Combination cathode bias and fixed bias

[tubeswell]

Yes, I mean running a combination of both types together. This is why it's called "combination bias". With repect to the 5G9, I'd have to see the print in order to make a comment on that.

-g

http://www.ampwares.com/ffg/schem/tr..._5g9_schem.gif

[tubeswell]

Yep thanks MWJB - I'm experimenting with various V1 tubes besides the 12AY7. Yesterday I tried a 12AT7, 5751 and 12AX7. I haven't played with the amp yet for long enough yet with those different tubes to form a complete opinion. They all do something each progressively less subtle than the last, to the thickness of the sound.

But I have got in my head a sound whereby you pick the string harder and the note sings more in the amp. Kind of a bit like what I get from my 5F2A, but with more complexity. I realise they are biased using different methods, and that the SE amp is operating in Class A. I was looking for different kinds of distortion from the push-pull configuration.

Is getting the grid voltage swing higher the best way to get distortion? Or can the output tubes be driven into saturation in another way that doesn't blow fuses? I changed the bias to 22mA before trying out those other pre-amp tubes

Maybe if I changed the rectifier tube to a Sovtek 5U4G coke bottle I could get more sag? (At the moment I have an EH 5U4GB in there) I'll try that and see.

Otherwise I might have to try and obtain a pair of JJ 6V6s and put the bias back to 31mA.

[CitizenCain]

Actually, the variable power circuit that Dennis Kager had in his Sundown amps is a combination fixed/cathode bias, with the cathode bias set by an adjustable pot. Check the schematics floating around, you can see the cathodes of the power tubes hooked to ground through a 250R 5W pot, bypassed with a cap.

[stokes]

If you want a combination of fixed and cathode bias and not a switchable option,it is quite simple.Leave the fixed bias setup you have,lift the cathode from ground and add a smaller cathode resistor than you would use for a full cathode biased amp.You would need to make your fixed bias feed adjustable,just replace that 56k resistor in the bias circuit with a 100k bias pot,I like the multi-turn cermet pots for this.Your power tubes will be biased "hot" with the smaller cathode resistor,you then tame it by adding more negative bias voltage with the pot.Like I said earlier,I tried a range of values with the rhrostat and when I got the sound I liked I used a 75ohm 5watt resistor.

[tubeswell]

If you want a combination of fixed and cathode bias and not a switchable option,it is quite simple.Leave the fixed bias setup you have,lift the cathode from ground and add a smaller cathode resistor than you would use for a full cathode biased amp.You would need to make your fixed bias feed adjustable,just replace that 56k resistor in the bias circuit with a 100k bias pot,I like the multi-turn cermet pots for this.Your power tubes will be biased "hot" with the smaller cathode resistor,you then tame it by adding more negative bias voltage with the pot.Like I said earlier,I tried a range of values with the rhrostat and when I got the sound I liked I used a 75ohm 5watt resistor.

Yep thanks Stokes, this is what i was thinking about initially

I have already built the amp with adjustable 'fixed bias', and I already have 1R resistor across each cathode to ground to measure the cathode voltage and work out the current. So I know it would be a simple matter to change the cathode resistors.

What I was wondering was, can I bias the tubes higher this way, or should I bias them at say 70% or 80% (allowing for working out the difference with (say 75R) resistors (- meaning I would calculate cathode voltage/75R to get the current, when I was fiddling with the bias pot)?

Can the tubes run some of the time in Class A and some of the time in Class A/B this way? or is that not possible?

And does the cathode biasing part give me some more compression when I pick the strings harder? I think I am looking for a little bit more responsiveness in this regard.

The other thing that occurred to me was; wouldn't the negative bias need to be reduced if I was adding cathode resistance? I wonder this because, a cathode biased 6V6 just has a (Say 470R) cathode Resistor and a grid load resistor of (say) 220k, but if you build it fixed bias, it has no cathode resistor and a negative voltage of about 20-30V applied to the ground side of the 220k grid load resistor. So surely if you add a cathode resistor to a fixed bias setup, you need to reduce the negative voltage going to the grid, and not the other way around (- or what am I missing here?)

