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02-19-2025, 05:50 PM
Hi Guys
Another way to turn the tubes on/off is to incorporate a screen switch rather than a plate switch. In this scenario, the plates of the active and inactive tubes are always tied to the OT.
As we saw in TUT, if the screen is grounded (through its screen-stop), plate current turns off regardless of the DC bias on the control grid. In the case of an amp where the tubes only operate in tetrode/pentode mode, a single relay or switch pole can control any number of tubes. Kai only wishes to switch them in pairs, so it is easy enough to do this. If the tubes have triode/pentode/UL switching, then a separate pole per tube is needed.
The relay for this should have high voltage isolation between the contact and the coil. Current is less than a few hundred milliamps.
One side of the heater feed to the pair of tubes has to be controlled and this could be a fairly high current given some tubes pull up to 1A8 (KT88). Whether a pair of such heaters can be controlled using a single contact depends on how the tube heaters are wired. In a conventional 6V system, a 10A or higher contact could do this. The heater turn-on surge current is typically about four times the operating current, as reported in TUT2, so we should look for relays with a TV rating that accommodates such surges.
If the heaters are wired using the 6V/12V system of TUT4, then each heater may need its own relay contact if the related pairs are on opposite sides of the heater CT.
This thread has prompted the design of a small relay board that can be used for both high-current and high-voltage switching, RLY-10.
Another way to turn the tubes on/off is to incorporate a screen switch rather than a plate switch. In this scenario, the plates of the active and inactive tubes are always tied to the OT.
As we saw in TUT, if the screen is grounded (through its screen-stop), plate current turns off regardless of the DC bias on the control grid. In the case of an amp where the tubes only operate in tetrode/pentode mode, a single relay or switch pole can control any number of tubes. Kai only wishes to switch them in pairs, so it is easy enough to do this. If the tubes have triode/pentode/UL switching, then a separate pole per tube is needed.
The relay for this should have high voltage isolation between the contact and the coil. Current is less than a few hundred milliamps.
One side of the heater feed to the pair of tubes has to be controlled and this could be a fairly high current given some tubes pull up to 1A8 (KT88). Whether a pair of such heaters can be controlled using a single contact depends on how the tube heaters are wired. In a conventional 6V system, a 10A or higher contact could do this. The heater turn-on surge current is typically about four times the operating current, as reported in TUT2, so we should look for relays with a TV rating that accommodates such surges.
If the heaters are wired using the 6V/12V system of TUT4, then each heater may need its own relay contact if the related pairs are on opposite sides of the heater CT.
This thread has prompted the design of a small relay board that can be used for both high-current and high-voltage switching, RLY-10.
