Soliciting some help with AC tube heaters in home made preamp

[JW]

Hey folks,

So, I was given a power supply description for a two bottle, EF86/12AU7 preamp. I drew up a schematic based on the description, and the dude that gave it to me okay'd it, but I haven't been able to get a hold of him since, and after some research I'm wondering if it is indeed correct in regard to the way the heaters are wired.

So, anyway, he has an artificial center tap to raise the heaters above ground a bit to reduce hum.

The green schematic is the one he okay'd initially, but after some research, I'm wondering if it shouldn't look more like the purple schematic, where the dual wires are maintained for the heater pins.

A side note: I've seen some schematics that have the fake center tap (the junction after the 100R resistors)  grounded instead of attached to the top of that 40uF filter cap (via the 270K resistor)

Anyway, I don't have much experience drawing schematics, so I may have something weird on there.
But I'd love to hear what you folks think. . . .

 

[ruffrecords]

The green one is closer to correct.  The arrow marked  'heater pins' is wrong. The heaters should be wired direct to the 6.3VAC winding of the transformer where the two 100 ohm resistors are connected.

Two things are going on in this circuit.

1. Heater elevation. The 270K and 10K resistors form a decoupled potential divider across the HT that raises the heaters a few volts above ground. This helps reduce hum coupled from heaters to cathodes.

2. Heater balancing. The pair of 100 ohm resistors do this as you say by creating an artificial centre tap.

Both techniques help reduce the effect of hum.


Here is a link to a schematic of my own showing how I do heater elevation. In this case the heater supply is not balanced:

https://www.dropbox.com/s/uxfcd0fbhtdnnp7/PMTGMUcct.jpeg?dl=0

Cheers

Ian

 

[JW]

Thanks Ian,

So, only one heater leg has the parallel cap and resistor to ground? (In your case the 47pF and 75K)

Pardon my newbie-ness. I guess I'm partly confused as to why there are always two twisted pairs of heater wires when they are connected at the tube anyway. Hum reduction again?

I redrew the schematic again to verify

 

[joaquins]

Thanks Ian,

So, only one heater leg has the parallel cap and resistor to ground? (In your case the 47pF and 75K)

Pardon my newbie-ness. I guess I'm partly confused as to why there are always two twisted pairs of heater wires when they are connected at the tube anyway. Hum reduction again?

I redrew the schematic again to verify

The parallel RC to ground should be connected to the node where the two 100Ω and the 270k resistors goes, not to the winding.

JS

 

[ruffrecords]

As JS already pointed out the pair of 100 ohm resistor connected to the centre point of the potential divider.

To put this all in context, in simple design the transformer has NO centre tap. One side of the heaters is therefore connected to the HT 0V (earth, ground, analogue 0V or whatever you want to call it). In slightly more sophisticated designs, a pair of 100 ohm resistors are connected across the heaters and the junction of the two resistors is connected to ground; alternatively a heater winding with a centre tap can be used instead of the two resistors.

This is all completely separate from heater elevation. heater elevation uses a pot divider across the HT to produce an elevated heater voltage and connects the heaters to it instead of HT 0V. As before, you can connect one side of the heaters to the elevation voltage, or you can connect a pair of 100 ohm resistors across the heater supply and connect the junction to the elevated voltage or you can connect the centre tap of a heater winding to the elevated voltage. In the example I gave I just connected one side of the heaters to the elevated voltage.

Cheers

Ian

 

[JW]

Thanks Ian,
for the very helpful info. I deleted the other schematic pics that I had posted above and redrew it again from what I understand thus far.

One of the first 10K resistors (after the 40uF cap) started smoking though.

I'm wondering if one of the B+ legs is pulling too much current, or do I still have the heaters wrong.? It sounds like from your description, what I have going on is consistent with what you're saying here:

"or you can connect a pair of 100 ohm resistors across the heater supply and connect the junction to the elevated voltage"
Another thing. All these resistors are 1 watt. Too low?

 

[PRR]

> resistors are 1 watt. Too low?

Do math?

The 100r each feel just 3.15V. 3.15V^2/100r = 0.1 Watt. Double that for long life. 0.2W specified.

The other bits can NOT be math-ed until we know stuff. What is the overall voltage and current??

A 6X4 suggests we are not over 450V. Often used nearer 300V. In fact I will take 280V because it makes the math simpler.

270K+10K= 280K. 280V across 280K (simple math) is 1mA or 0.001A.

0.001A in 10K is 10V.

The lower resistor gets 10V*0.001A = 0.01 Watts. Please check my math on the 270K (0.27W).

10K should be at-least 0.02W. (But 1/4W is cheaper, and easier to find on the floor.)

270K might be 1/2W (not quite double what it cooks).