Lastly - In doing the setup this way would I need a bypass cap for the cathode resistor?

Cheers

[Paul P]

If I were you I'd try it one way or another and then play your guitar as hard
as you can while monitoring the current through the tubes. Set the bias so
that the dissipation never goes above what you feel safe with (probably
somewhere around 13-14 watts). You have to also monitor the plate voltage
under load to get your calculations correct. I have my (now cathode biased)
Tung-sol 6v6s idling at 13 watts, but I don't crank the amp.

Since your tremolo is done with the power tubes you also have to take into
account what happens when the intensity is set to max.

Paul P

[hasserl]

Can the tubes run some of the time in Class A and some of the time in Class A/B this way? or is that not possible?

Just to clarify this, the class of operation is determined by what's happening at the point of max clean, undistorted power. If the tubes are counducting for the full 360 degrees at this point it is a class A amp; if they are conducting for less than 360 degrees but more than 180 it is class A/B. Most class A/B amps will have the tubes conducting for the full 360 degrees when not driving the amp hard but when you start driving it harder at some point before it reaches max clean power they will start to cut off for a period. Sometimes people describe that as operating in Class A at low power and Class A/B at high power, but that is not an accurate way to describe it. Class of operation is determined as I described above.

And does the cathode biasing part give me some more compression when I pick the strings harder? I think I am looking for a little bit more responsiveness in this regard.

Yes, the cathode bias will give the amp some compression and some volume swell. I don't know how much it will be, obviously it will be less than a straight cathode bias amp, but more than a straight fixed bias amp. I assume the larger the cathode resistor, the stronger the compression effect.

The other thing that occurred to me was; wouldn't the negative bias need to be reduced if I was adding cathode resistance? I wonder this because, a cathode biased 6V6 just has a (Say 470R) cathode Resistor and a grid load resistor of (say) 220k, but if you build it fixed bias, it has no cathode resistor and a negative voltage of about 20-30V applied to the ground side of the 220k grid load resistor. So surely if you add a cathode resistor to a fixed bias setup, you need to reduce the negative voltage going to the grid, and not the other way around (- or what am I missing here?)

Yes, you would need less negative voltage from the fixed bias voltage supply. Note that the total negative voltage amount wouldn't change, but you would get some of the negative voltage from the cathode resistor, and some from the fixed bias supply.

Lastly - In doing the setup this way would I need a bypass cap for the cathode resistor?

Cheers

I don't think it is ever required, but yes you could add the cap.

[BackwardsBoB]

Tubeswell - The grid bias has to go more positive if you add the cathode resistor. Let's say you've got 25mA of bias current, and your 100 Ohm bias resistor. There's .025*100 Volts, or 2.5V on the cathode resistor right? So the voltage from the plate to the cathode changed less than 2%, and it will take about the same voltage from the grid to the cathode to generate the 25mA current as it did when the grid was grounded. So the new required DC grid voltage is about 2.5V higher with respect to ground, since the cathode is 2.5V higher.

Let me try again. The cathode resistor raises the cathode voltage, and the required grid-to-cathode voltage stays about the same, so the grid voltage needed for the same bias current goes up by the amount the cathode is raised.

You can go all the way to full cathode bias if you want to. Just disconnect the end of the depth pot from the bias divider, and ground it. Insert the cathode bias networks, and you're done. The tremolo will still work.

But look at something else. If that cathode resistor isn't bypassed with a cap, it's going to severely reduce your gain, since it's providing negative feedback to the voltage from the grid to the cathode when you try to increase the current through the tube.

If you bypass your cathode resistor, what's the cathode? It's a virtual ground, only a few Volts above ground. Performance will be very similar to fixed bias.

