[ARC5] No Doubt Dumb Tube Question

[Geoff]

Since the 5R4WGB and 5AR4 have the highest PIV rating of full wave 
rectifiers and are indirectly heated their construction isnt the answer. 
There are many other indirectly heated rectifiers used over decades.
There are also several half wave indirectly heated HV tubes with 6.3V 
filaments that have very high PIV ratings that were designed for color TV.

Generally the cathode type tube has a fairly low voltage drop with the 5R4 
being a very noticable exception. I use that often to replace the 5U4 and 
5Y3 in order to drop the DC voltage a bit with todays line voltages and 
reduce the filament load a hair in the case of the 5U4. In many sets using a 
5Z3 I either change sockets or rebase the 5R4 based upon customers 
instructions. In consumer radios I dont like to go SS since the hard 
switching adds additional stress to old transformers that were marginal when 
new plus reducing the now higher DC gives a negative benefit when using heat 
generating resistors. Adding a bucking transformer under the chassis or 
using the now unused 5V winding are options but the latter brings its own 
potential problems.

The 6X5 has likely destroyed more transformers than any other tube until the 
manufacturers got the design correct after a few years. Ask anyone that is 
involved in pre WW2 Zenith restorations about them. Unfortunately there are 
still many 6X5G and GT versions of the older design available as 
NOS......these are fine at a much lower plate to plate AC voltage than shown 
in the "typical" use in the tube manuals. I use only 6X5WGT for replacements 
BUT their current ratings must not be exceeded. Not all GT's are bad, it is 
all about the plate configurations and that is a visual thing discussed for 
many years on other forums. The 6X4 also has a spotty reputation and easily 
shorts when leaky filter caps push the current ratings just a hair over 
spec.

OTOH, the 5V4, which was used in many Nationals, survived fine even when 
leaky caps took out the transformers.

Carl



----- 

> AFAIK, all of the 5.0 VAC heater 4-pin and octal base rectifiers have
> directly heated cathodes.  The reason (or one reason) is voltage rating. 
> It's
> easier to get an efficient cathode with a high voltage rating by putting 
> the
> requirement on the transformer and not on the tube.  Several of the tubes
> have PIV ratings above 1400 volts.
>
> Robert Downs - Houston
> wa5cab dot com (Web Store)
> MVPA 9480
>
> In a message dated 06/17/2013 11:12:37 AM Central Daylight Time,
> releazer at earthlink.net writes:
>> I am reading some in an early 50's USAF training manual, Radar Circuit
>> Analysis, because it gives a good description of vacuum tube circuit 
>> basics.
>> What I saw there led me to dig out my Sylvania tube manual and look up
>> some data.
>>
>> Last year I was rebuilding the ham-built power supply of a BC-348 and was
>> surprised to find that the transformer HV went straight to the filament 
>> of
>> the rectifier tube.  I had never thought about it until I had to wire a
>> rectifier tube socket, but had assumed that such tubes (e.g., 5Y3) used
>> indirectly heated cathodes.
>>
>> But now I see in the Sylvania book that there are not only rectifier 
>> tubes
>> like the 5Y3 that use just a filament but also those like the 5V4GA and
>> 5AR4 that use indirectly heated cathodes.  Furthermore, such tubes seem 
>> to
>> still have the filament hooked to the cathode internally.
>>
>> So why do some rectifier tubes have indirectly heated cathodes and others
>> do not?
>>
>> Thanks,
>>
>> Wayne
>>
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