Disco Volante
Well-known member
Staying in b7g, the ECC91 might replace the 6C8G, seeing as the cathodes are directly connected..
Collins 26C Analysis
- Thread starter DaveP
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Yes you are right, it would do the job, but I was sticking to the same type, mu20 ra10k.
The replacement for the 6J7 was the single ended 6SJ7 and that has the same triode characteristics as the 6J5 and half of a 6SN7, the ECC82 comes into that class as well.
best
DaveP
Disco Volante
Well-known member
Point taken, I need to look at *all* the parameters when looking for subs. I guess in this case Ra is of particular interest since we're driving transformers. ECC82 has mu17, but higher Ra (47k if I read the spec sheet right).
This would be a fun and useable project to recreate! Following with great interest!
Cheers, V!
This would be a fun and useable project to recreate! Following with great interest!
Cheers, V!
The 6C8G tube is rather special, no-one tried to copy it and it looks like it was only made by RCA and TungSol. Each section has this characteristic:-
mu 36, ra 22.5k, gm 1600. It was introduced in 1937. It contains two independent triodes, albeit that one has the top electrode for its grid. Its heater current is the usual 0.3A whereas its closest cousins are the 6A6 and 6N7 which both draw 0.8A, the characteristics of these tubes are mu 35, ra 11k, gm 3200. My guess is that the internal dimensions are similar but the higher heater current increases emission and this increases the gm and reduces the ra. The 6N7 was initially used as the resistance tube in the Collins 26W, but it was replaced by the 5814A which is a special quality ECC82/12AU7.
The successor tube to the 6C8G was the 12AY7, but as the chart below shows, it does not have the same characteristic curve:-
I have plotted two examples of 6N7 and they are very close to each other and pretty close to the 6C8G as well. The 6J6, 6DT8 (ECC81) and 12AY7 are not close at all. I have some research to do on other possible replacements and will post that next.
best
DaveP
mu 36, ra 22.5k, gm 1600. It was introduced in 1937. It contains two independent triodes, albeit that one has the top electrode for its grid. Its heater current is the usual 0.3A whereas its closest cousins are the 6A6 and 6N7 which both draw 0.8A, the characteristics of these tubes are mu 35, ra 11k, gm 3200. My guess is that the internal dimensions are similar but the higher heater current increases emission and this increases the gm and reduces the ra. The 6N7 was initially used as the resistance tube in the Collins 26W, but it was replaced by the 5814A which is a special quality ECC82/12AU7.
The successor tube to the 6C8G was the 12AY7, but as the chart below shows, it does not have the same characteristic curve:-
I have plotted two examples of 6N7 and they are very close to each other and pretty close to the 6C8G as well. The 6J6, 6DT8 (ECC81) and 12AY7 are not close at all. I have some research to do on other possible replacements and will post that next.
best
DaveP
6J7G: I'd stay w/this original tube family, as they are still plentiful, and are great tubes, esp. the metal 6J7 (and using vintage metal top caps for excellent shielding), IIRC european EF37A is also similar. Even Soviet 6J7 copies -- 6Zh7 -- aren't bad and are cheap. If I'm not mistaken, later development lead to 6BR7/6BS7 special quality noval tubes, but those are available in much smaller numbers.
6C8G: Very odd tube, hardly anything like it, 6N7 is much more plentiful and would be a better choice, at least on paper. That said, 6C8G are still somewhat affordable.
The output stage is probably not that critical so the 6K6s (if you don't have them) can be substituted/altered for 6F6 or 6V6 (or 6AQ5 which are also plentiful, but I'd stay w/octals, personally)?
6C8G: Very odd tube, hardly anything like it, 6N7 is much more plentiful and would be a better choice, at least on paper. That said, 6C8G are still somewhat affordable.
The output stage is probably not that critical so the 6K6s (if you don't have them) can be substituted/altered for 6F6 or 6V6 (or 6AQ5 which are also plentiful, but I'd stay w/octals, personally)?
I am exploring other tubes because the original transformers are no longer available. If we use modern transformers with less inductance, then we need a lower output impedance from the tubes to maintain good low frequency response. More on this later.
Some B7G Pentodes have a good triode performance that is reasonably close to a doubled 6C8G:-
The curves I have drawn are a bit wobbly because it is difficult to get the tangent right at such low currents. In fact, where the curves reach zero current, this may not have been recorded so accurately as the main lines. The main criteria for the resistance tube is to present a low resistance at one end (to join the transformers) and a sufficiently high resistance at the other end to isolate the transformers. It goes without saying that there should be a nice smooth transition between the two.
There will be more on B9A tubes to follow.
best
DaveP
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