That's ass-uming 280V. If the voltage is higher, the power climbs faster than the voltage. 1.4X the voltage is *double* the power. 1.4 times 280V is 400V, and it is not impossible you could be in that area. The 0.01W part is still 1/4W, but the 0.27W 270K needs to be a Whole Watt or even 2W for absolute best life.

The 10K B+ filter resistors can be big-Watt or tiny-Watt. If each feeds a thin 12AX7 then it is only 1mA each and 1/4W is plenty for the running current. If these 10K are expected to drop big voltage into a hungry load, say 400V to 9V 20mA, then almost 200V each and 4 watts waste heat (use 10W and big louvers).

But also at turn-on the 10K resistors WILL find "zero" at the far end. The cap starts from zero and it takes a while to fill it up. In the first tenth-Second the power in the 10K could be 4 Watts. Will a 1W part take 4W for fraction-second? They often do. But for many resistor-types it is a bit outside their rating. (Especially in modern days when they try to use the least material per part.) And long-term we do see failures in B+ droppers that "don't heat up" in steady cruising, which may be accumulated start-up damage.

So it is like: How big a beam do I need under my dog-porch? Well, how many dogs, how big are they? When I had a 2-pack of 20lb Corgis, a 2x2 stick would do. Now I also have a 110lb (and growing) Pyr, and we keep joking(?) about mini-horses (who would probably use the dog-porch too). A 2x6 may not be enuff for that herd. And if instead of mellow dogs, what about goat-kids or kangaroos? Shock-load! So don't just show the porch-sticks, show the load.

 

[JW]

Thanks PRR,

I suppose I should just put the schematic up for the preamps. I'm trying to build this guy's design: http://xfmr.blogspot.com/2012/06/brute-force-of-jekyll-hyde.html (the first one, called the "Brute Force") He has it up on his blog page for the world to see, so I suppose it's okay to post it. I thought it would be a good thing to try because I have some Akai M7 amps.

The power supply is powering two preamp channels.

So, two EF86's and two 12au7's. Voltage should be at 180-230V for the B+ legs. EF86's pull max 6mA (at the anode?) and quickly searching, 12au7's maybe pull 10mA max? (not talking heaters)
Don't quote me on these specs.

By the way, it takes a few seconds for that 10K to start smoking, like maybe 7 seconds.  And amazingly, after about 3 or 4 times, it still specs at 9.3K. Hmmmm.

 

[JW]

Eh!

Figured it out. During very limited power ups, I found that something was actually shorting. The sound of it reminded me of the crackle you hear when a resistor burns out. But then I realized it didn't have the same smell! It turned out to be the first 20uF cap's coating had been comprimised/cut into by a sharp piece of the tag board where I had modified it. It was intermittently shorting the case of cap to the + side.

Anyway, thanks for the help guys.

If I could ask one more question, does the application of the choke look okay? (in the schematic three posts back) I don't know how many Henries it is.

But I'm getting this symptom now:
The output of the 6X4 (unloaded) is something like 250VDC, which I think is okay because the AC windings for the 6X4 tube are 270VAC.  However, in circuit, the 6X4 output goes up to around 350V (before I hastily power down)

If that power supply schematic looks okay, I'm guessing it's a matter of adding resistance to drop the B+ down to 180-230V?

 

[PRR]

> The output of the 6X4 (unloaded) is something like 250VDC, which I think is okay because the AC windings for the 6X4 tube are 270VAC.  However, in circuit, the 6X4 output goes up to around 350V

What do you mean, "unloaded"?

The rectifier *alone* will give an ugly pulsating wave. For 270VAC we expect 243VDC, which agrees with your 250.

We never really work a rectifier that way. (Maybe for chrome-plating, or melting steel-- not audio.)

When you connect a suitable *capacitor* on the rectifier, we "catch the peaks" of that ugly wave and hold them. For sine-wave power (the only kind you should get in home), the peaks are 1.414 times the AC RMS. 270V AC should give 381V DC. A bit less for rectifier loss, a bit more if load is light. This agrees with your 350 "before I hastily power down ".

If the B+ caps are all 400V or more, leave the darn thing on for a minute! Depending on rectifier and other-tube warm-up rates, it may stay zero for a few seconds, rise up near 400V, and probably come down near 350V.

The voltage at the far ends of those 10K resistor strings will be even less.

Do not worry about burning-up your "300V" tubes. The do not drop-dead at 301V. They will not have full B+ across them when running. V1 will have maybe half of B+ from plate to cathode (more-or-less). V2a V2b split B+, so could stand 600V total (though they may run too hot for 480V total).

Yes, a 350V raw B+ through a couple 10K resistors will drop well down when loaded with this amp.

The actual limit is heater-cathode voltage on the upper half of V2. However a million Fender amps and clones of the Bassman run quite high heater-cathode voltages. Keep calm and carry on. It isn't a tube amp unless it smokes and sparks occasionally.

 

[abbey road d enfer]

It isn't a tube amp unless it smokes and sparks occasionally.

We should make it a sticky!