Choke Input Load

[Blind Lemon]

How much current carrying capability would a choke have to have for 4 6L6GC and 2 preamp (12A*7) for a choke input load?

BL

[HotBluePlates]

Enough for the cap charging pulses.

I had a book with the formula for that in an easy format, but it's in another state. Maybe PRR can help. It's a lot more than you'd think, if you figure from average current draw for the circuit.

[jeff]

Just curious, why would you want to do that? If it's 4 6L6s I assume it's push pull. I was always under the impression that it's unneeded it a PP amp.

[Blind Lemon]

I've got a 200ma choke and wanted to play around............just thinking.


BL

[HotBluePlates]

If the choke is rated for 200mA, you won't come close to what you need for choke input, as you'd more likely require a 1A rating or more. The actual size needed would depend on average current draw and the size of the cap after the choke.

For choke input, bear in mind your rectified voltage is something like vRMS * 0.9 - diode drop, instead of vRMS * 1.414 - diode drop. So output voltage is lower with a given PT, because a good part of the pulsating dc input is being dropped across the choke's reactance.

For a 200mA choke, you'd be looking at a cap-input power supply; cap -> choke -> cap, with the cap after the choke being your plate node. You could have an additional, smaller choke if you want between that node and the cap feeding the output tube screens. Obviously, that choke only needs to be rated for the screen and preamp current.

[PRR]

Try it. Watch voltage and temperature.

If VDC is nearer 1.4*VAC than 0.9*VAC, then it isn't choking.

What I suspect is: it will under-choke (run nearly cap-input) at idle, choke at full roar. That may mean voltage drops from 1.3+ at idle to 0.9 hard-worked... that is a LOT of sag.

Also check the core temp at 10, 20, 40 minutes. You probably want to be able to hold it "forever". (Some modern windings can run too-hot-to-hold, but I don't like it.)

[jeff]

"What I suspect is: it will under-choke (run nearly cap-input) at idle, choke at full roar. That may mean voltage drops from 1.3+ at idle to 0.9 hard-worked... that is a LOT of sag."

So is the advantage of using a choke input sag? This kinda makes it sound like too smaller choke will but the right sized one shouldn't. Or does this mean the right sized will sag but and undersized will sag too much?

 I always thought the choke was used to reduce the AC ripple but isn't needed in a PP amp because the PP OT cancels out any ripple by the winding of each side of the CT being out phase.

Sorry, bear with me please but I'm still trying to understand why a choke is useful on a PP amp.

[HotBluePlates]

"What I suspect is: it will under-choke (run nearly cap-input) at idle, choke at full roar. That may mean voltage drops from 1.3+ at idle to 0.9 hard-worked... that is a LOT of sag."

So is the advantage of using a choke input sag? This kinda makes it sound like too smaller choke will but the right sized one shouldn't. Or does this mean the right sized will sag but and undersized will sag too much?

That's not normally the intent of a choke input supply. PRR is describing a "misuse" of a not-quite-big-enough choke, because Lemon has one on hand that he wants to use.

Ordinarily, the choke is impacting the power supply current and dropping voltage at all times. But like a chicken-egg scenario, you often find a common choke in a choke input supply when the load current is relatively constant (a class A amp). Ripple is lower at the first cap than you find at the first cap of a cap input supply. That's really because the ripple is being reduced two ways (choke then cap), rather than just one (cap only) in the cap input case.

You might argue that if the load current is fairly constant to begin with, then the supply voltage is unlikely to sag due to the load, which makes the choke a needless complication. And that's true. Once upon a time, high capacitance values were more expensive than iron and copper; that changed, and much of the choke-input supplies went away as well.

Of course, Lemon could forget choke input, and have a cap to ground, the choke, then another cap... the plate node for the output tubes connected to that second cap. Now the plate current flows through the choke, then biggest cap charging pulses are largely at the first cap, and he still uses the 200mA choke. That's probably the best way to use this thing without odd results.

[sluckey]

Of course, Lemon could forget choke input, and have a cap to ground, the choke, then another cap... the plate node for the output tubes connected to that second cap. Now the plate current flows through the choke, then biggest cap charging pulses are largely at the first cap, and he still uses the 200mA choke. That's probably the best way to use this thing without odd results.

Get's my vote.

[Blind Lemon]

This is fun, thanks HBP. I was actually going to use it to help drop the voltage from a 500+ transformer I was wanting to use..........might be big enough for 2 6L6s but not 4. Plan "B" use one of my other lower voltage PTs.

BL

[sluckey]

Plan "B" use one of my other lower voltage PTs.

That should be plan "A".   

