Hi Jim, the math is well known so I won't repeat it here.
You are 100% correct about the THOUSANDS of SB200/220 units in service.
The discussion had addressed replacing BLOWN diodes and it only makes good
sense to replace them with a more robust selection while doing the repairs.
I have to confess that the last time I was inside one of those amplifiers
was in 1968 and my recollection of the equalizing resistors and capacitors
is fuzzy at best. I erroneously assumed that since it was so long ago they
were used. You know what they say about making assumptions and this surely
proves that. My bad.
The principles do apply though, to not needing the equalizing parts when
controlled avalanche type diodes are used for repairs or in new construction.
Thanks for all of your good thoughts seen here!
73,
Gerald K5GW
In a message dated 3/27/2011 9:48:29 A.M. Central Daylight Time,
w7ry@inbox.com writes:
Think about this again...
Please send your math that supports your claim below as to current draw (3
to 6 amps 60 times per second)?
Also, please take a look at the schematic of both the SB-200 and the
SB-220 and substantiate your claim of resistors and capacitors across the
diodes.
And... The original diodes supplied with both amplifiers are rated at
600mA.... And there are THOUSANDS still working with them.
73
Jim W7RY
From: TexasRF@aol.com
Sent: Sunday, March 27, 2011 6:00 AM
To: w7ry@inbox.com ; 4cx250b@muohio.edu ; amps@contesting.com
Subject: Re: [Amps] SB-200 power supply diodes
Think about this again. The SB200/220 both use a voltage doubler capacitor
filter hv supply. We know that peak currents in these type supplies can be
five to ten times the average current.
So, with a .6 to .7 ampere average, those diodes are taking a 3 to 6
Ampere hit 60 times every second.
For the miniscule added cost it makes a lot of sense to use 1N5408 diodes.
Reaming the mounting holes for the larger lead size is a trivial effort.
If 1N5408 diodes had been available when those amplifiers were designed no
doubt they would have been used. Especially so when you consider the
equalizing capacitors and resistors would not be needed. I would remove them
now
as the parts that are not there can't fail. Who knows what those resistor
values might be after upwards of 50 years use?
73,
Gerald K5GW
In a message dated 3/26/2011 11:25:25 A.M. Central Daylight Time,
w7ry@inbox.com writes:
Agreed Jim! 3 amp diodes are an overkill on the SB-200 and SB-220.
73
Jim W7RY
--------------------------------------------------
From: "Jim Garland" <4cx250b@muohio.edu>
Sent: Saturday, March 26, 2011 6:24 AM
To: <amps@contesting.com>
Subject: Re: [Amps] SB-200 power supply diodes
> Kathy,
> I'd advise replacing the diode string in your SB-200 with 1N4007 diodes
> instead of 1N5408. Not that there's anything wrong with the 1N5408, but
> the
> holes in the circuit board will have to be drilled out to make them fit.
> The
> 1N4007 will be a perfect fit, and with a 1A/1000V rating, will be more
> than
> adequate for an SB-200. You should probably also replace the elderly
> electrolytics in the amplifier while you have it apart.
> 73,
> Jim W8ZR
>
>> From: Kathy Bookmiller <wb2aio@yahoo.com>
>> Subject: [Amps] SB200 diodes
>> To: amps@contesting.com
>> Message-ID: <795630.36304.qm@web59714.mail.ac4.yahoo.com>
>> Content-Type: text/plain; charset=iso-8859-1
>>
>> Thanks to everyone for their replies. I'm going to order them from
Mouser
> and replace them
>> all as that is the consensus of the replies I received both on-list and
> off. Did take a run over
>> to my local RS store and they had some that were close to the 1n5408s in
> spec, but only a 3
>> pack of them, so will order them all from Mouser.? That seems to be the
> norm for RS. It
>> seems about every time I go in there for some parts, they'll have them
in
> those drawers in
>> the back, but only one or none is inside the compartment.
>> While waiting for them to show, maybe I'll finally get into my old
>> NCL2000
> that is loosing
>> it's drive and has been sitting on the repair bench for a year:) That is
> how I ended up buying
>> the SB200 when it showed up on Craigslist here last summer, while
waiting
> for a new set of
>> tubes to show up.
>> So I'll spend the weekend looking for the odd band fill in the WPX
>> contest
> and checking the
>> capacitors in the SB200 and cleaning off the board of the old diodes.
>> 73,
>> Kathy
> *************
>
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