current and cathode bias SE

[lowell]

Ok I need Yall to help me clear some things up. Is it true that in cathode bias the B+ current draw does not change from no signal to full volume? If so how is this possible? Also is it different for SE vs PP? I'm confused if this is true because as a plate voltage swings down, the current through the tube increases right? So how then would there not be more draw from the B+... regardless of fixed vs cathode bias? Let's start with that.

[Wilder Amplification]

Ok I need Yall to help me clear some things up. Is it true that in cathode bias the B+ current draw does not change from no signal to full volume?

No...this is true of Class A single ended. The bias method has nothing to do with it.

If so how is this possible?

I think you're getting instantaneous current and average current mixed up. Yes the instantaneous current will increase and decrease. However, assuming instantaneous current increases/decreases above/below idle current by an equal amount, the instantaneous current ends up averaging out to the idle current. In the world of AC, you mainly deal with average values rather than instantaneous values.

That being said, in Class A amplifiers, average current draw does not change whether under zero signal condition or max signal condition. Only the voltage drop across the load changes.

When under zero signal conditions, voltage drop across the load is almost nothing while the voltage drop across the valve is at maximum and the valve is dissipating its max rated dissipation (100% dissipation).

When under signal, the plate of the valve is pulled down towards ground, which increases the V drop across the load while the V drop across the valve does the opposite. Plate dissipation drops to about 50% while the other 50% is transferred to the load due to the increased V drop across the load. Thus the dissipated power has been handed off from the valve to the load due to the voltage drop being handed off from the valve to the load (power is the product of voltage and current).

[lowell]

Thanks Jon. May take me a while to get that. Another question. I think I'm right in thinking screen current doesn't change as well, because it usually is some lesser ratio of plate current. SO, introducing sag via a large screen resistor is not possible?

[redelephant]

In SE class-A the tube swings up and down around it's quiescent operating point in voltage and current. So if the qI is set so you symmetrical voltage swing, the current will actually vary +-100%. Lets say b+ is 300V, and u have a 5881 set to dissipate 23watts at idle, the quiescent current will be 76mA. This current can swing down to 0 and all the way to 150mA. So SE amps do vary the current drawn from the supply. (In real life tubes wont go fully +-100%)
A 'true' class-A PP amp is the only amp that does not vary the current drawn from the supply, since as one tube goes up, the other tube goes equally down. Average draw from supply will be zero.

[Wilder Amplification]

Since average screen current is dependent on plate current and plate voltage, you would be correct.

If you're trying to achieve sag, Class A is not for you.

[Wilder Amplification]

In SE class-A the tube swings up and down around it's quiescent operating point in voltage and current. So if the qI is set so you symmetrical voltage swing, the current will actually vary +-100%. Lets say b+ is 300V, and u have a 5881 set to dissipate 23watts at idle, the quiescent current will be 76mA. This current can swing down to 0 and all the way to 150mA. So SE amps do vary the current drawn from the supply. (In real life tubes wont go fully +-100%)

Again you're getting instantaneous current and average current confused. All the way down to zero and all the way up to 150 averages out to 75mA, which in this scenario is identical to our quiescent current. Therefore, average current in a Class A SE amplifier never changes whether at idle or under signal.

So yes, instantaneous current changes...however average current does not.

A 'true' class-A PP amp is the only amp that does not vary the current drawn from the supply, since as one tube goes up, the other tube goes equally down. Average draw from supply will be zero.

The only way you could ever average to zero would be for one valve to hit positive peak current at the same time that the other valve hit negative peak current and the two were identical in value but opposite in polarity. This would require a dual polarity supply, which we obviously don't have in valve amps.

That being said...current averages out to 1/2 the peak current just like in a symmetrically biased Class A SE amp. One valve hits zero current at the same time the other hits peak current. Thus average current = 1/2 peak current. In a perfectly symmetrically biased Class A amp quiescent current = 1/2 peak current so the same rules apply for Class A PP as do for Class A SE.

