Many commercial amps use a 25-ohm @ 25W resistor in the B+ line between the
filter cap and the anode(s). If regulation as a result of IR drop was an
issue with R placed in front of the filter C, (and as long as the filter C
can quickly recharge), wouldn't the same regulation issue also be present
with the resistor placed after the filter cap?
It seems to me as long as the filter C has ample capacity to stay charged
under all normal loading conditions, regulation should be a non-issue and
perhaps better regulation would result since no further IR drop occurs
between the filter C and the anode.
Paul, W9AC
----- Original Message -----
From: "Jim Garland" <4cx250b@muohio.edu>
To: <amps@contesting.com>
Sent: Sunday, February 07, 2010 10:25 AM
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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