Just off the top of my head (no time to look at data sheets at the moment):
12BZ7- resembles a pair of 12AX7 in parallel, same mu, 1/2 the plate resistance, twice the transconductance. In practice, I've found their linearity to be terrible.
6U8- Similar to 6AN8, combo pentode/triode, suitable for Dyna-type circuits. Not very linear.
EF86- Excellent pentode, makes an excellent triode, too, suitable for power amp input stages or line amps.
0A2- Voltage regulator tube, looks nicer than a Zener, but touchier and much worse performing.
12BZ7- resembles a pair of 12AX7 in parallel, same mu, 1/2 the plate resistance, twice the transconductance. In practice, I've found their linearity to be terrible.
6U8- Similar to 6AN8, combo pentode/triode, suitable for Dyna-type circuits. Not very linear.
EF86- Excellent pentode, makes an excellent triode, too, suitable for power amp input stages or line amps.
0A2- Voltage regulator tube, looks nicer than a Zener, but touchier and much worse performing.
Hi there, you will find data on these tubes at http://tdsl.duncanamps.com/tubesearch.php
OA2 and OG3 are voltage regulators (screen grid supply maybe?)
EL86 and 6cl6 are useful output pentodes
6BH6 and EF800 are small pentodes which might be useable triode strapped
Depending on the quantities you have of each, a 6cl6 pp (2 per channel) amp might sound quite nice
OA2 and OG3 are voltage regulators (screen grid supply maybe?)
EL86 and 6cl6 are useful output pentodes
6BH6 and EF800 are small pentodes which might be useable triode strapped
Depending on the quantities you have of each, a 6cl6 pp (2 per channel) amp might sound quite nice
trancy said:
Hmm, signal pentode, no? Probably a lot like 6AU6. Doesn't get much audio use, but pentodes tend to be irrationally shunned anyway.
Dual triode maybe like 12AT7?
Hmm come to think of it, I drew a circuit using it, and I think BH too? Lemme see here...
http://webpages.charter.net/dawill/tmoranwms/Circuits/Stereo_6L6_PP.gif
Yep!
Good driver if you need to push something stiff, like oh I don't know, a few hundred watts triode output. Or just to make things "look right" on a few hundred watt pentode amp. Otherwise, looks good pentode (nice Gm) and triode, use at will. Kinda low on plate dissipation and current capacity so not the best output tube - hey, it's a video amp after all, made for chunky resistance coupled circuits!
Or if you have an interest in radio, it'll do HF quite nicely.
I used one in Frankenhouse (check my sig). Sounds just fine!
Beware worn out types- being a TV tube it has a higher chance of being not only used but really hell *** used. I've had a few 6GH8s (similar) that crackled and hissed at me!
Dual double 12AX7 as SY said, but I don't know what his experience is coming from. The curves look sexy. Would be good for say, 6L6 PP UL class A driven by one LTP stage.
I've done a little graphing with this too, might be neat to make a 5 to 10W PP amp with a pair, just because it's metal and different. Otherwise, being a video amp, the same goes as for 6CL6, except less Gm and perveance (leading to higher saturation voltage).
150V, 5-35mA and 85V, 1-10mA respectively (I've heard 1-5mA quoted for 0G3 also). 0A2 and its cousins are best for regulating low current, or low current change, supplies, typically medium voltage screens (150-250V) or noise-sensitive amplifiers (particularly instrumentation: note a mere phono amp is suspect if it needs more than RC filtering!).
The 0G3 is used exclusively as a voltage reference, particularly in regulated supplies where other tubes take over the current-carrying capacity and, by amplifying error, gain much better performance as well as efficiency. Besides that, you could use it for regulating screens on a 6AU6, or something...but geez...why bother...
I don't 'do' British toobs, so I'll leave these to other posters.
ALWAYS!
Tim
6BQ7A: Dual triode maybe like 12AT7?
Not exactly, in terms of amp factor, it comes between a 12AU7A and a 12AT7. The 6BQ7A is intended for use as a small signal, VHF amp -- usually cascoded (spec sheet gives characteristic for this configuration). It includes an internal shield for RF isolation of each section. However, the characteristic for one section looks quite linear, and it'll probably do good in audio apps. Since these were frequently used in TV front ends, there shouldn't be any problem so far as availability is concerned.
