>Richard said:
>
>"* No tuned input with g-g is not hardly engineering practice".
>
* Pse strike the "not".
>Well, perhaps. Let me throw some round numbers out - just for discussion.
>I do not claim they are researched numbers, nor that the conclusions based
>upon these numbers are correct. But, I do hope to raise some questions at
>generalities spread. I recall the third order distortion level of the
>4CX250 tube is about -24 db.
* The way Hamdom measures distortion, that's 6db better, or 30db down.
I have built 4cx250 amps that produced under 32db down total distortion.
The hat-trick is to run minimal screen-current, zero grid-current and
maximal anode potential. The signal can additionally be cleaned up by
using euphemisms for 'dirty' words.
>That is not adequate.
* 32 db down total imd is c. average for Ham transceivers.
>However, driving the
>cathode has inherent degeneration and may improve the third order
>distortion level by perhaps as much as -6 db.
* agreed
>That would equal -30 db.
>And, while I try to build amplifiers cleaner than that, that is still
>acceptable and cleaner than most transceivers. There is no question that
>tuned circuits in the cathode will improve the third order distortion
>products by acting as a flywheel. Maybe there is a 3 db improvement or
>maybe a 5 db improvement.
? depends on the Chebyshev filter in the radio, the freq., and the
length of input coax.
> Without the tuned cathode, the load on the
>transceiver will vary substantially. But, still, the output signal may be
>acceptable. I do not know for sure and have not tried it with all
>transceivers. But, I am not prepared to say that it necessarily will sound
>rotten.
>
* On the Ham bands, according to one paragraph of Part 97, imd products
must be confined to the authorized band segment in use. For example 7.15
- 7.3MHz. I know of no case where anyone has been cited for rotten
splatter in the last 4 decades. P. J. O'Rourke said giving a gov't
beaucrat authority is like giving a teenage boy your car-keys and a
bottle of whisky.
>My Alpha 76PA uses cathode drive with no tuned input. It is not a poorly
>designed amplifier
* The 76 uses:
-1w carbon-comp resistors for equalizing the potential on the
electrolytic caps: Such resistors change value somewhat randomly.
-Medium-Q vhf parasitic suppressors: Although a 76 or a 78 is not as
unstable as a TL-922 or a SB-220, they occasionally osc at vhf. An
autopsy photo of a gold-sputtered 8874 from an Alpha appeared in
"Parasitics Revisited"/Sept.-Oct., 1990 *QST*.
- For the QSK-option, it does not use Jennings' speedup circuit for the
RJ-1A relay.
>and continues to run fine after a quarter century. The
>distortion products are acceptable and I have never had a complaint of
>splatter and it looks clean on my Tektronix spectrum analyzer.
* how many dog biscuits down @5kHz with voice modulation?
>It does have some resistive loading which no doubt does help a bit.
>
* Which would help more if there was a fast-switch 1A steering diode in
series with it to apply the R-loading on the neg. half of the drive and
disconnect on the pos. half.
- note - if you know how to solder, you are willing to try this mod.
make the measurements, and publish the results here, I will send you a
freebee diode in the interest of R and D.
>I did not look at the circuitry - just the mechanical assembly.
>
>I did build an amplifier once with cathode drive, no tuned input, and no
>grid meter. It ran 4 x 6AG7 tubes at a couple of hundred watts input. The
>problem, however, was that the 6AG7 was black and I could not see the
>plates. They lasted for over a year, when the amplifier was finally
>retired.
* 6AG7-mavens built amps with the tubes base-up and the tops submersed
in a pan of water. For contesting, add ice cubes.
> The CTI amplifier looks much better than my 4 x 6AG7 amp.
>
>At least you guys are reading these posts.
>
cheerz, Colin
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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