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

[Bill Kirkland]

forgot one other item. With solid state rectifiers you will be applying the 
B+ almost
instantaneously to the tube plates, well before they have had a chance to 
heat up.

I am not a tube expert but I can buy into arguments that this is not good 
for the
tube.

In essence if you just replace the tube with solid state rectifiers, you are 
not
replacing the rectifier tube with an equivalent system.
- rectifier tube has to heat up before it applies B+ to the rest of the 
tubes
    start up surge will put more stress on the transformer and filtering 
circuit
- rectifier tube has a huge voltage drop compared to a silicon diode, e.g. 
40V.
   at 250V plate voltage, this is > a 15% increase in the operating point of 
the circuit.


Will the system work, quite possibly/probably.
Is it working within its intended  parameter range, probably not.
Is it fatal - don't know, but I would expect it to shorten the life of the 
tube - which will get harder to find

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
>>>
>>>_______________________________________________
>>>
>>>
>>
>>______________________________________________________________
>>
>
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f