Not exactly, in terms of amp factor, it comes between a 12AU7A and a 12AT7. The 6BQ7A is intended for use as a small signal, VHF amp -- usually cascoded (spec sheet gives characteristic for this configuration). It includes an internal shield for RF isolation of each section. However, the characteristic for one section looks quite linear, and it'll probably do good in audio apps. Since these were frequently used in TV front ends, there shouldn't be any problem so far as availability is concerned.
Tube is tube. Anything can be used in audio.
But "The Tube Sound" is based on tubes that work well in audio: designed-for audio like 300B 2A3 6L6, or that happen-to-be good for audio like 6SN7, 6AU6, 6U8, and the TV V-sweep tubes, or for some uses 12AX7, 12FX5....
The TV tubes differ from the standard audio tubes in giving more gain and better performance/power at the expense of linearity and simple biasing. Like the difference in engines: truck, sedan, race-car. TV set circuits mostly run flat-out full-power. A race engine will make big power but won't go slow smoothly. TV sets had short warranties, so the tubes are not built like trucks that cost more but can run big power for many-many miles.
> 6BH6 is used in QUAD FM1 tuner.
That's, I think, the only "special" tube here. All the others were MASS produced for TV sets. I think they are all available in as-new condition for $4-$8, or $1/free used from someone's stash-pile. The 6BH6 isn't itself very special, just better than average, which means it was used in very few top-grade boxes and is not widely available today. I think the best use for a 6BH6 is to sell it to someone with one of the super FM tuners for a spare. However, TheTubeStore.com and TubeDepot.com only want $7-$9 for 6BH6; it isn't "valuable".
> Location: Slovenia
Ah, that makes a difference. I can get $5 tubes from Canada in a couple of days, but outside the USA, Canada, and west Europe it may be harder to get tubes.
> 6BQ7A - Dual triode maybe like 12AT7?
Actually... that 6DJ8 is a 6BQ7 with decades of minor improvement.
There is a whole series of these, what I call "TV Tuner Tubes". They were invented for RF stage use in VHF radio receivers. That meant radar and some special military radios, but the HUGE market turned out to be TV sets. Gotta have our Milton Berle! 12AT7 works for pulling TV out of the air, but gets weak at the top of the dial. The high Mu is not an advantage at higher VHF frequencies, and a higher Gm is an advantage, even if higher current is needed to get the high Gm. Hence 6BQ7. That was enough when Uncle Milty was the only thing on, but as more stations came on the air and as people moved out of the city we needed better performance (at least in the high-price TVs; 6BQ7 or an odd-volt odd-pinout equivalent stayed common in low-price TV sets). Wide-range TV inputs need variable-gain, but don't need low 2nd harmonic: 6ES8 seems to be the snazziest TV tuner tube. Some designs did not emphasize widest possible range of input signal (or got it other ways) and the 6DJ8 is a lot like a 6ES8 without the variable-gain nonlinearity.
The performance advantages of 6ES8/6DJ8 over 6BQ7 show clearly at high current, up around 10mA. We do a lot of 6DJ8 in audio but usually at 1mA to 3mA. The differences are much less down there.
I don't think a 6BQ7 is a drop-in in all 6DJ8 plans. But it will probably "work" in most. If you trim the cathode resistor to give the same plate voltage as the 6DJ8 version, 6BQ7 may work "about the same" as 6DJ8.
Yes, it is listed for small-signal work, but RF front-ends have to stay linear on strong signals, while fighting large losses in tuned circuits. They are small power tubes. 30 years ago I found some surplus 20K:200 servo transformers several-watt, on chassis with 9-pin ceramic sockets. I wired-up a push-pull 6BQ7 headphone amp, and it really kicked the old Koss stereophones. Working the tuner-tube to its voltage and dissipation limits, almost a watt is possible, though matching difficulties will usually lead to less power. I didn't used the 6BQ7 much because I was working it so hot the life was short, under 100 hours. There was another (now forgotten) tube that was rated a little higher. It didn't actually last longer, the over-rating was BS, but it was (at the time) cheap enough to replace every few months.
