This article describes using two CCS and capacitors to "balance" PP amp
https://tubejack.nl/wp-content/uploads/2021/10/6AS7-Current-Balanced-PP-Amp.pdf
Oh I see. So that allows each 6BQ5 to be at a specific cathode current and makes one not need matched tubes.
You need to decide if the amp shall run in class A for max. "hifi" or in class AB for max. power.
Cathode CCS only makes sense with pure class A, because when the amp leaves class A the cathode current needs to increase.
The CCS will prevent this, resulting in clipping above class A.
In class A the output power cannot exceed the plate dissipation limit of a single tube, i.e. 12W.
In reality output will be somewhat less than 12W.
In class AB something like 17W is possible. In either case the OT will "steal" between 5% and 10% of power.
24W, have been there, done that. With 6.8 k transformer slightly less, 20-22 W and a bit less harmonic distortion. Please note that g2 max is 300 V in pentode connection for 6P14P and 7189.
Oh ok. So I suppose that I can move the screens to the same B+ the phase splitter is connected to in order to lower it then I can increase the B+ some along with lowering the value of the 196 ohm resistor so that I get more cathode current.
Or I can just swap the screen connection and leave the rest as is.
Now to get a higher B+ I'd need to ditch the 5R4 and I sort of like the tube rectifier.
What I may do is measure a 400Hz sinewave on a scope with a 4 ohm non-inductive load just to see about how much power I am getting out of the amp before I modify it again.
I had always thought the tube was equal to a 6BQ5 not a 7189 and that possibly explains why the normal voltage slightly above 300Vdc was ok when I had the amp in use with the receiver drawing its B+ from it.
Or I can just swap the screen connection and leave the rest as is.
Now to get a higher B+ I'd need to ditch the 5R4 and I sort of like the tube rectifier.
What I may do is measure a 400Hz sinewave on a scope with a 4 ohm non-inductive load just to see about how much power I am getting out of the amp before I modify it again.
I had always thought the tube was equal to a 6BQ5 not a 7189 and that possibly explains why the normal voltage slightly above 300Vdc was ok when I had the amp in use with the receiver drawing its B+ from it.
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Not with a 6BQ5 sure, but I have the Russian version which is equivalent to a 7189.
Also due to the choke, I am limited to a B+ current of 100mA so I might try dropping the value of the 196 ohm resistor some just to see what the B+ voltage does.
I suppose a 5AR4 would drop less voltage than a 5R4 would. Plus it would have the same warm up time as the rest of the tubes since it has a cathode.
The voltage across the 196 ohm resistor is -10.07Vdc measured to chassis ground.
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With 5AR4 instead if 5R4G you will gain over 30 V at full power. More important, with Class AB1 B+ voltage needs to be well-regulated, which 5R4G is not suitable for because it has large voltage drop.
So with the 5AR4 I'll likely have to increase the value of the 1.2k resistor so that I keep the screens at 300Vdc, right?
I made the change to move the screens to after the 1.2k resistor and also the change to grounding the heater winding center tap. If there is any hum I cannot hear it now.
Here's the voltages.
B+ 308.8Vdc
B+ after choke 299Vdc
7189 Plate 297.7Vdc
7189 screen 290.9Vdc
Grid bias -10.13Vdc
Cathode current 65.5mA
The only reason the choke is there is because when I powered a receiver from the B+, I had issues with the receiver without the choke (the main reason I used a ground buss in the amp made out of 14 gauge solid copper wire) and given the ripple voltage is so low after the choke (less than 100mV I believe) I just left it in place when I made a separate supply for the receiver.
Something I just thought of.
Is it ok to use a switched isolated 1/4" jack in place of the cathode resistor with a 1/4" plug connected to the milliammeter which would be in a separate box with the switch keeping the cathode connected to ground when meter is not plugged into it or is that just asking for trouble?
Now if I increase the B+ current to closer to 100mA I might not want to use the meter since it is only rated to 100mA.
I made the change to move the screens to after the 1.2k resistor and also the change to grounding the heater winding center tap. If there is any hum I cannot hear it now.
Here's the voltages.
B+ 308.8Vdc
B+ after choke 299Vdc
7189 Plate 297.7Vdc
7189 screen 290.9Vdc
Grid bias -10.13Vdc
Cathode current 65.5mA
The only reason the choke is there is because when I powered a receiver from the B+, I had issues with the receiver without the choke (the main reason I used a ground buss in the amp made out of 14 gauge solid copper wire) and given the ripple voltage is so low after the choke (less than 100mV I believe) I just left it in place when I made a separate supply for the receiver.
Something I just thought of.
Is it ok to use a switched isolated 1/4" jack in place of the cathode resistor with a 1/4" plug connected to the milliammeter which would be in a separate box with the switch keeping the cathode connected to ground when meter is not plugged into it or is that just asking for trouble?
