[Amps] Warm up and on time question...

[Patrick Barthelow]

Here is an idea to help instant on Tubes, reduce the cold to hot current shock cycles.
I was going to say install a Variac  in the filament circuit.  Variacs to handle 200 watts are not so expensive and can be found at ham flea markets.  A way to do that is to route primarys to  the rear chassis of the amp through bananna jacks.  have the Variac on the op table somewhere and when you begin ops, simply bring up the filaments, with a turn of the variac to full voltage, say  1-2 seconds which will eliminate the high current pulse while the filaments gain heat and resistance.  Maybe a bypass switch on the Variac once fils are hot, to directly feed the primaries.   Or if you cannot isolate the filaments from the HV, then simply run a modest power 240 V variac to bring the whole amp up No need for a 2-3KW variac.  After the amp comes up, flip a bypass toggle switch to run direct to the AC mains.

Best Regards,   
Pat Barthelow;  AA6EG

> From: fxb1982 at hotmail.com
> To: dave at nk7z.net; amps at contesting.com
> Date: Tue, 13 Dec 2011 23:38:34 -0600
> Subject: Re: [Amps] Warm up and on time question...
> 
> 
> This is what I gather:
>  
> The AL-82 manual specifies warm up time as "approximately 30 seconds" while the AL-80B manual states "10 seconds".  I am guessing that the chimney type cooling VS the muffin fan cooling method accounts for the different warm up times?
>  
> W8JI states that to obtain maximum life from an 8877 tube, not to cycle the heater (filament) excessively.  http://www.w8ji.com/8877_failures.htm  It stands to reason that this would apply to a 3-500 tube as well.  
>  
> The QRO HF-2000 manual states:  "After prolonged operation, let the Amplifier run for several minutes without drive applied so the fan will cool the tubes before you turn the Amplifier off."
>  
> When I plan on operating an amplifier for 5 hours I leave it on for 5 hours.  I don't think turning an amplifier off and then back on after 30 minutes constitutes excessive cycling(subjective?), but I consider a tube filament to be like a light bulb and it is said that the life of a light bulb is shortened by cycling.
>  
> I as well warm up my tube amplifiers for about 5 minutes and let them cool down to what physically feels like idling temperature to me before shutting down.  Whether this is necessary or to what extent the tube needs to cool down may likely be a topic of debate but common sense tells me it does not hurt anything.
>  
> I'm looking forward to more info about this topic.
>  
> 73, BJ
> k0cwo 

> > From: dave at nk7z.net
> > To: amps at contesting.com
> > Date: Tue, 13 Dec 2011 07:30:30 -0800
> > Subject: [Amps] Warm up and on time question...
> > 
> > Hello,
> > 
> > I have an AL-82, my habit is to turn it on, allow it to warm for about 5
> > minutes prior to use, use the amp, then turn it back off. Sometimes I
> > find that I will be using the amp again within 10 or 15 minutes, maybe
> > 30 minutes. Is it better to leave it on for the half hour, or turn it
> > off then re power it 30 minutes later. 
> > 
> > Does this cause any issues over the long term? Should I just leave it
> > on for the half hour? Worried about repeated heating and cooling of the
> > tubes.
> > 
> > Dave
> > NK7Z
> > 
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