[Amps] Cheap auxiliary amp cooling AND IDLING PLATE CURRENT

[Gene May]

> On 3/30/2012 11:08 AM, Vic K2VCO wrote:
 I often wonder why so many amplifiers are designed so that the full ZSAC flows when the user isn't actually transmitting. . . . .
> >
This applies to VOX or PTT voice and data modes, too, although the heat reduction there is smaller.  A 3-500Z that is idling at 3kV with ZSAC of 100 mA is dissipating 300 watts just sitting there!
> 73
 
> Roger (K8RI)
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Roger and Vic,
 
I agree; it is a waste.  Pardon the following cynicism, but one of the reasons -- I am not saying I agree with it -- has to do with the fact that almost all QRO amps have capacitative-input filters.  When the load is taken off them, their voltage rises to 1.414 times their voltage under load.  This could be above the tube's rating, and in the case of marginally-designed power supplies, above the sum of the voltage ratings of the filter caps.  Another reason probably has to do with cost:  it would be possible to design cutoff for the plate voltage when there is no RF drive, but the decent high-power relay this would require would be relatively expensive.  Choke input filters on QRO amps now would probably be significantly pricier due to the current cost of copper.
 
Old audio buffs will recall that good old tube amplifiers (e.g., Dynaco, MacIntosh) had choke input filters, which had much better voltage regulation than cap-input filters, with lower idling current.  I think the old Collins 30S-1 had choke-input filters for its plate and screen supplies.
 
Gene May
WB8WKU