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

Tue Jun 7 19:50:00 EDT 2022

At this point, I wouldn't use the amp and get something else.

73! Mark KA6WKE - DM34st
Youtube: https://www.youtube.com/ka6wke
Website: https://www.ka6wke.net
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On Tue, Jun 7, 2022 at 4:43 PM <donroden at hiwaay.net> wrote:

> how bout an RF Choke .... internal or external to the input to the amp ?
>      W4DNR
>
> On 2022-06-07 6:25 pm, gudguyham--- via Amps wrote:
>
> > That does nothing at all to save finals from blowing due to a plate
> > cathode short in the tube..
> >
> > Sent from the all new AOL app for iOS
> >
> > On Tuesday, June 7, 2022, 6:34 PM, John Bodine <john.bodine at gmail.com>
> > wrote:
> >
> > I apologize if this is repetitive but have you used a method of
> > isolation
> > in the keying circuit such as an Ameritron ARB-704 to protect your
> > equipment from nasties?
> >
> > On Tue, Jun 7, 2022, 6:14 PM jim.thom jim.thom at telus.net
> > <jim.thom at telus.net>
> > wrote:
> >
> >> 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
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