[Hallicrafters] Re: [Boatanchors] Replacing rect tubes withdiodesquestion

[Bill Kirkland]

I am curious as to why you indicate that a higher B+ is ok without any 
indication
of how it affects the bias conditions and operating points of the tubes.

Bill


>From: "jeremy-ca" <km1h at jeremy.mv.com>
>To: "Rodger" <wq9e at dtnspeed.net>,WØQFC <erastber at tampabay.rr.com>
>CC: boatanchors at mailman.qth.net,HALLICRAFTERS 
><hallicrafters at mailman.qth.net>
>Subject: Re: [Hallicrafters] Re: [Boatanchors] Replacing rect tubes 
>withdiodesquestion
>Date: Wed, 13 Feb 2008 11:15:42 -0500
>
>A dropping resistor simply generates more heat and provides no useful 
>function. The tubes are all rated at a higher voltage than is used, 
>recapping takes care of that area and the output transformer has sufficient 
>insulation. The extra B+ current drain is well under 10W.
>
>Benefit of a resistor in a rebuilt radio =  Zero.
>
>Benefit of a SS surge protector =  Zero also since they function at the 
>zero crossing of the sine wave.
>
>Carl
>KM1H
>
>----- Original Message ----- From: "Rodger" <wq9e at dtnspeed.net>
>To: "WØQFC" <erastber at tampabay.rr.com>
>Cc: <boatanchors at mailman.qth.net>; "HALLICRAFTERS" 
><hallicrafters at mailman.qth.net>
>Sent: Wednesday, February 13, 2008 9:36 AM
>Subject: [Hallicrafters] Re: [Boatanchors] Replacing rect tubes with 
>diodesquestion
>
>
>>Hi Gene,
>>
>>You also need a dropping resistor to reduce the voltage to close to what 
>>would normally be supplied by the tube rectifier.  You can get a decent 
>>idea of what the resistor should be by looking at what the typical tube 
>>voltage drop is in normal operation, this information is available in the 
>>tube data books or the NJ7P online data base is handy: 
>>http://www.nj7p.org/Tube.php  I would imagine a 270 ohm 20 watt resistor 
>>connected in series with the common cathode connection would get you close 
>>enough.  Of course you could also use separate resistors for each 
>>replacement diode, in this case the resistor value would be doubled and 
>>the power rating would be halved.  I do replace a fair number of my tube 
>>rectifiers with SS rectifiers and some of the Halli power transformers 
>>definitely benefit from this replacement.  I always build the SS as a plug 
>>in and disconnect the 5 volt heater leads from the socket to remove the B+ 
>>from the 5 volt winding of the transformer since this is a fairly common 
>>failure mechanism in these older transformers.
>>
>>A couple of other considerations:  The B+ will come up instantly and will 
>>be higher than usual until the remaining tubes warm up and begin drawing 
>>current so if the filter caps are marginal this extra voltage will 
>>probably cause their rapid demise.  So replace any doubtful caps first. By 
>>the way, even with a tube rectifier the voltage is much higher just after 
>>turn on as the directly heated rectifier cathode heats up almost instantly 
>>in the existing tube type rectifier.  I would also consider adding an 
>>inrush current limiter to your set to further reduce the stress on the 
>>transformer and tube filaments, suitable ones are available from Mouser or 
>>Digikey for around a couple of bucks and they are installed after the fuse 
>>and in series with the transformer primary.
>>
>>73, Rodger WQ9E
>>
>>WØQFC wrote:
>>>I want to replace the # 80 in my S-40A with diodes so I can reduce heat 
>>>by less draw on the tranny.
>>>
>>>Can I just use two diodes that will handle the current, or is there 
>>>something else I should do ~ additional parts, etc?
>>>
>>>Also, I would like to replace the 6H6 for the same reason, but what 
>>>diodes should I use?
>>>
>>>I know that these items are available but in the interest of saving 
>>>money, I would like to do it myself. . . .
>>>
>>>Comments?
>>>
>>>73
>>>
>>>Gene, WØQFC
>>>
>>>www.w0qfc.com
>>>
>>>_______________________________________________
>>>
>>>
>>
>>______________________________________________________________
>>
>
>______________________________________________________________