Separate names with a comma.
Discussion in 'Amplifiers' started by KD7UGY, Jun 9, 2005.
Narrow the problem down
What would happen if you let it set cold for a day or two?
While it is setting on standby, are the plates of the 572B's turning orange (drawing current all the time) or are they gray (not drawing current until transmit)?
Also, do you have a dummy load so that you can eliminate the antenna as a problem and, also, what does it do on the other bands like 20 or 40 meters?
The bias is supplied by diodes D-19 thru D-24
The bias is established by the diodes D19 thru D24 in the B-minus lead (center-tap of the filament transformer). Check the diodes; they should all be the same.
Looking at the schematic, point W6 (B-minus) is connected to W5 when the relay RLY-1B closes. B-minus continues up thru S-4 (this switch could be flakey), thru the current meter, M1. R-10 is a 330 ohm resistor in series with the negative side of the meter (make sure it’s good) and, finally you have a 0.6 ohm resistor R-9 between B-minus and chassis ground.
I am not sure how the conversion to 572B’s were done; are there any resistors on the tube sockets themselves (maybe 47 ohms at 2 watts?), if so check value of all and make sure that they all are connected.
I cannot believe that the amplifier is eating tube with 1800 volts and no color showing; something is stopping you from drawing current. Replace the DC Blocking capacitor, C-13.
I'm confused by a couple of your statments:
First you said:
Okay, I checked it out last night and here are my results, everything was done into a dummy load with a bird 43, with 10 watts carrier in on am I got 40 watts out, plate current-75ma grid current-30ma.
Then you said:
increasing the drive to 25 watts got 60 watts out but the grid current won't go over 80ma and the plate current won't go over 35ma.
Are you sure that is right because more power output should result in more plate current,not less.Either way something is wrong with the bias because the plate meter should read around 110mA +/- 25 % (according to the manual) with the amp switched inline and with no drive applied.
Try reducing the loading control somewhat and see if that raises the grid current.It may be loaded too heavily.
Check R12 and R13 in the ALC circuit as well as the setting of R14 which is the ALC adjust pot.If it is set wrong it will limit the amount of drive from the radio regardless of what the radio is set at.That is of course if you have the ALC line connected to your driver radio.
After that :?: :?:
With adequate filament voltage on that tube, a 572B should last more or less forever in that box. It's like running a box meant for the 572B on "low" side all the time.
If you're running it on 120 Volts, and the outlet circuit isn't stout enough, you'll see the color of the filaments inside, dim noticeably with full drive.
This would indicate that the line voltage was falling under load, causing the 6.3 Volts on the filaments to fall, as well. This dimming of the yellow-white filament color indicates that the filament temperature is falling.
Running that filament at too low a temperature can and will cause the tube to go "soft", and appear to be worn out and weak.
If this is in fact the root of the trouble, you won't see those filaments dim when you key it now. If you remember seeing that happen when they were 'fresh', that would be a clue.
And if you're running it from a stout 240-Volt circuit, this is probably not the cause.
I'm glad we have a handle on the situation however, I have a problem with this conversion; there are circuit differences in the amps constructed to run 572B's and the amps constructed to run 811's
Heathkit SB-200. 572 B amp
Yaesu 2100B w 572B's
All the 572B amps have a grid leak resistor in the control grid circuit to provide "self bias' for the tube.
The 811 circuit has the control grid grounded; if a 572B tube is put in that circuit without the proper modifications done for the grid leak resistors then that tube (the 572B) IS gonna draw more current than it should and it's gonna draw current when it should n't.
When the RF sensing keyer was put in there I wonder what was done to the center point of the filament circuit; I wonder if it is still switched thru a relay or is it hard grounded.
I wonder why it was only doing 100 watts when it was sold to him?
You can run 120 Volts, so long as you hold the drive down to about half-throttle. If you draw less current from the wall, the voltage drop won't be that bad.
A 20-Amp 120-Volt circuit will be wired with #12 wire. A 15-Amp circuit with #14, usually. The fatter wire will have a smaller voltage drop at the same current draw. The reason the problem is worse on a 120-Volt circuit is that you need TWICE the amperage on a 120-Volt circuit as you need on the 240. This causes twice the voltage drop in the same size wire, for the same output wattage on the linear. Turn the drive down, and the current demand drops with it. So does the voltage drop on the outlet wiring.
If the 811A tubes haven't had this same (mis)treatment yet, try them on a 240-Volt circuit, if you can. Or hold back to about half of full power. Either strategy will work.
The old textbooks describe a procedure to "rejuvenate" the thoriated-tungsten cathode in a tube like that. I have had pretty poor results on the ones I have tried to bring back to life. Mostly it involves turning up the filament voltage on a test rig. More often than not, the filament just burns out.
You DO have to blow air on them when trying this trick. The heat from the filament alone gets out of hand when you turn up the filament voltage, even for 15 or 20 minutes.
If you placed larger wires in the wall, and fed your 120-Volt outlet with #8 or #6 wire, the problem would be partly fixed. Trouble is, most building codes forbid using wire larger than #12 for a 120-Volt circuit. Might void your homeowner's insurance if they discovered it after a house fire.
Besides, wire that large won't fit under the screws on a 120-Volt outlet.
Here's hoping the 811A tubes haven't been "flattened" yet.