High Gid Current

Overall it was interesting to reverse engineer the metering circuit and B- system Sue, even though I'm disappointed I did not add the voltage drops through the circuit right away. When that is done, it becomes very obvious the circuit is unreliable and will have grid current measurement problems.

It should have been a ten minute job, but the time-sink was the assuming the design was initially reliable. I try to not make assumptions like that even when I look at my own work.

 

When I worked as an electronics tech, I've made such assumptions, name brand equipment with shoddy designs and documentation full of errors.

I've learned that if the numbers don't add up in my head the first time around I had better take a second look at the circuit or my my perception in how it works, Usually (not always) it's some glaring error and it's obvious the second time through.

There are lessons in this thread everyone can come away with.

As usual Tom your contribution to this thread and the Zed was beyond price.

 

This has been a great thread to follow especially while studying the practial circuits portion of the Extra exam. Thanks guys.

 

Why is D106 in the circuit to begin with? What does it do? ( diode in series with resistor from B- to Ground ). I have been looking at other amplifier power supply schematics and most only have the resistor direct to ground off the B- side. There is no diode. Is there an advantage in having a diode off the B- to ground over having only a resistor?


73s, Thomas NE7X…
http://ne7x.com

 

Command Tech amps have the resistor in series with the diode too. Maybe one of them copied the other?

 

I wired the mod per W8JI's instructions, minus the HV limit resistor.

Remove F101/R109/D107/R119
Connect the Cathode of D106 to D101/C112/R108/R112. Leave C114 in
Connect junction of R115/R116 to D101/C112/R108/R112

Ip meter = key no drive, 600ma, with 50 watts or greater, pegs, full scale.
Ig meter = key no drive, 0ma. With 50 watts drive, 350ma. 100 watts drive, pegs meter, full scale

Just for fun, I replaced the .2 ohm Ip shunts with .1 ohm

Ip meter = key no drive, 110ma, with 50 watts drive 300ma, greater than 50 watts drive, pegs, full scale.
Ig meter = same as above, no change.

Before the mod the Ip meter was working perfect.
Only the Ig meter was reading off, 50ma key no drive and full scale with 50 watts or greater drive.

73s, Thomas NE7X…
http://ne7x.com

 

I'm not sure what that all says Tom, are you saying it still does not work?

Why the heck are you changing the shunt? Was it off value? Was the multiplier resistor off value?

There is no reason on earth to change the shunt value unless the shunt is defective. If you are changing component values just to get the meter back in scale, then this will never get fixed correctly.

If it does not work with that mod I gave you, then you have something wired wrong. My mod is based on correcting the amplfier using figure 7 page 24 of the manual.

It absolutely has to work, so trace through the wiring and don't change shunt values.

 

OK, more work for Tom and his Henry.

The mods I posted will work fine. They are 100% correct. Let's look at the meter scaling.


The meter M102 is listed as a 1 mA Beede meter. I managed a competitor of Beede meters, so I saw a lot of Beede meters we bought to compare to our meters. Most of the Beede 1 mA meters were 50 mV movements. TOM, you will have to measre the resistance of the meter out of circuit!!! But my best guess is it is a 50 ohm movement.

The shunt, R118, is 0.2 ohms. That resistor is .02 volts per 100 mA. If you want 400 mA FS, that is .08 volts. With a 1mA movement the meter has to have a total resistance of 80 ohms.

If R118 is a 0.2 ohm resistor, the correct multiplier resistor R117 would be 30 ohms. If R118 is 0.2 ohms, and if R117 PLUS the meter movement resistance is 80 ohms, you will have a 400 mA grid meter. That is just how it is.

The problem is the plate current meter. Henry lists it as a 1 amp meter. In that case the shunts make no sense at all. They have R115 multiplier as 0.2 ohms, and R116 shunt as .2 ohms. With R116 being 0.2 ohms, voltage across the shunt would be .02 volts per 100 mA of plate current. That means the combination of R115 and M101 must be 0.2 volts full scale if the meter drew just a few mA of current. It cannot be a 1 amp movement with the resitor values given for R115 and R116, so the question is what is M101 really?

I'll post a schematic in a few minutes....

73 Tom

 

Ignore this

Ignore this, I can't delete it and I have an error.

 

Tom You might be able to delete that if you go into your settings (for the Zed forum) and go to the managing attachments section.