[Amps] Blowing out finals on a Icom 7300 using a Ameritron AL80B. Help Please!

Tue Jun 7 18:13:43 EDT 2022

Date: Mon, 6 Jun 2022 18:26:59 +0000 (UTC)
From: Alton McConnell <nu8l at yahoo.com>
To: "amps at contesting.com" <amps at contesting.com>
Subject: [Amps] Blowing out finals on a Icom 7300 using a Ameritron AL
        80B. Help Please!

Need some help please.
I am trying to repair an AL80B amp for a friend.? Initially. there was no
plate voltage on the plate HV meter.? The ham that sold him the amp said it
worked but the plate HV meter did not work.? He tried to use the amp and it
blew out the Icom 7300 final transistors Q131 and Q132.??
He sent the exciter back to the manufacturer and asked me for help.? We
tracked the no plate voltage issue down to a missing pin #9 on J101A ribbon
connector plug and an open R110.? Repaired these issues. Meters were
working properly.
After the repair, he was low power tuning the amp on 40 meters using 43
watts from exciter.? He was in the process of tuning the amp for higher
power when the amps meters dimmed and the Icom transistors blew (again!).?
The HV plate voltage meter is still working.? I have not tried to operate
the amp with another radio.? He was running a plate current of 375 mA and a
70 mA grid current when the issue occurred.
I am at a loss to understand what is going on here.
I had removed the front panel of the amp while tracing the no HV plate
meter reading issue a couple of weeks ago as discussed above.? Today I
wondered if I may have gotten the band switch incorrectly set when I
reinstalled the front panel.? I put the output of a MFJ 269 SWR analyzer
into the amps input.? Held the T/R relay closed (With the amp un-plugged
and caps discharged, of course!) and verified that the signals for the
various ham bands were going to the correct tuned inputs and to the 3-500Z
(Eimac) tube correctly using a scope.? We did not remove the input board
but I inspected what I could see and nothing appeared to be smoked!? The
green 10m wire was clipped but it had taped ends.? We removed as much of
this wire as we could.? (I don't like taped wires hanging around.)
He has sent the 7300 out for repair again.
Other details:
He is running an 8 band off center fed dipole.? The swr is low.
I looked at the coax he was using to feed the amp.? He soldered the center
pins of the coax but not the shield.? The coax braid/shield was just
mechanically connected by twisting the connectors onto the braid which was
placed over the cable jacket.? I always solder the coax braid/shield to the
PL259 body.? Today, we re-terminated the exciter/amp coax with soldered
shields and center pins using new PL259s.? No shorts were found before or
after the soldering process.? He is in the process of changing all his PL
259s to soldered shield and pin connections.
If the coax braid was intermittent, could a sudden loss of shield
continuity cause an momentary high SWR that blows out the 7300's final
transistors??
I am looking for explanations and other things to check.
Of course, we cannot continue blowing out 7300 finals while we continue to
work on the amp..? I do not know if there is any type of protection circuit
that can be installed between the exciter and the amp.? I do not know if a
tuner between the two would help.? Perhaps a 1:1 UnUn?
I suspect that we might need to find a HW 100 or 101 (tube type finals) to
use as an exciter until we get this issue tracked down.? What do
professionals do about exciters for problem amps?
How do you test a 3-500Z to see if it is a tube issue?
Are Icom 7300's compatible with AL80Bs??
Does anyone have any thoughts?

Thanks and 73!
Alton, NU8L

###  per W8JI, who designed the Ameritron  AL-80B amp, the problem with
blowing up the xcvr is because of defective chinese 3-500Z tubes.
Internally, the grid does not come up high enough at one or both ends.

##  What happens, is the B+  from the anode arc across the grid..and goes
straight to the cathode.  Then all hell breaks loose.  With full B+ on the
cathode,  the B+ can damage the tuned input, tuned input bandswitch, TR
relay in the amp, and also back to the xcvr, damaging the LP filter, and
also LP relay's...and finally, the final push pull transistors in the Xcvr.

##  This has happened numerous times in the last several years.. on the
AL-80B, and also the 811 and also 572B amps. .

##  His  'fix'  for this is to install  150-170 vdc GDT's ( gas discharge
tubes), one on each cathode lug..to chassis..right at the socket.  Only 2 x
GDT's required, even on  the 3-4  x  811 amps.  He also installs a 100K  @
3 watt MOF resistor in parallel with each GDT.

## I asked him what the 100k resistors were for.   He said they
prevent..'ticking' noises on  RX.... go figure.

##  look at any current production  AL-80B amps, and their  811/572b
amps..and they all now come with the pair of GDT's installed..+ pair of
100k resistors.   The 100k resistors look like a high Z..and don't affect
input  SWR.

## ameritron has been using the GDT's for several years now.  The GDT's may
or maynot be in the current schematics.  They are not shown in the older
schematics.

##  Plenty of different brand xcvr's have been blown to hell.... all caused
by defective chinese  tubes.   Not all are defective, just some of them.
Depends which month it is. Good batches, followed by bad batches.
Sometimes several good batches,  sometimes several bad batches.  It's all
over the map.

##  You can get the correct  GDT's  directly from ameritron.  Dunno how
many  'hit's' they will take, before they fail.  Dunno if the GDT's  fail
shorted, or blown open.  Typ they fail shorted.   With any short, ur drive
power goes straight to the chassis.

Jim   VE7RF