Confused on the 12BY7a

[1blueheron]

I was looking through my stash yesterday morning looking for a 5987 and I ran across a 12BY7a.   Not sure exactly what it was, I pulled the data sheet.  Looking at the data sheet, it says it is a sharp remote cutoff power pentode for video amplifier output.   That would lead me to think that it would be best used as an output power tube.  Then a little more poking around and I find it was the tube of choice for preamps in the HK Citation II and the Carver Silver 7 and also used as a PI.  Supposed to be ultralinear. Clean gain, no coloration.

Has anyone used this as a guitar preamp or PI?  What would the pro's and cons be of using this in a geetar amp and where would you put it?   Is it possible this tube could be used in all three positions as preamp, PI and output?

[DummyLoad]

sharp remote cutoff...

if a pentode, then it's either a sharp or remote cut-off - not both, in this case it's a video output power pentode.

harman kardon used them in the citation II power amps. there were other, can't recall those at this time.

--pete

[PRR]

Not remotely "sharp remote".

It is half of a HIGH gain 6V6. Feed 250V, load 10K, bias/drive about 3 Volts, get like 2.5 Watts. Up to a point, cleaner than 6V6 (they had those too, and 6V6 was sometimes used as a video-amp), though at couple-watts you can exceed the clean zone without blowing down the walls.

[1blueheron]

So it was obvious after reading the replies and re-reading my question that I needed to study what the term "cutoff" means. 

So if I now understand it, there are two types of "cutoff".   Remote and Sharp,  "Remote" being a gradual fading away of plate voltage as grid voltage rises past a certain point and "Sharp" being an immediate drop in plate voltage once a certain point is reached.  Is this correct?   As such, the 12BY7A is a sharp cutoff and my OP calling it both remote and sharp was a newbie mistake and lack of diligence on my part in attention to the data sheet.  It clearly states sharp cutoff pentode.  I failed that exam miserably.

So in practical use, the 12BY7A could fill any of the roles of small signal preamp, Phase inverter, or output power amplifier and in theory you could make a very clean, low watt SE amplifier using two 12BY7's  or a PP amp using 4 correct?

Why then is it seldom seen if it is so versatile?  What does it do badly that makes it fall from favor?  Was it merely a cost factor that limited its application?

[PRR]

It's expensive, and a pig for heater power. Who needs such a beast? Especially for preamps.

The power you could get from two 12BY7, you could get from one 6V6, without a PP driver.

Two 12BY7 is 2*6V*0.6A= 7.3 Watts heater power, plus maybe 0.5W for a cathodyne driver; one 6V6 is 2.8 Watts heater power.

> if it is so versatile?

So? It's not like we need to hire one man who can play piano AND shortstop. Tube (and transistor!!) production has tended to many-many models each just good enough for one job. Hundreds of types, pick what you need.

[PRR]

It's expensive,..... blablablah...

Here's prices from 1960. Yes, these are nominally "used and pull-out", but had one full year guarantee. The more I look at it, this may really be a back-door outlet (prices for tubes of different types but same number of bits inside are too uniform to be used-market pricing).

And radio/amplifier factory buyers did sniff around the margins of the market looking for a deal so they might not go broke this season. They might not use Rad-Tel as supplier, but they would know Rad-Tel's prices when hammering on more direct distributors.

12BY7 is almost 150% the price of a 6AU6 which will do 95% of your audio pentode work.

Two 12BY7 are near 3 times the price of a 6V6 or 6AQ5. Not counting PP driver.

50L6 will whup a 12BY7 and with a cheaper OT. Save thirteen cents!

If 12BY7 was $0.74 in 1960 and $16 today, if you put one away then and sold it today you made 5% per year on your investment, before garage and advertising costs. That seems OK today but there have been years when paper made more (or less!) return.

[1blueheron]

My scrounge/hoard of tubes bears witness to the truth of your post,  only 3 or 4  of the 12BY7A and a whole bag of 6AU6. 

So with those thoughts in mind, what rare virtues of the 12BY7 would have caused H K, & Carver, to choose them?  Pure snobbery?  Thoughts of marketing the amps in a secondary market as the world's most expensive and inefficient space heaters?  The challenge of using an oddball and less common tube that they could relabel and sell at 10X market price for replacement?  Or was/is there a legitimate technical advantage?

Thanks for taking the time to school me.  Interesting to see tube prices from 1960.

[sluckey]

I have about 30 never used, original box, NOS MILSPEC 12BY7s. 16 of them are GE brand special matched pairs for Tektronix. All of these were spare stock at an old FAA radar station I worked at. Need some?

[DummyLoad]

So with those thoughts in mind, what rare virtues of the 12BY7 would have caused H K, & Carver, to choose them?  Pure snobbery?

looking at the transfer characteristic curves, the 12BY7 seems to have better linearity over other typical "preamp" pentodes. so on technical merits, perhaps for lower distortion?

OTOH, there are triodes that exhibit much better linearity and probably would have been a better choice. they seem to be well regarded in h-fi circles as a citation II can fetch around ~$3 in decent shape. as for the others that also used 12BY7: someone led, others followed? 

--pete

[HotBluePlates]

... what rare virtues of the 12BY7 would have caused H K, & Carver, to choose them?  ...

... the 12BY7 seems to have better linearity over other typical "preamp" pentodes. so on technical merits, perhaps for lower distortion?

OTOH, there are triodes that exhibit much better linearity ...

Harmon Kardon Citation II Service Manual

HK tells you first-thing on Page 2 of the Service Manual: wide bandwidth & low distortion.

Why?  They wrap feedback around the entire amp.  They also talk about using an amount of feedback close to a limit imposed by stability in the amp (which is related to frequency-response roll-offs of items inside the loop).  HK makes a big deal about frequency response wider than the range of hearing, and touting an output transformer with very wide bandwidth.

See, feedback reduces distortion & widens bandwidth.  Except you get "more bang for your feedback buck" if the basic amplifier has "low distortion & wide bandwidth" before the feedback is applied (it gets more-better after applying feedback). 

So it seems HK felt cost/power-hungry-nature were acceptable tradeoffs if very wide bandwidth before (and after) feedback was a key goal.  The point of all that is to keep listening artifacts minimal in the audio range.

The 12BY7 pentode is more-gain than using a triode (which means ore feedback can be used and still drive the output tubes).  Using a wasteful, high-plate-current power tube as a preamp tube enabled low-Ω plate load resistors, moving the treble roll-off frequency higher (as it works against output capacitance, that was also lowered by having a pentode).  All about that wide-bandwidth thing.

... Has anyone used this as a guitar preamp or PI?  What would the pro's and cons be of using this in a geetar amp ...

It's A Sound, but it might not be an Exciting Sound.

Have you ever played a guitar through a very clean hi-fi, or similar?  If you have, you'll notice how "guitar amp clean" usually has some amount of distortion before it's "obviously distorted" that you never noticed until it was compared to "stark clean."  You may also find your guitar's pickups don't sound as good as they do in more-colored, more-distorted amps.

[1blueheron]

Thank you all for the thoughtful and informative replies.  I think I get it.