A couple of technical questions

[kagliostro]

I've a couple of questions

The first one is about the indication of Mutual Conductance

I've read about this argument but I forgot what I've read about  

Mutual Conductance may be expressed in mA/V or in µ℧

Which is the correspondence between the two ? How to convert mA/V into µ℧ ?

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The second question is about something I've read in the STC807 datasheet but also in the STC6L6 datasheet and is about the situation in which the grid is at a lower voltage than the plate

in this situation the council is to use a potentiometer (bypassed by a capacitor) between B+ and ground instead of an in series resistor (see attached image from this datasheet http://www.retrovox.com.au/STC807.pdf)

I would like to know your opinion about

to me is a system as to have a more constant voltage on the grid using the pot as a voltage divider

a shunt stabilizer ?

Is my supposition right ?

if so, as the current flowing in the grid is low, a normal pot can be used or a special pot is required ?

Also, I didn't understand in which way the bypass cap is to be connected ??

Many thanks

K

[HotBluePlates]

... If I'm wrong ...

You're extraordinarily close.

V/A = Ω   (ohm),

So A/V = ℧   (mho, or "ohm" backwards, or "conductance"),

and mA/V = m℧   (if Amperes/Volts = mhos, then Milliamps/Volts = Millimhos),

therefore mA/V = 1000 * µ℧   (milliamps/volts = 1000 micromhos).

So 2.2mA/v is the same as 2200 micromhos (and also 2.2 milli-Siemens or 2200 micro-Siemens, to use the new terminology for the unit).

[kagliostro]

Thanks Drgonzonm & HotBluePlates

So the first question is solved 

and only the second question is still on

Thanks

K

[HotBluePlates]

The second question is about something I've read in the STC807 datasheet ...

Wire G2 (screen grid) of these tubes the way you always do with every other output tube in a guitar amp.

... to have a more constant voltage on the grid ...

Pentode plate curves look different than triode plate curves, right? But if you connect the screen grid to the plate (triode mode), they are now exactly the same.

The thing that makes a pentode have the shape of characteristics that is has is the screen voltage being connected to an unchanging d.c. voltage. This is why the data sheet says not to use a series resistor.

But guitar amps use a series resistor. Why?

Because the resistor should be large enough that if too much screen current flows, then screen voltage drops, and screen dissipation (G2 volts * G2 milliamperes) is kept from exceeding the rated limit.

But if you make the screen resistance too, too large then the dropping screen voltage causes reduced plate current and reduces maximum power output.

Which is why hi-fi (which expects no abusive use) has no series screen resistor, so that it can make maximum output power. And why guitar amps (which ought to expect abusive use) have a series screen resistor, to protect the screen grid from damage. You can intentionally make the screen resistor "too large" to cause compression at the limit of output power (like you might have with a class AB amp using a tube rectifier), even if you use a solid-state rectifier and stiff power supply.

... the council is to use a potentiometer (bypassed by a capacitor) between B+ and ground ...

This is old terminology. You & I think of "potentiometer" as a "volume knob"; they meant "voltage divider".

And the typical guitar amp power supply could be re-drawn as a voltage divider with multiple taps for the different voltages provided. If you also include a filter cap (what they call a "bypass cap") at a "voltage output tap", then the comparison to a guitar amp power supply is complete.

Compare the redrawn Twin Reverb power supply below with the schematic (the 10kΩ resistor takes the place of the preamp tube stage; old-style "potentiometer" supplies would likely have a resistor to ground here to draw more current than the preamp stage, to keep the B+ voltage steady).

[HotBluePlates]

HBP, I hope you used an SS nail.  (stainless not solid state)

No nailing!  Your post was already 99.9% on the money; I only gilded the lily.

[kagliostro]

Many thanks for this articulated explanation HotBluePlates

You can intentionally make the screen resistor "too large" to cause compression at the limit of output power (like you might have with a class AB amp using a tube rectifier), even if you use a solid-state rectifier and stiff power supply.

This is interesting

my eye fell on the STC807 datascheet about the reuse of the PT I recently recovered

because of the high B+ it has

in the datasheet the indication is plate 500v and screen 200v

someone can say if to drop 300v from B+ for the grid is feasible via a common resistor or if a zener or a mosfet (as Voltage Regulator) is required and if the presence of this "stabilized" grid voltage can "demage" the tone in a SE amp ?

is a doable solution to use a Voltage Regulator with an in series resistor to supply the grid as to give a fluctuation on the voltage that arrive to the grid ?

Many Thanks

K

[HotBluePlates]

someone can say if to drop 300v from B+ for the grid is feasible via a common resistor ...

No.

... someone can say if to drop 300v from B+ for the grid is feasible via a ... zener ...

It would take several, and I wouldn't personally go about it that way.

... someone can say if to drop 300v from B+ for the grid is feasible via a ... mosfet (as Voltage Regulator) ...

This is the best of the bad alternatives.

Historically, you'd have an entire separate power supply for a screen voltage that much lower than the plate voltage. Or you'd do like the SVT and have a 1/2 voltage output on a bridge-rectified supply to have a screen voltage at 1/2 of plate voltage.

But an 807 is the same as a 6L6GB, but with a plate cap. It's not a mystery; run the tube like a 6L6GB. If screen voltage is as high as the plate, you simply have more G1 bias voltage to counteract the increased plate current.

... use a Voltage Regulator with an in series resistor to supply the grid ...

Sure, you can do that if you must have the screen voltage well below plate voltage.

[kagliostro]

Thanks HotBluePlates

... use a Voltage Regulator with an in series resistor to supply the grid ...

Sure, you can do that if you must have the screen voltage well below plate voltage.

The idea was to use a Voltage Regulator like in the attached schematic
(not necessarily with zener diodes)

and using a large grid resistor as to force fluctuation on the grid

as to emulate the operation of a standard PS

K

[HotBluePlates]

That will work.

I'd recommend starting with a small resistance to get things working. Then experiment with using a larger screen resistor.

Even a screen resistor of 2-3kΩ will only cause sag if the power output is high enough to cause significant screen current variation.

[kagliostro]

Thanks HotBluePlates

Even a screen resistor of 2-3kΩ will only cause sag if the power output is high enough to cause significant screen current variation.

Therefore such a system can only be used with amplifiers PP and not SE, is that correct ?

Thanks

K

[HotBluePlates]

Even a screen resistor of 2-3kΩ will only cause sag if the power output is high enough to cause significant screen current variation.

Therefore such a system can only be used with amplifiers PP and not SE, is that correct ?

No, it works for either. The requirement for sag due to a large screen resistor is a changing screen current.

[kagliostro]

Many Thanks HotBluePlates

That info is useful to me

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excuse me if I reply only now, I spent a pair of days in Venezia with my wife (and dog) 

Thanks again

Franco