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24.27.41.140
In Reply to: RE: Thomas stereo organ amplifier 6973 to 6cz5 conversion posted by joshie on January 14, 2022 at 19:21:11
The RCA data sheet states 500K maximum for fixed bias, not 100K. In any event, when fixed bias is used, it's assumed the grid resistor will be connected to a low resistance node of the negative bias supply, not ground.
About dropping the anode supply, you might want to consider a small string of Zener diodes. Adding series resistance will degrade supply regulation.
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Buy Chinese. Bury freedom.
Follow Ups:
By low resistance node do you mean the dropping resistors on each output tube that the negative bias voltage comes into the grid on ? I thought grid stoppers are connected to ground? I'm converting this amp to use the 6cz5 tube not 6973. This is the post I'm getting my information from.
https://www.audioasylum.com/messages/tubes/51265/re-6973-any-substitute
If would be good to avoid confusion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
or it may be way off.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I will replace the 220k with some 100k.
As for the zener diode string from what I have read this will be in series with the center tap and not the b+ ?
I also have a problem with the negative bias supply the selenium stack I replaced with a silicon diode I was to inexperienced at the time to account for the voltage or less of a voltage drop and now it will only let me adjust down to -32 which gives me about 18ma on each of the output tubes. Would it be OK to put a zener in the bias supply to shed some volts?
Thanks for all the help!
Rather than "regulating" the bias supply, just change a resistor or two to get the voltage into range. Better to have the bias, plate and screen voltages change together with line voltage - if bias voltage is truly fixed, dissipation and operating point vary with supply voltage. They'll "track" if all are left unregulated (regulate screen and grid voltages and it will be even better, but that gets complicated...)
Ok I played with the divider in the bias supply and now I'm able to get -17 output tubes are drawing about 35 ma I don't know how accurate this old weston 911 is though.
Thanks for all the help guys especially Tom who has helped me with many amps from magavox to my heathkit aa-81 mono blocks.
I now suspect that the 10watt 560 ohm resistor I put in front of the input filter is really making some heat so should I find some 1k or 1.2k 10 watters and double up?
Ok nevermind about the paralleled resistors now that the output tubes are drawing proper current there's no need for the dropping resistor to be in there.
Looking at the grid and working "backwards"...
There should be a resistor about 1KΩ connected directly to the grid pin. This is the grid stopper. I always use carbon comp for this, which is non-inductive. The purpose of this resistor is to reduce the Q of grid circuit inductance, some of which is inside the tube. The other end of the grid stopper is connected to A) the coupling cap from the previous stage, and B) the grid resistor (the one I mentioned above). I see below that Tom has found a 100KΩ spec for this, so that's what you should use. The other end of the grid resistor connects to the output of the negative bias supply. This replaces the resistor that is connected to ground in a self-bias amp.
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Buy Chinese. Bury freedom.
Just so OP knows grid "resistor" full name is "grid leak resistor" most common newby mistake is not understanding this and not connect it right so no output. O p definitely needs to swat up on some beginners books thinking the grid stopper goes to ground. No offence meant, basic understanding keeps people alive.
Edits: 01/16/22 01/16/22
I think the name "grid leak resistor" should only be used when the tube is biased by means of the grid leakage current with an over size "grid resistor". Otherwise it should simply be called a "grid resistor".Just my opinion.
Hey, I found someone who agrees with me.
"Grid leak resistor
A very large resistor from the grid of a tube to ground, which is used to generate the bias voltage for the tube. See "grid leak biasing" for an explanation of how this works. This term is sometimes incorrectly used when referring to the grid-to-ground resistor in a cathode biassed configuration, which is used to provide a DC ground reference for the grid circuit."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/16/22
I had a conversation about this with someone last year. It was (and still is) my contention that "grid leak" is a misnomer unless the resistor's value is so high that grid leak bias is produced. But wouldn't you know, in the course of researching this further after the fact, I proved myself wrong. Don't remember the source now, but I came across an instance of a tube manufacturer using "grid leak resistor" for a simple, low value grid resistor to ground. The reference was from the '60s or earlier. Go figure.
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Buy Chinese. Bury freedom.
The problem is newbies arent about nuances of expression juggling by experts. They see a resistor going directly to the grid so think people are talking about this when they refer to "grid resistor". You have to go back in thinking to the time you didnt know what the differently positioned resistors were about and then consider how to talk about them so the ignorant dont mix things up. Because they do mix them up which is the point! Not which expert uses the cleverest name.
A grid resistor implies fundamentally different functionality than a grid leak resistor. Skewing the meaning of technical terminology to suit those who don't understand it does everyone a disservice.
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Buy Chinese. Bury freedom.
I can already see some pretty steep price increases for the 6cz5 on the bay!
For those that read this post looking for some help here it is..
In cathode bias a small value resistor is placed between the cathode and the ground so it develops negative voltage on the cathode thus achieving bias. In fixed bias a separate circuit provides negative voltage to the grid of the power tube and the cathode is simply grounded.
In cathode bias a small value resistor is placed between the cathode and the ground so it develops ***positive*** voltage on the cathode thus achieving bias. The grid is at zero volts DC and the cathode is at a positive voltage so the grid is negative with regard to the cathode. That is what bias is, the grid being negative vs. the cathode.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
New choke!
RCA lists the maximum grid resistor as 500K under "Vertical Deflection Amp". Sylvania does the same, but adds "Class A Audio Amp" with a max of 100K (6973 allows 500K). This is the resistor from grid to negative supply.
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