Calculating cathode resistor for cathode biasing.

[Alan0354]

Alan,

I've posted this a few other places but repeat it here .

Powerscaled 6V6 ( a quad) with fixed/cathode bias switching on each pair and and triode/pentode switching on each pair.

For your possible interest here are my power amp set to work notes:

Set to Work of the Power Amp.

Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).

Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts

Set Power Scale Control to 10
Set Compression (SAG) Control to 0
Set Fixed Bias
Set pentode Mode

A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.

Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.

Set the 4 bias contrls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.

Checks:
B+ = 345 Volts
Bias Current/Voltages
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V

Switched to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switch back to Pentode Mode

Set Power Scale Control for B+ = 250V
Checks:
B+ = 250V
Bias Current/Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V

Set Power Scale Control for B+ =150V
Checks
B+ = 150V
Bias Current and Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V

Set Power Scale Control back to 10 => B+ back to +345 Volts

Wind Compression Control from 0 to 10 => B+ drops to +105V
This looks a bit "drastic" but actually sounded OK.

Set Compression back to 0 and switch to Cathode Bias (each 6V6 pair has a common, unbypassed 470R 5W).

Checks:
B+ = 343V
Bias Currents / Voltages
Pair#1 Cathode Volts = +23.9V
25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V
25.2mA, 25.4mA tube currents

Cheers,
Ian

Hi Ian,

Can you tell me how the amp sounds when you turn down to 150V? I understand the bias current decrease from 24mA to 10mA. But still the bottom line is the sound.

The Fixed bias approach keep the bias current almost the same regardless of voltage if you make the grid voltage tracking the +B. But it is a lot more complicated. It sounded good when I did it. I just want to see how the cathode bias sound if you don't compensate the cathode to get more constant current.

Thanks.

[Alan0354]

In case anyone was wondering, here is the Fisher cathode bias adjustment method mentioned earlier by JoeM:

[ATTACH=CONFIG]29549[/ATTACH]

So this is to compensate in case of large signal pumping the cathode voltage higher? So when the cathode voltage increase, this will bring the grid voltage up. This will make the grid to cathode voltage vary less?

Is this to prevent the tube that is in the decreasing current cycle from turn off sooner when the cathode voltage increase?

[Gingertube]

Alan,
If you do not want a tone change as you scale down the voltage then you want to keep the transfer function the same shape as you dial down the power. For fixed bias is is essential to use a tracking bias regulator.
For Cathode bias this takes care of itself.

The amp sounded the same at 150V as it did at 345V except not as loud - that is the whole point.
The SAG Control is a current limit in the feed to the main B+ filter caps. It emulates the high internal resistance a tube recifier.

Lets see if I can embed some stuff here:
1st a bit on the SAG Control from a PM I sent someone.
2nd - 2 page (max legal here in Oz) extract from TUT5 for education purposes.See the pdf.
I ditched the "Body" Control and used a dual gang pot for the "Limit" Control.
I also changed the Triode/Pentode Switching to operate on a per output tube pair basis (2 switches 1 for each PP Pair). ditto the Cathode/Fixed Bias switches.

Cheers,
Ian

[Dave H]

So this is to compensate in case of large signal pumping the cathode voltage higher? So when the cathode voltage increase, this will bring the grid voltage up. This will make the grid to cathode voltage vary less?

I don't think the grid to cathode voltage will vary less than the usual simple cathode resistor circuit without the potential divider. If you put a divider across the cathode resistor the cathode resistor will have to be increased in value so that its voltage can be divided down to set the same grid to cathode voltage and bias current as the simple circuit. I think it was done that way to avoid using a high power rheostat in the cathode.

[Alan0354]

Alan,
If you do not want a tone change as you scale down the voltage then you want to keep the transfer function the same shape as you dial down the power. For fixed bias is is essential to use a tracking bias regulator.
For Cathode bias this takes care of itself.

The amp sounded the same at 150V as it did at 345V except not as loud - that is the whole point.
The SAG Control is a current limit in the feed to the main B+ filter caps. It emulates the high internal resistance a tube recifier.

Lets see if I can embed some stuff here:
1st a bit on the SAG Control from a PM I sent someone.
2nd - 2 page (max legal here in Oz) extract from TUT5 for education purposes.See the pdf.
I ditched the "Body" Control and used a dual gang pot for the "Limit" Control.
I also changed the Triode/Pentode Switching to operate on a per output tube pair basis (2 switches 1 for each PP Pair). ditto the Cathode/Fixed Bias switches.

Cheers,
Ian

Thanks Ian

It's very kind or you to share. I have to study the sag control. I was faking it by just using a 100 to 200 ohm resistor.

