sharp cutoff pentode

[Mr. Triatic]

what are the disadvantages of using SCP tubes in audio?
i was thinking of using either 6AS6 or 6CB6 in a linestage.. do you think they are useable or should i go with the ordinary boring ones like ECC88 or 12AX7.. ?

[nanana]

man,
you guys ask the most difficult questions!
first of all, it all depends on what you are trying to accomplish... there is nothing at all wrong with using pentodes and the onesz you picked are pretty decent ones, too. but for a line stage? what does a line stage do? how much gain do you need? what impedance level are you working into? what source impedance are you receiving? before one even asks a question like, "are sharp cut-off pentodes any good for a line stage" you might want to ponder some other questions first?

the co**** answer is "yes, of course"! sharp cutoff pentodes are the ones you need for linear amplification and that is probably what you are more interested in. remote cutoff pentodes are better suited for things like compressors and AGC circuits or voltage controlled volume controls (yes you can make them with tubes...). these are all jobs that take advantage of the non-linear gain thats available in a remote cut-off tube. a line stage doesn't need much gain and there is a lot of gain available with the tubes you mentioned. they can be used in feedback circuits, there are some very interesting local feedback schemes for pentodes and they also make great cathode followers... useful for impedance matching. there are also some interesting direct coupled possibilities and you don't even have to put signal into the grid if you don't want to... there are 4 different input points on a pentode and they all have their pros/cons.

so now i hope you're satisfied?! you have opened a can of worms with your supposedly simple question...
happy new year!
jc

[Mr. Triatic]

Gee... i guess i should have specified me a little bit... 8-)
yes a linestage with lowish gain, in the order of 6-8 dB maximum.. more like a buffer that is.
well i was thinking "problem" more in terms of biasing, finding a linear workingpoint and other similar problems?
and maybe if someone have some input on the soundquality of these tubes, mine are Mullard ones. (that means not automatically good sound....)

thanks!

[nanana]

i may be the only one on this forum to say it but i will keep saying it... as far as the "sound" goes, its NOT the tubes... or at least not mostly. among the special or selected tubes this will vary.... its is what you DO with them. mullard (which is to say philips), made some good tubes... the best "mullards" were sometimes from other factories in other places... (like the 1970's "made in britain" matsushita EL-34s that everyone loves so much!) chances are you have some good stuff. so what are you going to make with them?

jc

[cogsncogs]

(like the 1970's "made in britain" matsushita EL-34s that everyone loves so much!)

Yeah, like the Westinghouse EL34's made in Japan Matsushita ones that I loved soo much!

Wayne

[nanana]

yeah,
matsushita and NEC made some nice tubes! the most beautiful tubes i have ever laid eyes on are NEC 211's, metal plates, 75 watts like the westerns... so well made and sooooo gorgeous!
they made some of the best radio pentodes by the way... i love the 6AU6s.
jc

[cogsncogs]

I lost neg bias on one of a pair of Westinghouse EL34's. Turned a bright orange and fried (opened) the opt and the tube was still "good" after that abuse! Lasted another 6 or 7 years!
That's what I call craftmanship!

Wayne

[cogsncogs]

@ Mr. Triatic

In staying on topic, the 6CB6 is a good choice as also a 6EJ7 and a 12BY7 video pentode. And there are alot of the 4V and 5V versions which are plentiful and fairly inexpensive. Microphonics may a problem though. Using sharp cutoff pentodes, there will be a lot of gain and a high-ish output impeadance. You could "strap" them into triode mode, lower gain, less noise, and lower output imp. Either way you may want/need to use a cathode follower (dirty word around here!) with some, especially in pentode mode.

In that case you could use one of those made for TV use triode - pentodes with a little NFB around them. Such as 6U8, 6KD8, 6GH8, 6EA8, 6HG8, 6BR8, 6BL8, 6CL8, 6KE8 and the 4 and 5V versions etc. Hey IMO all tubes can be used for audio. It's just that some are better suited for audio than others. I think it would be fun to see what you can come up with! That's what DIY is all about!

Cheers
Wayne

[cogsncogs]

yes a linestage with lowish gain, in the order of 6-8 dB maximum.. more like a buffer that is.

Missed that! Well in that case you would definitely need to strap them into (pseudo) triode mode. I would suggest a EL84/6BQ5, 6AQ5, 12BY7 and the like. A 6AQ5 strapped as a triode would have a mu of around 9 with a plate voltage of 250V and plate current of 49.5mA at -12.5V grid voltage! That's quite a bit of current! Of course you could go with a lower plate voltage with the resultant lower plate current. Don't have the exact numbers in front of me at the moment. I'd have to draw me a load line for this. Could do a simulation with an EL84 to get you in the ballpark. I'll try to do one and post back the results if you are interested.

Wayne

[Mr. Triatic]

hi, thanks for the replies, i have decided to go for the 6as6, just for the size of it, if that wont work i aim for a configuration with 6cb6..
but i'm not really sure about the right way to "strap" a pentode into triodemode..?
due to these tubes small size i will be able to make a compact "module" to put in my existing passive preamp 8-)