You can go all the way to full cathode bias if you want to. Just disconnect the end of the tremolo depth pot from the bias divider, and ground it. Insert the cathode networks, and you're done. The tremolo will still work. I just don't think it's going to get you where you want to go.

Plate dissipation doesn't go way up on class AB fixed bias push-pull when you play loud. The plate voltage drops when you're sinking current with the tubes on peaks, and, on the other end of the waveform, when the plate voltage is high, the tubes are in cutoff. The tubes actually run a bit cooler when you play loud, though the peak current is much higher than the bias current.

You've got a bad case of Fender clean.

I reviewed the schematic, and Fender really didn't leave much gain on the table, except for the 12AY7 instead of a 12AX7. There's a bit here and there, but probably not what you're looking for. There are a few ways to get where you want to go:

1. Reduce the plate voltage. Can be done, but then you'll get something less than Fender clean when you turn it down. The advantage is that you'll get the distortion you crave at a slightly lower output.

2. Increase the drive to the tubes. The traditional way, which works well, is to cascade the preamp triodes on the two input channels. You can do this pretty easily. You may want a bit of attenuation between the stages. If you do it right, you can leave the a jack in between the two triodes, and have it switch to drive only the second input triode when you plug into it using the switch on the jack, which gives you a clean input, maintaining the original character of the amp.

Bias you 6V6s at <= 25mA unless you drop the plate voltage somehow. JJ's 6V6s sound kind of sterile.

[tubeswell]

Thanks hasserl.

The only question that remains not quite full answered in my view is how changing the circuit to a part cathode-biased circuit would affect the trem in this amp.

Or is the trem only taking the input from the bottom of the output tube grid resistors (which happen to be negatively biased, but which probably benefits the trem anyhow because it puts the trem input at a different ground voltage potential to the rest of the grounds in the circuit?)

Or does having the output stage partially cathode biased (and hence lowering the negative grid bias to compensate for the right tube current) therefore reduce the signal going into the oscillator circuit thereby having some sort of impact on the first oscillatr triode and thereby changing the intensity of the screens switching on and off (possibly meaning that I might need to lower the 1M resistor in series with the depth pot ground to something like 470k to keep a stronger signal in the first oscillatro triode)?

Or am I theoretically over-complicating a simple experiment?

[tubeswell]

Tubeswell - The grid bias has to go more positive if you add the cathode resistor. Let's say you've got 25mA of bias current, and your 100 Ohm bias resistor. There's .025*100 Volts, or 2.5V on the cathode resistor right? So the voltage from the plate to the cathode changed less than 2%, and it will take about the same voltage from the grid to the cathode to generate the 25mA current as it did when the grid was grounded. So the new required DC grid voltage is about 2.5V higher with respect to ground, since the cathode is 2.5V higher.

Let me try again. The cathode resistor raises the cathode voltage, and the required grid-to-cathode voltage stays about the same, so the grid voltage needed for the same bias current goes up by the amount the cathode is raised. .

Hi Backwards Bob

so just to be sure all this advice is in alignment, you're saying I need to increase the grid voltage if I put a cathode resistor in? Do you mean increase voltage as in 'decrease negative voltage' because that would align with what hasserl is saying.

Quoting backwards Bob "You can go all the way to full cathode bias if you want to. Just disconnect the end of the depth pot from the bias divider, and ground it. Insert the cathode bias networks, and you're done. The tremolo will still work."

So what do I connect the grid load resistors to? (- surely not straight to ground?, because then the trem depth pot wiper would be straight to ground, and I don't think that would work, or is there something I'm not understanding here?).



Quoting backwards Bob again "But look at something else. If that cathode resistor isn't bypassed with a cap, it's going to severely reduce your gain, since it's providing negative feedback to the voltage from the grid to the cathode when you try to increase the current through the tube.

If you bypass your cathode resistor, what's the cathode? It's a virtual ground, only a few Volts above ground. Performance will be very similar to fixed bias.

