GB> Tubes in Series was (Re: [Heathkit] Question on 12AU7) LONG

[N2EY at aol.com]

In a message dated 2/2/08 4:00:04 PM Eastern Standard Time, 
ckepus at comcast.net writes:


> what I am confused about is the requirement to have all tubes in
> the string to have *identical* current draw requirements. I know (or thought
> I did) that in a series string, the current is the same throughout the
> circuit.  

That is true. 


Take the case above: put two tubes in the series circuit, one with
> 
> a current rating of 0.175A at 12.6V while the other is rated at 0.15A at
> 12.6V.  If the tube rated at 0.175A causes the tranny to deliver 0.175 amps
> to the circuit, and since the tube rated at 0.15A will be "exposed" to that
> current flow, will it "live" at that current level? (filament wattage
> dissipation rating??)?

No.

What happens if you put dissimilar-current tube heaters in series is that 
they won't get the right voltages across them. The low-current tubes will get too 
much voltage and the high-current tubes won't get enough. 

For example, consider what would happen if we put a 6SN7 (6.3V 0.6A heater) 
in series with a 6SL7 (6.3V 0.3A heater) and put the whole thing across 12.6 
volts.

(for simplicity, we'll just use the hot resistances for the first go)

The 6SN7 heater has a hot resistance of  6.3/0.6 = 10.5 ohms. 
The 6SL7 heater has a hot resistance of  6.3/0.3 = 21 ohms. 

The combination has a hot resistance of 31.5 ohms. With 12.6 volts applied, 
the current will be 0.4 amps. The voltage across the 6SN7 heater calculates as 
4.2 volts and the voltage across the 6SL7 heater calculates as 8.4 volts! Not 
good.

In real life, things will be even worse because a tube heater's hot 
resistance is more than its cold resistance. So the under-heated 6SN7 will be even more 
underheated than the above simplified calculation shows, and the 6SL7 will be 
even more overheated.


 Are "balancing resistors" something that could be
> 
> used if necessary? 
> 

Yes. For example, consider the BC-348, which uses 8 tubes that have 6.3 volt 
heaters and was meant to be used with 24 volt supply. All the tubes except the 
6K6 (42 in some models) have 0.3 amp heaters, but the 6K6/42 has 0.4 amps. So 
there's a resistor to pass the extra 0.1 amp.


> If the answer is, "That's just the way it is per the laws of electronics -
> the current draw requirements MUST be identical for tubes in series"  - then
> someone say so and I won't use any more of the list members time or
> bandwidth on the subject.  
> 

It's a good question IMHO and worth discussing.

> I pulled up the GE 12AU7 data sheet at this tube data site
> [http://www.tubedata.org/] (Carl, KM1H provided info about this incredible
> site...thanks, Carl!).  The only special reference to filament current I
> could find on the sheet was a comment that the 7AU7 (equivalent to the
> 12AU7) "..is specially suited for use in television receivers that employ
> 600 millampere series-connected heaters."  Hmmmm... So they all HAVE to be
> 600 millampere??
> 

There are two things going on there.

The first is that some TV makers made TVs with no power transformer, and 
needed tubes for them. But conventional tubes had heater current ratings all over 
the place - just compare the 6C4 to the 6DQ6! So versions of those tubes were 
developed that had the same heater current - 450 mA, 600 mA, etc. That's where 
all the odd tubes like the 25DQ6 and 4BZ6 came from. Designers could find all 
sorts of tubes with the same heater current.

The second is that simply having the same heater current isn't always the 
complete solution. If one tube in a series string heats up fast and another heats 
up slowly, there can be some pretty serious over- and under-voltages on the 
various tubes until things calm down. So the tubemakers came up with 
controlled-warm-up technology for heaters on the tubes used in those sets. 


> If there is an explanation of this on the web or in a text, please direct 
> me
> to it so I can read it. 
> 

I learned a lot from reading the technical parts of the RCA Tube Manuals. 
Many different editions can be downloaded from various sites. 

When I built this receiver:

http://www.qsl.net/k5bcq/Jim/jiminfo.doc
http://www.qsl.net/k5bcq/Jim/SilverRX1.jpg
http://www.qsl.net/k5bcq/Jim/SilverRX2.jpg
http://www.qsl.net/k5bcq/Jim/SilverRX3.jpg
http://www.qsl.net/k5bcq/Jim/SilverRX4.jpg
http://www.qsl.net/k5bcq/Jim/SilverRX5.jpg
http://www.qsl.net/k5bcq/Jim/SilverRX6.jpg

back in the 1970s, I used old TV tubes because I had them. The terminal strip 
is used to connect the various heaters and balancing resistors in series or 
parallel combinations across the 12.6 volt heater line. 


73 de Jim, N2EY


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