Re: My first VJ mod

On 29 May 2007, Jim <askme@xxxxxxxxxxxxxxxxx> wrote in
alt.guitar.beginner:

Since a tech recently posted that the resistor is often there as
part of the filtering circuit, not just as a bleed, I wouldn't
recommend taking the 220K out of the Valve Junior without looking
at any change with a scope. It could induce a bit more ripple.

Paul P wrote:


I just happened across an interesting article about how to
have your power supply capacitors discharge automatically on
power-off without the use of a (constantly energy wasting)
bleeder resistor.

The idea is to use a dpdt main switch and have the capacitors
discharge through the power transformer and a couple of
resistors when the power is turned off.

For details see the last article in the following :

http://www.edn.com/contents/images/70501di.pdf

Paul P

That article is excessive to make a point.

If you take the dropped output voltage of 150V (as they show on the
output), then you get: 150/1000 => 0.15 (150 mA), and 150 x 0.15 =>
22.5 Watts. Even a 50W tube amp, usually has a power supply delivering
a max of about 190 mA total (at 400VDC). This article is just plain
excessive to use a bleed resistor like that, in order to make a point.

I don't think many tube amps will be found designed quite like that.

You don't need an immediate bleed anyway. That is just plain excessive.
You could put a 10K resistor in that circuit, and drop the bleed
current to 15 mA, and it will still discharge quick enough. The power
wasted would drop way down, to a mere 2.25 Watts.

* BUT *

In _that_ circuit shown (which is abnormal for a tube amp, IMO), the
input voltage is 200V, and the load of the bleed resistor causes the
output to sag to 150 V. So changing or removing that bleed resistor
will have consequences to the rest of the circuits being fed from it.
Eliminating the bleed in that circuit would raise the voltage to the
amp, from 150 V to 200 V. Most tube circuits are very tolerant of that
(bias wise), but there are other factors like the voltage ratings of
capacitors used to consider.

The stock VJ has no parallel bleed resistor. Adding a 1K bleed would
guarantee a smoking power transformer, since you would demanding an
additional ~ 20 Watts of power from it (I'm not able to determine the
output voltage of the OPT from the schematic I'm looking at). If you
want to add one, be careful to not exceed the rating of the
transformer. It will also have the effect (depending upon how much
bleed you use), of lowering the output voltage.

Assuming a voltage of about 200 V, you could add a 200 / 0.005 =>
40,000 (40K) resistor, and only waste 5 mA (1 watt @ 200 V). Using the
RC calculator here:

http://ourworld.compuserve.com/homepages/Bill_Bowden/rc.htm

(note that the calculator shows "charge time", so be sure to set the
"Instantaneous voltage" box to 170 (200V-30V), and leave the "Time" box
blank before you click "calculate").

That would discharge to a safe level of 30 Volts within 4.553 seconds
after turning off the power (assuming a total of 60 uF of capacitance
in the power supply for the VJ). It will actually take slightly longer,
maybe 5 seconds, because I didn't take into account the series
resistors (4.7K and 10K) between the three filter capacitor sections.

But this should be fairly close. It is possible to calculate it
precisely, but I'll leave that as an exercise for the reader. ;-)

Snark.

--
Posted via a free Usenet account from http://www.teranews.com

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