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On 15 Apr 2005 12:44:00 -0700, 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

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