In TV RF duty, the RF tube will work at full power ONLY on a no-signal or super-weak signal condition. Any watchable signal brings up the AGC, reduces RF stage current. Since people mostly watched watchable signals, TV tuner tubes did not really sit at maximum dissipation very many hours per decade, and I don't think they really want to be worked at full rating. Even just watching moderately weak signals, I remember the tuner-tubes dying almost as often as the over-worked H-sweep tube.
Used TV tubes are dubious. The factory set was super low-bid junk. Replacements were better but rarely the best. Some stages in a TV are worked as close to the ratings as they can get away with and not have sets dying in the showroom. A tube that "came out of a TV" (and that line-up sure smells like a TV) may be very tired. Since these are all five-buck tubes, I would not waste a lot of time fooling with them. You can easily waste $5 of time and aggravation wondering why an old sick tube isn't working like the book says.
But "The Tube Sound" is based on tubes that work well in audio: designed-for audio like 300B 2A3 6L6, or that happen-to-be good for audio like 6SN7, 6AU6, 6U8, and the TV V-sweep tubes, or for some uses 12AX7, 12FX5....
The TV tubes differ from the standard audio tubes in giving more gain and better performance/power at the expense of linearity and simple biasing. Like the difference in engines: truck, sedan, race-car. TV set circuits mostly run flat-out full-power. A race engine will make big power but won't go slow smoothly. TV sets had short warranties, so the tubes are not built like trucks that cost more but can run big power for many-many miles.
> 6BH6 is used in QUAD FM1 tuner.
That's, I think, the only "special" tube here. All the others were MASS produced for TV sets. I think they are all available in as-new condition for $4-$8, or $1/free used from someone's stash-pile. The 6BH6 isn't itself very special, just better than average, which means it was used in very few top-grade boxes and is not widely available today. I think the best use for a 6BH6 is to sell it to someone with one of the super FM tuners for a spare. However, TheTubeStore.com and TubeDepot.com only want $7-$9 for 6BH6; it isn't "valuable".
> Location: Slovenia
Ah, that makes a difference. I can get $5 tubes from Canada in a couple of days, but outside the USA, Canada, and west Europe it may be harder to get tubes.
> 6BQ7A - Dual triode maybe like 12AT7?
Actually... that 6DJ8 is a 6BQ7 with decades of minor improvement.
There is a whole series of these, what I call "TV Tuner Tubes". They were invented for RF stage use in VHF radio receivers. That meant radar and some special military radios, but the HUGE market turned out to be TV sets. Gotta have our Milton Berle! 12AT7 works for pulling TV out of the air, but gets weak at the top of the dial. The high Mu is not an advantage at higher VHF frequencies, and a higher Gm is an advantage, even if higher current is needed to get the high Gm. Hence 6BQ7. That was enough when Uncle Milty was the only thing on, but as more stations came on the air and as people moved out of the city we needed better performance (at least in the high-price TVs; 6BQ7 or an odd-volt odd-pinout equivalent stayed common in low-price TV sets). Wide-range TV inputs need variable-gain, but don't need low 2nd harmonic: 6ES8 seems to be the snazziest TV tuner tube. Some designs did not emphasize widest possible range of input signal (or got it other ways) and the 6DJ8 is a lot like a 6ES8 without the variable-gain nonlinearity.
The performance advantages of 6ES8/6DJ8 over 6BQ7 show clearly at high current, up around 10mA. We do a lot of 6DJ8 in audio but usually at 1mA to 3mA. The differences are much less down there.
I don't think a 6BQ7 is a drop-in in all 6DJ8 plans. But it will probably "work" in most. If you trim the cathode resistor to give the same plate voltage as the 6DJ8 version, 6BQ7 may work "about the same" as 6DJ8.
Yes, it is listed for small-signal work, but RF front-ends have to stay linear on strong signals, while fighting large losses in tuned circuits. They are small power tubes. 30 years ago I found some surplus 20K:200 servo transformers several-watt, on chassis with 9-pin ceramic sockets. I wired-up a push-pull 6BQ7 headphone amp, and it really kicked the old Koss stereophones. Working the tuner-tube to its voltage and dissipation limits, almost a watt is possible, though matching difficulties will usually lead to less power. I didn't used the 6BQ7 much because I was working it so hot the life was short, under 100 hours. There was another (now forgotten) tube that was rated a little higher. It didn't actually last longer, the over-rating was BS, but it was (at the time) cheap enough to replace every few months.
