[ARC5] Replacing the 12v tubes with 6volt types

[WA5CAB at cs.com]

I'm surprised, considering how long this sub-thread has continued, that no 
one has mentioned the obvious.  The first criteria for s 6 volt tube to 
replace the 12A6 is that it draw the same filament current as a 6SR7, which is 
what it will be in series with.  

6SR7     300 mA
6C5       300 mA
6F6       700 mA
6K6       400 mA
6L6       900 mA
6V6       450mA
6W6      1200 mA

There are several later double-letter octal beam tetrodes and power 
pentodes but they all have heater currents of 450 mA or more.

Maybe that's why A.R.C. apparently plugged a 6C5 into a receiver, 
presumably to get a quickey 14 volt receiver for some reason.  :-)

Of the available choices, the 6K6 comes closest to working right.  To a 
first approximation (ignoring the fact that tube heater resistance is pretty 
current sensitive), the series string current would be 343 mA.  There would be 
7.2 volts across the 6SR7 and 5.4 volts across the 6K6 (with 12.6 VAC/VDC 
applied).  Actually, because of the resistance variations with delta-current, 
it would be a little worse than that.  But I've done it for a few hours 
(until I got the 28 volt system working in my International Scout) and it works 
OK.  If I hadn't been building a 28 volt add-on system, I would have tacked 
a resistor across the 12SR7 socket to balance the drop.

With a 6F6, the imbalance is so great that using the rated currents to 
calculate heater resistance would be 'way off but using them, you get 8.8 volts 
across the 6SR7 and I would bet in practice, you would get closer to 10 
volts.  The 6F6 would certainly run cool enough to pull out while hot with your 
bare fingers.  :-)

In a message dated 10/18/2012 15:12:47 PM Central Daylight Time, 
kgordon2006 at frontier.com writes: 
> 12A6 has a 7AC base.
> 
> So does the 6V6....and the 6K6 and 6F6 and...
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480