Studio Projects T3 circuit

[Emmathom]

Hi @LevinGuitar ! I just got a Studio Project TB1 for a very good price because it was sold without a PSU. This is not a problem since I will build a new one. I found a schematic that says 200v for B+
I'd like to ask you please if you know the plate current ? with Ra 100KΩ and Rk 3KΩ it must be around 1,5mA... ?

BTW I don't understand why such a power dissipation for R2 100KΩ (5w? 3w?) in the mic ? (1,5mA^2)*100KΩ = 225mW... I see brown-black-black-orange on the pict attached so 100KΩ
Same with the 2 zeners 100v to set HV out : they need more than 1,5mA to be active and this 1MΩ load does nothing about it... so the tube current must be higher than 1,5mA ?

I intend to do some mods :
- set heater voltage to 12,6v DC and elevate it with a divider from HV (+ 25v on ground ref.) > I will use the one unused wire of the seven for the "minus elevated"
- change the capas' where improving quality is needed
- change all resistors for metal
- regulate HV and add a switch to turn it on/off
- regulate heater > I don't understand either why heat is set to 6,3v DC

Regards

 

[Khron]

- set heater voltage to 12,6v DC and elevate it with a divider from HV (+ 25v on ground ref.)

Why?

I don't understand either why heat is set to 6,3v DC

Why shouldn't it be?

 

[Emmathom]

Why elevate heater voltage range ? I will quote Thor who adviced me to do so :

The glowing filament makes a diode cathode and the cathode makes an anode, not great, but then you have rectifier tube build into the cathode circuit. Rectifiers pass noise, hum and clicks into one direction with high efficiency. It blocks the other direction. The heater elevation is functional only. It works ok without, it is just to reverse bias the parasitic heater cathode diode. The more positive the diode cathode relative to the anode, the more the diode is reverse biased and the more voltage is needed to make the unwanted diode conduct unwanted current into the cathode.

About 6,3v DC : with 6,3v I will need twice the current (300mA) and since I've got 15vAC available for heater, why not go to 12,6v@150mA ?

 

[mjmajilton]

Why elevate heater voltage range ? I will quote Thor who adviced me to do so :

The glowing filament makes a diode cathode and the cathode makes an anode, not great, but then you have rectifier tube build into the cathode circuit. Rectifiers pass noise, hum and clicks into one direction with high efficiency. It blocks the other direction. The heater elevation is functional only. It works ok without, it is just to reverse bias the parasitic heater cathode diode. The more positive the diode cathode relative to the anode, the more the diode is reverse biased and the more voltage is needed to make the unwanted diode conduct unwanted current into the cathode.

About 6,3v DC : with 6,3v I will need twice the current (300mA) and since I've got 15vAC available for heater, why not go to 12,6v@150mA ?

 

[mjmajilton]

You are confusing the 6922 heater with the 12AX7 heater. The 6922 heater is rated for 6.3v between pins 4 and 5. There is no center tap.

 

[Emmathom]

the SP TB1 tube is supposed to be a 12AY7 / 6072

 

[mjmajilton]

You are confusing the 6922 heater with the 12AX7 heater. The 6922 heater is rated for 6.3v between pins 4 and 5. There is no center tap.

On elevating the heater, the potential for the heater on that half of the tube is from 6 to 3 volts (remember they are in series).
With the cathode at 1to 1.5 volts you already have reverse bias.

 

[Emmathom]

On elevating the heater, the potential for the heater on that half of the tube is from 6 to 3 volts (remember they are in series).
With the cathode at 1to 1.5 volts you already have reverse bias.

I don't understand : the heater concerns the whole tube, 2 half triodes, either you use one or two. If you don't plug the center tap you feed pins 4&5 (in serie) with 12,6v DC and that's it... and as I said I'm gonna mod. the 2 circuits (mic & PSU) to make my own version...

 

[thor.zmt]

- set heater voltage to 12,6v DC and elevate it with a divider from HV (+ 25v on ground ref.) > I will use the one unused wire of the seven for the "minus elevated"

Only one half of the tube is used, the second half has all pins to GND, so only 1/2 the heater is used, so 6.3V heater voltage.

In the interest of longer life, better consistency of gm over time and reduced noise, the old trick from amplifiers for transoceanic telephone cable repeater amplifiers can be used, that is operate the heater with a tightly regulated 5.7V instead of 6.3V.

