I have seen a few older commercial amps with a resistor on the transformer
side of the filter capacitors. I have also seen it in older handbook power
supply projects. I haven't seen that done on anything in a long time.
I have an old TMC amplifier that had dual 25 ohm resistors between the
transformer secondary and the diodes. The regulation was terrible. I shorted
them out and it works much better.
If the supply delivers say 1 amp of DC to the tubes and you have a 25 ohm
resistor on the transformer side of the filter capacitors the voltage drop
is not just 25 volts. The problem is that you have to be concerned with the
PEAK charging current which is many times the DC output current.
The filters charge up to peak AC voltage. As current is drawn from the
supply the current that recharges the capacitors is drawn from the
transformer in very narrow pulses as the diodes only conduct near peak
voltage. That makes conduction time very short. In order to supply the
amount of energy needed with these short pulses the current has to be much
higher. So voltage drop across the series resistor (or transformer
resistance) is much higher than the DC current out of the supply.
I agree with the fellow that says it is a waste of expensive transformer if
you put a resistor on that side of the filter.
The reason for a glitch resistor AFTER the filter capacitors (between
filters and the tube) is to absorb the energy held in the filter capacitors
in the event of a short. Even if the primary is disconnected, when there is
a fault the filter capacitors store enough energy to do damage. The glitch
resistor limits the amount of energy available.
Using too large a filter capacitor presents a problem too as the amount of
glitch energy will be higher.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Jim Garland
> Sent: Sunday, February 07, 2010 10:26 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Vac Relays for B+
>
> Great comments, Jim and Roy. Two points. First, my 8877 Amp, described
> in
> several editions of Orr's book, was designed and built in the late
> 1970s. In
> it, as Jim noted, I used a vacuum relay to interrupt the HV under
> overcurrent conditions. That was a bad idea, which I learned years later
> the
> hard way when I had a flashover and ruined the relay and other circuit
> components. I redesigned the HV overload circuit in the mid-80s to
> eliminate the vacuum relay. Now the overcurrent circuit monitors cathode
> current and trips the amp off-line for currents in excess of 1.3A. It
> uses
> an optically isolated relay to do this and has worked well for many
> years.
> Unfortunately, there was no way to correct the published design. (I
> also
> upgraded other aspects of that 8877 anp: it now has QSK capability and
> covers all WARC bands. I can send a revised schematic to anybody who
> want
> it.)
>
> Roy questions whether it is appropriate to use a current-limiting
> resistor
> (in my case, 25 ohms) between the transformer/rectifers and the filter
> capacitor (50uF in my 8877 amp). His point is that doing so is
> equivalent to
> using a cheap transformer with a 25 ohm "Equivalent Series Resistance"
> and
> thus negates the benefits of using a stiff (and expensive) transformer.
> This
> is a reasonable objection, but I don't fully agree. Here is the
> counterargument. First, there are two benefits of the resistor; it
> limits
> the inrush current when the HV supply is switched on, since without it
> the
> rectifiers would see a dead short from the uncharged capacitor. (Of
> course,
> a step-start circuit in the xfmr primary is another way of limiting the
> inrush current.) Second, it protects the transformer and rectifiers
> from a
> shorted filter capacitor. Many amps use banks of electrolytics for the
> HV
> capacitor and a short circuit in one can lead to a cascade of failure
> along
> the entire bank. (My 8877 amp now uses oil caps, but the original design
> used electrolytics.).
>
> The second point is that the effect of the resistor on power supply
> voltage
> regulation is negligible, On PEP peaks, the current is supplied by the
> charged filter capacitor, which is not limited by the resistor. Under
> key
> down conditions, when the power supply is delivering, say, 1 Amp of
> current,
> the drop across the resistance is only 25V, or less than a one percent
> drop
> in power supply output. By contrast, an inexpensive tranformer, with an
> underrated core, will not only overheat in sustained use, but will have
> very
> poor voltage regulation -- in effect an ESR much greater than 25 ohms.
> Note
> also that the resistor has a negligible effect on the transformer's
> ability
> to recharge the filter capacitor. A 25 ohm resistor in front of a 50uF
> capacitor has a time constant of only 1.25 mSec, which is much faster
> than
> the pulsed DC from the diode rectifiers that replenish charge on the
> filter
> capacitor.
>
> 73,
>
> Jim Garland W8ZR
>
> > ------------------------------
>
> > Date: Sun, 7 Feb 2010 03:45:43 -0800
> > From: "Jim Thomson" <Jim.thom@telus.net>
> > Subject: [Amps] QRO-RG-142
> > To: <amps@contesting.com>
> > Message-ID: <030B78EE98BC44B3A4EBCA14168AB551@JimboPC>
> > Content-Type: text/plain; charset="iso-8859-1"
> >
> > ## I just looked at the W8ZR 8877 amp in Orr's last book [23rd ed]
> > ## I was also going to use the vac relay as intended... to open off
> > B+ UNDER LOAD... . In that case... it was part of the W8ZR
> > plate over current setup. IE: If plate current exceeds say 1000 ma
> > during tune up, vac relay opens off the B+. Problem is, during
> > a fault condx.. and only a 25 ohm WW for a glitch R... fault current
> > is 4000 V /25 = 160A !! Which will FRY vac relay contacts !
> >
> > ------------------------------
> >
>
> > Date: Sun, 7 Feb 2010 06:37:02 -0600
> > From: "Roy" <royanjoy@ncn.net>
> > Subject: [Amps] B+ relays
> > To: <amps@contesting.com>
> > Message-ID: <6F3629C315AD4D3B88B36541561ABD81@RoyPC>
> > Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> > reply-type=original
> >
> > If you really mean the resistor is located between the rectifier and
> the
> > filter cap, that defeats all efforts to design and build a plate
> transformer
> > with low ESR. Just go buy a cheap transformer instead.
> >
> > 73, Roy K6XK
> >
>
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