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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 3rd June 2011, 05:44 AM   #1
sprinter is offline sprinter  United States
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Default Should I use a B+ switch for SSE?

Apparently, using a switch to bring up the B+ after filament warm-up has caused some problems. I'll likely use a SS rectifier, but I'm reluctant to use a switch. Can the typical EL34 or 6L6 take it if B+ comes up right away? I see more and more tube-type guitar amps without switches and I'm beginning to think maybe it's not such a bad problem anymore.
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Old 3rd June 2011, 08:18 AM   #2
rknize is offline rknize  United States
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It's always best to let the heaters come up first. There are a few reasons, such as cathode stripping and so forth. But as you say...many amps past and present did not concern themselves with it and worked just fine.
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Old 3rd June 2011, 11:57 AM   #3
Ty_Bower is offline Ty_Bower  United States
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I would forgo the standby switch, although I'll admit my experience with solid state rectifier amplifiers has been limited to designs with B+ of 400 volts or less. I've had no problems that I could attribute to cathode striping. I've run both fixed bias and cathode biased amps this way.

I've heard many stories of problems associated with the standby switch. Often the standby is placed between the PT center tap and ground. With the switch open and the amp first powered on, you will have a floating high voltage secondary on the PT. You can avoid this situation by installing a large valued resistor (1 meg ohm) across the poles of the standby. Negligible current will flow, but the PT stays referenced to ground.

The other big problem you can get with the standby switch is inductive kick if the switch is opened while the amp is powered up. There is a lot of energy stored in the field of the PT, and when the current flow is abruptly cut off by the switch you may develop very high voltages (several kV) at the secondary. This can often blow solid state rectifiers, and then the fuse will blow the next time you try to turn the amp on. Avoid this problem by never opening the standby switch until after you've powered off the main switch.
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