In TV RF duty, the RF tube will work at full power ONLY on a no-signal or super-weak signal condition. Any watchable signal brings up the AGC, reduces RF stage current. Since people mostly watched watchable signals, TV tuner tubes did not really sit at maximum dissipation very many hours per decade, and I don't think they really want to be worked at full rating. Even just watching moderately weak signals, I remember the tuner-tubes dying almost as often as the over-worked H-sweep tube.
Used TV tubes are dubious. The factory set was super low-bid junk. Replacements were better but rarely the best. Some stages in a TV are worked as close to the ratings as they can get away with and not have sets dying in the showroom. A tube that "came out of a TV" (and that line-up sure smells like a TV) may be very tired. Since these are all five-buck tubes, I would not waste a lot of time fooling with them. You can easily waste $5 of time and aggravation wondering why an old sick tube isn't working like the book says.
tubes
I agree with one of the previous posts that some TV tubes are junk. I also have found that some are very good. I have a collection of over 100000 "useless TV tubes" and I have been testing them to find out what works for audio. I have been analyzing mostly tubes that I have large quantities of, to find out what they are good for. I have found that most tubes designed for linear amplification can be used for audio. it may take some experimentation to find its "sweet spot". There are tubes that were never designed for linear operation. These generally do not make good audio amplifiers. Examples of this are "remote cutoff pentodes" that were designed for gain controlled amplifiers. Beam switching tubes, sync separators, variable reactance tubes and the like are useful only in very specialized situations for audio.
Most of the American type numbers that you mentioned are generally acceptable for some audio use. I am not familiar with all of the European numbers. As far as the quality of TV tubes, some are excellent and some are junk, most are in between. The brand name tubes made in the USA or Canada are generally pretty good. Their quallity did start to decline in the later years of tube TV. Since all of the type numbers start with 6, I can assume that these are from 1965 or before. The short life time mentioned in the previous post was often related to the TV manufacturer operating the tubes at or above their maximum ratings. Some TV's were notorious tube eaters. Anyone remember a Muntz TV, half of the tubes operated with a dull red glow to the plates.
I have audio experience with the following types that you mention:
6BH6 this was designed as an IF amplifier in an FM radio or TV. It makes a good high gain amplifier connected as a pentode (some are quite microphonic) and an excellent medium gain amplifier connected as a triode. Connect both G2 and G3 to the plate. It likes a plate supply of 200 - 250 volts with 125 - 150 volts on the plate and about 5 mA of current.
6BQ7 A TV tuner tube. This makes a good voltage amplifier tube. Also useful for a phase splitter. I have also used this one for a differential amplifier (long tailled pair) but I don't remember the operating conditions.
6CL6 A video amplifier tube that was also used as an RF driver tube in radio transmitters. I have a lot of these so I have tested some of these under "severe conditions". A push pull pentode connected pair produces about 10 watts without abusing them (280 volts B+). 15 watts is possible by operating at 330 volts. No red glow or other bad effects were noted. Turning the power supply up to 360 volts produced almost 20 watts, the tubes were beginning to glow, and the tube current was unstable and tended to run away. These can also be triode connected. Connect G2 to the plate and G3 to the cathode. As a single ended amp you will get about 3 watts in pentode mode and just over a watt in triode mode.
6U8 This tube was common in TV sets from the 1950's and 60's. The triode makes a decent voltage amplifier or phase splitter. The pentode can be used as a voltage amplifier, but it is not terribly linear at large signals. I have used the pentode for the input stage and the triode as a phase splitter in a 6L6 push pull amp before. There are several similar tubes with the same pinout, so that if you use this tube, you can experiment with others later. I finally settled on a 6BL8 in the push pull amp, but it was only a small improvement ofer the 6U8.
6AG7 Another video amp tube. I also have lots of these but I have not spent much time with them. My first guess is that I like the 6CL6 better.
