[AMPS] Bias for SB-220

[Tom Rauch]

OK let's play number games.

From:           	yo3ctk@qsl.net
Date sent:      	Wed, 02 Jun 1999 17:03:06 +0300
To:             	amps@contesting.com
Subject:        	RE: [AMPS] Bias for SB-220

> 
> W8JI:
> 
> The real problem is wasted power and heat. Letting the bias run  in
> full-tilt on condition is like putting a 600-1000 watt space heater
> the room.
> 
> Ok, let me see: 1500 out at 60% efficiency means 2500 in. If the bias
> current is set at 20% max. plate current, zero signal plate dissipation
> would be 500W. You're 3dB off, Tom.

Sorry. 

600 watts is not 3 dB more than 500 watts. Wrong answer, no 
prize.

The common bias setting is generally where the quiescent dissipation 
is 50% of rated dissipation at full output. That's about 750 watts for
 a 1500 watt dissipation tube.

While it is true you can run the tube at 20% of maximum plate 
current, or any value you pick, the general rule of thumb is to use 
more. That's particulary true with glass tubes, like 3-500's where 
the gettering agent is coated on the anode. If you run the anode too 
cold, the tube will not degas properly and you may have periodic 
arcovers. For long life in the 3-500, you need a touch of anode color.
  
> Further, for 50% on, 50% off contest duty cycle, "wasted" dissipation
> would be just 250W. Not much of a heater.

Depends, most contesters I hear seem to use a 80% on time since 
they prefer to transmit more than listen to keep a frequency clear.
  
> On the other hand, I could understand the economics behind EBS approach:
> for a few dollars in parts, the manufacturer could use a smaller blower
> with corresponding cost reduction. Not sure if an economy in the cooling
> system department is a good idea, though.

Properly implemented electronic RF bias switching does not add 
distortion or splatter. Anyone who thinks so must not understand 
rise times and biasing, and the cause of IMD. They probably 
simply parrot what they hear from other people who don't 
understand the problem.

The filter in the SSB transmitter limits rise time rate due to the 
bandwidth of the filter, so it is quite easy to build a bias system 
that leads the envelope rise time by toggling quickly into a full 
operating state long before power levels reach a few hundred 
milliwatts.

We are really, once again, arguing a non-issue. Like the ferrite core.

The most common reason slower blowers are used is most 
customers whine about noise. In many amps, noise takes priority 
over cooling at rated static pressure.

The market generally dictates what is sold, not the other way 
around. Building an amplifier that sells well is a juggling act 
between many factors. 

If an amp was built entirely the way any one person on this 
reflector views "properly", the total sales would be somewhere just 
over one unit. He and his buddies would be the only ones really 
happy.







 
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73, Tom W8JI
w8ji@contesting.com

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