Protection for Flex 6500 with tube Amplifier

James Del Principe

I am really enjoying my new 6500 and would now like to connect it to my Ameritron AL572 amplifier.   Here is my concern.   I had a 572b tube snap over in this amplifier a while back and took out the finals in my FT2000; an expensive lesson.   Is there any device or recommendation for protection for the output (RF) of the Flex transceiver. I know about the MFJ ARB-704 but that only protects control lines. I am concerned about a high transient feeding back to the Flex via the coax.    Any thoughts?        73, Jim

Jim Gilliam

Perhaps a small tuner placed between the radio output and amplifier input would give further isolation for protection.

Jim, K6QE

wg c

Ameritron ARB 704 might provide the protection.  I have one, never used, I'd sell for $50...I know this is not a classified section but thought it might address your issue.
73, Bill
AH6FC

SteveM

With ingenuity this could be accomplished by running the RF signal through a lightning suppressor installed between the radio and amp. You will need to change out the gas tube for one with lower arc-over voltages moderately above those produced by the radio. Note that if the gas tube misfires, it'll produce terrible intermod. Various gas tubes can be found at DigiKey.

James Del Principe

Jim, I can't see how a tuner would provide transient protection except by arcing between plates which is still highly undesireable.  I need something that would shunt a high transient to ground but not have high insertion losses.

Bill, the ARB704 protects the control lines like amp keying and ALC but not the RF on the coax between the transceiver and the amp. This is the route that killed the finals in my FT2000.

Thanks for your responses.  73, Jim

James Del Principe

Steve, that's a good thought and one I will investigate.    The normal voltages for 100 watts are fairly low, 70 volts RMS and about 99 volts peak, not enough to trigger most gas discharge tubes in normal operation.  A transient from an arc fault would be many times higher than that so a gas discharge tube looks promising.   Many Thanks.   (why didn't I think of that?  Hi HI )     73, Jim

YV5WZ

My humble experience: I have an Ameritron 811 ...Last May I was working Spain stations, I changed from band  40m to 17m "BOOM!" Its good to know by default  any time you change band RF power go to 100 Watts.....Sadly it blowed on of the 811 Bulbs......It still is at technician´s home cause replacement bulb just arrived yesterday....I open a post here asking for an advise about new amp.....and winner was Elecraft KPA500....a rock....solid state....auto by pass "just in case".....change to right frequency without tune needs.....and 500 watts power free of worries....

Question to Jim: you mean  to use 2 antenna tunners?

Greetings from Venezuela

George

YV5WZ

James Del Principe

George, I do NOT want to use two antenna tuners.    I just wanted to protect my Flex 6500 from any harm if a 572b tube arcs in the future.    The idea of a lightning suppressor sounds like the best so far. It requires no intervention, no tuning and should have minimal effect on the system.  The only question becomes, at what point does it 'fire' to protect the Flex.     73, Jim

YV5WZ

James del Prince  I apologize if I express my question improperly, actually my question was for Jim, K6QE, cause his comment: 

Perhaps a small tuner placed between the radio output and amplifier input would give further isolation for protection.  

k3Tim

When the tube "snaps over" what does that mean?  A DC arc caused high voltage DC pulse to appear on the SO-239 connector? 

Assuming yes to above, I pulled out a copy of Radio Handbook (23rd ed) by Bill Orr.  The HF amps all have a high voltage "transmitting" cap before the PI tuning.  This is 0.001pf at 15Kv-DC.  On the output side of the tank circuit, at the S0-239 a 1.0 millihenry RF choke (0.30 amp) is placed. Between the DC blocking caps and the DC shunt to ground. 

If tube arcs over, the cap should block the DC and the Choke will also.

Checking my Ten Tec Titan 425 schematic, it has the same configuration.  It never hurt my SS rigs.

Best of Luck...

k3Tim


* Transmitting Cap means one intended for high power RF and can handle the high RF currents it will experience, aka. expensive thoguh cheap insurance.