EL86- I looked this one up and found that it is equavilent to an American type 6CW5. I have lots of these too, and it is the best output tube on your list. It was used in the Hewlett Packard 200CD audio oscillator. It is similar to the EL84 - 6BQ5 except that it requires less plate voltage. Screen voltage is 200 MAX. Since this is a low impedance tube it wants a lower than usual load impedance, about 3000 ohms for a push pull amplifier. 20 watts from a push pull pair with 225 - 250 volts of B+ is possible. I have not tried it in single ended configuration, but it should work well.
Since the history of these tubes is unknown and they could have lived a long hard life in a TV somewhere, it would be a good idea to test them somehow before investing much time with them. Se if you can find someone with a good tube tester who will test them for you.
If I had to build an amplifier using only these tubes, I would use the 6BQ7's for voltage amplifiers (and a phase splitter if P-P) and the EL86's or 6CL6's for output depending on which type I had the most good ones of.
I agree with one of the previous posts that some TV tubes are junk. I also have found that some are very good. I have a collection of over 100000 "useless TV tubes" and I have been testing them to find out what works for audio. I have been analyzing mostly tubes that I have large quantities of, to find out what they are good for. I have found that most tubes designed for linear amplification can be used for audio. it may take some experimentation to find its "sweet spot". There are tubes that were never designed for linear operation. These generally do not make good audio amplifiers. Examples of this are "remote cutoff pentodes" that were designed for gain controlled amplifiers. Beam switching tubes, sync separators, variable reactance tubes and the like are useful only in very specialized situations for audio.
Most of the American type numbers that you mentioned are generally acceptable for some audio use. I am not familiar with all of the European numbers. As far as the quality of TV tubes, some are excellent and some are junk, most are in between. The brand name tubes made in the USA or Canada are generally pretty good. Their quallity did start to decline in the later years of tube TV. Since all of the type numbers start with 6, I can assume that these are from 1965 or before. The short life time mentioned in the previous post was often related to the TV manufacturer operating the tubes at or above their maximum ratings. Some TV's were notorious tube eaters. Anyone remember a Muntz TV, half of the tubes operated with a dull red glow to the plates.
I have audio experience with the following types that you mention:
6BH6 this was designed as an IF amplifier in an FM radio or TV. It makes a good high gain amplifier connected as a pentode (some are quite microphonic) and an excellent medium gain amplifier connected as a triode. Connect both G2 and G3 to the plate. It likes a plate supply of 200 - 250 volts with 125 - 150 volts on the plate and about 5 mA of current.
6BQ7 A TV tuner tube. This makes a good voltage amplifier tube. Also useful for a phase splitter. I have also used this one for a differential amplifier (long tailled pair) but I don't remember the operating conditions.
6CL6 A video amplifier tube that was also used as an RF driver tube in radio transmitters. I have a lot of these so I have tested some of these under "severe conditions". A push pull pentode connected pair produces about 10 watts without abusing them (280 volts B+). 15 watts is possible by operating at 330 volts. No red glow or other bad effects were noted. Turning the power supply up to 360 volts produced almost 20 watts, the tubes were beginning to glow, and the tube current was unstable and tended to run away. These can also be triode connected. Connect G2 to the plate and G3 to the cathode. As a single ended amp you will get about 3 watts in pentode mode and just over a watt in triode mode.
6U8 This tube was common in TV sets from the 1950's and 60's. The triode makes a decent voltage amplifier or phase splitter. The pentode can be used as a voltage amplifier, but it is not terribly linear at large signals. I have used the pentode for the input stage and the triode as a phase splitter in a 6L6 push pull amp before. There are several similar tubes with the same pinout, so that if you use this tube, you can experiment with others later. I finally settled on a 6BL8 in the push pull amp, but it was only a small improvement ofer the 6U8.
6AG7 Another video amp tube. I also have lots of these but I have not spent much time with them. My first guess is that I like the 6CL6 better.
EL86- I looked this one up and found that it is equavilent to an American type 6CW5. I have lots of these too, and it is the best output tube on your list. It was used in the Hewlett Packard 200CD audio oscillator. It is similar to the EL84 - 6BQ5 except that it requires less plate voltage. Screen voltage is 200 MAX. Since this is a low impedance tube it wants a lower than usual load impedance, about 3000 ohms for a push pull amplifier. 20 watts from a push pull pair with 225 - 250 volts of B+ is possible. I have not tried it in single ended configuration, but it should work well.
