Critical resistors for s.e.x amp?

[Paul Joppa]

You have the version 3.0 S.E.X. amp.

The grid stoppers are the 220-ohm resistors attached to pins 3 and 10 of the 6FJ7, labelled "Rstop" on the circuit. They replaced the somewhat fragile carbon composition used in earlier versions and there is little to no benefit in changing them.

There are no 120-ohm resistors on the headphone socket in this version 3.0, so there's no point in trying to replace them  :^)

The 499-ohm resistors on the C4S board, labelled R1A and R1B on the PC board, set the current. Those resistors do potentially affect the sound, so bulk metal foil resistors can be considered an upgrade.  The board is a little tight there, so be sure they will fit before spending the big bucks!

 

[jufelling]

Alright, thanks for all the information.

I‘m really interested to swap those two resistors with something different.
How much ohm should those two resistors have?
Exactly the same or is there a span, e.g. from 300 - 600 ohm? If yes, how will different ohm values affect the sound?

I assume other factors of the resistors are probably more influential, right?

 

[Paul Birkeland]

Just to be completely transparent, the grid stopper resistors handle zero signal current

Ideal resistor types for this position are types with minimal inductance, which is why we used carbon composition originally.  Since these were fragile, we transitioned to carbon film for higher reliability. 

If you go up too high in value, you could potentially introduce some audible treble roll off, but 200-1000 ohms is certainly a fine range and I wouldn't be able to hear the difference between those values.

The coupling caps, parallel feed caps, and cathode bypass caps are far more influential to the overall sound of the amp.

 

[Paul Joppa]

PB is talking about the grid stoppers, which are not critical in value.

The 499-ohm current set resistors on the C4S board determine the driver stage operating point, and should be within +/- 5% of 500 ohms.