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Author Topic: Alpha 78  (Read 4516 times)
IK0PRG
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Posts: 4




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« on: May 05, 2005, 06:33:50 AM »

Hi OM!
I have recently purchased a used ETO ALPHA 78 amplifier: it work fine and I am satisfied. I have only a little problem. It is a "F" (suffix) model so don't put out the full power on 10M. I check in the elctrical diagram and found that the C34 capacitor on S1-B Load Capacitance switch is not installed on "F" models. Do anyone know the value and working voltage of C34?
Another question. How much is the max power output of this amplifier? I need this info to value the actual life time of the tubes. Note that my amplifier haven't the "low drive mods" on board. How much the power output would be increse with the "low drive mod"?
Thank you for your reply.
73 de Roberto IK0PRG
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K6AER
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« Reply #1 on: May 05, 2005, 03:34:06 PM »

Typically HF amplifiers put out less power on 10 meters than the lower frequencies. Power reduction could be as much as 15% lower on ten meters than 20 meters. This is because of stray inductance in the tank coil increases losses at ten meters.

For a specific legacy question send a E-Mail to Alpha Service at: service@alpaharadioproducts.com.
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W9GB
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« Reply #2 on: May 06, 2005, 09:32:43 PM »

Roberto -

Let's deal with your other questions about the Alpha 78.
http://www.alpharadioproducts.com/legacy.asp

IF operate this amplfier PROPERLY the 8874 tubes will give years / decades of amatuer service!!
I have the Alpha 76PA, which similar to your amplifier -- but with out the "pre-set" tuning additions.  This amplifier was purchased around 1980 by one of my elmers - K9EWJ(sk).  It still has the original Eimac 3CX400A7 (8874) tubes -- and they are still operating at full output.

When this amplfier was built (1980s) "Legal Limit" as defined by the USA FCC was 1 KW DC INPUT (2 KW PEP INPUT).

You may have received an ampflier that was used heavily for government/military or contest usage -- in that case -- your tubes may be showing some age.  The 3CX400A7 (8874) tube are used in health care/medical and comemrcial trasnmitters - so a surplus market does exist.  New tubes are available from both Eimac and Svetlana.

w9gb
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W9GB
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« Reply #3 on: May 06, 2005, 09:39:13 PM »

You may find this download of interest
Service/info manual for the ETO Alpha 76 amplifier
(much of this also covers the Alpha 76, 76A, 374A & 78 )
http://www.qsl.net/kk5dr/ETO76PDF.htm

w9gb
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N3ZC
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« Reply #4 on: July 15, 2015, 04:19:31 PM »

How much power should my Alpha 78 be putting out in say 20M Bandpass Mode with say 60W drive. Low SWR (SteppIr) I'm trying to decide if my tubes need replacing...Thanks all Smiley The tubes are 2- 3CX800A7s
« Last Edit: July 15, 2015, 05:53:46 PM by N3ZC » Logged
AD4U
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« Reply #5 on: July 15, 2015, 06:09:50 PM »

I bought an Alpha 78 brand new in the early 1980's and used it for around 10 years. When driven by 100 watts from a Kenwood TS-930S I could get around 1200 watts out on most bands - a little less on 10M. As others posted the Alpha 78, even with 3-8874 tubes was never designed to run 1500 watts output.

I understand that 8874 tubes are extinct. If you are happy running around 1200 watts and if your tubes are good, you should get many years (decades) of use. If you drive it to 1500+ watts output, you will abuse the tubes and probably get much less tube life. When the tubes are shot, you may as well scrap the amp or modify it to use other tubes that are available.

Dick AD4U
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N3ZC
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« Reply #6 on: July 16, 2015, 03:30:58 AM »

Thanks..mine is using the 3CX800A7 tubes
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KM4AH
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Posts: 599




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« Reply #7 on: July 16, 2015, 04:03:05 AM »

How much power should my Alpha 78 be putting out in say 20M Bandpass Mode with say 60W drive. Low SWR (SteppIr) I'm trying to decide if my tubes need replacing...Thanks all Smiley The tubes are 2- 3CX800A7s

It depends on whether it has the low drive mod and how it was done. My 374 A will do about 900 out with 50 watts drive, but somebody had modded the input at some point.
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N3ZC
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« Reply #8 on: July 16, 2015, 04:04:58 AM »

Mine has the low drive mod.
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KM4AH
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« Reply #9 on: July 16, 2015, 08:04:14 AM »

Well, I'm thinking the gain should be about the same although the 3Cx800's are capable of more power. So, if you can get a KW or so with 50 watts drive and not exceed 50 ma on the grid current you should be fine. The 3cx800's will out run your power supply if it is stock so don't expect twice the output of the 8874's or anything like that. I'm talking about SSB PEP BTW.
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KM1H
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« Reply #10 on: July 18, 2015, 08:01:08 AM »

The 76/78 series came out when the FCC limit was 1000W INPUT on CW and 2000W INPUT PEP on SSB. Once the 1500W OUTPUT rule was in effect many tried for more and often fried tubes and transformers.

My stock 76PA (3 holer with EI style transformer) will do 1800W 160-15 with 100W drive but I run it at 1200W for longevity as Im in no hurry to do a 3CX800 conversion.

With the optional hypersil transformer the output is still limited due to the low HV but 2000-2200 is possible but Id also add a muffin fan outside the cabinet where the factory optional one went. Since I run mine on AM at times I use the extra fan and the 1200W available tube dissipation allows for a decent signal.

Carl
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