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[AMPS] Alpha 78

To: <amps@contesting.com>
Subject: [AMPS] Alpha 78
From: k7fm@teleport.com (Lamb)
Date: 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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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<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>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>I converted a 76PA (3 x 8874 tubes) to =
2 x 3CX800=20
tubes.&nbsp; I think the main reason I did it was just to see if I could =
do=20
it.&nbsp; It really is not difficult if you are not frightened with a=20
project.&nbsp; 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.&nbsp; </FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>The 78 is close to the 76PA for =
conversion=20
purposes.&nbsp; I think you have the hypersil transformer, which mine =
does not=20
and I think your transformer is a bit smaller.</FONT></DIV>
<DIV>&nbsp;</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.&nbsp; New silicon chimneys need to be made.&nbsp; The filament =
voltage=20
needs to be increased.&nbsp; I found two surplus transformers to use in =
series=20
with the existing filament voltage and stacked them along the right side =

transformer corner.&nbsp; I build a bracket using existing holes, so =
thee was no=20
holes drilled.&nbsp; I used the two transformers because of size=20
limitations.&nbsp; Voltage was a bit high and dropped with a dropping=20
resistor.&nbsp; 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.&nbsp;=20
</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Next, the startup delay must be =
increased from 2=20
minutes to 3 minutes.&nbsp; This simply involves a capacitor in the =
timing=20
circuit.&nbsp; </FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>That is about it.&nbsp; Works =
fine.&nbsp; I have=20
seen one mod that has an external transformer and that looks =
stupid.&nbsp; 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.&nbsp; But, I did not change tubes.&nbsp; The reason is that the =
"new" amp=20
takes 3 minutes to warm up, while the "old" amp takes 2 minutes.&nbsp; I =
prefer=20
2 minutes and bought some used 8874 tibes which should last a =
lifetime.&nbsp;=20
</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>That is about it.&nbsp; If nothing =
else, the=20
conversion proves you are more than an appliance operator worthy of an =
extra=20
class license.&nbsp; </FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>73,&nbsp; Colin&nbsp;=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>&nbsp;</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>&nbsp;</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>&nbsp;</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>&nbsp;</DIV>
  <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</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>&nbsp;</DIV>
  <DIV><FONT face=3DArial size=3D2>any input would be =
helpful</FONT></DIV>
  <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
  <DIV><FONT face=3DArial size=3D2>Thanks</FONT></DIV>
  <DIV>&nbsp;</DIV></BLOCKQUOTE></BODY></HTML>

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