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    Building, troubleshooting and testing of these amplifiers should only be
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Those Magnificent Television Tubes

'' 12BH7 and 6CW7 etc. are actually designated for TV-sets''

I am just a musician, but as far as I understand, the design was for TV sets, but for audio use you should get the ones made for audio, such as JJ Tesla, etc.
I mention this because I remember Dan Torres wrote in his book : 'If you need 12bh7, make sure you get one made for audio'.

I would assume he's correct....for example, I also play classical guitar, and many believe that classical guitar strings are the same as fishing lines, but they are not, although they look the same.

Just my worthless 1 cent :)
 
I hope you have more than just that one cent. Takes a lot of them to buy an “audio” tube.

And they (the sellers) know it. You may indeed find a certain brand of tube whose particular characteristics you just like (as say, a guitar amplifier) and that would be a valid reason to use them. But it is just as likely to happen on a tube originally intended and sold for TV service. Why? None of them are designed to do anything special with the sound. They are designed to transport electrons, meeting a set of objective specifications per a given type number (that have nothing to do with audio). Don’t get caught up in marketing hype. Try some, listen for yourself, then decide where to spend your money.

A couple other things about “TV” tubes. There are a good many that nominally have identical characteristics to say a 6L6, that may hook up differently or have different ratings. Many more will work in similar circuits with just a bit of modification. Some require ground-up redesign, and perhaps give better performance. Stuff like 12BH7 have “twins” too - the 6GU7. The 50C5 and 6W6 run the same withing their ratings (and heater voltages). This just opens up a LOT of possibilities for those of us who actually build amplifiers. And you don’t shell out inflated prices to do it.


Of course it also occurred to me you may have been joking.
 
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The devil is in the details as they say. Knowing the exact application can help to pick out the useful ones. For example a horizontal sweep tube in a TV is essentially a low distortion audio amplifier. Thus 6EM7 and similar are often used for very nice audio power amplifiers. Similarly Vertical sweep tubes are high power amplifiers that can be used quite well in applications where something like a EL34 would be used.

Some TV and radio tubes are used for very high frequency use but with care work great at audio frequencies (they aren't even breaking a sweat in such applications). Just be on the lookout for remote cutoff tubes which are intentionally non-linear for use in automatic gain control and the like. However even these can be useful for limiters or even for very small signals like phono or mic preamp inputs. Over very small grid voltage swing they may be just fine.
 
'' The devil is in the details as they say. Knowing the exact application can help to pick out the useful ones.''. Yeah, which is what Torres probably meant. Or even I :). Again, he wasn't marketing any brands or tubes, in his book (except historically famous brands such as RCA, etc). Made for audio could also mean 'selected', as far as I know that is what audio specialists do for microphonic tests, for example, but I assume there's a lot more than that. Then again, I am just a musician. But what Torres probably meant was: ''don't just plug any 12bh7 in your amp, get a good one from a reputable brand or shop that sells tubes for amps, not televisions.''. :)
 
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but anyways: unless I am mistaken, JJ Tesla is a tube manufacturer, right? And it sells 12bh7s. It would be safe therefore to assume that the tube is made for audio, not just selected, since JJ makes its own tubes. The basic construction might be the same, but as you say, the devil is in the detail. It would be reasonable to assume that JJ takes extra steps to make the tube sound as good as possible, not merely work into a circuit where audio is irrelevant or not top priority.
 
The targeted "audio" useability 60-70 years ago was clearly practical-economical question, not audiophile oriented. For example: "audio" power pentodes and beam tetrodes must be equal U plate and U g2 (simpler power supply) - see EL84, EL34. The little tubes must be high gain (ECC83), because fewer tubes enough, and must be low hum level at AC heating (for example EF86), because the rectification and filtering is expensive. Low microphonic level (tubes and speaker built in common box). The low microphonie and hum are good things for audiophiles too, but not made directe for us, only coincidence...
 
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I am just a musician, but as far as I understand, the design was for TV sets, but for audio use you should get the ones made for audio, such as JJ Tesla, etc.
I mention this because I remember Dan Torres wrote in his book : 'If you need 12bh7, make sure you get one made for audio'.
Just to say it in a few words: That Mr. Torres told you snake oil BS. The only purpose in advertising tubes for »audio« is to draw stupid money from the stupids' wallets. Rest has been said by wg.ski.
The devil is in the details as they say. Knowing the exact application can help to pick out the useful ones. For example a horizontal sweep tube in a TV is essentially a low distortion audio amplifier. Thus 6EM7 and similar are often used for very nice audio power amplifiers. Similarly Vertical sweep tubes are high power amplifiers that can be used quite well in applications where something like a EL34 would be used.
I guess it's just vice versa. Vertical deflection amplifiers need to be as linear as possible, while horizontal sweep power tubes more or less act as switches.

Best regards!
 
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I think it is valid to say that not all tubes with a particular designation could be useful for all purposes. Having measured a box of 100 12AT7WAs that I was hoping to sell, using a uTracer so that I could match the curves over a range of operating characteristics for both halves of the tube, there are not so many that have similar curves on both sides - maybe 10% that have a close match.

Depending on how those tubes will be used, that may or may not be important. If you use two sides of an 12AT7 in a headphone amplifier, then you want both sides to be close. If you use one half for gain, and the other half as a phase splitter, as is common in PP amps, it does not make a lot of difference.

Some tube testers have a small amplifier so you can listen to the tube under test wit headphones. Some are just noisier than others; some are more susceptible to shock than others. Different brands can have different variations in manufacture. That can dictate where you use the tube, or even if the tube is useable at all.

It is horses for courses.
 
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I'd add that it is also important to make the design as much tolerant as possible to tube mismatch. About microphonic tubes, not sure how important it is for a power amplifier. For phono stages, yes, it is, also for guitar amps, but then a decent 12AX7 could probably handle those tasks.
 
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I fully concur. A clever amplifier design takes care of tube mismatch, tube wear etc. right until the end of tube life, due to carefully implemented NFB. In another current thread we - hopefully - have learned that for example in a LTP phase inverter with a CCS as the tail even severe tube mismatch doesn't hurt etc.

Best regards!
 
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1961 Philips data sheet for EL500 on Frank’s has curves, and over a wider range of Vg2. Gm is a bit lower than HB5, but not game changer lower. I got a bunch of 6GY5s as test mules (6HB5 with anode cap). Haven’t had a chance to play with them yet. The guts inside look identical to 6/21LG6, but they’re running 75% of the heater power. I’m hoping they’ll still take 28W without red plating. I normally only run 120-150V on g2, with Ik peaking at half an amp or a bit less. But on 600V you end up with a lot of plate dissipation.