Okay - so are SE amps alot different when cathode biased? The 5F2A I built before this one has a 25uF cathode bypass cap in combination with a 470R cathode resistor. I thought that the cathode bypass cap function was pass A/C to ground without affecting the DC bias of the tube. What am I misunderstanding here?


Quoting Backwards Bob again - "You can go all the way to full cathode bias if you want to. Just disconnect the end of the tremolo depth pot from the bias divider, and ground it. Insert the cathode networks, and you're done. The tremolo will still work. I just don't think it's going to get you where you want to go."

So I put the depth pot wiper to ground? Where does the oscillator circuit get its signal? Sorry I'm not quite following you

Quoting Backwards Bob again - "Plate dissipation doesn't go way up on class AB fixed bias push-pull when you play loud. The plate voltage drops when you're sinking current with the tubes on peaks, and, on the other end of the waveform, when the plate voltage is high, the tubes are in cutoff. The tubes actually run a bit cooler when you play loud, though the peak current is much higher than the bias current.

You've got a bad case of Fender clean."

I love the Fender clean, what I think I was wanting was to get more pick responsiveness esp at high volumes and get that driven 'syrupy-thick' sound when I play hard.

Quoting Backwards Bob again - "I reviewed the schematic, and Fender really didn't leave much gain on the table, except for the 12AY7 instead of a 12AX7. There's a bit here and there, but probably not what you're looking for. There are a few ways to get where you want to go:

1. Reduce the plate voltage. Can be done, but then you'll get something less than Fender clean when you turn it down. The advantage is that you'll get the distortion you crave at a slightly lower output.

2. Increase the drive to the tubes. The traditional way, which works well, is to cascade the preamp triodes on the two input channels. You can do this pretty easily. You may want a bit of attenuation between the stages. If you do it right, you can leave the a jack in between the two triodes, and have it switch to drive only the second input triode when you plug into it using the switch on the jack, which gives you a clean input, maintaining the original character of the amp.

Bias you 6V6s at <= 25mA unless you drop the plate voltage somehow. JJ's 6V6s sound kind of sterile"

Thanks for the tips about increasing drive into the output tube grid. I guess another way I could go also is to change the LTP into another driver stage and cathodyne inverter. Putting a 5751 and or 12AX7 in V1 definitely helps, along with patching the two input channels together.

Cheers

[MWJB]

"But I have got in my head a sound whereby you pick the string harder and the note sings more in the amp. Kind of a bit like what I get from my 5F2A, but with more complexity. I realise they are biased using different methods, and that the SE amp is operating in Class A. I was looking for different kinds of distortion from the push-pull configuration." Well, I think this is hitting the nail on the head, your 5F2A is cathode biased, you 5G9 is fixed bias...you have built a different kind of amp, that will sound different. Push-pull vs SE is not the issue, you seem to be describing the cathode biased character as what you find desirable.

Fixed bias has harder dynamics, with typically less envelope of sag & bloom of the note, less interesting harmonics.

Forget class, forget a specific negative grid voltage at pin 5. If 31mA gives you the sound you like fit sturdy 6V6s (JJ) and run them at 31mA, maybe even try a saggier rectifier (5Y3GT, 5R4) to increase sag & drop plate voltage so that you can push the current up somewhat?

Or, convert to cathode bias.

[BackwardsBoB]

Hi Backwards Bob

so just to be sure all this advice is in alignment, you're saying I need to increase the grid voltage if I put a cathode resistor in? Do you mean increase voltage as in 'decrease negative voltage' because that would align with what hasserl is saying.

Yes. If your bias votage at the grid with fixed bias was -22V (for example), with 25mA of bias current and a 100 Ohm cathode resistor, you'll need about -19.5V (-22V + (2.5ma X 100R)) at the grid to maintain 25mA bias current. I sensed confusion based on previous answers. If you decrease negative voltage, does it get more negative or less negative? I don't know. You'd asked twice about how the grid bias voltage would change with your 100 Ohm cathode resistor, and hasserl responded while I was writing. I think we're in agreement.

