Id still use a 2nd resistor as a HV glitch resistor for the tube. The one at
the PS input has no effect on a tube arc at the critical moment.
Ive yet to see any commercial amp, ham or otherwise use a resistor in the
secondary AC
Carl
KM1H
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: <amps@contesting.com>
Sent: Sunday, February 07, 2010 10:55 AM
Subject: Re: [Amps] Vac Relays for B+
> 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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