We have probably beat the screen regulation issue to death - but the very
nice Collins S-line and M-2 series did not use regulated screen voltage on
their final 6146's.
As a matter of fact, they were poorly regulated - but the Collins always
sounded superb - partly because they used rf feedback which improved the imd
by 10 db and actually reduced distortion.
Bill Orr ran an article in Ham Radio and scrutinized the 3rd order imd of
the Collins. He noted that by removing the screen from the original
configuration of a series resistor and attaching it to the low voltage
supply (which had less variation), he reduced imd by a few db.
Unless the driver transceiver uses feedback or high voltage transistors,
almost any amp (except a sweep tube amp) will have better imd specs than the
driver - meaning the results will be irrelevant no matter how the screen is
regulated if a proper tube is used. The 4CX1000 is a poor tube to use
without feedback. The 4CX1500 is excellent. Today, many of the high end
transceivers have superb 3rd
order imd specs, meaning that the amp is now becoming the weak link.
Amateurs are still way behind commercial units. The standard for ham gear
is a -30 db whereas I believe most commercial equipment is a -45 db. If you
listen on the ham bands, you can tell the difference.
The bottom line is that amateurs are experimenters. Excellent tetrode amps
can be made without a regulator - if the builder is prepared to test and
document the amp. under actual conditions of use. Without the testing, it
is better to follow the lead of someone else.
A real advantage of the screen regulator circuit is that it provides
absolute protection in case the plate voltage is lost for some reason. I
have built amps without electronic regulation on the screen that had
excellent imd performance - but there was no protection for loss of plate
voltage until I used a relay operated by the plate bleeder to give some
protection. However, if the rf choke had opened, I would have been in
trouble. There are some fine regulator circuits and boards available and
they are highly recommended.
Colin K7FM
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|