Since the history of these tubes is unknown and they could have lived a long hard life in a TV somewhere, it would be a good idea to test them somehow before investing much time with them. Se if you can find someone with a good tube tester who will test them for you.
If I had to build an amplifier using only these tubes, I would use the 6BQ7's for voltage amplifiers (and a phase splitter if P-P) and the EL86's or 6CL6's for output depending on which type I had the most good ones of.
> type 6CW5. ... best output tube on your list. ... similar to the EL84 - 6BQ5 except that it requires less plate voltage.
I never noticed that. The heater is the same. Pd is the same, voltage a bit lower, current a bit higher. The biggest real difference is that 6BQ5 amplification factor is 17, 6CW6 amp factor is 8. You could simplify and say the 6CW5 is a low-Mu 6BQ5.
For unlimited supply voltage and load impedance, 6BQ5 has better power sensitivity. For limited supply or low-Z loads, the 6CW5 does the same things if you run about 70% of voltage and about half the load resistance. In hot-chassis TV with 150V B+, the 6CW5 does more work than 6BQ5 would.
6BQ5 versus 6CW5 curves
TungSol didn't give triode curves for 6CW5, but Rp at nice current will be around 800 ohms, a good value for damping iron-core windings.
I never noticed that. The heater is the same. Pd is the same, voltage a bit lower, current a bit higher. The biggest real difference is that 6BQ5 amplification factor is 17, 6CW6 amp factor is 8. You could simplify and say the 6CW5 is a low-Mu 6BQ5.
For unlimited supply voltage and load impedance, 6BQ5 has better power sensitivity. For limited supply or low-Z loads, the 6CW5 does the same things if you run about 70% of voltage and about half the load resistance. In hot-chassis TV with 150V B+, the 6CW5 does more work than 6BQ5 would.
6BQ5 versus 6CW5 curves
TungSol didn't give triode curves for 6CW5, but Rp at nice current will be around 800 ohms, a good value for damping iron-core windings.
PRR said:
Yeah I sat there scratching my head remembering my schematic, pulled up the data sheet and I thought, hmm, mini-6DJ8. It looks more like 12AU7 (the tube I compare *everything* with
) than AT, but VHF service suggests otherwise.Hey, if it works, it works. With used tubes, it's a good idea to have a tube tester on hand, and a few spares of the type - not a problem with such cheap tubes - when checking out the amplifier. Like I said, some suck, others work just fine. In fact...lemme see here... ha! I have a 6GH8A in Frankenhouse right now, not the 6U8A it was designed for.
Works just fine!Tim
> mini-6DJ8. It looks more like 12AU7 (the tube I compare *everything* with ) than AT, but VHF service suggests otherwise.
12AU7 is a good reference tube. It is the central General Purpose Triode, the grandchild of the '00 and '01 types. Several mA of current, and Rp not too high, 10K or so.
If Rp is higher than that, you can't get good response out of a coupling transformer.
Since a small cathode has a limited Gm, and Rp is Mu/Gm, a limited Gm and a limit on useful Rp sets a limit on Mu. The '01 amplifier uses Mu=8, so its 1/Gm= 1.2K times its Mu gives Rp of 10K.
With better cathode-stuff and better grid windings, Gm was larger so a higher Mu was useful as a General Purpose Triode (i.e., one that could drive a coupling transformer acceptably). This leads to the 12AU7.
If you leave out the bit about coupling transformers and use R-C coupling into grids, a higher Rp is workable. This leads to many hi-Mu triodes, most famously the 12AX7. It gives more voltage gain, but less peak output current.
But as you move up in frequency, universal capacitance forces the effective load impedance down. Even if you can resonate it away, tank losses limit the working impedance. Up through the FM band, you can work 10K-20K, so the 12AT7 gives good gain and good output power. As you go to the top of the TV band, impedances drop to a few K. A higher Mu gives less peak current which reduces your signal before distortion (typically getting two stations mixed against each other). So you drop the Mu from ~60 towards ~30. This is the 6BQ7, and nearly all the VHF input tubes are in this ballpark.
