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

Tue Jun 7 21:17:53 EDT 2022

AL-80B has EBS, so there is no idle current until a small amount of RF 
is applied. Unlike the Emtron amps, it does not have a switch to defeat 
the EBS.

73, Alek, VK6APK

On 8/06/2022 8:19 am, David Gow wrote:
> So, just curious, with no transceiver connected and with the antenna relay
> activated by grounding the connector center pin, does the amplifier show
> normal idling current?   Or was there obvious damage to the amplifier?
> Dave W7VM
>
> On Tue, Jun 7, 2022 at 4:23 PM gudguyham--- via Amps <amps at contesting.com>
> 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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