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The K9ZW Shack Alpha 78 Amp Story 23 - January - 2007

Posted by k9zw in Amateur Radio, K9ZW Shack.
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An interest in adding power, or going to QRO operations as they say, lead to the question “What amp would best balance needs, wants & budget?”

I’d spent many hours behind an Alpha 87a, Alpha Radio Product’s benchmark setting auto-band switching amp. These computer controlled amps are impressive in their ease of use and reliability. They also push the budget a fair bit, with the updated model the Alpha 9500 being roughly three times what my maximum amp budget was.

So it was off to do research, reading on-line reviews and talk with other amateurs with HF amp experience.

One amp that was recommended over and over, was an older Alpha Radio Products amp, the Alpha 78, a manual & bandpass tuning model using three 8874 tubes.

The 8874 is far from the cheapest watt per buck, but they have a decent life and are readily available.

Bandpass tuning is an interesting system where the Alpha 78 has pre-tuned settings for the main bands, one for SSB and CW. When a Bandpass setting is selected the Amp is switched so the front Load & Tune controls are out of circuit and a preset (internally accessable) Load & Tune value are brought on-line.

This means for most bands you can select Bandpass and the Alpha 78 is ready to go with about 90-95% of available power. If milking the last bit of output is important, or if tuning to a WARC band or 160m, there is a Tuned Setting for each band.

The Alpha 78 is by design quite tollerant of reasonable mismatches & minor blips. In some ways the Alpha 87a models seemed rather touchy kicking off-line sometimes seemingly for no reason at all.

 The Bandpass with CW/SSB selection goes a long ways towards reducing high power on-air tuning, or switching to a dummy load to tune.  Often you will hear an amateur who is using an Alpha 87a “blip” his amp after changing bands to trigger the 87a automatic tuning feature.  Bandpass tuning options let me skip the “blip.”

 For the WARC bands on a 78 one selects the manual tuning of a near band and manual tunes.  I found I can tune 17m most easily from the 15m manual setting, though it does work from the 20m manual as well.

Topband is manual tune only, being advertised as “Semi-Bandpass.”  I’ll have to ask Alpha what that actually means when I next talk to them.

The Alpha 78 also is quite a bit faster from off & cold to QRO operations compared to an Alpha 87a. Where the 87a takes something like 3 to 4 minutes to heat & offer QRO output, the 78 version is ready in about 1/2 the time. If you’ve just heard a country you need in a rare opening, and QRO is the only way you’re going to heard, 4 minutes is an awfully long time to wait for a 87a to come online. Your DX may have weel gone or the window closed while you’re druming fingers watching the front panel of an 87a.

In the end I found two Alpha 78s which each were sent to Alpha Radio Products for bench checking, installation of all safety & performance modifications and to have new front panels & cases installed. The Alpha 78 shown on Alpha’s website with new panels I’m told was mine. I sprung for both the new face & wrap on each to have amps that looked as nice as they work.

It looks like this Alpha 78 amps are keepers. Only an instant-on solid-state or the rare instant-on tube amp would be quicker to power up from cold, the output is more than adequate & is in balance with my station’s “ears”, and the budget wasn’t pushed two hard, of course excepting that I bought two 78 amps in the end.

There are other similar Alpha Amps – the Alpha 374 with the same three 8874 tube setup, the Alpha 374a with two 8874 tubes doing duty.  Both the 374 and 374a are single bandpass tuning per band, lacking the bandpass cw/ssb selector.  The 374a looks like a 78.   There is also a whole raft of manual tune models and of course great amps by other manufacturers.

It is possible to go QRO effectively without draining the bank account completely. It seems that an Alpha 78 is one way to meet those goals.

73

Steve
K9ZW

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1. For Sale - Alpha 78 Amp (K9ZW Station Backup Unit) « With Varying Frequency - Amateur Radio Ponderings - 12 - July - 2007

[...]  I had written up the Alpha-78 amps I had redone by Alpha Radio Products at “The K9ZW Shack Alpha 78 Amp Story” [...]


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