Elevation of the heater voltage over ground makes sense if parts of the heater are negative with respect to the cathode.

On elevating the heater, the potential for the heater on that half of the tube is from 6 to 3 volts (remember they are in series).
With the cathode at 1to 1.5 volts you already have reverse bias.

You are wrong. Please analyse the circuit shown, it will become clear why.

Thor

 

[mjmajilton]

the SP TB1 tube is supposed to be a 12AY7 / 6072

You are correct. I misread the drawing. With a connection from pin 4 to pin 5 you would need 12 volts.

 

[Emmathom]

Ok I didn't pay much attention to the wiring, sorry... Heater 5,7v must be connected to pin 4 with pins 5+9 to ground so half... so is it now 5,7v@150mA ? since half heater is used ?
I always thought tha the heater filament is one and only part, not 2 parts one for each 1/2 tube...

 

[thor.zmt]

You are correct. I misread the drawing. With a connection from pin 4 to pin 5 you would need 12 volts.

But that is not how it's connected, innit?

Pin 9 is center tap, pins 4/5 are the two triodes heaters.

6.3V are connected to pin 4, pin 5 & 9 are both connected to ground. So only the heater from 4 - 9 is used, the second triode has all pins (including heater) to ground and is thus totally disabled.

Thor

 

[thor.zmt]

I always thought tha the heater filament is one and only part, not 2 parts one for each 1/2 tube...

In a dual triode there are always separate heaters*. In triodes the way they Ard connected varies.

For the ECC88/6DJ8 style tube the pinout RETMA 9AJ, both heaters are 6.3V nominal and connected in parallel.so heaters are only 6.3V.

For 12A...7 style tube the pinout is RETMA 9A and heaters are 6.3V nominal and are series connected with a center-tap, so heaters can be connected in series (12.6V nominal) parallel or individual (6.3V).



Thor

 

[LevinGuitar]

The best and cheapest mod for the TB1 that comes to my mind is to change the capsule position to an end adress mic, Rogs have designed a 3D printed capsule mount for this application.
https://groupdiy.com/threads/rode-broadcaster.85703/post-1118000


If you can install some kind of deemphasis for this capsule, it will be way nicer mic, but there are not much space inside.

If you can change the capsule for a good CK12 or K47, it will also make you smile.

Using the stock 12at7 tube, try lowering anode resistor to 47K. Also you can lower a bit the polarization, not sure if it's set to 75v by the R3/R4 divider.

Some of the original TB1 PSU I had were noisy btw.

 

[Emmathom]

Thanx @LevinGuitar for your answer...

Speaking about capsule bias I found a schem. of the PSU which says 200v for B+ (see earlier post please). This schem. also shows a 12AY7, not a 12AT7 (!)

If so then B+ 200v goes to a 560K + 2M divider, so (200v*560K)/(560K+2M) = 43,75v for capsule bias... Seems low huh ? Usualy it's more 60v to 80v...

About swaping the capsule I'd rather try first to install a deemphasis
Regards

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[LevinGuitar]

The schematic shows 12ay7, but the real mics came with chinese 12at7 tubes.
The divider is R3 1M and R4 560K. The PSU loaded maybe was something around 185v or so, so that would make around 65v. With 200v -> 71v. Pretty safe anyway.

 

[Emmathom]

Ok I read R3 2M...
185v loaded even with 200v zeners ? I think my schematics don't fit reality...
185v with 560K & 1M means ~66v for caps. bias

Will check all this when I receive the mic...

 

[LevinGuitar]

The schematic of your reply #41 shows R3 1M

And besides the tube designation, it's an accurate schematic

 

[LevinGuitar]

About swaping the capsule I'd rather try first to install a deemphasis
Regards

I once tried a Ruud's u67ish mod with the original capsule and a neumann K47 with the circuit flat and I liked both versions. Different characters of course but both were at similar or even same level, so really worth to try.

 

[Emmathom]

I once tried a Ruud's u67ish mod with the original capsule and a neumann K47 with the circuit flat and I liked both versions. Different characters of course but both were at similar or even same level, so really worth to try.

I'll look for Ruud's mod thread to see how he sets his demphasis. Thank uou