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Old 26th November 2009, 08:31 AM   #1
Alkis is offline Alkis  Greece
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Default Delay relay

Hi all!Regarding the OPT is there any problem if use a delay relay connected to the secondarys,in order to avoid the charging noise?
According an article from some manufacturer,if the primaries don't see any current at the secondaries the V+ can destroy the primaries.Is it true?
Delay is about 12 sec.
Thanks.
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Old 27th November 2009, 03:37 AM   #2
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It's not a good idea to operate a tube amp with the OPT secondaries open, since without current flowing, B+ voltage will be high (1.41 x power transformer secondary voltage) and can damage caps and transformer windings if their dielectric strength is exceeded.

There are also inrush current and voltage issues due to transformer inductance that can exceed the dielectric strength of the OPT windings that are beyond the scope of my knowledge, but perhaps one of the experts will help out here and enlighten us.

check out this thread:

self-oscillation in Dynaco ST-70
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Old 27th November 2009, 04:01 AM   #3
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Have the relay switch an 8 ohm resistor in or out.
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Old 27th November 2009, 06:44 AM   #4
Alkis is offline Alkis  Greece
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Yes,I think that I could use an alternative 8 ohm resistor when the relay is off.
Or to bypass the relay at all.
What about your projects guys?Does anybody use some delay or not?
The primary of the OPT is allready fried and I 'd prefer to hear those charging noises instead of destroying a new OPT and maybe the tubes/caps.
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Old 27th November 2009, 12:30 PM   #5
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I agree that I too, would rather hear the charging noises so can reach for the off switch.
Would not a switch on delay for the HT [B+] be better, less stress on components; do a search on this forum. On my latest amp, after switch on the HT smoothing caps etc slowly charge through a 10k resistor until shorted out of circuit by a 555 timer/relay circuit after 30-40 seconds.
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Old 30th November 2009, 06:56 AM   #6
Alkis is offline Alkis  Greece
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There must be some excessive currents in the primaries when the amplifier starts,even with a tube rectifier and as the B+ rises from 0V up to the nominal, some res.load should be used.And I think that the tubes of the primary stages see the rising voltage as a signal as well.Otherwise why should we have the OPT damaged in absence of input signal?
An 8 Ohm resistor seems to be a simple solution.
Thanks for the answers to my newbies thoughts.
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