On the long head, you might be able to sneak a mirror between the PT and the bottom of the cabinet. The big transformer on the left (PT) will be stamped MESA and then either of these codes underneath it.
100-162318 - "100" PT
105-162318 - "105" PT
X101-162318- export PT
I have seen a couple of the export PT's in amps that did not have the voltage selector switch. :shock:
The 100 and export PT should put 450-460v to the plates of the tubes, where the 105 will put out 480-485v. Then you have to consider the bias voltage: (reprinted without permission)
"Negative bias voltage has been permanently set during amplifier burn-in and should never need
adjustment. Correct bias voltages varies according to amplifier model: Measured at pin 5 of each 6L6, the
correct bias for Standard Hundred Watt = -52 (or -47 for Hundred
Export) "
The 105 PT 60/100 C+ will need Red or Yellow color coded STR-440's or some other brand that falls into the same idle current draw range.
The 100 and export PT's will allow for a Green or Grey color code in addition to the Red and Yellow coded STR-440's mentioned above.
Or just go all out and buy the Sylvania STR-415's that came stock in these amps.
Everyone likes something different regarding preamp tubes, so feel free to experiment with different brands.
