>
>On Wed, 03 Dec 1997 20:56:03 -0500 "Joe Subich, W8IK" <w8ik@ibm.net>
>writes:
>>At 10:54 AM 12/3/97 EST, KM1H wrote:
>>>
...snip...
>> ... ... The tube is rated for 300 watts plate
>>dissipation ... so I wouldn't push them.
>>
>AMEN Joe. Too many hams think and use the 8875 as if it was the 8874.
>Heath SB-230 owners (8873) are even worse!
However, Denny's cooling must have been semi-OK in the MLA because I have
never heard about an 8875 in one failing from overheated ceramic to metal
seals. Stripped cathodes from CB-ish overdrive, yes. Gold-sputtered
grids, yes. . .
- I have never heard about an 8873 failing in a SB-230 due to overheated
seals.
>
>>R1 was a 100 ohm wirewound resistor ... with it in place, the input
>>impedence of the MLA-2500 is about 30 Ohms with 40 pF shunt
>>capacitance, making the amplifier difficult to drive with solid state
transmitters.
......snip ...
>>The amplifier requires a non-inductive swamping resistor of aproximately
>>50 Ohms/25 watts (to suck up about half of the drive from they typical 200
watt rig)
>> and a 2:1 un-un (to bring the drive impedence back into the 50 Ohm range).
>
Another way to arrive at a match is to add shunt and series input
resistors. To do this: Put approx. 40 ohms in series with each cathode
-- and add a approx 100 ohm shunt R in parallel with the input. The
series/parallel combo provides close to 50 ohms -- without using a
toroidial un-un.
>Even 100W and the resistor is pushing it...cut the 1000D WAY back to
>70-80W.
The rated drive is 27w per cathode.
>
>> Even then, the shunt capacitance (tune input) can
>>make the exciter/amplifier system unstable on some bands depending on
>>coax lengths.
The instability problem has a lot to do with the 'abundanza' of VHF
resonances in the extraordinarily long and winding lead between the tune
C and the anodes. This is a good example of how not to lay out an HF
amplifier. If Denny had put the tuning capacitor next to the tubes via
an extension shaft from the front panel, and moved the tank inductor
forward, the stability would have been better.
>> ... Joe Subich, W8IK ex-AD8I
>> <w8ik@ibm.net>
>
>
>That was part of the excitement with Dentron products...it actually
>required a few brains to get them 100% . Not Plug n Play !!
Dentron was more like Plug 'n' Pray, Carl.
- The ironic thing is that Eimac tried to help Denny fix the problems in
the MLA prototype, and he didn't do a thing they recommended.. . . .
Are amplifier manufacturers hubristic, or what?
Rich...
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|