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Old 25th July 2003, 04:58 AM   #1
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Question Switching circuit help needed

Hi everyone,

I have a question: Does anyone know a simple way to wire a circuit so that a SPST switch can interrupt the HT B+ (to act as a standby switch) while simultaniously activating a pilot lamp (kind've like a "standby on" indicator)? The lamp runs on 6.3V so I don't think I can use the HT to light it... Any suggestions? I know a DPDT switch would be ideal but I would really really like to use the SPST I have....

Thanks!
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Old 25th July 2003, 05:42 AM   #2
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Put the neon light (with current-limiting resistor!) across the switch contacts.

Hmm, you say your light is 6.3V. Then you'll need either a new light or a new switch. Your pick (psst, neons are <$.30 each).

Tim
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Old 25th July 2003, 05:42 AM   #3
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Use a small neon indicator instead- just use a bigger dropping resistor than usual. Easy. The only other idea is to use a relay with multiple sets of contacts.
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Old 25th July 2003, 11:27 AM   #4
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Quote:
Originally posted by Sch3mat1c
Put the neon light (with current-limiting resistor!) across the switch contacts.
...
Tim
I am not an expert by any means on tubes, so perhaps this is a moot point, but wouldn't this still allow a current of near-normal magnitude to flow through the plate circuit(s) anyway?

Perhaps the additional resistance afforded by the series neon lamp (Vf ~90V) and its current-limiting resistor (~1k/V) would be enough to achieve the desired effect, but I would think the amp would still operate... just not very well.



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Old 25th July 2003, 01:06 PM   #5
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The best way to switch HV on/off is by switching the filament of the rectifying tube. With an SPDT switch you can switch between the filament of the rectifying tube and the lamp. But be very careful, there is dangerous HV on the lamp and switch then! Take care of good isolation! A better way is to use a relay for switching the rectifying tube’s filament, powered from the 6.3V heater supply for the amplifying tubes

If you have silicon rectifiers, use a relay and switch between the relay and the lamp.

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Old 25th July 2003, 01:13 PM   #6
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Hi,

Quote:
The best way to switch HV on/off is by switching the filament of the rectifying tube.
If you use a valve as a rectifier then what's the point of delaying the B+?

Unless it's a guitar amp I fail to see the advantage.

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Old 25th July 2003, 02:36 PM   #7
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Hi Frank,

The advantage is indeed slow turn on and slow turn off of the HV, avoiding large clicks in the loudspeakers and transient conditions in the OT. Note that normally at power on the heaters are not heated up causing a natural slow turn on. When heated up and hard switching the HV it is somewhat different, especially with a large smoothing choke in the HV (overshoot). Also you avoid possible (too) large inrush currents through the rectifying tube. But anyhow the “lamp problem” is easily solved.

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Old 25th July 2003, 03:05 PM   #8
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Hi Pjotr,

Quote:
Also you avoid possible (too) large inrush currents through the rectifying tube.
In real life that just can not happen as the rectifier can't pass current before it's cathode is conducting.

In my OTL amps the output tubes ( 10 double triodes), as is everything else, are switched on straight from a bridge rectifier and a smoothing bank of 10.000µF/150V.

At first it got me worried, so I monitored the B+/- and the heaters simultaneously. What happened is that it took about the same time to bring the cathodes to operating temperature as it did for the caps to charge.

I've seen some people using no bleeders on their caps, leaving B+ on the anodes for weeks if the amp's not used.
Now that is going to harm the tubes I reckon, not to mention that it dangerous if you're unaware of it and want to work on the amps.

As you can see, while I like a slow start wherever possible, I'm no big fan of switched warm up...sooner or later someone is going to forget to apply the correct sequence and that's where trouble starts.

A good thing "lampen" are pretty tough animals.

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Old 25th July 2003, 03:21 PM   #9
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Default Sequencing is art of life......

Quote:
A good thing "lampen" are pretty tough animals.
It's a shame we can't type /pick Cyrillic directly.

Cheers,
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Old 25th July 2003, 07:45 PM   #10
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Quote:
Originally posted by jeffreyj
I am not an expert by any means on tubes, so perhaps this is a moot point, but wouldn't this still allow a current of near-normal magnitude to flow through the plate circuit(s) anyway?
I suppose it might on a preamp, where the current is in the mA range; but you only have 1mA flowing. No problem.

A power amp might appear as from 5 to 1kohms (to the power supply), so between that and the 100k+ (yes 1k per volt) limiting resistor, there's negligible voltage across the amp.

Oh, and put a small cap, say .01 across the switch to reduce popping.

Tim
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