My DIY home power amp I built in the early '70s is built into an old western electric chassis (32C or something like that from the 1930s?).merlin said:The 5E6 Bassman was the first amp with a standby. There are two plausible explainations for this: Fender used smoothing caps after the standby switch that were lower rated than the main reservoir cap before the standby switch, which must have saved money and space. A standby switch was therefore needed to protect the lower rated caps from overvoltage, until the heaters warmed up enough for the valves to pull the HT down to normal working levels. The later 135 Bassman schematic even shows labelled voltages exceeding the cap ratings during standby.
The other possible reason is that the 5E6 was the first amp where Fender used a DC-coupled cathode follower. This stage will sometimes arc between grid and cathode at switch-on if the cathode has not yet warmed up. (These days you should put a diode or neon-lamp between grid and cathode to prevent this, not rely on the user).
Marshall copied Fender without questioning it, and everyone else copied them both.
PRR said:Aside from electronic concerns, there is a real operational reason for standby.
A mute switch is indeed a useful feature on a guitar amp, but a standby switch is the most egregious way to implement it. It demands an expensive high-voltage switch, the absence of anode current for too long leads to interface resistance (= tube death), if designed very badly it results in hot switching of the rectifier tube (= tube death), and it can cause an annoying thump in the speaker. If a mute switch is all that is required then it is far easier, cheaper, and safer to short one of the grids to ground. Even switching off the screen grids is an improvement on cutting the HT to the whole amp.Gene Pink said:I have always been of the opinion that a standby switch has two purposes.
merlin said:Even switching off the screen grids is an improvement on cutting the HT to the whole amp.
clintrubber said:Would be good if their product manuals also tried to 'educate' people w.r.t. the standby-switch. The manuals I saw gave the same impression as the usual other brands.... perhaps they gave up...
clintrubber said:Aah, FWIW, here's from the Classic 20 MH manual:
STANDBY switch
* Excessive time off (more than one hour) in "STANDBY MODE" can damage OUTPUT TUBE by "poisoning
the cathodes".
isophase said:Most tube amps that uses tube rectifier don't have a stand-by switch, why?
The old Western Electric amp/PS that I got the 3 position (1st position warm up) switch from had vacuum tube rectifiers, 4 huge ones sitting in external tube sockets, poking out the front of the main chassis. But this was a design from the 30's so who knows?mjrippe said:Because there is no high voltage until the rectifier warms up?
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