[redelephant]

Good to learn something new

I meant zero difference in current draw, not just zero...my bad. In other words, constant current draw from the supply.

[lowell]

What do yall mean by "instantaneous" and "average?"

[Wilder Amplification]

"Instantaneous" is the amount of current being drawn at any given point in time.

"Average" is the instantaneous current measured at the peaks of a 1 cycle period of an AC sine wave, then averaged out.

For example, if the amplifier draws 150mA instantaneous under signal at the high peak and 0 at the low peak, the average is 75mA. Since Class A amps idle at 1/2 the peak current, the idle current is identical to the average current.

As current is constantly alternating (hence the name "alternating current") during the alternation period of a sine wave (it is never constant), you take the peak and the minimum instantaneous currents and average them to calculate the average amount of current drawn over 1 cycle.

So to conclude, on a textbook perfect Class A amplifier, the instantaneous current is always changing. But the AVERAGE current is constant at all times.

[lowell]

So instantaneous current draw does not draw from the B+ then?

[MWJB]

Most commercially available SE guitar amps are biased closer to cut off, nowhere near ideal "textbook perfect class A", so "no" it is not true that current does not change from idle to full signal in SE amps.

If current cannot rise, the B+ voltage wil be pulled down under signal.

Cathode bias will compress the front of the note compared to fixed bias, different values/presence of a cathode bypass cap will affect this. Changing rectifiers for models with increased internal resistance will increase sag. SE amps tend to sag less than PP, because of the lower current draw of one tube compared to 2 (SE amps tend to have adequate power transformers, whereas a lot of budget PP amps can be/were borderline), they are less affected by the internal resistances of the rectifier & power supply.

[Wilder Amplification]

All current is drawn from the supply. Instantaneous current draw is the current measured at one point of the output signal sine wave. Average current is the current drawn over a period of time, in this case being 1 cycle of the output signal sine wave.

When you're dealing with an AC signal, current is always alternating in value linearly (hence the name "alternating current"). This why we deal with average current most of the time.

Most commercially available SE guitar amps are biased closer to cut off, nowhere near ideal "textbook perfect class A", so "no" it is not true that current does not change from idle to full signal in SE amps.

Yes we know the amp must be biased symmetrically for average current not to change. We also know that we live in an imperfect world...but the textbook concept still applies.

[MWJB]

Indeed the textbook concept may well apply, but the OP asked about "amps" not ideals/concepts. If the larger proportion of SE amps that he and everyone else has ever heard don't conform to the ideal then the ideal is of limited practical value. If the amps that sound good & are revered for their sound are "imperfect", then who wants "perfection"? It's often the fact that they are "imperfect" that makes them desirable.

What an amp "should" do is one thing, but we can only hear what it actually does & that is the real world template.

[lowell]

Amen. So in general class A, in the world of guitar amps, is imperfect. In the world of perfectly designed, Ideal class A amps average current applies more so. This answers my question whether I stated it well or not. Which is: Can I get some sag from a single-ended class A amp. If its imperfectly biased... the answer is yes. Thanks Jon and MWJB.

[MWJB]

As well as my previous suggestions for increasing sag, note also that things like tone controls will give the feel of increased sag, an amp without any tone control (like a 5F1, Epi V Jr) will sound tighter, whereas cutting some highs & most importantly mids may give the effect of a softer envelope.

Drawing more idle current (to increase the effect of any internal resistance with the power supply, NOS 5Y3 rectifier & PT), though still keeping the power tube(s) biased somewhat nearer to cut off will enhance any sag effect, though using a tube with a harder dynamic response (stiff 6L6GC like the JJ, & 6550/KT88) will, of course, lessen this...so maybe a couple of squashier sounding 6V6s, or a softer sounding 5881 like the TAD 6L6WGC, or a KT66?

Paying attention to filtering too may help, don't go so small on the input filter as to enhance any hum (30uf+), preamp filters might be kept to 8/10uf.