FWIW, I like a shared RC bias network, in each channel. Matching requirements are not particularly restrictive. Each pair must, as is always the case, be very close in gm. However, minor differences in cathode current will be compensated for automatically. Also, 2X matched pairs cost less than a matched quartet.
Buy O/P tubes from a reliable dealer who matches pairs under real world operating conditions. Factory "matching" leaves much to be desired in more than 1 way.
The "Cheap Charlie" method of DC heaters SUCKS.
Do you want O/P stage tube cathode current modulating phono section cathode temperature? Obviously, that is an undesirable state of affairs. Building a 300 mA. regulated 12 VDC PSU is not at all challenging.
Buy O/P tubes from a reliable dealer who matches pairs under real world operating conditions. Factory "matching" leaves much to be desired in more than 1 way.
The "Cheap Charlie" method of DC heaters SUCKS.
Do you want O/P stage tube cathode current modulating phono section cathode temperature? Obviously, that is an undesirable state of affairs. Building a 300 mA. regulated 12 VDC PSU is not at all challenging.Eli Duttman,
Quote:
FWIW, I like a shared RC bias network, in each channel. Matching requirements are not particularly restrictive. Each pair must, as is always the case, be very close in gm. However, minor differences in cathode current will be compensated for automatically. Also, 2X matched pairs cost less than a matched quartet.
Question:
For a circuit with coupled cathodes and a single self bias resistor plus a bypass capacitor:
"minor differences in cathode current will be compensated for automatically"
How?
What is the method of difference current correction?
How is it automatic?
Quote:
FWIW, I like a shared RC bias network, in each channel. Matching requirements are not particularly restrictive. Each pair must, as is always the case, be very close in gm. However, minor differences in cathode current will be compensated for automatically. Also, 2X matched pairs cost less than a matched quartet.
Question:
For a circuit with coupled cathodes and a single self bias resistor plus a bypass capacitor:
"minor differences in cathode current will be compensated for automatically"
How?
What is the method of difference current correction?
How is it automatic?
An increase in the total cathode current results in a greater negative bias voltage, which acts to lower the total cathode current and vice versa. The 2 tubes have to be reasonably well matched, from the outset. Otherwise, 1 of them will be dominant.
Stability of "idle" conditions is an inherent characteristic of RC self (cathode) bias.
Stability of "idle" conditions is an inherent characteristic of RC self (cathode) bias.
It is not always easy to test the individual currents of 'matched' tubes in an amplifier that uses common self bias, or that uses common fixed bias.
The total current of a pair of tubes with a common self bias circuit is somewhat self adjusting.
The balance of the two currents in the same circuit is not self adjusting.
Verifying Balance:
Install individual low Ohmic value resistors in each cathode circuit.
Install individual low Ohmic value resistors in each plate circuit.
Then measure the voltage across the resistors.
Calculate the individual currents, V/R.
Or, a small resistor, R, in series from the plate to the lowest DCR 1/2 winding of the push pull primary, where such resistor plus the lower DCR equals the higher DCR of the other 1/2 of the primary.
Then measure the differential DC voltage of the plates.
Add R + lower DCR + higher DCR = Rtotal.
Calculate the differential current V/Rtotal.
I use 100 Ohm resistors in series with the screen of Beam Power tubes, and Pentode tubes. That way I can know screen current too (measuring cathode current only tells you what the total screen current plus plate current is).
Only measuring cathode current works for Triode wired Beam Power tubes and Pentode tubes.
I generally do not use push pull transformers that have a recommended maximum rating for differential DC current in the primary.
But I want the maximum unsaturated performance I can get at low frequencies with those transformers.
That is why I pay attention to the individual currents of each tube.
An additional benefit is that after thousands of hours of operation, I can easily check if one tube is starting to get weak.
The total current of a pair of tubes with a common self bias circuit is somewhat self adjusting.
The balance of the two currents in the same circuit is not self adjusting.
Verifying Balance:
Install individual low Ohmic value resistors in each cathode circuit.
Install individual low Ohmic value resistors in each plate circuit.
Then measure the voltage across the resistors.
Calculate the individual currents, V/R.
Or, a small resistor, R, in series from the plate to the lowest DCR 1/2 winding of the push pull primary, where such resistor plus the lower DCR equals the higher DCR of the other 1/2 of the primary.
Then measure the differential DC voltage of the plates.
Add R + lower DCR + higher DCR = Rtotal.
Calculate the differential current V/Rtotal.
I use 100 Ohm resistors in series with the screen of Beam Power tubes, and Pentode tubes. That way I can know screen current too (measuring cathode current only tells you what the total screen current plus plate current is).
Only measuring cathode current works for Triode wired Beam Power tubes and Pentode tubes.
I generally do not use push pull transformers that have a recommended maximum rating for differential DC current in the primary.
But I want the maximum unsaturated performance I can get at low frequencies with those transformers.
That is why I pay attention to the individual currents of each tube.
An additional benefit is that after thousands of hours of operation, I can easily check if one tube is starting to get weak.
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I leave the resistor(s) in place, not just for testing, but for operation.
By using a DMM that can read down to 1mV or less, you can use small resistors that will not adversely affect the operation of the amp.
I like using the method of a resistor in the plate.
Suppose 1/2 winding of the center tapped output transformer DCR is 90 Ohms, and the other 1/2 winding DCR is 100 Ohms. Connect a 10 Ohm resistor from the '90 Ohm lead' to the Plate of the tube that is driving it. Then read the DC voltage from one plate to the other plate.
