6SN7 tube question

Ralph (Atmasphere) beat me to it, as I was just about to post the same comment. The GTAs and GTBs are rated to handle considerably higher plate voltages and power dissipations than the GTs.

But be aware that there are some current production (not vintage) tubes identified as 6SN7s, with no suffix, which meet GTB specs, at least supposedly.

Regarding the original question, I would go for matched/balanced/low noise/etc., as that could very conceivably make a difference.

Also, FWIW, I've been very pleased with ca. 1950s Sylvania 6SN7GTBs in my amp, which I believe requires GTAs or GTBs.

Best regards,
-- Al

How do you check the filament voltage?

It would have to be checked by touching or connecting the probes of a voltmeter (or a multimeter set to measure volts) to the filament pins of one or more of the tube sockets, under the chassis, or to some other circuit points that are connected to those pins. (To assure a meaningful result the measurement should be done while the tubes are in their sockets, as opposed to removing the tubes and accessing the socket pins from above the chassis). Lethal voltages will be present on some of the other pins on those sockets, as well as elsewhere under the chassis. And if the probe tips were to momentarily come into contact with two adjacent pins simultaneously damage to the component may occur. So it’s a task that is best left to a qualified technician.

Also, regarding the references to 6.3vac and 7vac, many designs provide dc voltages to the tube filaments rather than ac. In those cases, of course, the meter function should be set for dc volts rather than ac volts, and the probes should be connected with appropriate +/- polarity.

Best regards,
-- Al