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[AMPS] Ameritron AL-572 Difficulties with Gas Arcs

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Subject: [AMPS] Ameritron AL-572 Difficulties with Gas Arcs
From: e.briggs@snet.net (Ed Briggs)
Date: Sat, 24 Feb 2001 21:02:25 -0500
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I'm having troubles with my new Ameritron 572-B, and I thought I'd pose =
some questions here.
Essentially, I'm having persistent problems with gas arcs in the tubes =
which frequently take
out the bias transistors. Since I've had the amp, about two months, I've =
had to replace the
transistors about 6 times.  I'm on my second set of tubes, and the third =
are on their way.
Sometimes, the tubes arc in standby (HV is applied, but the tubes are =
biased to cutoff). =20

I hasten to add that Ameritron has been very helpful, and I believe them =
when they say these
problems are rare with the 572, but alas here I am.  So here are my =
questions which I hope
may help shed some light on getting this solved. The amp is a very nice =
unit, and for me
it represents a great 'bang for the buck'  once I get the present =
difficulties ironed out.

First, can I reasonably expect this to get ironed out eventually? I'm =
not using Svetlanas.
My HV is 2800v no load dropping to about 2400 under load and I have only =
been operating
into an 50 dummy load, can very carefully at that (no stress testing!!)

I guess what I'm asking here is if the design of the amplifier places =
too high an operating voltage=20
on the tubes.

Secondly, if the gas arcs are something I can expect to continue, is =
there a method of
protecting the bias circuitry?  I'm concerned that after some number of =
transistor replacements,
I'll start to cook the traces off the power supply board.

Finally a couple general questions about 572s. =20

First, when the bias transistors go, the tubes draw about 200ma at about =
2500 v, or appx 500w with
no drive. Within 20 seconds or so, the anodes begin to glow red, and =
they will grow redder and redder until
I place the amp in standby.  Since the literature (e.g. Svetlana) says =
the tubes can dissipate 160
watts ICAS without turning red, I'm wondering why 125w would cause this. =


Secondly, I'm toying with the idea of disabling the dynamic bias =
altogether in an attempt to put
an end to constantly having to replace the transistors.  Assuming I were =
to use the T/R relay
to switch between cutoff and operating bias, what would be an =
appropriate zero signal current and
bias voltage?  Reading the manual suggests that 100 ma is the resting =
current (I conclude this
from the operating manuals notes on dynamic bias).  Svetlana suggests =
something closer to
200 ma, but my experience (above) is that this resting current causes =
the anodes to get red,
and that's not supposed to happen to these tubes (from what I've read).

So, if there are any other thoughts or suggestions on what to do, please =
let me know.=20

And again, I'm not bashing or interested in bashing the manufacturer. =
They've been very responsive and we're
working these issues together. And the amp itself seems to be a very =
nice unit. There are many apparently
in use, so this note should not be construed as a general problem with =
all AL-572s.  Just something
that's causing me accelerated hair loss.

Thoughts/suggestions much appreciated.

Ed Briggs
N1TS=20

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<META content=3D"text/html; charset=3Diso-8859-1" =
http-equiv=3DContent-Type>
<META content=3D"MSHTML 5.00.2920.0" name=3DGENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial size=3D2>I'm having troubles with my new =
Ameritron 572-B,=20
and I thought I'd pose some questions here.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Essentially, I'm having persistent =
problems with=20
gas arcs in the tubes which frequently take</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>out the bias transistors. Since I've =
had the amp,=20
about two months, I've had to replace the</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>transistors about 6 =
times.&nbsp;&nbsp;I'm on my=20
second set of tubes, and the third are on their way.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Sometimes, the tubes arc in standby (HV =
is applied,=20
but the tubes are biased to cutoff).&nbsp; </FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>I hasten to add that Ameritron has been =
very=20
helpful, and I believe them when they say these</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>problems are rare with the 572, but =
alas here I=20
am.&nbsp; So here are my questions which I hope</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>may help shed some light on getting =
this solved.=20
The amp is a very nice unit, and for me</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>it represents a great 'bang for the =
buck'&nbsp;=20
once I get the present difficulties ironed out.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>First, can I reasonably expect this to =
get ironed=20
out eventually? I'm not using Svetlanas.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>My HV is 2800v no load dropping to =
about 2400 under=20
load and I have only been operating</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>into an 50 dummy load, can very =
carefully at that=20
(no stress testing!!)</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>I guess what I'm asking here is if the =
design of=20
the amplifier places too high an operating voltage </FONT></DIV>
<DIV><FONT face=3DArial size=3D2>on the tubes.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Secondly, if the gas arcs are something =
I can=20
expect to continue, is there a method of</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>protecting the bias circuitry?&nbsp; =
I'm concerned=20
that after some number of transistor replacements,</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>I'll start to cook the traces off the =
power supply=20
board.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Finally a couple general questions =
about=20
572s.&nbsp; </FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>First, when the bias transistors go, =
the tubes draw=20
about 200ma at about 2500 v, or appx 500w with</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>no drive.</FONT><FONT face=3DArial =
size=3D2> Within 20=20
seconds or so, the anodes begin to glow red, and they will grow redder =
and=20
redder until</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>I place the amp in standby.&nbsp; Since =
the=20
literature (e.g. Svetlana) says the tubes can dissipate 160</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>watts ICAS without turning red, I'm =
wondering why=20
125w would cause this.&nbsp;</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Secondly, I'm toying with the idea of =
disabling the=20
dynamic bias altogether in an attempt to put</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>an end to constantly having to replace =
the=20
transistors.&nbsp; Assuming I were to use the T/R relay</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>to switch between cutoff and operating =
bias, what=20
would be an appropriate zero signal current and</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>bias voltage?&nbsp; Reading the manual =
suggests=20
that 100 ma is the resting current (I conclude this</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>from the operating manuals notes on =
dynamic=20
bias).&nbsp; Svetlana suggests something closer to</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>200 ma, but my experience (above) is =
that&nbsp;this=20
resting current causes the anodes to get red,</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>and&nbsp;that's not supposed to happen =
to these=20
tubes (from what I've read).</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>So, if there are&nbsp;any other =
thoughts=20
or&nbsp;suggestions on what to do, please let me know. </FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>And&nbsp;again,</FONT><FONT =
face=3DArial size=3D2> I'm=20
not bashing or interested in bashing the manufacturer. They've been very =

responsive and we're</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>working these issues together. And the =
amp itself=20
seems to be a very nice unit. There are many apparently</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>in use, so this note should not be =
construed as a=20
general problem with all AL-572s.&nbsp; Just something</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>that's causing me accelerated hair=20
loss.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Thoughts/suggestions much =
appreciated.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Ed Briggs</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>N1TS</FONT>&nbsp;</DIV></BODY></HTML>

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