Eddystone 888A: Turn it OFF, or leave it ON?

[Herald1360]

Now there's a nice new can full of wiggly critters just waiting for a good airing

[G6Tanuki]

Originally Posted by M0ALK Richard

Valve amplifiers often have a standby switch.
The valve heaters have power going to them but the high voltage supply to the amplifier is not connected.
If it is the thermal stress of warming up and cooling down that is the problem then I wonder if there is a case for leaving the receiver's heaters on even though the HT supply is off.

That's a recipe for "Cathode poisoning" which can significantly reduce the gain of small-signal RF valves (and CRTs: there's been a recent thread about this in the context of 'instant-on' CRT TVs where the tube heaters were continually powered). The correct way to implement a "standby" switch in a valve receiver is by introducing a high-ish value resistor in the cathode return path of the RF/IF stages - which biases the gain of these stages back to the point where the receiver's sensitivity is massively reduced. You leave the audio-stages running so the current-draw from that big 6V6, 6BW6, N78 or whatever in the output provides a significant current-draw which stops the HT + rail soaring to some silly voltage and stressing the smoothing capacitors (designers of the RCA AR88D power-switching please take note!).

"Communications" receivers often have this standby-resistor variable [a pot on the back panel] so you can reduce the sensitivity 'just enough' so that the receiver can provide sidetone through your headphones when transmitting.

As to valve robustness to thermal shock: they are surprisingly robust as regards switching on and off: at least one design of juke-box feeds the heaters of a push-pull pair of 6L6 initially with about 10V on each filament to get them warmed-up in time to give full output before the record-selector has dropped your chosen beach Boys or Buddy Holly 45 onto the platter and lowered the needle. Once the valves are passing current the 10V is cut back to the more-usual 6.3V The valves didn't seem unduly traumatised.
There are also "Quick Heat" RF output valves like the 2E24 and the YL1030 which were used in some mobile-radio transmitters: pressing 'transmit' energised the filaments.

To get back on-topic, I don't leave any of my valve communications radios switched-on when they're not being used.