BTW: the second "miniature" tubes were VHF twin triodes, adaptable to general purposes. They were not thinking of 12 Watt plate dissipations when they blew the mini bottle! They seemed to think that a couple watts was all the package was good for, and that they had enough use of small few-Watt bottles to recover the investment in new base-molding machinery.
6DJ8 adds more heater power and improved grid winding techniques and can hold a low Rp at Mu~=30. The improvements give ample peak current and high Gm for low noise.
There are oddballs. There is a ceramic triode with Gm around 50,000uMho and Mu around 100. The Rp is like 2K, low enough for the microwave system it was made for, and the super-Gm gives low-low noise for good point-to-point microwave signal at less system cost than building bigger transmitters. Makes a cute mike-amp, but today (if you can find one) it costs more than a six-pack of 6BQ7 or a dozen 12AX7 which would give similar performance.
12AU7 is a good reference tube. It is the central General Purpose Triode, the grandchild of the '00 and '01 types. Several mA of current, and Rp not too high, 10K or so.
If Rp is higher than that, you can't get good response out of a coupling transformer.
Since a small cathode has a limited Gm, and Rp is Mu/Gm, a limited Gm and a limit on useful Rp sets a limit on Mu. The '01 amplifier uses Mu=8, so its 1/Gm= 1.2K times its Mu gives Rp of 10K.
With better cathode-stuff and better grid windings, Gm was larger so a higher Mu was useful as a General Purpose Triode (i.e., one that could drive a coupling transformer acceptably). This leads to the 12AU7.
If you leave out the bit about coupling transformers and use R-C coupling into grids, a higher Rp is workable. This leads to many hi-Mu triodes, most famously the 12AX7. It gives more voltage gain, but less peak output current.
But as you move up in frequency, universal capacitance forces the effective load impedance down. Even if you can resonate it away, tank losses limit the working impedance. Up through the FM band, you can work 10K-20K, so the 12AT7 gives good gain and good output power. As you go to the top of the TV band, impedances drop to a few K. A higher Mu gives less peak current which reduces your signal before distortion (typically getting two stations mixed against each other). So you drop the Mu from ~60 towards ~30. This is the 6BQ7, and nearly all the VHF input tubes are in this ballpark.
BTW: the second "miniature" tubes were VHF twin triodes, adaptable to general purposes. They were not thinking of 12 Watt plate dissipations when they blew the mini bottle! They seemed to think that a couple watts was all the package was good for, and that they had enough use of small few-Watt bottles to recover the investment in new base-molding machinery.
6DJ8 adds more heater power and improved grid winding techniques and can hold a low Rp at Mu~=30. The improvements give ample peak current and high Gm for low noise.
There are oddballs. There is a ceramic triode with Gm around 50,000uMho and Mu around 100. The Rp is like 2K, low enough for the microwave system it was made for, and the super-Gm gives low-low noise for good point-to-point microwave signal at less system cost than building bigger transmitters. Makes a cute mike-amp, but today (if you can find one) it costs more than a six-pack of 6BQ7 or a dozen 12AX7 which would give similar performance.
6bq5 (el84) vs 6CW5 (el86)
I forgot to mention that simply plugging a 6CW5 into an amp made for a 6BQ5 is really not a good idea. I got a bunch of 6CW5's from dismantling a few hundred pounds of HP oscillators. These looked like 6BQ5's so without looking them up, I plugged a pair into a home made amp. Bad idea, no matter where I set the bias the tubes glowed. Before I could shut it off, one of the tubes arced over, lots of sparks inside the tube, loud zapping sounds in the speaker. Fortunately, nothing was permanently damaged. Even the tube survived. Operating this amp on a variable power supply revealed that it works ok with a plate (and screen) voltage of 220. Any more than that yielded a tendancy toward bias runaway. Yes this happens with tubes too. With the 6.6k transformer made for the 6BQ5's, power was only 10 watts. Attaching the 8 ohm load to the 16 ohm tap gave 20 watts. It sounded good too, but slightly different than the 6BQ5's.