Now if I increase the B+ current to closer to 100mA I might not want to use the meter since it is only rated to 100mA.
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Bad idea with switched plug. Poor contact (which could happen) will break cathode circuit and cause a powerful transient that may destroy the speaker. But small value resistor can be soldered in for DC current sensing.
Figured it wasn't a good idea. I currently have a 1 ohm resistor and a pair of banana jacks that goes across it so that I can use a DMM to measure the voltage and convert it directly into current.
I found the 5AR4 and I'll need to replace the 1.2k resistor with a higher value as screen B+ is about 320V. I've got a 2k resistor that I will try. If that doesn't do it I'll put it in series with the 1.2k resistor.
I found the 5AR4 and I'll need to replace the 1.2k resistor with a higher value as screen B+ is about 320V. I've got a 2k resistor that I will try. If that doesn't do it I'll put it in series with the 1.2k resistor.
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So I tried the 5AR4 and the 2k resistor in series with the 1.2k resistor.
Here's the voltages,
B+ 344.3Vdc
B+ after choke 333.5Vdc
7189 plate 331.5Vdc
7189 screen 312.3Vdc
7189 grid bias -10.94V
7189 cathode current 54.8mA
Thought the cathode current wasn't right, but I then remembered the 1 ohm cathode resistor was barely touching the ground buss so that likely caused the meter to read higher.
Are the voltages and current ok?
Here's the voltages,
B+ 344.3Vdc
B+ after choke 333.5Vdc
7189 plate 331.5Vdc
7189 screen 312.3Vdc
7189 grid bias -10.94V
7189 cathode current 54.8mA
Thought the cathode current wasn't right, but I then remembered the 1 ohm cathode resistor was barely touching the ground buss so that likely caused the meter to read higher.
Are the voltages and current ok?
If I'm doing the measurement right:
1. Audio generator to amplifier input
2. 4 ohm non-inductive load on the 4 ohm output.
3. Scope and DMM across the load.
I'm only getting 6.47 watts before the amp starts to distort and it looks like the phase splitter stage is running out of steam before the output stage is.
So I need a different phase splitter tube such as a 6J5.
1. Audio generator to amplifier input
2. 4 ohm non-inductive load on the 4 ohm output.
3. Scope and DMM across the load.
I'm only getting 6.47 watts before the amp starts to distort and it looks like the phase splitter stage is running out of steam before the output stage is.
So I need a different phase splitter tube such as a 6J5.
Tried the 6J5, but same issue so I did more probing and I discovered a huge flaw in the way I have derived the bias.
As the B+ current increases with an increase of audio signal, the voltage drop across the 196 ohm resistor gets larger which increases the - bias voltage on the 7189 tubes and is likely causing my issue.
So given I know the bias voltage requirements, that 10 volt center tapped transformer will provide the proper bias voltage.
By doing that the bias will be made adjustable which will allow me to vary the cathode current.
I'll use a bridge rectifier and a larger value 25V cap so that there is no ripple voltage on the bias supply.
Doing the math 10Vac will get me an unloaded voltage of 14.14Vdc.
What I may do is put a resistor between the pot terminal that would be grounded and ground so that I cannot adjust the bias voltage so low as to damage the tubes. That would also make the pot not as sensitive when adjusting it. I'll wire the pot so that CW rotation increases the cathode current.
As the B+ current increases with an increase of audio signal, the voltage drop across the 196 ohm resistor gets larger which increases the - bias voltage on the 7189 tubes and is likely causing my issue.
So given I know the bias voltage requirements, that 10 volt center tapped transformer will provide the proper bias voltage.
By doing that the bias will be made adjustable which will allow me to vary the cathode current.
I'll use a bridge rectifier and a larger value 25V cap so that there is no ripple voltage on the bias supply.
Doing the math 10Vac will get me an unloaded voltage of 14.14Vdc.
What I may do is put a resistor between the pot terminal that would be grounded and ground so that I cannot adjust the bias voltage so low as to damage the tubes. That would also make the pot not as sensitive when adjusting it. I'll wire the pot so that CW rotation increases the cathode current.
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I did a quick simulation of your circuit, with an ideal fixed bias source and B+, looks like you should get ~17W indeed. The quiescent EL84 currents, for some reasons, are much higher in my simulation, I get ~40mA per tube. The usual caveats for simulations, it is not the real thing.
I will se if I can find the model, but I do not expect miracles. Can't find 6P14P-specific models either. In any case, I think the way to max power is to increase B+, keep G2 at 300V, keep the plate and G2 dissipation in check.
I can see around 21W with B+ at 360V and G2 at 308, bias -12.5V, with 14V peak as input signal to the splitter.
I can see around 21W with B+ at 360V and G2 at 308, bias -12.5V, with 14V peak as input signal to the splitter.