Tube layout of the /13 SJT 10/20 ?

[rh]

Just picked up a used /13 SJT 10/20.

Anyone know:

1) The tube layout? I may have a bad reverb tube, but I don't know which one to swap.

2) What the little toggle switch between the 4ohm and 8ohm speaker jacks does? (It's not the fan -- that switch is clearly marked, on the other side of the amp.)

Any help greatly appreciated.

[Bo Faulkner]

the toggle is a low freq damping switch.. either loose or tight response

[Neon]

Just picked up a used /13 SJT 10/20.

Anyone know:

1) The tube layout? I may have a bad reverb tube, but I don't know which one to swap.






The tube layout (looking from the back) is:5AR4/GZ34

5879 12AX7* 12AT7 12AX7* 12AX7 6V6 6V6


Mine has two tubes that are unmarked (*asterisk above), but I believe they are 12AX7s. The reverb driver is likely the 12AT7. One of the 12AX7s is probably involved in the reverb return path too.


2) What the little toggle switch between the 4ohm and 8ohm speaker jacks does? (It's not the fan -- that switch is clearly marked, on the other side of the amp.)

Any help greatly appreciated.
Bo answered that one. I would add that the "loose" response has more high-end and could be described as a more aggressive tone.

Hope that helps.