I forgot to mention that simply plugging a 6CW5 into an amp made for a 6BQ5 is really not a good idea. I got a bunch of 6CW5's from dismantling a few hundred pounds of HP oscillators. These looked like 6BQ5's so without looking them up, I plugged a pair into a home made amp. Bad idea, no matter where I set the bias the tubes glowed. Before I could shut it off, one of the tubes arced over, lots of sparks inside the tube, loud zapping sounds in the speaker. Fortunately, nothing was permanently damaged. Even the tube survived. Operating this amp on a variable power supply revealed that it works ok with a plate (and screen) voltage of 220. Any more than that yielded a tendancy toward bias runaway. Yes this happens with tubes too. With the 6.6k transformer made for the 6BQ5's, power was only 10 watts. Attaching the 8 ohm load to the 16 ohm tap gave 20 watts. It sounded good too, but slightly different than the 6BQ5's.
can i add these tubes, pfc802, 8br11? 8fq7?
got these tubes also, pcf802 is a triode-pentode, 8br11 is a compactron dual pentode, 8fq7 is like 6fq7/8cg7.
i am thinking line amps, triode input with pentode constant current source load/cathode followers.
what do you think?
any schematics i can use?
thanks a lot!
got these tubes also, pcf802 is a triode-pentode, 8br11 is a compactron dual pentode, 8fq7 is like 6fq7/8cg7.
i am thinking line amps, triode input with pentode constant current source load/cathode followers.
what do you think?
any schematics i can use?
thanks a lot!
Sure. Can do a lot with those. If the compactron is signal pentode, you could make, oh heck let's be creative and make a LTP pentode stage, enough gain in *one stage* that it'll go from line level to 6L6 PP drive plus enough spare for the usual 20dB gNFB. If it's a beefier dual pentode, I smell a one-tube PP output.
I'm guessing PCF802 is kinda like 6GH8/6U8 small signal triode-pentode deal? Those are good for massive gain + cathodyne like in my Frankenhouse, or say for phono, massive gain + CF output. (Some people apparently don't like "noisy" pentodes for phono.. I don't know and I don't care - I have far better signal sources than some cruddy old spinning plastic!)
And FQ/CG is always fun, it's like 6SN7, so line stage, driving 6L6 SET, etc. Nice thing about the 8xx is it's close enough to run on 6.3V (which is usually higher, about 7V, anyway), with extended life to boot.
Tim
If it's a beefier dual pentode, I smell a one-tube PP output.I'm guessing PCF802 is kinda like 6GH8/6U8 small signal triode-pentode deal? Those are good for massive gain + cathodyne like in my Frankenhouse, or say for phono, massive gain + CF output. (Some people apparently don't like "noisy" pentodes for phono.. I don't know and I don't care - I have far better signal sources than some cruddy old spinning plastic!
)And FQ/CG is always fun, it's like 6SN7, so line stage, driving 6L6 SET, etc. Nice thing about the 8xx is it's close enough to run on 6.3V (which is usually higher, about 7V, anyway), with extended life to boot.
Tim
More tubes
I agree with Sch3mat1c that the 8FQ7 will usually work in the place of a 6CG7/6FQ7. I have a large box full of these also, and I have tried several of them in my 300Beast amplifier, which uses 4 6CG7's as SRPP drivers. Most of them worked just fine and sounded good. A few worked, but sounded lifeless, and a couple distorted badly.
If you were designing fron scratch, use a full wave schottky bridge and a large cap on the 6.3 volt winding. Depending upon the transformer you will get close to 8 volts. If you don't have a suitable schematic, use a circuit for a 6SN7. Some resistor value tweaks may be needed. You could use these to drive some 8BQ5's, they are cheap alternatives to 6BQ5/EL84.
I am not familiar with the other two numbers, and they are not in any of my tube manuals.
I agree with Sch3mat1c that the 8FQ7 will usually work in the place of a 6CG7/6FQ7. I have a large box full of these also, and I have tried several of them in my 300Beast amplifier, which uses 4 6CG7's as SRPP drivers. Most of them worked just fine and sounded good. A few worked, but sounded lifeless, and a couple distorted badly.
If you were designing fron scratch, use a full wave schottky bridge and a large cap on the 6.3 volt winding. Depending upon the transformer you will get close to 8 volts. If you don't have a suitable schematic, use a circuit for a 6SN7. Some resistor value tweaks may be needed. You could use these to drive some 8BQ5's, they are cheap alternatives to 6BQ5/EL84.
I am not familiar with the other two numbers, and they are not in any of my tube manuals.
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