Quoting backwards Bob "You can go all the way to full cathode bias if you want to. Just disconnect the end of the depth pot from the bias divider, and ground it. Insert the cathode bias networks, and you're done. The tremolo will still work,"

So what do I connect the grid load resistors to? (- surely not straight to ground?, because then the trem depth pot wiper would be straight to ground, and I don't think that would work, or is there something I'm not understanding here?).

The grid load resistors still go to the wiper of the pot. The connection of the end of the pot that goes to the bias divider would change to a connection to GND. There will always be 0V DC on the wiper of the pot, plus the AC from the tremolo, divided by the pot and 1M resistor, coupled through the 0.1uF cap above the pot on the other side of the 1M resistor. Stock, that 0.1uF lets the circuit reference to the bias voltage on the bias divider. It could just as easily be referenced to GND by connecting the bottom end of the pot to GND instead of the bias voltage, and then the grid bias voltage would be 0V, as is common for cathode biasing. I'm showing you how to keep the tremolo and go to straight cathode biasing instead of "combination" biasing, which I thought you were forced into because you couldn't get the tremolo to work with the cathode at ground.

Quoting backwards Bob again "But look at something else. If that cathode resistor isn't bypassed with a cap, it's going to severely reduce your gain, since it's providing negative feedback to the voltage from the grid to the cathode when you try to increase the current through the tube.

If you bypass your cathode resistor, what's the cathode? It's a virtual ground, only a few Volts above ground. Performance will be very similar to fixed bias.

Okay - so are SE amps alot different when cathode biased? The 5F2A I built before this one has a 25uF cathode bypass cap in combination with a 470R cathode resistor. I thought that the cathode bypass cap function was pass A/C to ground without affecting the DC bias of the tube. What am I misunderstanding here?

You're right about the cathode bypass cap, and you need it to keep the 470R cathode resistor from cutting the gain and linearizing the circuit with its negative feedback. 25uF is marginal with 100R resistance though. at 100 Hz, its still 63 Ohms. An AC30 uses 250uF, which is overkill.

Yes, I'm claiming that the 5F2A would have the same behavior as it has with cathode biasing if the output tube(s?) were fixed bias, and the plate and screen voltage were reduced by the voltage on the 470R resistor in cathode bias. My point is that the 100R cathode resistor you propose won't change the sound significantly if you bypass it, and if you don't bypass it, you'll lose gain and non-linearity. Hasserl may disagree with me.

Quoting Backwards Bob again - "You can go all the way to full cathode bias if you want to. Just disconnect the end of the tremolo depth pot from the bias divider, and ground it. Insert the cathode networks, and you're done. The tremolo will still work. I just don't think it's going to get you where you want to go."

So I put the depth pot wiper to ground? Where does the oscillator circuit get its signal? Sorry I'm not quite following you

Sorry. I find that I have an uncanny ability to understand what I write, and I sometimes forget that others are less skilled. You don't change the wiper connection. It still goes to the grid load resistors, now biasing them to ground through the bottom end of the pot, which used to go to the negative bias voltage. If you turn the pot so that the wiper is connected to ground, you don't get tremolo. If you put it somewhere else, you get tremolo. In either case, the DC voltage that the grid load resistors go to is ground.

"You've got a bad case of Fender clean."

I love the Fender clean, what I think I was wanting was to get more pick responsiveness esp at high volumes and get that driven 'syrupy-thick' sound when I play hard.

Oh, very good. You wrote that you wanted "nice distortion when cranked" with more compression and sustain. I though you wanted at least crunchy, not syrupy. Wondered why you built the wrong amp. Maybe you're gifted at reading your own writing too!