James Del Principe

K3TIM in the amplifier you describe, the DC blocking capacitor blocks the B+ from the PI network output. The RF choke to ground is there in case the cap fails. It will ground the B+ and keep it from appearing on the output of the amplifier.   My issue is looking in the other direction. That is, a transient that goes from the amplifier back to the transceiver. This is what caused the damage to my transceiver. An AL572 also has these components but they do nothing to protect a transceiver.   73, Jim

elan

hello Jim 
yes the best buffer is from http://k4avu.webs.com/apps/webstore/products/show/1298607
better then the ABR704  AND IT IS OPTOno noise never ever use any amp with out a buffer even new one , btw i am only happy customer i bought some buffer from him 
73 elan g0uut

James Del Principe

Thanks, Elan.   I will look into this today and see if it is what I need.   Very Best, Jim

k3Tim

Sorry for my confusion,  I was looking at the output side of the amp. I'll put some more thought to this and if anything interesting is found will post back.

Sure would have to **** up a 6k due to a microsecond transient event, EMP included.

elan

yes Jim no problem Paul great guy and it will save your radio by using it , i have atom 1000 and 2000 and use this buffer very quit since it is OPTO good price as well 
73 now elan g0uut

W9OY

I looked at the 572 schematic and the input is protected by the blocking caps connected to the fil choke. Unless these are shorted you should be good. Makes me wonder if the arc really caused the problem

James Del Principe

Elmer, the problem with blocking caps are they block DC but a transient is really an AC event because of the steep slope of the waveform. A capacitor  will pass such a transient.

W9OY

Yes I took EE AC circuits back in 1970 at Purdue.  Seems I remember something about that.  My name is Lee by the way.  I think Elmer is some patchwork elephant.

To the OP  My guess is the arc was not the "cause" and may have been caused by the Yaesu.  Arcing readily happens by over-driving an under loaded amp as in the case of overshoot into an under loaded amp.  Many Yaesu's are known to overshoot.  So the best protection is to make sure your amp is properly tuned toward overloaded.  You can tell this by watching the grid current in a GG triode amp (screen current in a tetrode).   If it soars you need to unmesh the loading cap till the grid current reduces.  Flex 6xxx's as far as I know do not overshoot

73

James Del Principe

Thanks, Lee.   You make good points and I will keep in mind the need to over couple the amp. I could actually hear the arc in the tube and most 572b's have been poorly made in the past years. There is little in the way of quality control. Ameritron has acknowledged this and made some recommendations just now via e-mail. They suggest to remove the grid resistors and ground the grids directly to chassis via short lengths of braid. I have already do this. They also suggest to add gas discharge tubes at the tube sockets on the filament pins. Ameritron carries these with part number 304-6015.    I am going to add these before trying out the amp with the Flex.

W9OY

Welcome   Also if you're going to run lower power say 500w instead of 1300, I would tune for 1300w then turn down the power.  Also run the amp hard enough to get some color on the plates.  Another cause of arcing is gassy tubes and color will tend to assure gettering occurs. I think for a tube amp of that class the AL80B is probably a better choice, or better yet ALS-1300.  The ALS-1300 has excellent fault protection.  Not sure I like the idea of entrusting my $4500 radio to a gas discharge tube.

73

Ned K1NJ

    In the 572 manual, Ameritron discusses the possibility of internal arcing
because of debris or gassing.  While it's good to try to work around these
problems, it might only be a matter of time before the fixes prove inadequate.
It may be wise to consider a replacement like the ALS-1300.  As Lee points
out, you have a lot at stake.
     The only way to see if the fixes work, is to wait until another tube arcs.
I hope one doesn't.  I also hope my 5 mph bumper works, but I think I'll
try to avoid that situation.

Ned,  K1NJ

James Del Principe

Lee/Ned, I could switch over to my SB200 which puts out 600 watts and has never snapped over in the 25 years I have owned it.  The AL572 seems to stress these 572b tubes with 2700 volts on the plate.   If I could afford an ALS-1306, I would certainly do just that. These tubes use carbon anodes and do not normally glow like a 3-500 might in service.      Jim       KD1I