Hello folks I am in the build process of a point to point 6lu8 push pull amp. This is my most complicated point to point yet, and it is a real rats nest. I am following a previously published design by DIYAudio member Jane. (Circuit attached) This is my first PP build. I am now at the point where I am checking DC voltages. I am pretty confident I have it wired correctly. The voltages are in the right zone now I need to refine them to my conditions. I understand, that my transformers are not the same as the ones he chose and some adjustment may be needed.
Tube1 Tube2 Comment
pin# voltage Voltage
1 80 80 referenced to 1/4 B+
2 108,2 187.5 B+, Tube 1 is high, tube2 is low compared to the schematic. should be 90v and 200v
3 - -
4 311 311 output plate, seems about right
5 - -
6| 1.8 1.6 not sure, I think it is bias voltage from balancing network circuit
7| - -
8 115.6 115.6 screen voltage a bit low but the best I could do with the Zener diodes I have for now. should be 120v
9 14.2 14.2 Seems real low should be 18.
10 0 108.3 tube 1 is correct. Tube 2 seems wrong but is as per diagram for Cathodyne inverter should be 90v?
11 1.4 109.1 tube 1 is as per diagram. tube 2 is high compared to diagram should be 92v
12 80 80 B+ reference
Now for my questions:
1.) When trying to get things sorted out and bring the voltages more in line with the diagram, do I start at the output stage to bring the cathode bias up to the 18v target. I do not seem to have 5v on the 10k adjustment pot I only have 3.8v. I am only pulling 40ma through to pin9. I think I should change the 82R/220R but not sure which one or how much?
2.) The voltage on the Cathodyne is too low. The only thing I can think of is to change the pair of 39K resistors, or the 3.9k B+ resistor in front of the phase splitter. If I change the 3.9K then the voltages on the triode section down stream will also need adjustment to bring the triode B+ down to 90v.
I am expecting a few other things. Right now I am not real sure how to proceed. Some encouragement and help is needed.
Thanks for the help
Tube1 Tube2 Comment
pin# voltage Voltage
1 80 80 referenced to 1/4 B+
2 108,2 187.5 B+, Tube 1 is high, tube2 is low compared to the schematic. should be 90v and 200v
3 - -
4 311 311 output plate, seems about right
5 - -
6| 1.8 1.6 not sure, I think it is bias voltage from balancing network circuit
7| - -
8 115.6 115.6 screen voltage a bit low but the best I could do with the Zener diodes I have for now. should be 120v
9 14.2 14.2 Seems real low should be 18.
10 0 108.3 tube 1 is correct. Tube 2 seems wrong but is as per diagram for Cathodyne inverter should be 90v?
11 1.4 109.1 tube 1 is as per diagram. tube 2 is high compared to diagram should be 92v
12 80 80 B+ reference
Now for my questions:
1.) When trying to get things sorted out and bring the voltages more in line with the diagram, do I start at the output stage to bring the cathode bias up to the 18v target. I do not seem to have 5v on the 10k adjustment pot I only have 3.8v. I am only pulling 40ma through to pin9. I think I should change the 82R/220R but not sure which one or how much?
2.) The voltage on the Cathodyne is too low. The only thing I can think of is to change the pair of 39K resistors, or the 3.9k B+ resistor in front of the phase splitter. If I change the 3.9K then the voltages on the triode section down stream will also need adjustment to bring the triode B+ down to 90v.
I am expecting a few other things. Right now I am not real sure how to proceed. Some encouragement and help is needed.
Thanks for the help
hmmm
95 views and no comments.
I guess I am not doing this right.
Well I made some progress in the 1 hour I had today.
I eliminated the 3.9k B+ resistor upstream from the cathodyne splitter and brought the cathodyne plate to ~ 200v. (197v). The cathode is still quite high @ 115 but I think that is okay as the plate voltage on the driving triode is 114v coupled to the cathodyne grid via a 3.9k resistor.
Tomorrow I hope to find time to hook it up to the O-scope and test things out.
I really could use some discussion
I think the cathodes on the output tubes are only drawing 40ma and are at 14.7v. Is this too low? How do I do the calculation with the input signal balancing network?
Hoping to get some help
95 views and no comments.
I guess I am not doing this right.
Well I made some progress in the 1 hour I had today.
I eliminated the 3.9k B+ resistor upstream from the cathodyne splitter and brought the cathodyne plate to ~ 200v. (197v). The cathode is still quite high @ 115 but I think that is okay as the plate voltage on the driving triode is 114v coupled to the cathodyne grid via a 3.9k resistor.
Tomorrow I hope to find time to hook it up to the O-scope and test things out.
I really could use some discussion
I think the cathodes on the output tubes are only drawing 40ma and are at 14.7v. Is this too low? How do I do the calculation with the input signal balancing network?
Hoping to get some help
hello Jeff:
B+ on the output plates is 311v
I agree it is a bit light. The circuit has a balancing network that gets me confused.
