[HBR] 6CX8 substitutes

[match at ece.utah.edu]

On 17 May 2010 at 22:18, Carlos wrote:

> Thanks Darrel, for the information.
> 
> My question was why after researching similar tubes, all have a higher 
> amplification factor on triode side. Possible substitutes that I found 
> (6AW8, 6LY8, 6EB8, 6GN8 and so on) have this factor in the 70 to 100 range, 
> while in the 6CX8 is 40.

If you look at the various Heathkit schematics, you'll find that they 
used most of the tubes in this family as audio outputs in one radio 
or another...

> I'm wondering if I can use one of those substitutes, without change some 
> component values, to reduce the stage gain and prevent overload.

Uh... what am I missing? A higher amplification factor means higher 
gain, which means easier to overload, doesn't it? You really don't 
need the higher amplification factor here.

> Moreover, in these four examples, the 6AW8 seems the most adequate, but its 
> pentode plate current has a tipical value of 13mA, while in the others this 
> value is 25mA, like the original 6CX8, meaning more power output. So, what 
> is the best substitute, since I don't have details of the respectives 
> impedances loads, as audio amplifiers?
> Sorry any annoyance.

Why not just use a 6CX8? It's actually a really good choice.

On to your other question... There are ways to figure out what the 
"best" output transformer impedance should be, after you decide what 
"best" is! You'll find that there are two "best" configurations. One 
will maximize output power, the other will minimize distortion. This 
is found in texts like The Radiotron Designers Handbook, but I can 
tell you that with any of these tubes in this family, the output 
impedance will be rather high, like 7k-12k.

Now then, does it matter? Well, for our purposes it matters but not 
much. Unless the output transformer is really low for the tube, like 
2k-3k, I seriously doubt you'd notice much difference in actual use, 
between output transformers of say 7K compared to 10k, so why bother 
to do the calculations? The load you present to the output stage 
(speaker) is very reactive and varies with frequency anyway. A 
speaker with an 8 ohm impedance marked on it can vary by a factor of 
as much as 5 or even more over the voice frequency range.

Marvin
KA7TPH