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I need help to connect this Amp please

Robalo

Robalo

Well-Known Member
Aug 21, 2006
598
246
53
California
1st time playing with fire .I hope you can help me on this experience. The amp has 3 new bulb A811a and wend i turn on. I have 1.8 plate voltage on STBY. Thank you in advance.Also I need to know is the ARb-702 box run with 9-12 volts .o if I used de grey cable only with pin 2 ground,pin 4 red , 6 pin center white and shield ground.
 

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My Arb 704 runs on 12 to 13.8 volt, i bet your 702 does as well.
The 702/704 can switch high voltage relais, or low voltage, most transceiver only have 50 milli amp switching capabillety, these deliver much more switching power.voltages.
I just replaced the ARB704 with a home made PTT switch having dual separate switching contacts one switching the FT991A the other the Heatkit SB1000 no need for the ARB anymore.
 
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Robalo said:
I have 1.8 plate voltage on STBY.
Click to expand...
That's 1800 vdc...normal...looks as it should.
The relay box does use 12vdc...make sure you get polarity correct, with the plus 12vdc to the center pin.
ALC cable on interface to ALC on the rear of the amp.
Relay to relay...Adjust the ALC control on amp till you reach spec I list below.
Now the Amp: The AL811 is a nice little amp if you follow certain drive levels.
I recommend keeping the FT-920's drive level to about 50w PEP on SSB.
This produces and output of around 500 watts PEP on 160 - 20 meters.
15/10 meters adjust the drive to output level to produce PEP output of around 400 watts.
YES the book says it will do more!!! However todays 811A tubes(CHINA) are not the robust tube they once were. The small difference in output from my suggestion to books "FULL" rating nobody will ever notice the difference.
This will with proper tuning will keep the "Grid" current in the correct range on "peaks".
This will save the tubes and give you good service for many years.
I have the same amp. I have run both 811A tubes and 572B tubes. Presently I have 572B's installed, I do have have 2 sets of 811A's I keep for back-up.
The only advantage truly to running 572B's is the "grids" in these tubes are much stouter built, and IMHO require a little less monitoring on the grid current.
Using the ALC function, which is a "feedback" circuit designed to reduce the rigs output so that overdriving the amp is less of and issue. (when adjusted correctly)
I still hold true to my suggested drive specs and the ALC circuit use just helps keep the amp from overheating, "splattering" or flat topping" on voice peaks.
I do NOT recommend using this amp for AM service!
This said, if you plan to do this PEP OUTPUT should be adjusted to HALF(50%) the sideband PEP. These China 811A's just won't hold up to that duty cycle service they will self destruct quickly.
My AL811 was made in 1994 and still working. I have beat the pants off it at 500 watts PEP on 160-20 meters and about 400 watts on 15/10 meters on "Field Days" and it just keeps going.(I keep it around because the only HF amp I have that does 160m)
I have owned it for like 20+ years, I think it may be on it's third set of tubes. So good service IMHO. The Svetlana 572B's have been in there at least 8-10 years and have survived at least 2-3 "Field Days" and are still about 85-90% of rated output.
So my 2 cents worth.
Enjoy...Treat her with common sense and she will give you many years of good service.
All the Best
Gary
 
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See manual on Radio. Relay must be activated with a switch . However I bet it's already done. The interface should connect to the DATA plug (8 pin din socket on back).(and 12 vdc power) Be careful to get that plug turned the correct way, it's keyed but don't wabble it in. You can easily bend other pins. You should be good to go, as I am guessing whom ever had it was using it that way. When you key the rig in AM (POWER turned down) and the amp pre-tuned to manual specs and in OPR, you should hear relay in the amp key. Should be all that is needed. I use a CW straight key and using CW mode to set power level at about 25 watts to tune the map. You can use AM (mic gain full down) and power level at about (Leave Amp in STD-BY till power level set correctly)
(9 O'clock on the RF Power knob) and go from there. See AMP manual about "DIPPING" the Plate. Just keep power level set low until you get use to the correct way to tune. Then bring the power up slowly till you see the proper level on the Grid current meter and the external wattmeter. The give it a Very quick "tweak" on the Amp tuning, and then adjust the power level back to correct output.
GL
All the Best
Gary
 
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BJ radio nut .Thank you for the help I keep post it the result .I love that radio .like new the coolant fan is clean
 
Last edited:
ok here we go, amp is conneted to the dummy load, on 10M. radio on ssb(rf pwr knob at 2:00 oclock) with 60 watts pep just talking ,making sure the signal passes:) the amp,now that the amp is on I see about 180-200 watts on MTA-20 meter from the radio shack and when I pushed the proc key on I saw 565 watts pep is this ok? So now I need to drive the amp with 60 watts to see 180-200 dk with a 565 pep. Grid current is about 115 ma,plate voltage 1.7kv and IP is about 300 ma. I think that this radio internally is pre setup for no more than 60 watts dk.:)
 
Robalo said:
I think that this radio internally is pre setup for no more than 60 watts dk.:)
Click to expand...
:eek:
Guess someone has "screwdrivered" the AM power level, because AM carrier power is factory set/rated at 25W!!:(
Per Yaesu specs:
RF output power: AM: Max 25 W carrier
SSB/CW: Max 100 W

Per AL811 Specs: You even get 400 watts on
demanding continuous modes like RTTY, SSTV, FM and AM.

PS: Doing a Search the Push-Pull Motorola MRF 355 FET finals in the FT-920 are no longer in production, and no substitutions are listed. So be careful!
Maybe some of the Service Tech's (RF Guy's Here) know of a
source or replacement but I could not find anything!:eek::(

UPDATED INFO: BAD TYPING SKILL...The correct Part # is MRF255 FET...Still availible part #
MY BAD
 
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