See the first PDF above
Thanks
B+ on the output plates is 311v
I agree it is a bit light. The circuit has a balancing network that gets me confused.
See the first PDF above
Thanks
You can lower either cathode resistor, though reducing the 82 Ohm one will reduce the range of the balance control. You can also increase the screen voltage, with a small value Zener in series with the 120V one. But measure it as-is, so you can compare with the results after any change.
Hello Tom:
Thanks,
How does the balance network impact on the cathode resistor? is it borrowing power for biasing the grids?
Does the ratio of the 2 cathode resistors matter? Right now my total resistance is 302r.
I did not have a 120v zener but I am getting 115 from a 100v zener. How close is good enough with screen voltage.
Sorry for all the questions. As I say this is my first Push pull.
Thanks,
How does the balance network impact on the cathode resistor? is it borrowing power for biasing the grids?
Does the ratio of the 2 cathode resistors matter? Right now my total resistance is 302r.
I did not have a 120v zener but I am getting 115 from a 100v zener. How close is good enough with screen voltage.
Sorry for all the questions. As I say this is my first Push pull.
The balancing network : Part of a small portion of the cathode voltage (5 V) can be added to the grids of the output tubes. The pot determines how much. Turning the wiper upwards adds a little positive voltage to the upper tube increasing its cathode current, turning it downwards results in more positive voltage of the other tube ( resulting in an increase of its cathode current) and at the same time lowering the small positive grid voltage of the other tube thereby lowering its cathode current. Unfortunately your schematic doesn't offer a testpoint to measure the respective cathode currents.
You can solve that by adding two 10 ohm resistors to the respective cathodes. If the tubes draw 40ma each it would result in 0,4 V across the 10 ohm resistors. It doesn't have to have that value exactly as tubes have a certain spread and that's nothing to worry about. The only thing that is important is to make the cathode currents equal. And that can be done with the pot.You simply turn the wiper until the voltages across the 10 ohm resistors are equal.
You can solve that by adding two 10 ohm resistors to the respective cathodes. If the tubes draw 40ma each it would result in 0,4 V across the 10 ohm resistors. It doesn't have to have that value exactly as tubes have a certain spread and that's nothing to worry about. The only thing that is important is to make the cathode currents equal. And that can be done with the pot.You simply turn the wiper until the voltages across the 10 ohm resistors are equal.
Some years ago I built the exact same amplifier 100% following the schematic.
(output transformers and choke are cheap Chinese ones).
It should work without a problem if there are no errors made.
Search "6LU8" in the tube forum to see my version.
(output transformers and choke are cheap Chinese ones).
It should work without a problem if there are no errors made.
Search "6LU8" in the tube forum to see my version.
Hello KeesB:
Thank you for the explanation. I will add 1 ohm resistors.. So far I have been balancing the DC voltage. I will balance the tubes based on a low voltage 1K sine wave input signal.
Hello 968driver:
Thank you for your comment. I will double check my implementation again for errors. My circuit works, I am trying to figure out why I can not replicate the voltage values more closely.
I will review your version to see if there is something I missed.
Sometimes it is hard to see your own mistakes.
Thank for the comments
Thank you for the explanation. I will add 1 ohm resistors.. So far I have been balancing the DC voltage. I will balance the tubes based on a low voltage 1K sine wave input signal.
Hello 968driver:
Thank you for your comment. I will double check my implementation again for errors. My circuit works, I am trying to figure out why I can not replicate the voltage values more closely.
I will review your version to see if there is something I missed.
Sometimes it is hard to see your own mistakes.
Thank for the comments
Hi Scotty,
Since the input voltage amp tube is direct coupled to the grid of the cathodyne, it sets the cathodyne operating point. It the cathodyne voltages are off you first need to get the input stage voltages sorted.
John
Since the input voltage amp tube is direct coupled to the grid of the cathodyne, it sets the cathodyne operating point. It the cathodyne voltages are off you first need to get the input stage voltages sorted.
John
hello again 968driver:
Very nice implementation. The use of dual tag boards really cleans things a lot.
Very nice implementation. The use of dual tag boards really cleans things a lot.
Screen voltages have a significant effect on plate current. Try to get the output screen voltages where they need to be and you may be ok with the cathode resistors as specified. Do you have any low voltage zeners? A 5 or 6Vish zener in series would bring the screens up a bit. Possibly a resistor is series with the zener would raise the voltage as well.
Hello John:
I have been working my way down stream from B+. Today I will focus on getting the input voltages more in line.
And, first I will change the Zener diodes to get closer to 120v. That should be quick. I will do that before adjusting the output cathode resistors.
Thanks for the help
I have been working my way down stream from B+. Today I will focus on getting the input voltages more in line.
And, first I will change the Zener diodes to get closer to 120v. That should be quick. I will do that before adjusting the output cathode resistors.
Thanks for the help
