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Old 4th February 2014, 01:23 AM   #1
Abszero is offline Abszero  United States
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Default Shorting or Non-Shorting Rotary Switch: Hi-Low Z tube headphone amp

Shorting or Non-Shorting Rotary Switch: Hi-Low Z tube headphone amp
I am in progress of building a tube headphone amp. I am using a custom Electra-Print OT that has two 32 ohms secondary windings. I plan to use a 2 position 3 poles rotary switch.
Dilemma is which kind of switch, shorting or non-shorting:
shorting ("make before break") means momentarily short position H and L when rotate. In this case during the action of rotation both secondary windings are short-circuited and therefore the final tube sees a Zero ohms load.
non-shorting ("break before make") means that connection is broken for the moment of rotation. In this case, during the action of rotation, both secondary windings are disconnected from output and therefore the final tube sees infinite load.
Now the question is : "what is less destructive: short circuit or open circuit?". I would like to make this amp idiot proof and not to worry that I am going to destroy the OT or prematurely wear out the final tube. Or I should not worry at all considering that the action of switching is more likely less than 1 second.
Anybody out there who encountered this issue ?
Picture below showing the connections of the switch. "H" is for high Z( 128ohm) and "L" is for low Z (32ohm).
Best,
Radu
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Old 4th February 2014, 05:21 AM   #2
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Radu,
You have analysed the problem nicely and reduced it to "what is the best between a momentarily shorted output (make before break) and a momentary open circuit (break before make) output during switching".

Can you tell us what is on the primary side of the transformer?

We could then make more informed advise.

Without seeing that I would GUESS that break before make (momentary open circuit) would be the better choice as that "open circuit" can be managed by permanently wired "minimum" load of say 470 Ohms across each 32 Ohm winding.
Cheers,
Ian

Last edited by gingertube; 4th February 2014 at 05:26 AM.
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Old 4th February 2014, 12:20 PM   #3
Abszero is offline Abszero  United States
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Quote:
Originally Posted by gingertube View Post
Radu,
You have analysed the problem nicely and reduced it to "what is the best between a momentarily shorted output (make before break) and a momentary open circuit (break before make) output during switching".

Can you tell us what is on the primary side of the transformer?

We could then make more informed advise.

Without seeing that I would GUESS that break before make (momentary open circuit) would be the better choice as that "open circuit" can be managed by permanently wired "minimum" load of say 470 Ohms across each 32 Ohm winding.
Cheers,
Ian
Hi Ian,
Sorry I was not clear enough.
This is tube amp using 4P1L triode strapped at 250V, 35 mA,-21V filament bias.
I am not as much concerned about the tube as I am about the transformer. The OT is 30 times more expensive than the tube.
Also having additional resistors connected parallel across the secondary will probably change the sound. I am just guessing.
Thanks,
Radu
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Old 4th February 2014, 04:11 PM   #4
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Interesting thread. I am also building a tube headphone amp with output transformer. I am putting locking 1/4" stereo headphone jacks so that it can't come unplugged and un-load the transformer while it is powered up. I am also putting a standby switch labled somthink like "Change Headphones" so it means put it in standby before changing headphones.

Last edited by mark02131; 4th February 2014 at 04:34 PM. Reason: change some to come
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Old 4th February 2014, 05:39 PM   #5
TheGimp is offline TheGimp  United States
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Based on the fact that many Guitar Amplifiers have shorting output jacks, I would suggest the make-before-break switch.

Tubes driving output transformers tolerate momentary shorts better than they handle discontinuities in the output load.
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Old 4th February 2014, 05:51 PM   #6
Abszero is offline Abszero  United States
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Quote:
Originally Posted by mark02131 View Post
Interesting thread. I am also building a tube headphone amp with output transformer. I am putting locking 1/4" stereo headphone jacks so that it can't come unplugged and un-load the transformer while it is powered up. I am also putting a standby switch labled somthink like "Change Headphones" so it means put it in standby before changing headphones.
I like the idea of having a locking jack. I was thinking of having that standby switch. However, I want it simple, too many switches can create confusion.. But thanks for the idea.
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Old 4th February 2014, 06:03 PM   #7
Abszero is offline Abszero  United States
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Quote:
Originally Posted by TheGimp View Post
Based on the fact that many Guitar Amplifiers have shorting output jacks, I would suggest the make-before-break switch.

Tubes driving output transformers tolerate momentary shorts better than they handle discontinuities in the output load.
Totaly agree with you. Don't want to have any transients and high voltage spikes in the OT.
I was also thinking of having a mute switch instead of having to turn the volume down every time i need to switch. I want to be able to mute the input signal before switching.
I don't recall that the headphone tube amps on the market have switching safety features.
I know in some designs there is a resistor placed in parallel with the secondary winding. In that case I can break before make switch.
I hope that I am not over analyzing this....
Thanks,
Radu
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Old 4th February 2014, 09:39 PM   #8
45 is offline 45  Italy
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Quote:
Originally Posted by Abszero View Post
• shorting ("make before break") means momentarily short position H and L when rotate. In this case during the action of rotation both secondary windings are short-circuited and therefore the final tube sees a Zero ohms load.
• non-shorting ("break before make") means that connection is broken for the moment of rotation. In this case, during the action of rotation, both secondary windings are disconnected from output and therefore the final tube sees infinite load.
None of the two is correct. Those are only true for an ideal transformer.
A real transformer has a DC resistance, finite inductance, leakage and core losses that set limits in any case.
It is rather an transient issue.
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Old 4th February 2014, 10:29 PM   #9
Abszero is offline Abszero  United States
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Originally Posted by 45 View Post
None of the two is correct. Those are only true for an ideal transformer.
A real transformer has a DC resistance, finite inductance, leakage and core losses that set limits in any case.
It is rather an transient issue.
45,
Are you saying that I should not worry, and choose the shorting one?
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Old 4th February 2014, 11:29 PM   #10
45 is offline 45  Italy
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Originally Posted by Abszero View Post
45,
Are you saying that I should not worry, and choose the shorting one?
No I was not saying this.
I wouldn't choose it without that fixed resistor in parallel. I have never tried but I guess that if you have DC current + signal and short the secondary long enough without that resistor you might damage the insulation because all the power will be dissipated in the transformer. For how long you can short it without causing damage depends of course on the transformer specs as well. That resistor in parallel helps and in such case I think the transformer has no special requirements to fulfill.

Last edited by 45; 4th February 2014 at 11:39 PM.
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