[Amps] Alpha 78

[Carl]

 A pair of 3CX800's will last a long time even in a RTTY contest at that 
roughly 50% reduced power. The blower is more than adequate.

The EI transformer in the 76PA is not as rugged or as forgiving as the 
Hypersil in the CA. Both are 3 holers and I know several who have run them 
at 1500W in a RTTY contest. Over several decades the EI in the PA has failed 
for some.

Since the 78 is just a 76CA with a no tune section added its performance 
should be the same when in Manual. I dont believe an EI transformer was used 
in the 78 but Ive never seen one.

The key is to use the louvered section on the rear panel to mount an 
external muffin fan blowing on any/all of the 8874 amps that are being run 
hard.

The 3CX800 conversions should have the top cover opening increased even at 
the reduced power, that is a big restrictor up there. Ive seen several with 
a section cut out and perforated sheet used, it sure helps since the 
original design is pretty poor even with 8874's.

Adding some holes to the cover to the side and above the tank circuit can 
only help, especially on the highest bands and in bandpass mode.

It is also a good idea to recap the PS every 15-20 years as well as tossing 
the old carbon equalizing resistors for 3W MOX.  The leakage current just 
keeps getting worse as the caps age and it doesnt show up on the meter but 
the transformer knows all about it.

There are also other parts on the circuit board needing attention such as 
caps and out of tolerance carbon resistors.

Many years of hard use might crystalize the solder at the switch and 
elsewhere in RF paths, it should be removed and fresh solder flowed on.

Carl
KM1H



----- Original Message ----- 
From: "Roger (K8RI)" <k8ri at rogerhalstead.com>
To: <amps at contesting.com>
Sent: Sunday, August 26, 2012 4:22 AM
Subject: Re: [Amps] Alpha 78


> On 8/25/2012 4:06 PM, Carol Richards wrote:
>> I am using an alpha 78 with a pair of 3cx800A's  instead of the hard to 
>> get and afford  8874's.  I get about 1200w output on cw.  Not bad since 
>> the power supply only provides 2000V  and I am only able to drive it with 
>> 100w..   My question is   when I operate RTTY is it still safe to run the 
>> alpha at max output or should I cut back on the drive...This is 
>> especially important if I plan to operate a RTTY contest.
>
> Just give it the smell test, or stick a thermocouple onto the core and
> keep track of the temp.  Bout I'd settle for about 40C rise over room
> temp as good for 24 X 7.  The hams on here who really know transformers
> probably can give a limit higher than that for the different
> transformers use in the Alphas, or at least more accurate.
>
>
> 73
>
> Roger (K8RI)
>
>>
>>
>> Thanks,,,,
>>
>>
>> Carol
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>>
>
>
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