A short while ago I acquired 2 Alpha 78 amplifiers. Both were pretty dirty
and in need of some serious attention. One of these amplifiers is now
cleaned up and working, sort of, it puts out about 1300 watts with about 70
watts of drive on 20m and below. I have yet to try 15 and 10 but before I
try 10 I need to fix the broken strap going from the coil to the switch, I
presume regular solder is not appropriate!
I manufactured chimneys out of silicone rubber sheet and fitted an extra fan
to the back of the power supply area. I can blow a sheaf of about 10 sheets
of 80gsm paper off the top now. With just the internal fan I could barely
move 2 sheets. My understanding is that the type of fan fitted has its speed
controlled by the AC frequency, in this case 50Hz, giving a speed 20% less
than it would be in the US operating on 60Hz, is this not significant?
The Alpha 78 wires the fan across one of the 120VAC primaries together with
a 25VAC buck winding setting the fan voltage to 95VAC, why would this be?
I didn't check with the internal fan only but with both fans running and
using my cheap Chinese multimeter with the supplied temperature probe taped
to the top of the case in the airstream I get the following. Normal room
ambient varies around 27 C depending on how much airconditioning is running.
Amp on and in standby, after the 1 minute warm up about 30 C. Normal speech
at about 1200 W out it goes to around 78 C and with the processor on I have
seen about 95 C. Second question, can I relate these figures into something
useful to know if or not the tubes have enough air through them?
Thanks
Martin HS0ZED
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