[AMPS] Alpha 78
Lamb
k7fm@teleport.com
Sat, 23 Feb 2002 08:36:17 -0800
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Hello:
I converted a 76PA (3 x 8874 tubes) to 2 x 3CX800 tubes. I think the =
main reason I did it was just to see if I could do it. It really is not =
difficult if you are not frightened with a project. I do not think it =
devalues the amplifier, although I must say that I purchased a second =
amp real cheap and have left the 8874 tubes in it. =20
The 78 is close to the 76PA for conversion purposes. I think you have =
the hypersil transformer, which mine does not and I think your =
transformer is a bit smaller.
With the conversion, the middle socket is plugged and a black piece of =
cardboard is taped to the underside of the top to cover the hole. New =
silicon chimneys need to be made. The filament voltage needs to be =
increased. I found two surplus transformers to use in series with the =
existing filament voltage and stacked them along the right side =
transformer corner. I build a bracket using existing holes, so thee was =
no holes drilled. I used the two transformers because of size =
limitations. Voltage was a bit high and dropped with a dropping =
resistor. If you use a dropping resistor, each tube should have its own =
resistor so the voltage does not soar when one tube is pulled. =20
Next, the startup delay must be increased from 2 minutes to 3 minutes. =
This simply involves a capacitor in the timing circuit. =20
That is about it. Works fine. I have seen one mod that has an external =
transformer and that looks stupid. On the second Alpha, I actually =
moved my power transformer toward the center about 3/4" to give me more =
clearance to add a new board for some sophisticated extra stuff. But, I =
did not change tubes. The reason is that the "new" amp takes 3 minutes =
to warm up, while the "old" amp takes 2 minutes. I prefer 2 minutes and =
bought some used 8874 tibes which should last a lifetime. =20
That is about it. If nothing else, the conversion proves you are more =
than an appliance operator worthy of an extra class license. =20
73, Colin K7FM
----- Original Message -----=20
From: W4KEN=20
To: amps@contesting.com=20
Sent: Saturday, February 23, 2002 7:57 AM
Subject: [AMPS] Alpha 78
has anyone done a tube conversion on a 78 ?
=20
I have been told it is a real pin in the a** .
=20
I know it will devalue the amp but I plan on holding on to it for a =
long time
=20
I was thinking about using/converting it to 3cx800a7's.
=20
=20
Im thinking it would be cheaper to convert it that to buy 3 new 8874's
=20
any input would be helpful
=20
Thanks
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<HTML><HEAD>
<META content=3D"text/html; charset=3Diso-8859-1" =
http-equiv=3DContent-Type>
<META content=3D"MSHTML 5.00.2614.3500" name=3DGENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial size=3D2>
<DIV><FONT face=3DArial size=3D2>Hello:</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>I converted a 76PA (3 x 8874 tubes) to =
2 x 3CX800=20
tubes. I think the main reason I did it was just to see if I could =
do=20
it. It really is not difficult if you are not frightened with a=20
project. I do not think it devalues the amplifier, although I must =
say=20
that I purchased a second amp real cheap and have left the 8874 tubes in =
it. </FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>The 78 is close to the 76PA for =
conversion=20
purposes. I think you have the hypersil transformer, which mine =
does not=20
and I think your transformer is a bit smaller.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>With the conversion, the middle socket =
is plugged=20
and a black piece of cardboard is taped to the underside of the top to =
cover the=20
hole. New silicon chimneys need to be made. The filament =
voltage=20
needs to be increased. I found two surplus transformers to use in =
series=20
with the existing filament voltage and stacked them along the right side =
transformer corner. I build a bracket using existing holes, so =
thee was no=20
holes drilled. I used the two transformers because of size=20
limitations. Voltage was a bit high and dropped with a dropping=20
resistor. If you use a dropping resistor, each tube should have =
its own=20
resistor so the voltage does not soar when one tube is pulled. =20
</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>Next, the startup delay must be =
increased from 2=20
minutes to 3 minutes. This simply involves a capacitor in the =
timing=20
circuit. </FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>That is about it. Works =
fine. I have=20
seen one mod that has an external transformer and that looks =
stupid. On=20
the second Alpha, I actually moved my power transformer toward the =
center about=20
3/4" to give me more clearance to add a new board for some sophisticated =
extra=20
stuff. But, I did not change tubes. The reason is that the =
"new" amp=20
takes 3 minutes to warm up, while the "old" amp takes 2 minutes. I =
prefer=20
2 minutes and bought some used 8874 tibes which should last a =
lifetime. =20
</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>That is about it. If nothing =
else, the=20
conversion proves you are more than an appliance operator worthy of an =
extra=20
class license. </FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>73, Colin =20
K7FM</FONT></DIV></FONT></DIV>
<BLOCKQUOTE=20
style=3D"BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; MARGIN-RIGHT: =
0px; PADDING-LEFT: 5px; PADDING-RIGHT: 0px">
<DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV=20
style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>=20
<A href=3D"mailto:w4ken@tampabay.rr.com" =
title=3Dw4ken@tampabay.rr.com>W4KEN</A>=20
</DIV>
<DIV style=3D"FONT: 10pt arial"><B>To:</B> <A =
href=3D"mailto:amps@contesting.com"=20
title=3Damps@contesting.com>amps@contesting.com</A> </DIV>
<DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Saturday, February 23, =
2002 7:57=20
AM</DIV>
<DIV style=3D"FONT: 10pt arial"><B>Subject:</B> [AMPS] Alpha 78</DIV>
<DIV><BR></DIV>
<DIV><FONT face=3DArial size=3D2>has anyone done a tube conversion on =
a 78=20
?</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>I have been told it is a real pin in =
the a**=20
.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>I know it will devalue the amp but I =
plan on=20
holding on to it for a long time</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>I was thinking about using/converting =
it to=20
3cx800a7's.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Im thinking it would be cheaper to =
convert it=20
that to buy 3 new 8874's</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>any input would be =
helpful</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Thanks</FONT></DIV>
<DIV> </DIV></BLOCKQUOTE></BODY></HTML>
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