[HotBluePlates]

The problem with plan A is that it only works if the choke can handle those charging pulses.

The choke, if it's within its current rating and choking, will result in a rectified voltage of 0.9 * vRMS. But if the core saturates, inductance drops to zero and the choke stops choking, then the output voltage of the rectifier rises to 1.414 * vRMS. Basically, that's cause the choke's effect stops, and the cap takes over.

You can do a good choke-input plan for the circuit you're describing while reducing the output voltage from a too-hot PT, or you can use the part you have, but you likely can't do both. Knowing you have an over-volt PT to use adds a new wrinkle that wasn't in the mix before. You better rate the caps to take the full vRMS *1.414 plus some margin for high wall voltage. For a 500v PT, that's 700v+ of cap rating and you still need some margin.

I'm more firmly in favor of plan B now. Not just cause it's my plan... 

NOTE:
I found the reference online that I was wanting to cite earlier. It's from Valve Amplifiers, by Morgan Jones.

The proper rating for an input choke is Ichoke = Idc + Iac. Idc is your dc load current (while producing full output power).

Iac = Vin/1386L (for 60Hz operation). Vin is the RMS output voltage of the power transformer.

Assume your 4x 6L6GC is being run in class A (for my ease). The amp runs hottest at idle, with 120w of plate heat. Let's assume everything works right with your 500v PT dropping down to 500v * 0.9 = 450v. 120w/450v = 267mA idle current total, plus screens plus preamp. Let's make life easy and round that up to 300mA (probably close to the truth).

Already, we're over your choke's rating. You hadn't told us the number of henries of your choke. Already, we're looking at Hammond chokes rated for 500mA, cause there's no in-between above 300mA. You'd like to pick a high inductance rating, cause it would filter best, but 10H is the limit at this current... and that choke weighs 21 lbs!!

But there are smaller and lighter ones, so let's proceed. We've figured a d.c. load current around 300mA. The ac choke current = 500vRMS / (1386 * 10) = 36mA, for the 10H choke. But they also have 5H and 3H chokes at 500mA, which result in ac currents of 72mA and 120mA, respectively. So for the lower 3H choke, the total current rating of the choke must be 300mA + 120mA = 420mA. The 500mA unit suffices, and shows that the current rating may be well above the dc load current if the inductance is not very high. Fortunately for you, the 3H 500mA choke is only 5lbs. Big chokes with big current ratings will probably be bigger than your OT, and could be as big as your PT.

You might need to look at using only a pair of output tubes for choke input with this.

PRR will now point out that in class AB, the average dc load current is smaller than it is with a class A amp, and may allow you to skate by more easily, unless you have a choke rated under 10H. 

[PRR]

There's a minimum inductance to stay chooke-input mode.

Usually, this leads to a choke SO big that it is more reasonable to consider a different PT instead.

Push-pull is less affected by ripple, sure. But cancelation is never exact. And at FULL ROAR the ripple will appear on the output. Choke-input makes some sense. Especially when capacitors were expensive. Also when cost and weight are not-bad, or even "good", as in some industrial and church-organ units.

[HotBluePlates]

There's a minimum inductance to stay choke-input mode.

Usually, this leads to a choke SO big that it is more reasonable to consider a different PT instead.

Yeah, this was the other part I needed to add.

There is a more-exact formula, but the rule of thumb Morgan Jones provides is I_{min (mA)} = V_{in (RMS)} / L_(H)

The V is described in terms of RMS volts from the PT, so in your case, that's 500v / 10H = 50mA (for a 10H choke).

Or:
500v/5H = 100mA minimum
500v/3H = 167mA minimum

This is maybe another argument for class A operation (though all could allow class AB). It also suggests you don't want to implement a standby which cuts B+, and that on startup when the tubes haven't warmed up, you won't have the idle current draw to allow the choke to work properly and so the caps must be rated for full peak input voltage. We said that earlier, but now there's an additional reason.

[Fresh_Start]

Playing around with Duncan's PSU a while ago for a single-ended amp, I discovered that the value of the first "reservoir" cap in the Pi filter HBP described for Plan B isn't that important in terms of ripple reduction.  The value of the second cap (plate node) IS important and can balance out a lower choke inductance to a certain extent.

Based on the discussion above, it sounds like there isn't a good reason to consider a choke-filter input today unless you've got a situation like Blind Lemon's, right?  You need a higher voltage secondary on the PT, the minimum current rating for the choke is much higher for a given amp topology, and you have to meet the minimum inductance requirement HBP & PRR just mentioned.  Bottom line, electrolytic caps today are much cheaper.

Does that sum it up about right?

Cheers,

Chip