That is 200 Ohms from plate to plate.
If you get 0.2 Volts difference, then you have 0.2V / 200 Ohms = 1mA differential current in the output transformer primary.
By the way, with this correct value resistor installed, the two output tubes see a balanced load.
A 7408 that is triode wired may have a plate resistance of 2,000 Ohms.
A 7408 that is Pentode wired may have a plate resistance of more than 10,000 Ohms.
Putting a 10 Ohm resistor in series from the plate to the output transformer primary will not have much effect.
10/2,000 = 0.005, or 0.5% Even less effect for pentode wired.
Individual 10 Ohm resistors from ground to each cathode will not have much effect either.
Suppose you want 35mA current in the cathode. 0.035 Amp x 10 Ohms = 0.35V
Easy to read, and since you have one in each cathode, it is balanced.
My DMM can read down to 0.1mV (100uV).
But even if your DMM only can read down to 1mV, it is important to use resistors that are better than 1%, or at least 1% accuracy. Otherwise, it is hardly worth testing the DC current balance of the two output tubes.
By using a DMM that can read down to 1mV or less, you can use small resistors that will not adversely affect the operation of the amp.
I like using the method of a resistor in the plate.
Suppose 1/2 winding of the center tapped output transformer DCR is 90 Ohms, and the other 1/2 winding DCR is 100 Ohms. Connect a 10 Ohm resistor from the '90 Ohm lead' to the Plate of the tube that is driving it. Then read the DC voltage from one plate to the other plate.
That is 200 Ohms from plate to plate.
If you get 0.2 Volts difference, then you have 0.2V / 200 Ohms = 1mA differential current in the output transformer primary.
By the way, with this correct value resistor installed, the two output tubes see a balanced load.
A 7408 that is triode wired may have a plate resistance of 2,000 Ohms.
A 7408 that is Pentode wired may have a plate resistance of more than 10,000 Ohms.
Putting a 10 Ohm resistor in series from the plate to the output transformer primary will not have much effect.
10/2,000 = 0.005, or 0.5% Even less effect for pentode wired.
Individual 10 Ohm resistors from ground to each cathode will not have much effect either.
Suppose you want 35mA current in the cathode. 0.035 Amp x 10 Ohms = 0.35V
Easy to read, and since you have one in each cathode, it is balanced.
My DMM can read down to 0.1mV (100uV).
But even if your DMM only can read down to 1mV, it is important to use resistors that are better than 1%, or at least 1% accuracy. Otherwise, it is hardly worth testing the DC current balance of the two output tubes.
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in first time ,i have placed 4 10ohm resistors to reed a bias to the 4 6v6 and I have too many differences between the tubes.
I bought another quad 6v6gt and it's much better, I managed to mix the two quad to have a balance.
I think I will mount a 100ohm DC balance potentiometer as on the A500 to fine tune the setting.
I bought another quad 6v6gt and it's much better, I managed to mix the two quad to have a balance.
I think I will mount a 100ohm DC balance potentiometer as on the A500 to fine tune the setting.
in fact, mounting with 10 ohm resistors to read bias does not work.
we have to find out how to do it differently because with the original polarization system, the 4 resistances add up.
or then, I have to disconnect the heating of the phono tube, that I weld the 4 reistances 680 ohm + the 10 ohm and then make the measurements to see what it gives.
Or does anyone have an idea? to read the bias of each lamp with the original bias circuit?
we have to find out how to do it differently because with the original polarization system, the 4 resistances add up.
or then, I have to disconnect the heating of the phono tube, that I weld the 4 reistances 680 ohm + the 10 ohm and then make the measurements to see what it gives.
Or does anyone have an idea? to read the bias of each lamp with the original bias circuit?
stupide man + fingers salad +too many things at the same time= jackass.
now that I reconnected my brain, it works and I actually have a significant shift between the tubes, so I will continue and solder the 4 x 680ohm + 2x potentiometer DC 100ohm
i just say a big big thanks to 6A3sUMMER for his help and advice .
I finished the changes this morning and since then it's just happiness !!!
the bass are clean, the highs are fine and smooth, he looks released.
have found the signature of the a300 but everything is better, much better.
So I dismounted the old bias system, separated the two pairs of tubes,
mounted two potentiometer 1K resistance variable (because I could not have a good result with the resistances available) set the bias at 34mA, then I mounted two balance potentiometers dc 100r to balance in current the two pair of tubes.
I can just say it's night and day !!
it only remains to control the cathode resistances and it's over.
I still want to thank 6A3sUMMER especially for me in the right direction because thanks to him I learned a lot and I can fend for myself now.
I bought a heathkit AA32 but I did not keep it because I do not like its sound, I bought a Kenwood W41 and a lafayette LA-250 to see ...
to be continued in the next episode...
I finished the changes this morning and since then it's just happiness !!!
the bass are clean, the highs are fine and smooth, he looks released.
have found the signature of the a300 but everything is better, much better.
So I dismounted the old bias system, separated the two pairs of tubes,
mounted two potentiometer 1K resistance variable (because I could not have a good result with the resistances available) set the bias at 34mA, then I mounted two balance potentiometers dc 100r to balance in current the two pair of tubes.
I can just say it's night and day !!
it only remains to control the cathode resistances and it's over.
I still want to thank 6A3sUMMER especially for me in the right direction because thanks to him I learned a lot and I can fend for myself now.
I bought a heathkit AA32 but I did not keep it because I do not like its sound, I bought a Kenwood W41 and a lafayette LA-250 to see ...
to be continued in the next episode...
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