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Old 14th January 2013, 01:52 PM   #1
west_nm is offline west_nm  United States
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Default On/Off cycling

I try to respect the members of this forum by not asking too many stupid questions, so please bear with me.

Can lowering the incoming voltage via variac to an amplifier before shutting down help avoid any possible voltage spikes?
Someone suggested a thermistor, but that doesn't make sense to me for shut down. It does make sense for turn on AFAIK.


Steve
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Old 14th January 2013, 02:13 PM   #2
DF96 is offline DF96  England
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As you say, a thermistor may help limit current surges (not voltage surges) for switch-on.

For switch-off a suitable capacitor or snubber across the mains transformer primary will reduce clicks and lengthen the life of the switch.

A variac will not do anything useful. It could even shorten valve life by seriously under-running heaters.
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Old 14th January 2013, 03:43 PM   #3
west_nm is offline west_nm  United States
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Thank you.

I only use the variac because I'm auditioning old gear that needs anywhere from 110-117 ACV.
I thought quickly turning down the power to about 80 volts followed by complete turn off might help.

When I settle on which gear to keep, I'll install a proper bucking transformer, thermistor and snubber if needed.


Steve
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Old 14th January 2013, 03:46 PM   #4
DF96 is offline DF96  England
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A variation from 110-117V should not do any harm. Typically, valves and valve-era components had 10-20% tolerance on parameters.
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Old 14th January 2013, 03:51 PM   #5
cotdt is offline cotdt  United States
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You can use a computer system to raise B+ more slowly and automatically adjust the bias point. The computer can be used to keep a log to track the health of your tubes.
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Old 14th January 2013, 04:08 PM   #6
west_nm is offline west_nm  United States
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Quote:
Originally Posted by DF96 View Post
A variation from 110-117V should not do any harm. Typically, valves and valve-era components had 10-20% tolerance on parameters.
These are stock units from the 50's and 60's.
I really shouldn't be running them until out of spec components are replaced, but I need to verify their present operating condition.


Steve
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