[Heathkit] How about the SB-220?

Mon Aug 24 22:35:06 EDT 2009

The HV current limiting resistor is best installed in place of RFC-2. This 
eliminates the cavity effect of the RF cage that is excited by reradiated 
VHF RF from the choke. You get the resistor protection as a bonus. 20 Ohms 
at 25W is a good choice and it must be a real enamel wirewound. The nichrome 
wire is inductive and acts as a lossy choke to the original VHF energy and 
is no longer radiated or at least no longer creates a cavity oscillator

Carl
KM1H

----- Original Message ----- 
From: <sbjohnston at aol.com>
To: <heathkit at mailman.qth.net>
Sent: Monday, August 24, 2009 9:32 PM
Subject: Re: [Heathkit] How about the SB-220?

>
>> Once, there was a big bang in the amp -- some say it would have been
>> caused by a parasitic oscillation.  Rich Measures belongs to this
> school.
>
> I know he does love to blame VHF parasitics, but Measures does
> acknowledge that there can be other causes of the "bang" heard but not
> seen.  I personally feel the most likely to be an internal arc (not
> that uncommon event in transmitting tubes), but other possibilities
> include little insects getting into a gap, dust plus moisture in a gap,
> and the like.
>
> A glitch resistor in series with the B+ is good protection for all
> these events...
>
> Steve WD8DAS
>
> sbjohnston at aol.com
> http://www.wd8das.net/
> ---------------------------------------------------------
> Radio is your best entertainment value.
> ---------------------------------------------------------
>
>
> -----Original Message-----
> From: Carl <km1h at jeremy.mv.com>
> To: Mike Hyder --N4NT-- <Mike_N4NT at embarqmail.com>;
> mmoss at mindspring.com; Heathkit Reflector <heathkit at mailman.qth.net>
> Sent: Sat, Aug 22, 2009 8:58 am
> Subject: Re: [Heathkit] How about the SB-220?
>
>
>
>
>
>
>
>
>
>
> My view based on close to 40 years experience with servicing the SB-220
> family pus converting over 200 to 6M.
>
> See below for my interjected comments.
>
>
> ----- Original Message -----
> From: "Mike Hyder --N4NT--" <Mike_N4NT at embarqmail.com>
> To: <mmoss at mindspring.com>; "Heathkit Reflector"
> <heathkit at mailman.qth.net>
> Sent: Saturday, August 22, 2009 12:40 AM
> Subject: Re: [Heathkit] How about the SB-220?
>
>
>> Marvin, you're welcome.  I'm sending this to the reflector so as to
> expose
>> my ideas to flying barbs.
>>
>> I'll give you my experiences briefly.  When the FCC ordered that amps
> no
>> longer could cover 10 meters (approximately 1977), I ordered an
> SB-220
>> kit.
>>
>> Once, there was a big bang in the amp -- some say it would have been
>> caused
>> by a parasitic oscillation.  Rich Measures belongs to this school.
>
>
> Where he is a very small minority and has been proven to be wrong time
> and
> again.
>
> Common engineering sense as well as tube manufacturers realize that
> transmitting tubes will arc internally. This is due to either
> outgassing,
> grid alignment, or sharp points not caught in inspection. The cure is
> to
> first accept this as a normal occurance and do what is necessary to
> minimize
> the damage.
>
> The most important step here is to remove RFC-2 and replace with a
> 15-20 Ohm
> 25W real wirewound resistor. Do not use cement or aluminum encased
> versions.
> This resistor acts as a HV current limiter and is not sacrificial...it
> should survive an arc.
> RFC-2 is also self resonant close to the tubes parasitic frequency and
> adds
> to instability. The resistors resistance wire also kills any VHF that
> reaches it.
>
>
>
> I
>> installed new tubes (one was ruined by the bang) and installed
> parasitic
>> suppressors -- NiChrome coils.  Measures sells them.  Look here and
>> specify
>> your amp model when ordering: http://www.somis.org/Price-Info.html
>
>
> That site is good reading and mostly to see what not to do.
>
>
>>
