[R-390] LINE /AUDIO POTS

[Charles Steinmetz]

Steve wrote:

>If you need log pot response from a linear pot one method that works is
>connecting a resistor that's 20% of the value of the linear pot between the
>ground end solder lug and the center variable contact solder lug.

Jacques responded:

>Despite the trick presented by Steve works practically to "imitate" a log
>potentiometer from a linear one, it cannot be used on R-390/390A because
>the end to end value of both local and line potentiometers are a part of the
>follower tube bias system.

The potential problem Jacques is alluding to is that when you add a 
low resistance (let's say, equal to or less than the end-to-end 
resistance of the pot) from the wiper to the CCW terminal, the total 
resistance from the CW terminal to the CCW terminal now changes quite 
a lot depending on where the pot is set.  (Whereas, when the load on 
the wiper is just a grid resistor -- much larger than the end-to-end 
resistance of the pot -- the total resistance from the CW terminal to 
the CCW terminal remains very close to the pot's own end-to-end 
resistance, regardless of rotation.)  For some hard numbers, two, 
2.5k linear pots, each with a 500 ohm resistor from wiper to CCW (20% 
of the pot value), would present a load to the driving circuit of 
only ~208 ohms *if both pots were turned fully up*.  (Of course, 
almost never would both pots be turned fully CW on an R390A.  In 
practice, the load on the driving circuit would likely be no lower 
than 1k or so.)

This same issue arises if one substitutes audio-taper pots with 
values other than 2.5k for R104 and/or R105.  In this case, the load 
on the driving circuit would simply be the value of the two 
end-to-end resistances in parallel, and would not change 
significantly with pot rotation.

So -- what is the reality of changing the value of R104 and/or 
R105?  I simulated the V601B circuit (please refer to the schematic 
below) to demonstrate the effect of various potentiometer values.

R607, R608, and R627 make V601B operate as an approximation of a 
current source with respect to its DC bias conditions (recall that 
cathode-biased triodes make pretty bad current sources).  This means 
that R104 and R105 (the Line and Local Audio potentiometers) have 
relatively little effect on the bias of V601B.

The V601B plate voltage is approximately 200v.  With R104 and R105 = 
2.5k (net potentiometer resistance = 1.25k), the cathode voltage is ~ 
10.5v.  Increasing R104 and R105 to 10k each (net potentiometer 
resistance = 5k), the cathode voltage is ~ 17.7v.  And decreasing 
R104 and R105 to 1k each (net potentiometer resistance = 500 ohms), 
the cathode voltage is ~ 8.6v.  In all cases, V601B has from 180v to 
195v of voltage headroom, and has plenty of operating current to 
drive the output amplifier grids.  All of the coupling capacitors 
(C602, C604, and C607) are in high-impedance grid circuits, so the 
low frequency corner does not change.

For all practical purposes, changing R104 and R105 from 1k to 10k has 
no effect on the bias conditions of V601B.

There is one difference when you change the values of R104 and 
R105.  The combined (net) load of R104 and R105 forms a voltage 
divider with cathode resistor R627 at audio frequencies as well as at 
DC.  With R104 and R105 = 10k, the pots have about 5dB more audio 
voltage on them than with R104 and R105 = 2.5k, so you would need to 
use a slightly lower setting of the controls to get the same audio 
level.  Conversely, with R104 and R105 = 1k, the pots have about 5dB 
less audio voltage on them than with R104 and R105 = 2.5k, so you 
would need to use a slightly higher setting of the controls to get 
the same audio level.

Note that the effect mentioned in the first paragraph above -- the 
load on the driving circuit changing with pot rotation if one uses a 
low-value resistor from wiper to CCW of a linear pot -- makes that 
trick work even *less* well than normal in the V601B circuit.  As the 
pot rotation increases, just when you want the audio voltage on the 
wiper of the pot to start increasing faster and faster, the 
attenuation due to the reduced value of the pot load causes the audio 
voltage on the pot to *decrease* faster and faster.

All of that said, I'm astounded at the amount of effort people on 
this list devote to finding hacks to cobble up their radios in one 
butchery after another.  Jeez, people, JUST GET THE RIGHT PART 
!!   This whole discussion will have some meaning in a distant future 
when there are no longer ANY audio taper pots available, 
anywhere.  But when that time arrives, I suspect that boatanchor 
radios will have long ago ceased to be useful for anything at all.

Best regards,

Charles
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