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Stupid Standby Question

2/22/2000 1:34 AM
Mark Knapp
Stupid Standby Question
Has anyone ever turned off the rectifier tube heater to accomplish standby? This would leave the signal tubes warm and the bias voltage applied, though obviously it  
would still take time for the B+ to come up. Alternately, could you just disconnect one of the two high voltage AC rails from the rectifier and just lower the DC  
voltage? Or would one expect that to overstress the rectifier tube?  
Mark Knapp
2/23/2000 6:13 PM
Stephen Conner
actually quite a deep issue
Hi Mark,  
Turning off the rectifier heater would have the desired  
effect. However, and this is especially true with 'soft  
start' indirectly-heated rectifiers like the GZ34,  
every time you warm the rectifier up onto a load (ie with  
the power tubes already hot) you will damage the cathode  
coating a little. The life of your rectifier will probably  
be reduced.  
Having said that, with the usual standby switch  
configuration, the current surge when you flip the switch  
to 'operate' will damage the cathodes too. The question  
is, which way does the least damage? Who knows? Not me.  
I *do* know that disconnecting one of the AC rails from  
the rectifier to reduce the voltage is a no-no.  
You will not get much of a reduction in voltage, but the  
remaining half of the rectifier will be overloaded and the  
transformer will hate it too.  
Steve C.
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2/23/2000 7:59 PM

As long as the heater is warm, you won't strip the cathodes. Another way to go on standby is the way marshall does, which is to switch the low voltage node where the output tubes join and go to ground. This is better than switching the B+, which is high voltage, as on Fender amps, because of switch life.  
2/23/2000 8:01 PM

sorry for the terrible typing on my previous post.
2/24/2000 2:21 AM
Mark Knapp

I am not sure I understood, but are you saying they lift the poweer tube cathodes? That sounds like a good solution.  
2/24/2000 10:48 AM
Re: Stupid Standby Question
The rectifier filament supply is at a high voltage, with lots of current. High voltage, high current, going to a mechanical switch on the chassis - that's not too reliable.  
The usual options are:  
1) switch the B+ (altough it's still high voltage there's much less current)  
2) switch the CT (low voltage, low current - but if the negative bias is from a tap on the B+ winding, it will also switch bias of).  
3) switch the cathodes (low voltage, low current, bias OK - but preamp tubes are still on).
2/25/2000 2:01 AM

I was going to make the same comment but, if Mark were to use a separate rectifier filament transformer (remember, remember, separate cathode type tubes have "heaters" - most 5V rectifiers have "filaments") he could switch the 120v primary easily - a thought.  
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