The actual R the supply resistor is driving into is the screen resistance, analogous to a plate resistance of a triode. So the R in the RC time constant then becomes the parallel resistance of the screen & supply resistances. The problem is complicated by the lack of any published data with regards to screen resistance.

Fred Terman (And Wm. Hewlett) tell us that screen resistance for a pentode can be estimated as follows-

rs = ( ( Ib + Ic2 ) / Ic2 ) * Rp

From this we need to know 6AN8 triode connected pentode Rp, another spec not published for the 6AN8. For the attached 6AN8 pentode plate family draw a line from the origin to the intersection of the Ec1 zero bias curve where Ec2 & the plate are at equal voltage. In this example that would be at 150 plate volts.

Triode plate Rp for the pentode section at zero bias is approximately as follows-

Rp ~ Change of Eb / Change of Ib

From the 6AN8 Average Transfer page, Change of Eb = 150 Volts

& Change of Ib ~ 28mA (plate) + 9mA (screen)

So triode Rp ~ ( 150 / 37 ) K or 4.05K

Aside from the scale factors of voltage & current all common triode plate families look very much the same. As an example refer to the 6BQ7 plate family. When G1 bias is increased plate resistance increases. As an educated guess we can assume Rp for a triode connected 6AN8 pentode section to be something like 10K at the operating point.

Using the typical operating currents split on the 6AN8 pentode we get-

rs ~ ( ( 9 + 2.8 ) / 2.8 ) * 12K

or rs ~ 42.1K

That is the number we need to compute the rolloff frequency of the amplifier front end.

The R in the RC calculation now becomes rs in parallel with one M.

So R ~ 40.4 K, a factor of about 20 times different than where we started.

RC = 0.0040 sec F = 0.159 / RC Hz 39.4 Hz

Looks like it is a step built in as part of the NFB stabilization circuits.

Note 1- Wm Hewlett & David Packard of HP were Fred Terman’s Graduate Students at Stanford U.