Dear Martin,
The bucking winding serves to increase the speed of the fan motor when
the
power transformer is under load. Alpha used a variable resistor to reduce
the fan voltage as well in their '77 series. This was to quiet the blower
noise.
If you don't mind the noise, put the fans at maximum voltage and get the
most
airflow you can generate. The 50 volt bucking winding isn't necessary
with
that modification.
If you look up the tube specs and see the maximum envelope
temperature and airflow requirement you will notice that
the 95C figure is still okay, but having the maximum push all the
time means a faster recovery with your key up in RCV.
Hal Mandel
W4HBM
On Sat, 23 Jul 2005 19:57:13 +0700 "Martin Sole" <msole@loxinfo.co.th>
writes:
> 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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