I am going to try both ways. The hard part is putting in the big MOSFET and have to worry about the heat. Building the opamp for fixed bias or using a cathode resistor is the easy part. If it work out that I can just use a single cathode resistor and only adjust the +B with the MOSFET, it would be easy AND also this reduce the power dissipation on the MOSFET. This is because as you lower the +B, the bias current decrease when more voltage drop across the MOSFET. That is a big plus.

If doing the constant bias current fixed bias in your example, the total bias current is about 48mA. When you drop to +B=150V, about 200V is dropped across the MOSFET and power dissipation is 200V X 0.048A=9.6W!!! In your case using cathode bias, dropping 200V across but with only 20mA(total at 150V) W=200V X 0.02A=4W. That is a lot lower.

Alan

[jazbo8]

The Fisher cathode bias scheme is pretty clever - it's not just a pot on the cathode - a portion of the cathode voltage is also fed back to the grid leak resistor, so Vgk can be set accurately and be maintained even when the B+ sags. Only downside is that more parts are needed.

[Alan0354]

Alan,
If you do not want a tone change as you scale down the voltage then you want to keep the transfer function the same shape as you dial down the power. For fixed bias is is essential to use a tracking bias regulator.
For Cathode bias this takes care of itself.

The amp sounded the same at 150V as it did at 345V except not as loud - that is the whole point.
The SAG Control is a current limit in the feed to the main B+ filter caps. It emulates the high internal resistance a tube recifier.

Lets see if I can embed some stuff here:
1st a bit on the SAG Control from a PM I sent someone.
2nd - 2 page (max legal here in Oz) extract from TUT5 for education purposes.See the pdf.
I ditched the "Body" Control and used a dual gang pot for the "Limit" Control.
I also changed the Triode/Pentode Switching to operate on a per output tube pair basis (2 switches 1 for each PP Pair). ditto the Cathode/Fixed Bias switches.

Cheers,
Ian

I looked at your hand written circuit. I have a few questions:

1) I notice you don't put any filter caps between the rectifiers and the Drain of the MOSFET. The voltage at the Drain of the MOSFET can be pulled down by the Power Scale potentiometer in series with the 33K resistor. Are you relying on the diode BYT13 1000 right in front of the filter cap to allow the whole MOSFET and SAG circuit to be pulled down by the potentiometer and the 33K?

2) I am looking at the sag circuit. Looks like it's the voltage drop across the 15R and 0R33 that turn on the 2N3904. Which in turn pull the gate of the MOSFET down to lower the +B. But my question is this. The current drawn by the 2N3904 develop voltage across the 10K resistor and the resistance seen INTO the wiper of the power scale potentiometer. When the potentiometer adjust to the top, the voltage drop is only cause by the 10K resistor. BUT if the potentiometer is adjust somewhere in the middle. The resistance looking into the wiper of the potentiometer is very high. There would be a lot of sag even when the 2N3904 conduct very little current. This means the sag is a lot more severe if the Power Scale potentiometer is adjust in the middle. IS that true?

2) What is the difference between using a 100 or 200 ohm resistor in place of the SAG circuit to simulate the tube rectifier?


I am thinking about simplifying as shown, please comment. I use a 100 ohm as sag resistor. D6 and D7 is to protect the MOSFET from reverse of voltage.

[J M Fahey]

FWIW I remember a post a few years ago where somebody tested an amp with Power Scaling or some equivalent, recorded the amp with lower and lower +B , but took the precaution when recording of resetting recording mic gain every time so recorded level was exactly the same, so as to avoid human ear nonlinearity from affecting the perceived results.

So he started with, say, +B 450V ande went down step by step, 50V each.

It was chilling to hear that sound stayed the same, until incredible + 50V +B , where amp output could be measured in milliwatts.

Going back and forth you could hear very small differences, but sound was always good and natural.

Poster said *his* room perception was that beyond certain point sound became thinner and fizzier .... but recording proved it was not the amp's fault.

[Alan0354]

I can say my amp with power scaling using variac sounded very good at 1/3 +B. It was a Twin Reverb with two power tube pulled. I had the variac at about 1/3 and it sounded great. So the +B should be about 150V as variac is linear. This is the recording back in 78. The quality is really bad as it was recorded with a hand held cassette only. There was no pedal, I had a 3 stages cascade for the OD channel. I used a A/B pedal to do channel switching between clean and OD.

Two solo - YouTube

http://www.youtube.com/watch?v=ZViICRVzE0s

This is the Twin that I modified in 78 with separate transformers for filaments and preamp. Then put the variac on the original PT to adjust the voltage of the power tubes, PI and the grid bias only. Both the clean sound for rhythm and OD for solo was the sound of the amp, no effect pedal of any sort.