>> I use Ten-Tec rigs and run 35-45 WPM CW.  My main consideration is
> that
>> the
>> transmit relay in the SB-220 presents over 100 VDC across the
> back-panel
>> jack and current sufficient to cause pitting to the contacts in a
>> transceiver's relay.
>
>
> And totally destroys a SS switching transistor. The voltage is
> approximately
> +125-130VDC.
>
>
> You can buy a small circuit called the SK-220 Soft-Key
>> for the Heathkit SB-220 or SB-221 from this website:
>> http://www.harbachelectronics.com/main/page_products_sb220__sb221.html
>> This is a very worthwhile modification.
>
>
> I agree 100%
>
>
>
>>
>> They also sell a module called the SS-221 Soft-Start which I believe
> is
>> NOT
>> NEEDED AT ALL, don't waste your money (read about that on Measures'
> site).
>
>
> Here we go again with the king of junk science. First of all the power
> switch is not a common replacement item any longer. Secondly the inrush
> currents effect on the switch depends upon where in the AC cycle it
> happens.
> Since this is not a zero crossing controlled circuit it can be anywhere.
> The Harbach circuit is good conceptually but uses cheap cement power
> resistors which are known to fail. Replace them with real wirewounds.
>
>
>>
>> I have a full QSK amplifier but greatly prefer my SB-220.  Here's
> why:
>> The
>> Ten-Tec rigs have a drop-out delay on the relay which switches an
>> amplifier
>> to transmit.  That delay is to prevent the rapid clatter of the
>> amplifier's
>> relay.  But there is a most salutory effect to that.  When the
> amplifier
>> switches to transmit, there is sufficient signal coupled from the
> antenna
>> through the antenna relay and into the transceiver.  So when I'm
> working
>> QSK
>> at perhaps 45 WPM and start to transmit, my receiver becomes much
>> quieter --
>> but enough of the other guy's signal comes through for me to hear
> him,
>> even
>> if his signal is perhaps S4 or so (maybe much less) because the
> signal is
>> still well above the noise level.  I keep my transceiver's drop-out
> delay
>> set to maximum.
>
>
> The stock relay is way too slow for QSK and can even hot switch using
> VOX on
> CW or SSB. This is a major cause of arced bandswitches....not voodoo
> parasitics. A speed step-up circuit could be used or use PTT or a foot
> switch. If controlling with an old tube transmitter the sequencing is
> OK for
> VOX. Some modern rigs have adjustable delays but you really need a
> scope to
> verifiy sequencing. Many rice boxes come on with a full power spike
> which
> raise havoc with many amps, not just a SB-220.
>
>
>>
>> With the age of the your amp, here's what I'd consider doing to it:
>> First, remember that the 3-500 tubes must be operated vertically!  I'd
>> remove the front spacers on the feet to make the amp sit level on the
>> desk.
>
>
>
> Since the fan also cools the filament pins below the chassis this may
> not be
> a good idea. Pins have been known to overheat and melt their solder
> which
> causes an open filament until repaired, and destroys the socket temper
> resulting in poor tension.
> Its the old cause and effect of making changes that have not been
> completely
> thought out.
>
>
>
>
>> a. I'd oil the fan motor as suggested by Rich Measures
>
>
> Take it out, take it all apart, clean both the front and rear bearing
> surfaces and relube with a good grade machine/electric motor oil. Its
> extremely simple to do. Read the manual as to exactly where to mount
> the
> blade. If using graphite tubes go for the Harbach high speed fan and
> learn
> to live with the noise unless you want to install a 2 speed
> modification.
> Leave the amp on for at least a minute after transmitting before
> powering
> down.
>
>
>> b. I'd install that SK-220 Soft-Key circuit.
>
> See comments above
>
>
>> c. If the electrolytics are original, I'd replace them.
>
>
