of cradles if needed. Very happy.