[Alan0354]

Alan,
If you do not want a tone change as you scale down the voltage then you want to keep the transfer function the same shape as you dial down the power. For fixed bias is is essential to use a tracking bias regulator.
For Cathode bias this takes care of itself.

The amp sounded the same at 150V as it did at 345V except not as loud - that is the whole point.
The SAG Control is a current limit in the feed to the main B+ filter caps. It emulates the high internal resistance a tube recifier.

Lets see if I can embed some stuff here:
1st a bit on the SAG Control from a PM I sent someone.
2nd - 2 page (max legal here in Oz) extract from TUT5 for education purposes.See the pdf.
I ditched the "Body" Control and used a dual gang pot for the "Limit" Control.
I also changed the Triode/Pentode Switching to operate on a per output tube pair basis (2 switches 1 for each PP Pair). ditto the Cathode/Fixed Bias switches.

Cheers,
Ian

I like the inverter tracking circuit using Q12 and 13 in the pdf file. I was going to use a high voltage opamp to do it. Opamp is more accurate, the summing junction of this simple circuit has a 0.7V offset. But I don't think this matter much. I am going to order the opamp and the two transistors and try both ways.

[Alan0354]

The Fisher cathode bias scheme is pretty clever - it's not just a pot on the cathode - a portion of the cathode voltage is also fed back to the grid leak resistor, so Vgk can be set accurately and be maintained even when the B+ sags. Only downside is that more parts are needed.

Seems like it compensate the grid voltage when the cathode voltage get pumped up when a large signal hits. The increase of cathode voltage is partly feed to the grid leak resistors and raise the bias a little.

[Gingertube]

I looked at your hand written circuit. I have a few questions:

1) I notice you don't put any filter caps between the rectifiers and the Drain of the MOSFET. The voltage at the Drain of the MOSFET can be pulled down by the Power Scale potentiometer in series with the 33K resistor. Are you relying on the diode BYT13 1000 right in front of the filter cap to allow the whole MOSFET and SAG circuit to be pulled down by the potentiometer and the 33K?

2) I am looking at the sag circuit. Looks like it's the voltage drop across the 15R and 0R33 that turn on the 2N3904. Which in turn pull the gate of the MOSFET down to lower the +B. But my question is this. The current drawn by the 2N3904 develop voltage across the 10K resistor and the resistance seen INTO the wiper of the power scale potentiometer. When the potentiometer adjust to the top, the voltage drop is only cause by the 10K resistor. BUT if the potentiometer is adjust somewhere in the middle. The resistance looking into the wiper of the potentiometer is very high. There would be a lot of sag even when the 2N3904 conduct very little current. This means the sag is a lot more severe if the Power Scale potentiometer is adjust in the middle. IS that true?

2) What is the difference between using a 100 or 200 ohm resistor in place of the SAG circuit to simulate the tube rectifier?


I am thinking about simplifying as shown, please comment. I use a 100 ohm as sag resistor. D6 and D7 is to protect the MOSFET from reverse of voltage.

Question 1 above:
No the mosfet gate has a pulsating DC (just the rectified AC) voltage set by the Powerscale pot. When the source voltage is less than this then (by the Vgs threshhold) the MOSFET will switch on charging the down stream B+ caps. That is, the MOSFET is acting as a "switching regulator" if you like and B+ is set by that Powerscale pot setting.

The 0R33 Ohms sets a peak inrush current limit (working with the 2N3904) of 0.65V/0R33=2 Amps and that circuit should be retained regardless of if you add the SAG control or not. To delete the SAG (Compression) Control leave out the 15R 5W, the 25K lin SAG Control and wire the 100K back to the MOSFET Source. Note that that 10K on the Powerscale Control wiper can under certain conditions (at power on time) be subject to momentary high voltage peaks and a Power resistor is therefor recommended, schematic says 10K 5W (2 off 4K7 2W in series would be OK).

The SAG amount is determined ONLY by the amount of the current determined voltage drop across 15R 5W resistor that is tapped off by the 25K pot. As soon as there is 0.65 from base to emitter on the 2N3904 then the current will start to limit.

If you delete the SAG Control you can add the 100R or 200R power resistor instead. This should go after the 0R33 peak current limit sense resistor.

Your proposed circuit needs that peak current limit cicuitry consisting of teh 0R33 and the 2N3904 put back in, the diode from MOSFET gate to drain does nothing and can be left out. The diode in series with the output, before the B+ capacitors is required to prevent current back thru' the MOSFET bulk diode at power off and it can be a ultrafast soft recovery type which will also control diode switching noise from the rectifiers. That 1R + 1uF PolyPropylene cap shown is also to address diode switching noise.

