Octal sound with new production tubes?

[Colossal]

Suppose a fellow wanted to take an old classic amp design which used an all-octal preamp and PI and convert the design to use new production (octal or nonal) tubes. All philosophical arguments aside about NOS sounding better, nothing beats an old octal, you can't do that, or it'll never sound the same, what might be the selection criteria and process for doing so be?

Is it as simple as taking the data sheet for the old octal, consider the basic operating parameters such as VaMax, PaMax, Cgk, Cak, mu, ra, Ia et al and trying to find a modern production tube (in 8 or 9-pin format) that matches these as closely as possible? Assuming this is the case, the usual design process would then have to take place such as biasing the new tube to have similar behavior as the octal (i.e. if the octal was center biased to have a clean tone).

I am looking to be able to capture the darker/warm sound of the original octal preamp but not necessarily use NOS.

Thanks for any help on this.

[tubeswell]

Is it as simple as taking the data sheet for the old octal, consider the basic operating parameters such as VaMax, PaMax, Cgk, Cak, mu, ra, Ia et al and trying to find a modern production tube (in 8 or 9-pin format) that matches these as closely as possible?

I think it is most likely that simple. The trouble is finding the right sort of modern tube. So far I like the EH12AY7 for V1 (in almost every amp I've tried one in), it has lower plate resistance and more bandwidth than a 12AX7, and works well with the same RC circuit values. I guess a 12BZ7 would be better still in theory, because it has 2 x as much gm (as well as lower rp and higher BW) but I've never tried one.

The attached discourse from OTM is interesting in this regard

[labb]

6sl7 is in production and is pretty much = to 6sc7.

[Colossal]

Is it as simple as taking the data sheet for the old octal, consider the basic operating parameters such as VaMax, PaMax, Cgk, Cak, mu, ra, Ia et al and trying to find a modern production tube (in 8 or 9-pin format) that matches these as closely as possible?

I think it is most likely that simple. The trouble is finding the right sort of modern tube. So far I like the EH12AY7 for V1 (in almost every amp I've tried one in), it has lower plate resistance and more bandwidth than a 12AX7, and works well with the same RC circuit values. I guess a 12BZ7 would be better still in theory, because it has 2 x as much gm (as well as lower rp and higher BW) but I've never tried one.

The attached discourse from OTM is interesting in this regard

Tubeswell,

Thanks a lot for the reply; good to know I may be off on the right footing. I appreciate the attachment, very useful reading, and yes, I have been looking at plate resistance specifically in trying to sort out some equivalent modern tubes. Great to know about the EH12AY7. I was looking at that one specifically for the replacement of a 6N7 which is a octal dual triode with a common cathode. The 12AY7 seems to be quite close to the old 6N7.

Ok, I will just look at all of the tube sheets for the modern tubes and see what ones come up closest to what I'm trying to substitute for. My second question is: is it reasonable to say that it is possible to emulate the octal tone through substitution? So, in other words, is it safe to say that the tone is in the tube's parameters (e.g. Rp, gm, etc)?

Thanks for the help, I appreciate it.

[Colossal]

6sl7 is in production and is pretty much = to 6sc7.

Thanks Labb, I've been looking over what new octals might fit the bill and see the 6SL7 and 6SN7 are new production. Mostly wondering if the first place to start is finding tubes which have the closest application data and parameters as the ones I'm trying to substitute for.

[David Root]

Just use the 6SL7 and the 6SN7. If you want 12AX7 type gain use 6SQ7, which is sort of an ancestor of half a 12AX7.

6SN7 is a killer PI but has low gain. 6SL7 is actually pretty good as a PI.

Just factor the plateloads to suit the plate resistance, Rk and coupling caps are different too. Output impedance is lower too, (not the 6SQ7). Basically what you are thinking about.

I think the envelope size has a lot to do with tone. I've done amps with 7-pin tubes as well as octals, and of course the std 9-pins in between, and they are all three different in tone.

One tip, if you are looking for more gain in octal tube circuits than Leo used in the octal tweeds, make sure you use 6V DC on the heaters. Ask me how I know this!

[Andy Le Blanc]

second using the octals, its not just the electronic equivalence, its also the
envelope, vacuum and physical structure of the device.
Although 12ay7 isnt a bad sub for a 6sl7 and 6sc7 is a 9pin equivalent for a 6sn7.
I'd scab together a simple gain stage and audition different types.
It'd be a great addition to your bench stereo, bust out the old vinyl call it a phono pre....

[tubeswell]

One tip, if you are looking for more gain in octal tube circuits than Leo used in the octal tweeds, make sure you use 6V DC on the heaters. Ask me how I know this!

I'm game. What's the answer then? (42? )

No seriously - how do you know?

[Colossal]

Just use the 6SL7 and the 6SN7. If you want 12AX7 type gain use 6SQ7, which is sort of an ancestor of half a 12AX7.

David,

It's actually a 6SQ7 that I'm looking to replace. A wonderful sounding tube and I like it a little more than the GT (at least the ones I auditioned). But I am looking for a replacement for a 6SQ7 and its characteristics are a bit unique.

I'm also looking to replace a 6N7 (not 6SN7), an early dual triode with common cathode and a 12AY7 actually seems to be a fairly close match and might be a good candidate.

I think the envelope size has a lot to do with tone. I've done amps with 7-pin tubes as well as octals, and of course the std 9-pins in between, and they are all three different in tone.

second using the octals, its not just the electronic equivalence, its also the envelope, vacuum and physical structure of the device.

I was a bit afraid of this; that it's not just about equivalence on paper. It might be a bit more of a challenge to get the tone but if modern octals would be a better starting point despite more disparate operating characteristics then I'll start there (6SN7, 6SL7).

I basically need two triode stages similar to a 6SQ7 and a PI similar to a 6N7.

Thanks guys.

[David Root]

Tubeswell, personal hard experience with 6SL7s. If you start pushing the gain on them they hum like hell.

Heaters are not wired humbucking like 12A*7s so your only option is 6VDC. Elevating the heaters did not help at all. I am talking mostly JAN ECG 6SL7s, not new stuff. Paralleling triodes did not help.

Only thing I did not try is local feedback from plate to grid.

[Colossal]

David,

Interesting comments. After some analysis, I'm thinking about using 6SL7 and 6SN7 as replacements instead of trying to consider more closely matched noval tubes. The 12AY7 is a fairly close match to a 6N7 however.

Regards,
Dave