AL1200 Bias voltage

[357]

Does anyone know what the bias voltage should be? I get 125ma on ssb mode, I think it should be 150ma and my trapezoid of the signal has a slight sag in it which is not enough bias.
I measured the DCV on the zenners and get around 5.7v. I'm not sure where to take the measurement and the schematic gives nothing.

thanks

 

[linearone]

Put one lead on one leg of the cathode before the filament choke, one lead to gnd. Set meter to DC V.
Turn amp on, this is your bias cutoff voltage. Key amp. This is operating bias voltage

 

[linearone]

Does anyone know what the bias voltage should be? I get 125ma on ssb mode, I think it should be 150ma and my trapezoid of the signal has a slight sag in it which is not enough bias.
I measured the DCV on the zenners and get around 5.7v. I'm not sure where to take the measurement and the schematic gives nothing.

thanks

Also, aberrations like that can be caused by your pick up/sampler setup. Nobody can hear the difference between a sag in your trapezoid or a perfect sine wave on the air. I really wouldn't worry about it.

 

[357]

I did measure at the centertap of the filiment and it was 5.7 then dropped to 4.5 on key up.
thats at the terminal strip

 

[linearone]

I did measure at the centertap of the filiment and it was 5.7 then dropped to 4.5 on key up.
thats at the terminal strip

All those ameritron amps are biased quite lightly. That's because they really are designed for SSB, not am. All the broken ones have all been on am, almost invariably. If you want to run am first thing I'd do is double the bias. You'd have to play with it.

 

[357]

I'm just wondering what the spec is.
there is no voltage spec in the manual or schematic.
i know harbach ditches those stud diodes for a reason.
I just want it to be right.
I'm on ssb all the time and its a ab2

a tube is 3400usd on mfj so I want this thing running right. a new diode is $30

 

[357]

I did some more testing at the center tap.
The HV needs to be on to get the right readings.
CW 14.86VDC
SSB 7.49VDC
STAND-BY 24.89VDC

I think the SSB/CW switch is playing up

 

[linearone]

I did some more testing at the center tap.
The HV needs to be on to get the right readings.
CW 14.86VDC
SSB 7.49VDC
STAND-BY 24.89VDC

I think the SSB/CW switch is playing up

Uh...yeah the hv needs to be on. But do not measure this with RF drive, just key the box with a pedal.
And those readings sound fine.

 

[357]

yes I did with no drive. lsb mic unplugged.
So I guess thats ok?

I found that the cw/ssb switch is shot and was why I was getting cw power reading on ssb and why it would magically go back to a higher level. probably not ideal

 

[357]

first thing I'd do is double the bias. You'd have to play with it.

actually I'd like to lower it.
It says 0 bias for ab2 but I think 0 is too unstable up top.
Maybe 5.1v? I think if I swapped the 1 diode for a 5.1 and left the other one that gets shorted I'd be ok.
I'm going for clear ssb


I just happen to have one too:
sctrach that its toast

 

[linearone]

I think the datasheet quotes zero bias for an as in zero grid bias. I'm not sure that refers to a zero bias state when driving the cathode.
Wtf do i know, I could be wrong.

 

[357]

It seems to be ok. the pdf says +65v for class c but grid driven.
I guess 25v is enough for cut-off with grounded grid. I'm surprised theres not alot out there on it.
But I did see this:

 

[linearone]

So I was right. Why are you messing with it?

 

[357]

messing with what? I'm asking what the voltages should be.

 

[nomadradio]

The suffix numeral on this class of tube indicates the amplification factor or "mu". That's the name of the greek alphabet character that looks like an upside-down lower-case "n".

It's the ratio of plate voltage to require 1 Volt of negative grid bias to cut off anode current. The 3-500Z is shown with a mu of 160, by way of comparison. The higher this number, the better the tube would behave with zero bias.

Eimac uses the numeral 7 for this suffix when a tube has enough voltage gain to use for zero bias. The specs above show a mu of 200 for the "D7" version of the tube. Any Eimac "D3" tube was meant for pulse amplification, or as a power oscillator. Not enough gain for an RF amp.

The one pic of a "D3" shows just what the correct one looks like, but don't buy one of these to use in a linear. It just won't behave right.

73