That does nothing at all to save finals from blowing due to a plate cathode
short in the tube..
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On Tuesday, June 7, 2022, 6:34 PM, John Bodine <firstname.lastname@example.org> 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 email@example.com <firstname.lastname@example.org>
> Date: Mon, 6 Jun 2022 18:26:59 +0000 (UTC)
> From: Alton McConnell <email@example.com>
> To: "firstname.lastname@example.org" <email@example.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
> 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
> ## 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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