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

[donroden at hiwaay.net]

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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