exceeding heater to cathode voltage

Archived from groups: rec.audio.pro (More info?)

well obviously if you exceed the h-c rating you could cause a h-c
breakdown.

If you float the filament at 150 V, make sure you don't have any other
tubes connected to the same filament winding and make sure the
transformer filament winding is rated for 150 V.

But I don't understand why the cathode has to be at 150 V above ground
for a cathode follower, isn't the cathode a few volts above the grid
and the grid is near ground?

Of course what I really don't understand is why you want to do this at
all , but lets not go there.

Mark

Archived from groups: rec.audio.pro (More info?)

On 15 Apr 2005 12:44:00 -0700, seanbroderick20003@yahoo.com wrote:

>i'm actually modifying an existing mic pre, and one thing they did that
>i don't understand is they return each side of the AC heater supply to
>ground with a 1k resistor. what is this done for?

Hum reduction. Make a voltage divider (pulling about 1mA)
to make a 75 volt reference. Bypass with a capacitor and
move the heater ground reference up there.

Piece of cake.

Chris Hornbeck
6x9=42 April 29

Archived from groups: rec.audio.pro (More info?)

"Scott Dorsey" <kludge@panix.com> wrote in message
news3pd1l$4vn$1@panix2.panix.com...
> In article <1113594240.319495.28460@g14g2000cwa.googlegroups.com>,
> <seanbroderick20003@yahoo.com> wrote:
> >I'm wondering what nasty effects i can expect, if any, for exceeding a
> >tube's maximum heater to cathode voltage rating. the tube is a 6072,
> >and it's maximum claimed voltage is +- 90v peak. i'm using one triode
> >as a cathode follower, and so the voltage at the cathode is gonna be
> >around 150v above ground. i'm running AC on the heaters.
>
> Depending on the tube, it could be anything from hum to premature heater
> failures.

I've even seen some odd distortion behavior and oscillation.

> >so, why not cap couple before the follower you say? well, some think
> >even the expensive caps sound better when driven and loaded with lower
> >impedances. i'm saving the cap for the follower's output.
>
> So, raise the heater reference voltage. Hang the heater at +150V.

If you're limited to a single heater circuit for all the tubes, float it at
+75V. That'll mean the non-CF tubes will be at about -70V H-C and the CD
will be at about +75V. Everyone should be happy.

Peace,
Paul

Archived from groups: rec.audio.pro (More info?)

In article <1113807391.901521.296310@g14g2000cwa.googlegroups.com>,
<seanbroderick20003@yahoo.com> wrote:
>
>> Or you could use a different tube for the output stage.
>
>do you think the 6072 makes a poor follower? it'll just be driving the
>5 or 10k load of an AD converter, and i don't plan on using long cables
>between the two. do i need a higher current tube, like a 12AU7 or
>something? since i'm modding, i'm trying to minimize the
>pain-in-the-ass factor, and also not bludgeon the PCB too much. the
>preamp used 12ax7s before.

I don't know, I never did the math on the 6072. But if you look up the
follower section in the Radiotron Book, there's a method for calculating
basic linearity from the load lines, and there's a formula for figuring
the effective output impedance with a given cathode resistor. Your whole
goal is to balance voltage swing with output impedance and get the best
combination for the device you're using. My personal feeling is you
probably want the load to be at least four or five times as high as the
output impedance of the stage.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Archived from groups: rec.audio.pro (More info?)

"Scott Dorsey" <kludge@panix.com> wrote in message
news417b4$3p3$1@panix2.panix.com...

> I think Mr. Stamler is exaggerating a little bit about a load that
> is a factor of ten higher than the effective output Z being a problem.
> Note that when the effective output Z is calculated, the gain of the
> tube is taken into account... the more gain you have, the more the
> feedback is helping you.

Except that into, say, a 6k load, the tube has a lot less gain to begin
with, so a lot less feedback when used as a cathode follower.

I did some tests a couple of decades ago to illustrate the point. The tubes
weren't completely open-loop, since they had unbypassed cathode resistors,
but still. For a 12AU7 with a 100k plate resistor and a 1.5k cathode
resistor, idling at about 2.2mA:

Load / Gain / THD at +15dBu

240k / 10.0 / 0.22
92k / 8.7 / 0.26
48k / 7.2 / 0.32
20k / 5.0 / 0.52
9.6k / 2.9 / 0.90

Note that whereas at a 240k load there's 10x gain, so the chance to reduce
distortion by that amount, at a 9.6k load, there's only 2.9x gain, so you
can only reduce distortion by that much when you turn the tube into a
cathode follower. Hence my suggestion that the 12AU7 be used with an A/D
converter of -10dBV nominal sensitivity.

For a 6SN7 with a 27k plate resistor and 475 ohm cathode resistor, running
at about 5.4mA idle current, the numbers are:

240k / 13.0 / 0.072
92k / 12.0 / 0.082
48k / 10.0 / 0.10
20k / 8.0 / 0.18
9.6k / 5.6 / 0.27

Note that at 9.6k you have about twice the gain and less than 1/3 the THD,
so a 6SN7 should be about 6x cleaner as a cathode follower under those
conditions. It's a nice tube, although a power hog. But it's still happier
at higher load impedances; into 240k you have the possibility of around
..006% THD, and in fact that's what I've measured at that level into that
impedance.

By the way -- yes, I know I've written that I don't think THD is a good
measure of audio quality, and I stand by that. But I did these tests back in
the 1980s, when I was young and foolish -- and besides, the same pattern of
behavior will show up in IM tests.

Peace,
Paul

Archived from groups: rec.audio.pro (More info?)

On 15 Apr 2005 12:44:00 -0700, seanbroderick20003@yahoo.com wrote:

>I'm wondering what nasty effects i can expect, if any, for exceeding a
>tube's maximum heater to cathode voltage rating. the tube is a 6072,
>and it's maximum claimed voltage is +- 90v peak. i'm using one triode
>as a cathode follower, and so the voltage at the cathode is gonna be
>around 150v above ground. i'm running AC on the heaters.
>
>so, why not cap couple before the follower you say? well, some think
>even the expensive caps sound better when driven and loaded with lower
>impedances. i'm saving the cap for the follower's output.
>
>i'm actually modifying an existing mic pre, and one thing they did that
>i don't understand is they return each side of the AC heater supply to
>ground with a 1k resistor. what is this done for?

It guarantees the capacitive coupling of the AC from the heater to
the cathode is balanced: When one side of the heater supply goes
positive, the other side is going negative. If it were left to float,
it would probably be driven by the capacitive coupling of the
transformer winding, where one side likely has more capacitance to
ground than the other.

>seems like if it
>were left floating, i wouldn't have to worry about the voltage limit
>situation.

Does this tube have a heater winding separate from the other tubes?
If not, can you add a separate transformer to run its heater? I
presume any transformer will have at least 200 volts isolation.
If you have this tube's supply isolated, remove the ground side of
the two 1k resistors, and connect them to the cathode connection.
Even without isolation and using a single winding for all heaters,
you can perhaps 'bias' the heater voltage up to perhaps 80V DC so that
all heater-cathode voltages are within spec.

>it probably also causes ground to wiggle a tiny bit at
>60hz.

I doubt there's that much current to ground, it's just the
capacitively coupled 60 Hz from the secondary winding.

>
>thanks!
>
>SB

-----
http://mindspring.com/~benbradley