Standby switch for Quad II?

[Zombie]

Hello!
If anyone here has any ideas where to put a "standby" switch on a Quad II, please reply to a novice.
The idea is to shut off everything (B+?), except the heater voltage to keep the valves warm.
I saw smth here with ICs etc. (Jukka Tolonen's), but I'd rather just have a simple switch.
Thanx,
Tom

[7N7]

If your aim is simply to allow th valves to warm up longer, then a simple switch could be used, although with an indirectly-heated rectifier it's not really worth it.

You must not leave the valves on with full heater voltage for long periods; this will cause cathode-poisoning.

If you wish to leave the valves on permanently, then they must be run at about 60% of the heater voltage (i.e. about 4v).

This means that a "simple switch" will not do.

7N7

[fdegrove]

Hi,

This means that a "simple switch" will not do.

In a way a simple switch would do just fine, namely the ON/OFF switch.

Since the QIIs use the GZ32 valve rectifier it has a built in soft start, I can't really think of any reason why you'd want to change that.

What is your concern? Valve life? Warm up time?

Cheers,

[7N7]

Originally posted by fdegrove
Hi,



In a way a simple switch would do just fine, namely the ON/OFF switch.

Since the QIIs use the GZ32 valve rectifier it has a built in soft start, I can't really think of any reason why you'd want to change that.

Frank:

This is what I said in my message - in different words admittedly!

7N7

[fdegrove]

Hi,

This is what I said in my message - in different words admittedly!

Sorry for not being so tactful when it comes to potential valve destruction.

No offense,

[Zombie]

Thanx for the replies!
I mainly thought about extending the life of the valves and also to provide a soft-start every time I bring home a new batch of NOS GEC KT66s, of course
Fdegrove, 7N7: would you please elaborate the built-in soft start in the Quad II using GZ32 (GZ34?)? How does that work?
Cheers,
Tom

[7N7]

Originally posted by Zombie
Thanx for the replies!
I mainly thought about extending the life of the valves and also to provide a soft-start every time I bring home a new batch of NOS GEC KT66s, of course
Fdegrove, 7N7: would you please elaborate the built-in soft start in the Quad II using GZ32 (GZ34?)? How does that work?
Cheers,
Tom

Quad II is fitted with a rectifier valve. This is a GZ32, an indirectly-heated type. This rectifier takes perhaps 15 - 20 secs to warm up before it will pass current. The current increases as the valve warms up therefore a soft start is guaranteed.

Meanwhile all the other valves have been warming up too, and should have reached probably >80% of their operating temperatures, ergo no problem.

As I mentioned earlier, if you want to leave the valves idling, i.e. with heaters on but no HT, then you must arrange things so that they idle at about 60% of normal heater voltage.

7N7

[Zombie]

Thank you!
One learns something new every day
I assume it goes for the GZ34, too...?
Tom

[7N7]

Originally posted by Zombie
Thank you!
One learns something new every day
I assume it goes for the GZ34, too...?
Tom

Yes it also applies to GZ34. However rectifiers do differ; 5U4 for example is directly heated and current will flow sooner than with the indirectly-heated types.

7N7

[bournville]

Originally posted by 7N7

As I mentioned earlier, if you want to leave the valves idling, i.e. with heaters on but no HT, then you must arrange things so that they idle at about 60% of normal heater voltage.

7N7

If I remember rightly, (and I admit I'm desperately trying to remember a circuit I saw 20 years ago!) it is possible to have an idling voltage for the valves by placing a half-wave rectifier (a standard silicon rectifier) in the heater leads. So long as there is no cap smoothing, the rectifier will pass half cycles at RMS voltages only, so 3.15v x 1.414 = approx 4.5v, enough to keep the valves warm without risking cathode poisoning. Then a simple switch wired across the rectifier will give the normal/idling voltage control.

The explanation may not be brilliant but the use of a half wave rectifier as a 'voltage dropper' did enjoy some success in old AC/DC radios as it meant you could reduce heat by avoiding a resistor dropper in the heater leads. It has also been used as a method of having an idling voltage on soldering irons, to keep them warm in between use and reduce warm-up time.