Re: Sudden B+


"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote in message
news:47AE98C3.5EC3D71F@xxxxxxxxxxxxxxxxxxxxx


Bret Ludwig wrote:

On Feb 9, 6:02 pm, "West" <w...@xxxxxxxxxxx> wrote:
We have often discussed the merits of having the HV on a delay
circuit. It's
easy to ramp DC filament voltages with a transistor and a few parts
but
incorporating this type of design with high voltages is somewhat
impractical.
My point and concern is the sudden thump, assault, if you will, on
precious
($$$) valves when the HV kicks in. Having less experience then many
other
RATs, maybe my concern is unwarranted. It's just that I cringe when
all that
energy comes on so instantly. What do you think? Is there a concern
here or
am I picking fly *** out of pepper?


It can be a concern, yes.

A really elegantly sequenced supply first brings up the A (filament)
and C- (bias) voltages, then the B+, and if a separate supply for them
is used, the screen (D+) voltages last.

The use of sizable filter caps combined with the copper resistance of
the PT secondary and the rectifier series resistance will ramp the B+
in most applications but a series resistor bypassed by a relay, say,
could be implemented. There are also thermistors or inrush limiters
available. Mc used these a lot.

One design I knew of used a big rotary knob. As you turned it it
sequenced things from current limited filaments, to full filaments and
bias, current limited B+, then full B+. It used a multiposition stack
switch off commercial laundry equipment and relays. The B+ relay was
actuated by the heater voltage so if the heater voltage failed the B+
opened as well. One could use the bias for this too, though a lower
impedance bias supply than is sometimes used would be needed.

Is it worth the bother? Depends, but if I were building higher power
amps with lots of commoner power tubes or big transmitting types,
possibly.


Many old brand makers never bothered about the rise of the B+ to
a much higher value than the working value at turn on.

With tube diodes with DH cathodes, B+ would be a LOT higher than working
because most output tubes have indirectly heated cathodes and take time
to conduct.

The output tubes do not suffer if they have +500V applied when cold
which then falls
gradually to say 400V with full idle conduction, unless there is a delay
in applied bias.

Bias comes from the cathode resistors, or from fixed bias.

The danger is where a high B+ voltage is suddenly applied to hot tubes
without any grid voltage bias, so the Ia is temporarily the maximum
Ia or saturation current, which isn't good for cathode life.

In my amps, I have a delay on the B+ by having a well rated R in series
with the mains
to limit inrush current so that B+ is allowed to rise to about 2/3 full
value in a
few seconds before the R is shunted by a relay. The second inrush due to
shunting this R
is usually less than the first with the R, and either is less than 1/2
the inrush
without any series R. In this way mains fuses can be kept minimal, and
thus offer real protection
sensitivity.

The delay circuit also works if someone turns the amp off when its hot,
then back on
after after say 1 second; the delay sequence begins all over, and
re-establishment
of the Vdc and Idc levels is civilised.

My latest creations with 845 have been tested without delays and so far
no troubles.

The 845 are DH triodes, and they begin conduction withing seconds of
turn on
and at about the same time frame as it takes to charge up the total 12 x
470uF caps
used in the PSU, and the 2 x 470uF cathode caps.

With tubes, "hitting them with voltage" does little damage.

Just do please try not to hit them with excessive current demands.

Like all electronic things, tubes don't like being forced to conduct too
much current.

Thanks very much Patrick. Another gem. You not only understood my question
but you also experienced the exact same concern. A couple of ankle nipping
Philistines tried to make it sound like I was being irrational, but they're
the ones who missed the point.. The logic and intelligence of your post will
shut their ugly faces at least on this thread.
BTW: As previously mentioned, I'm interested in your 845 progress and hope
you will keep us all informed. I'm not sure if you mentioned it before, but
are you going to use an interstage xfr?


Patrick Turner.


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