----- Original Message -----
From: "Gene May" <gene-may@hotmail.com>
To: "Amplifier Mailing List" <amps@contesting.com>
Sent: Sunday, April 01, 2012 4:35 PM
Subject: Re: [Amps] Cheap auxiliary amp cooling AND IDLING PLATE CURRENT
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>> 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)
> ------------------------------------------------------------------------------------------------
> 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
I dont understand any of the logic for the above.
1 A tube has to be able to withstand the sum of the DC and RF voltages
applied to it plus a good safety margin so thats scratched. With tubes
designed for AM service that headroom is typically 3-4X the DC applied, with
pulse its much higher. Scratch 1
2. In standby the HV is at its maximum and the filtering is designed to
leave sufficient headroom. Scratch 2
3. Relay keying of the HV transformer primary during operation went out with
6V ignitions and flatheads except for those using or building vintage
circuitry AM amps. A good part of the reason that a choke input has good
regulation is the low value bleeder resistors that had to be used. A
200-225W resistor was very common even for a 2000V 1000W INPUT amp back
then. Scratch 3
4. For low power, chokes were cheaper than high value caps in the vintage
past. The last ham amp choke user is long out of that business and they had
reliability issues anyway. Scratch 4
The only reason for excessive idle current is poor tubes that werent
designed for linear service to start with and the few that were werent
concerned about IMD such as the Soviet military.
Carl
KM1H
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