The fixed resistor will give fixed SAG/COMPRESSION, The control show allows a settable comression from 0 to max determined by the 15R 5W sense resistor (note the 25K control just taps off more or less of the current determined voltage drop from that 15R). Want a bit more range then take the 15R up to 22R.

Hope this helps.

Ian

[Alan0354]

Question 1 above:
No the mosfet gate has a pulsating DC (just the rectified AC) voltage set by the Powerscale pot. When the source voltage is less than this then (by the Vgs threshhold) the MOSFET will switch on charging the down stream B+ caps. That is, the MOSFET is acting as a "switching regulator" if you like and B+ is set by that Powerscale pot setting.
Yes, that's what I figure.
The 0R33 Ohms sets a peak inrush current limit (working with the 2N3904) of 0.65V/0R33=2 Amps and that circuit should be retained regardless of if you add the SAG control or not. To delete the SAG (Compression) Control leave out the 15R 5W, the 25K lin SAG Control and wire the 100K back to the MOSFET Source. Note that that 10K on the Powerscale Control wiper can under certain conditions (at power on time) be subject to momentary high voltage peaks and a Power resistor is therefor recommended, schematic says 10K 5W (2 off 4K7 2W in series would be OK).
I studied the sag circuit, say if the 25K sag control pot is at the bottom, once the current reach 2A, the 2N3904 turns on and pull the gate of the MOSFET down. So this ONLY work for charging the filter caps on power up, not really for sag sounding like in tube rectifier?
The SAG amount is determined ONLY by the amount of the current determined voltage drop across 15R 5W resistor that is tapped off by the 25K pot. As soon as there is 0.65 from base to emitter on the 2N3904 then the current will start to limit.

If you delete the SAG Control you can add the 100R or 200R power resistor instead. This should go after the 0R33 peak current limit sense resistor.

Your proposed circuit needs that peak current limit cicuitry consisting of teh 0R33 and the 2N3904 put back in, the diode from MOSFET gate to drain does nothing and can be left out. The diode in series with the output, before the B+ capacitors is required to prevent current back thru' the MOSFET bulk diode at power off and it can be a ultrafast soft recovery type which will also control diode switching noise from the rectifiers. That 1R + 1uF PolyPropylene cap shown is also to address diode switching noise.

The fixed resistor will give fixed SAG/COMPRESSION, The control show allows a settable comression from 0 to max determined by the 15R 5W sense resistor (note the 25K control just taps off more or less of the current determined voltage drop from that 15R). Want a bit more range then take the 15R up to 22R.

Hope this helps.

Ian

Looks like the 2N3904 is only for current limiting during power up, I don't think it is necessary. The MOSFET usually can carry over 20 Amps, the one I use is 50A continuous. Instantaneous current is over 100A. With a 100 ohm resistor in series, worst worst case if the PT can provide unlimited current, the max current is only +B/100ohm which is only about 4A!!! In reality, the power transformer pretty much serves as current limiting. I never see any amp with SS rectifier using a transistor as current limiting. In fact, I have not seen amp using 100 ohm as current limiting resistor. I am more thinking about using it to sag the +B when the input signal is high to simulate the tube rectifier.

Yes, the diode across the gate and drain is not necessary. This is my circuit with power scaling on fixed bias. I need to try out the two transistor adjustable grid bias circuit. So at the mean time, I am still using the high voltage opamp powered by the 45VAC tap of the PT. The opamp is a little more expensive, but it's going to be more precise.

I delete the 10K resistor at the gate of the MOSFET. I don't think it's necessary as I don't use 2N3904 sag circuit. I use 150 ohm as sag resistor to limit the current to about 3A.


Thanks for you help.

Alan

[Alan0354]

One question. How important is the switching transient of the rectifier diodes? I don't see other amps with SS rectifier worry about having small value caps to suppress the switching transients.

[Alan0354]

Alan,

2nd - 2 page (max legal here in Oz) extract from TUT5 for education purposes.See the pdf.
I ditched the "Body" Control and used a dual gang pot for the "Limit" Control.

Cheers,
Ian

I am still reading the pdf attachment.

1) Is this from a book or you wrote this book?
2) The body control looks like it's a volume control for one side of the power amp. When you crank it down, input on one phase to the power amp is reduced and the push pull becomes unbalanced. What does that do to the sound.
3) In the notes, the IMA which is the PPIMV, you said it is replaced by a dual gang pot. How is this connected?

Thanks