Question on preamp section - Elka tube amp

[slidincharlie (Carlo P)]

I am doing some basic maintenance job in a friend's Elka TX20 amp. Cap job, tube pin cleaning, pot cleaning, the usual stuff.
This amp has got one of those PCB's that show the solder joints on the upper (inspectable) side and the components on the hidden (lower) side, which drives me crazy because it is very hard for me to track where each wire goes...
The amp has till some buzz (ground problem?) and what sounds to me like an oscillation problem, further it sounds definitely too weak for a two-EL84 amp.
I am writing because I noticed something odd (or maybe it's not that odd?) in the circuit:
(1) the Normal as well as the Vibrato volume pots go to the grid of V4 through one 100k resistor each;
(2) in the actual amp such resistors are 470k.
Now in any Fender blackface or silverface amp such resistors are missing, so I am wondering:
(i) what happens if I eliminate the two resistors from the circuit;
(ii) or else should I replace the current 470k resistors with two 100k as per the schematic?
I was also wondering: does it sound weak because of those resistors? According to the model name it 'might' be a 20W amp.

All the pots in this amp are linear taper pots. Should I replace them with audio taper pots, except maybe the vibrato intensity and frequency pots?

P.S.: The attached schematic is for a Hohner Kingstar 20 amp: my amp has got exactly the same circuit except for the reverb section, which is missing.

Elka TX20 (Hohner Kingstar 20).pdf

[Dave H]

Channel I looks like an AC30 Top Boost. It should have enough gain to produce full power even with 470k mixing resistors which could attenuate the signal by 4x. If you change both to 100k they will still attenuate by about 2.5x. I'd measure the power output to check what it is really doing.

Edit.
If the resistors are replaced by links turning either vol pot to zero will set both channels to zero volume.

[slidincharlie (Carlo P)]

Channel I looks like an AC30 Top Boost. It should have enough gain to produce full power even with 470k mixing resistors which could attenuate the signal by 4x. If you change both to 100k they will still attenuate by about 2.5x. I'd measure the power output to check what it is really doing.

Thus a smaller resistor is expected to yield a bit more volume, right?
Unfortunately I am not able to measure the power output.
The reason why I think that the amp sounds weak is that it has a massive output tranny, yet with two EL84 sounds about as much as half my 2-6V6 Fender Deluxe Reverb.

If the resistors are replaced by links turning either vol pot to zero will set both channels to zero volume.

Well, this should not be a big issue, right?

[Jazz P Bass]

Why are you not able to measure the output power?

A basic volt meter set to read volts ac will do.

Measure at the speaker leads.

Power= VxV/ Speaker resistance.

Note: without splitting hairs as to whether or not the reading is RMS or not (depends on the meter's capabilities)

[Dave H]

Well, this should not be a big issue, right?

The channel vol pots will be very interactive without mixing resistors.