Quoting Backwards Bob again - "I reviewed the schematic, and Fender really didn't leave much gain on the table, except for the 12AY7 instead of a 12AX7. There's a bit here and there, but probably not what you're looking for. There are a few ways to get where you want to go:

1. Reduce the plate voltage. Can be done, but then you'll get something less than Fender clean when you turn it down. The advantage is that you'll get the distortion you crave at a slightly lower output.

2. Increase the drive to the tubes. The traditional way, which works well, is to cascade the preamp triodes on the two input channels. You can do this pretty easily. You may want a bit of attenuation between the stages. If you do it right, you can leave the a jack in between the two triodes, and have it switch to drive only the second input triode when you plug into it using the switch on the jack, which gives you a clean input, maintaining the original character of the amp.

Bias you 6V6s at <= 25mA unless you drop the plate voltage somehow. JJ's 6V6s sound kind of sterile"

Thanks for the tips about increasing drive into the output tube grid. I guess another way I could go also is to change the LTP into another driver stage and cathodyne inverter. Putting a 5751 and or 12AX7 in V1 definitely helps, along with patching the two input channels together.

I gave you crunchy compressed. You have better taste than that. I still think you should take advantage of the MIC preamp triode if you're not using it to give yourself a drive channel.

To get compression and octave harmonics without buzzy saturation distortion, move the operating point on the load-line of the tubes closer to cutoff. That's where you get maximum non-linearity and compression without giving the waveforms a flat-top. You can increase the cathode resistor on the preamp triodes to 1.2K, change the 100K plate resistors to 68K (to increase the influence of plate resistance changes), and change the 12AY7 and the PI tube to high-gain 12AX7s to help compensate. Then cool your 6V6 bias current to 18mA/tube. This should add some compression and good harmonic distortion, at the cost of a bit of output level.

You wanted more pick-attack, which seemed strange when I though you wanted crunch/overdrive. The 5U4GB isn't exactly a stellar rectifier tube, and coupled with only 40uF of capacitance on the OT center tap, you're in sag heaven. I'm jealous, because I have a jazz box, but you can switch to a GZ34, pick up 25V of plate and screen voltage on the output tubes, and reduce that sag. If that isn't enough, the GZ34 has slow turn-on, so you can beef up that 40uF of capacitance between the rectifier and the choke all you want, along with the other 20uF caps shown on the schematics all the way to the triode supplies, and if you do, you can use a lower inductance choke. This will thin the syrup though. You could go with solid-state rectifiers, and you'll get lots of attack, but you'll steal the amp's soul.

I recommend something NOS, EH or Ruby for the 6V6s instead of JJ's if you're looking for warm, and don't forget the tube and speaker break-in, which will help.

Sorry for any misunderstanding. I wrote a reply early on that I didn't post, because I thougt it was low quality, but in it, I recommended that you try your combo bias mod. It's really simple, and it will be educational. I'd certainly like to hear about what you hear. All you have to do is put in the cathode loads and move your bias pot a couple of volts. Somebody else said the same thing, so I didn't bother with a re-write.

Cheers

Cheers back atcha.

[MWJB]

"You can increase the cathode resistor on the preamp triodes to 1.2K, change the 100K plate resistors to 68K (to increase the influence of plate resistance changes), and change the 12AY7 and the PI tube to high-gain 12AX7s to help compensate. Then cool your 6V6 bias current to 18mA/tube. This should add some compression and good harmonic distortion, at the cost of a bit of output level." This will thin out the tone & increase plate voltage & hi end in the preamp. Cooling off the bias to under 20mA will make the tubes sound raspy, harsh & scratchy, thinner harmonically.

[BackwardsBoB]

MWJB wrote:

This will thin out the tone & increase plate voltage & hi end in the preamp. Cooling off the bias to under 20mA will make the tubes sound raspy, harsh & scratchy, thinner harmonically.

Wow. That would be bad.

[mooreamps]

Is getting the grid voltage swing higher the best way to get distortion? Or can the output tubes be driven into saturation in another way that doesn't blow fuses?

The best and easiest way to PA distortion is electronic power scaling.

-g