> The Cornell-Dublier 381LX series 330 uF 450V 105*C fits the original
> blocks
> perfectly and adds a bit of dynamic regulation.
>
>
>> d. I'd REPLACE the equalization / bleeder resistors across each
>> electrolytic
>> capacitor per Measures, whether or not I replaced the electrolytics!
>
>
> I suggest 75K 3W MOX across new caps. Do not reduce the value for
> original
> caps as their age and even initial tolerance could seriously unequalize
> voltages.
>
> All parts that Ive mentioned are regular stock at Mouser.
>
>
>> e. I'd make sure I had installed the NiChrome parasitic suppressors
> from
>> Measures.
>
>
> Pure garbage. Instead use a pair of the same style 100 Ohm resistors in
> parallel using the original coil if the original resistors have
> increased
> more than 10% above 47 Ohms. RF and heat cooks carbon composition
> resistors,
> when the value increases too much they no longer absorb the parasitic
> RF.
> Ive also used a pair of 100 Ohm 5W MOX resistors (Mouser again) and had
> no
> problem with the slight added inductance.
>
>
>> f. I had trouble with the big zener diode used for PA bias.  The
> Harbach
>> RM-220 Rectifier/Metering Board fixes this problem.
>
>
> I prefer a string of robust 1N5408's for the bias. The 1N4007's on the
> Harbach blow almost as easy as the zener. Haven heard of a 1N5408
> blowing in
> the 25 years Ive used and recommended them.
>
> The diodes in the stock board should be replaced by 1N5408's also.
> Drill out
> the holes slightly to fit the thicker leads.
>
>
>> g. I'd take Measures' advice about removing the interlock from inside
> the
>> final cage.
>
>
> Ive been doing that since the amp was first sold!
>
>
> and I'd use "Stove Black" or as he said, "black liquid shoe
>> polish" to paint the inside of the final compartment (including the
> cover
>> to
>> same).
>
>
> More pure voodo science.
>
>
>
>> h. If I had lotsa money, I'd buy a spare fan & motor and a spare
> antenna
>> relay (just in case).  I was stationed in Ethiopia for some three
> years
>> and
>> developed a great fondness for spares.
>
>
> Fan motors are too low torque to fail but yes on the relay. Many have
> already lost their flexible arm temper from use and RF heat.
>
>
>>
>> But my main points are these:
>> Don't convert the amp to QSK.
>> Do install an SK-220 circuit (or build your own circuit).
>> Otherwise consider ALL of Measures' ideas.
>>
>> The SB-220 is a great amp.
>
> Absolutely.
>
> One other modification that may be a bit hard to source. Besides the
> reasons
> already mentioned the bandswitch can arc due to high circulating RF due
> to
> the shorted turn effect of the design. Again, no voodoo science
> parasitics
> are involved. This can be snubbed by installing a 10-12pf 1000V silver
> mica
> between the 20 and 80M contacts directly at the switch. It will affect
> the
> Tune control by that much capacity at 10M so the coil spacing may have
> to be
> stretched a very small amount. A 5Kv RF doorknob can also be used.
> Fair
> Radio, SSN and others sources may have suitable caps.
>
> Carl
> KM1H
> Amplifier Repairs and 6M conversions since 1964
> National Radio 1963-69
> Member of NCL-2000 Design Team
>
>
>>
>> 73, Mike N4NT
>>
>> ----- Original Message -----
>> From: "Marvin Moss" <mmoss at mindspring.com>
>> To: "Mike Hyder --N4NT--" <Mike_N4NT at embarqmail.com>
>> Sent: Friday, August 21, 2009 04:59 PM
>> Subject: Re: [Heathkit] How about the SB-220?
>>
>>
>>> That is a good one.
>>> Thanks.
>>> Marvin
>>>
>>>
>>>> [Original Message]
>>>> From: Mike Hyder --N4NT-- <Mike_N4NT at embarqmail.com>
>>>> To: <mmoss at mindspring.com>
>>>> Date: 8/21/2009 03:43:24 PM
>>>> Subject: Re: [Heathkit] How about the SB-220?
>>>>
>>>> Look here: http://www.somis.org/
>>>>
>>>> particularly here: http://www.somis.org/SB220ci.html
>>>>
>>>> 73, Mike N4NT
>>
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