OK-- so here's what I'm thinking:
The voltage for the D3a anode is being supplied by the voltage dropped after the 1K 300b cathode resistor, right? The voltage after the 1K resistor should be about 189V, some of which is dropped and supplied to the D3a anode (~177V, according to the schematic).
Now, if I change the 300b cathode boas resistor, this should change the voltage drop, but it will also change the bias point. Using a rudimentary calculation, I think that the 300b cathode would have to see a 1.2K resistor to have a grid-to-cathode voltage of -72V (72=.06 x 1200), but it looks like we only have a 1K resistor in this position.
But wait! the cathode could still be seeing a total of 1.2K as bias, its just that its not immediately obvious to me how this is the case, so a push in the right direction here would be great.
Back to the issue:
So, if I'm seeing 277V at the 300b cathode, wouldn't I need to increase the cathode resistor to accommodate a greater voltage drop for the D3a supply? If g-k voltage is -67 now, and it needs to come up to -72, I think I need about another 100R on the cathode, but since this is supplying the grid voltage through the added resistance drop, I would only be increasing the drop by another 6V, which would not reduce the D3a anode voltage enough.
Any thoughts on this?
Kofi
The voltage for the D3a anode is being supplied by the voltage dropped after the 1K 300b cathode resistor, right? The voltage after the 1K resistor should be about 189V, some of which is dropped and supplied to the D3a anode (~177V, according to the schematic).
Now, if I change the 300b cathode boas resistor, this should change the voltage drop, but it will also change the bias point. Using a rudimentary calculation, I think that the 300b cathode would have to see a 1.2K resistor to have a grid-to-cathode voltage of -72V (72=.06 x 1200), but it looks like we only have a 1K resistor in this position.
But wait! the cathode could still be seeing a total of 1.2K as bias, its just that its not immediately obvious to me how this is the case, so a push in the right direction here would be great.
Back to the issue:
So, if I'm seeing 277V at the 300b cathode, wouldn't I need to increase the cathode resistor to accommodate a greater voltage drop for the D3a supply? If g-k voltage is -67 now, and it needs to come up to -72, I think I need about another 100R on the cathode, but since this is supplying the grid voltage through the added resistance drop, I would only be increasing the drop by another 6V, which would not reduce the D3a anode voltage enough.
Any thoughts on this?
Kofi
Not having done a careful analysis, my impression is that the voltages you see are within the tube to tube variation you might expect to see. A schematic with the actual voltages labeled would tell the tale. Mere minutes to generate. As an old mentor was fond of saying, "when the facts are sufficient, argument is usless". BTW, your choke is about 1.2K DCR?
Sheldon
Sheldon
OK-- so the schematic with my voltages are attached (thinner font = my voltages). I know I could drop some voltage in series with the D3a anode, but its the 300b cathode voltage that troubles me. If I increase the value of the bias resistor to drop more voltage, it will also change the bias point.
I know there's something I'm not understanding here.
Also, the DCR of my choke is about 1.37K.
Any push in the right direction would be much appreciated.
Kofi
I know there's something I'm not understanding here.
Also, the DCR of my choke is about 1.37K.
Any push in the right direction would be much appreciated.
Kofi
Attachments
Kofi Annan said:
You need to help a bit more. Measure the voltage before and after each resistor in a series path and give us the DCR of the inductors (OK, bout 1.4k for the choke, how bout the OPT primary). I looked up the Lundahl OPT. It says 164R for the primary. A drop of 21V over 164R gives a current of 128mA. So not consistent with your other figures. So put em ALL in there. Not that hard, man.
Sheldon
OK-- so the DCR of the choke is 1.37K, but I would the actual bias formula be:
(0.06 * 1K) + (0.010 * 1.37K)
The 15mA in the schematic is the rating of the choke and not the actual current through it, but of course, I could be horribly, horribly wrong about this.
This means that the bias would be ~-74V, but I'm getting about -67V, so maybe the D3a is not properly biased? If so, does that boil it down to the 100R trimpot family below?
Kofi
(0.06 * 1K) + (0.010 * 1.37K)
The 15mA in the schematic is the rating of the choke and not the actual current through it, but of course, I could be horribly, horribly wrong about this.
This means that the bias would be ~-74V, but I'm getting about -67V, so maybe the D3a is not properly biased? If so, does that boil it down to the 100R trimpot family below?
Kofi
In a DRD the two sets of resistors in the cathode and the resistance of the choke all work together. Sort of like a cats cradle, when you pull one string the others move.
You need to drop about 30V at both the D3a and cathode. Increase the resistance of the lower most cathode resistor(s) Try disconnecting the pot and 10 ohm resistor and see were that gets you.
Don’t try to nail the voltage. There lies the path to madness. If you get the D3a below 190V or so and the bias in the neighborhood you’ll be golden…John
You need to drop about 30V at both the D3a and cathode. Increase the resistance of the lower most cathode resistor(s) Try disconnecting the pot and 10 ohm resistor and see were that gets you.
Don’t try to nail the voltage. There lies the path to madness. If you get the D3a below 190V or so and the bias in the neighborhood you’ll be golden…John
Of course, this is correct. So previously I was only adjusting the trimpot a bit here and there and was noticing no appreciable change in voltage. So I whaled on the ******** thing today and dropped the resistance a buttload and zzzzzzing! Voltage exactly where it needs to be!
The D3a anode / 300b grid are at about 177V and the 300b plate-to-grid voltage is 344V.
Good enough for government work.
This particular flavor of dumb-assedness is particularly frustrating as its Kofi making an easy thing difficult.
One question though-- my meter indicates that the current through the 300b is only about 56mA when it should be 60mA. I'm guessing that this is just the by-product of a $9.00 meter, but maybe not.
I'll measure the current with my DMM, but it is common that $9.00 meters are generally off like this?
Any comments?
Kofi
Cool! Let the music flow.
As to the meter, Check across the 1k resistor (how close to 1k is it ) to see just were you are . It is possible that the resistance of the meter (which should be low ) is adding it’s own mix to the party. Look for an adjustment screw, probably near were the needle pivots. Otherwise scratch out 56ma and write in 60ma with a red Crayon
Or you can take apart the meter and flip the scale card over and make your own face. It usually pays to add a few stern word in German so that people take it seriously.
Have fun, down the line you may want to try the 300b at a bit more current. Subbing a variable resistor here and there allows for easy experimentation…John
PS, How do you get your quotes in those nice little boxes?
As to the meter, Check across the 1k resistor (how close to 1k is it ) to see just were you are . It is possible that the resistance of the meter (which should be low ) is adding it’s own mix to the party. Look for an adjustment screw, probably near were the needle pivots. Otherwise scratch out 56ma and write in 60ma with a red Crayon
Or you can take apart the meter and flip the scale card over and make your own face. It usually pays to add a few stern word in German so that people take it seriously.
Have fun, down the line you may want to try the 300b at a bit more current. Subbing a variable resistor here and there allows for easy experimentation…John
PS, How do you get your quotes in those nice little boxes?
Will do.
Well, having already busted a meter, I know that there's a resistor in there, so maybe that's mucking things up. I'll check for an adjustment screw. Barring that as a solution, the crayon is likely something I'd only need minimal guidance on.
Wow! I may actually have an answer here!
When you are typing in the reply box to a message, there's a button called "Quote". Type your quote in there and watch the magic happen. For added effect, you can "bold" it by typing a ['b'] at the start of the quote and a ['/b'] at the end, but take the single quotes out when you do it. I needed to add the single quotes into the explanation otherwise you wouldn't see the code it would just look like this.
I helped!
Kofi!
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