Help trouble shooting Sonar BR-21

[cranked247]

Ok I have a Sonar BR-21 (12 volt) with 4 6JB6A's in it. When I turn it on everything looks good tubes warm up and it idles at a 7 amp draw like supposed to and plate voltage is at 500. Now when I key up the relays click like they are supposed to and the tubes energize, plate voltage drops to 460 and total draw is 17 amps. Only problem is I have 0 rf output, and the load and tune knobs don't have any effect. At idle the plate current meter shows 50 ma and under load it shows 200 ma. Please give me some trouble shooting tips so I can get this thing working. Thanks

[Night Crawler]

Check C7 100pf coupling capacitor to the PI network for an open circuit
also check RFC 2 the Plate choke. If C7 is bad it can be replaced with any value
100pf to .01uf it's not critical just make sure it has a 1000v rating. Check
your input circuit RFC 1 cathode choke and all connections to the relay and antenna
connectors. Check your tune and load caps for shorts to ground.

[cranked247]

Ok I figured it out. The previous owner had jumpered the tank coil for higher frequency use. All I had to do was put the jumpers back for the lower frequency range. Now it works how it is supposed to, but the output is pretty low. Could this be from old worn out tubes? I'm going to clean all the relay contacts to make sure it is getting full contact, also what other components should I check that could cause very low output?

[Night Crawler]

By Low Output what do you mean? The amplifier will only put out about 45 watts with the plate
voltage their using you can look up the schematic and manual on cb tricks
[Please login or register to view this link] Check the Tune and Load Caps and if they are
Fully Opened or Closed with Maximun ouput you could try Changing the tap on the Coil
They should be between 1/4 to 3/4 open when the amp is tuned properly.

[cranked247]

By low I mean at a 7 watt input it is only getting about 12 watts out. My radio swings to about 18 watts and that only drives the amp to 25 watts. My 2 tube sonar BR-2906 (with brand new 6KM6's), with the same input DKs at 30 and swings up to 50+. I've tuned both by using their process in the manual, including dipping the power for AM use with the antenna tune control.

[Night Crawler]

Sounds like it's time for new Tubes

[cranked247]

Thanks for all your help. I will post back and yet you know how it goes.

[cranked247]

The amplifier will only put out about 45 watts with the plate
voltage their using

I looked it up too, but isn't 45 watts low for 4 6JB6A's? I've heard of other people getting 100 to 150 watts out of this model. Or were they BSing? Or is the circuit limiting there output?

[Night Crawler]

If you look at the chart your getting 45 watts out for 10 watts in if you drive
it harder with about 20-25 watts then you will see about 100-150 watts out.
That is a class AB amp which is about 65% efficient. You could raise the plate
voltage to 700-800 volts which will give you more output but you would have to
keep your drive low about 1-2 watts with a high swing to keep the tubes from
over heating. The reason they keep the plate voltage low is to give you a 100%
duty cycle for continuous carrier. It is cathode driven so some your input is part of your
output. If you look at any of the old sweep tube ham amps Dentron Yeasu they have
a high plate voltage around 1000 volts but are limited to ssb or cw only they only
have a 50% duty cycle. The amplifier you have can be driven with 20 Watts DK
4 6jb6's are equal to 2 6lq6's the low plate voltage will keep it from over heating.
Voltage X Current = Wattage Input

[Av8r1]

I have a BR-21 also and get over 500W out.



Thanks

[Night Crawler]

I have a BR-21 also and get over 500W out.



Thanks

How did you get it to that with four 6JB6's
and 600 volts for plate voltage?

[Av8r1]

Just by tuning the external adjustments. Driving it with 7 watt dead that swings to 15-20watts.