altec 436c tube biasing ?

[skal1]

What is the cathode bias  of the 436c the v1 and v3

where do i connect my meter to read the grid bias on those tubes. is ground to pin

thanks

skal1

just 1 2 learn

 

[mich]

Grid bias for V1 is read at the input TX center tap to ground - its a negative DC voltage (about 0.6V for no signal and 30V for maximum GR)
cathode curent of V3 can be measured across the 220R resistor and shold be 10mA - (2.3V / 220 = 0.0104)

if im wrong someone will correct me...

 

[skal1]

Grid bias for V1 is read at the input TX center tap to ground - its a negative DC voltage (about 0.6V for no signal and 30V for maximum GR)
cathode curent of V3 can be measured across the 220R resistor and shold be 10mA - (2.3V / 220 = 0.0104)

if im wrong someone will correct me...

thanks  mich,



skal1

 

[skal1]

?@ mich or any valve guru.

Does this mean the v1  tube is self biased ,or is that 34 ohm resistor sitting across the meter holding the cathode at 3v .

Is the 220 ohm resistor the cathode of  v3 holding the grid at - 2.3v or 0v

skal1

 

[mich]

The resistor at V1 (resistor paralel to meter in this case) is biasing the tube to a certain point.then when the control voltage is hitting on the grids - this bias point changes and we've got less gain from this stage.
dont know if its called self bias - dont think so..

At v3 its kind of the same thing - the 220r biases both section of the tube to 2.3V.

 

[skal1]

hey thanks for that , so my thing was right , really getting into this tube stuff now just have to get my head around the maths. ;D


where is the control voltage coming from ?


skal1

 

[mich]

The control voltage is coming from (or through) the diode tube.
the music is beeing amplified by v3 and gets to v2 through capacitors c5 and c6.

then it leaves the diodes (after beeing rectified ) as your control voltage.
it charges the cap c4 , and when you stop feeding signal this cap discharges to ground trough p2+r9 wich makes 1270K for your slowest realese setting or just 270k for the fastest.

if you make this cap smaller you get even faster realese time.

 

[skal1]

The control voltage is coming from (or through) the diode tube.
the music is beeing amplified by v3 and gets to v2 through capacitors c5 and c6.

OK i understand that bit, now this is where i gets confused .Now how does the control voltage reduce v1s amplitude  , i can not see where it is connect to v1 to control it  .

i read you can connect a dc voltage to v1 directly to control it would this be a + or a - supply and where would i place this connection on v1 to simulate the action of the rectifier diode.

thanks

skal1

just need to learn

 

[MagnetoSound]

Now how does the control voltage reduce v1s amplitude  , i can not see where it is connect to v1 to control it  .

i read you can connect a dc voltage to v1 directly to control it would this be a + or a - supply and where would i place this connection on v1 to simulate the action of the rectifier diode.

skal, read mich's first post again. CV is fed to the grids via transformer CT and attenuator P1.

CV must be negative DC only, or the tube will have a short life.

 

[skal1]

so if i am reading this schem correctly , the cv is made up of  the signal coming from v3 into the v3, it is then rectified into dc which comes out of pin 2 and 7 of v3 throw r12 to the juntion of the tx  . correct me if i am wrong .


ok this is where it get fuzzy. so if v2 can not produce a large enough ac signal to drive v3 the cv will be weak. Am i correct in say that.


if i wanted to make the cv larger would i have to set up  v2  to produce more gain ?, or am i missing something.


skal1

 

[mich]

O.k, I made a mess with the numbers of the tubes...
now its clear:  V1 is the 6BC8 and is the GR stage.
V2 is the 6CG7 and is the output stage.V3 is the diode tube and is the rectifier.

so if i am reading this schem correctly , the cv is made up of  the signal coming from v3 into the v3, it is then rectified into dc which comes out of pin 2 and 7 of v3 throw r12 to the juntion of the tx  . c

yes.

ok this is where it get fuzzy. so if v2 can not produce a large enough ac signal to drive v3 the cv will be weak. Am i correct in say that.

yes.

 

[skal1]

O.k, I made a mess with the numbers of the tubes...
now its clear:  V1 is the 6BC8 and is the GR stage.
V2 is the 6CG7 and is the output stage.V3 is the diode tube and is the rectifier.

so if i am reading this schem correctly , the cv is made up of  the signal coming from v3 into the v3, it is then rectified into dc which comes out of pin 2 and 7 of v3 throw r12 to the juntion of the tx  . c

yes.

ok this is where it get fuzzy. so if v2 can not produce a large enough ac signal to drive v3 the cv will be weak. Am i correct in say that.

yes

Thanks mich , i have learnt something today.


just have to learn how to build a tube output  driver capable for  driving that v3


skal1

 

[skal1]

OK i have another ? trying to understand a bit more about this circuit.


i  do not under stand why v2 has no plate resistor , i thought all tubes need a plate resistor at their anode.

please can someone explain to , why v2 has not got a plate resistor, is the tx the anode rs ?.

thanks skal1

 

[MagnetoSound]

please can someone explain to , why v2 has not got a plate resistor, is the tx the anode rs ?.

Yup. B+ at CT, each half-winding provides the Ra.

 

[skal1]

Yup. B+ at CT, each half-winding provides the Ra.

Ok so how would you work out the  Ra of the tx..




thanks

skal1

 

[MagnetoSound]

I can hazard a guess according to my ropey math, looking at the GE data sheet ...

There's 150v at the plate.

Rp for the 6CG7 is given as approximately 7k, and the plate current to aim for in Class A operation as 10mA or so ... for B+ @ 150v you need to roughly double the Rp to get 10mA so, if I have got my head around this correctly, one might assume around 7k in the winding ...

150/14000=0.01

 

[skal1]

the math looks right, but wat do i know.


so the Rp is the internal resistance of the tube right,and you are adding another 7k  primary.


How do you that the plate current to aim for in Class A operation ?


could you add the link for the 6GC7 data sheet  ,can not find 1 on the net.

Thanks again

skal1

 

[MagnetoSound]

It's 6CG7, that's why. (My mistake, sorry. I've corrected the typo in my last post.  )

www.drtube.com/datasheets/6cg7-ge1956.pdf

It's on Page 3 ...

 

[skal1]

no wonder i could not find anything.


skal1

 

[PRR]

> the plate current to aim for in Class A operation as 10mA or so ...

WHY? (Hint: no, you could do "Class A operation" with much less than 1mA...)

> one might assume around 7k in the winding ...

And one would be wrong. You wish for zero DC resistance in the winding. You bias-down the tube somewhere between maximum and zero by setting grid-cathode voltage.

This gives terrible DC gain and output. But we don't want DC signals. The AC impedance is higher than the DC resistance.

This is rather basic transformer-coupled amplifier design. Well covered in dozens of classic texts (plus spotty essays on the Web).