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#41
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"Arny Krueger" wrote in message
... Slew rate and bandwidth. Those Philbrick and HP tube op-amps really did not have the bandwidth for anything approaching audio. --scott When I was an undergraduate back in the 60s, I did some time *programming* analog computers. My recollection is the Philbrick parts posted at http://www.national.com/rap/vacuumtubes.html were bottom-of-the barrel parts. The sort of vacuum tube op amps I'm thinking of were more on the order of this one: http://ed-thelen.org/dc_amp.gif There's really no excuse for vacuum tube amplifiers to be terribly slow, after all Tektronics made some pretty fast 'scopes out of bottles.... Right, but the Philbrock jobs were optimized for DC and low-frequency operation, not audio. As you say, they were for analog computing, and that usually didn't involve much in the way of high frequencies. Peace, Paul0 |
#42
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Pooh Bear wrote:
You should pop over to s.e.d and see what nonsence he's been posting there ( along with a couple of others ) about decibels. See the thread - 'the truth about decibels'. ....and make up you own mind about who said what. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#43
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"Paul Stamler" wrote in message
"Arny Krueger" wrote in message ... Slew rate and bandwidth. Those Philbrick and HP tube op-amps really did not have the bandwidth for anything approaching audio. --scott When I was an undergraduate back in the 60s, I did some time *programming* analog computers. My recollection is the Philbrick parts posted at http://www.national.com/rap/vacuumtubes.html were bottom-of-the barrel parts. The sort of vacuum tube op amps I'm thinking of were more on the order of this one: http://ed-thelen.org/dc_amp.gif This op amp was part of the analog flight control computer for a Nike system. This was one of those time-critical applications. It looks more like what I was used to seeing in higher-end analog computers (EIA, for example). There's really no excuse for vacuum tube amplifiers to be terribly slow, after all Tektronics made some pretty fast 'scopes out of bottles.... Right, but the Philbrick jobs were optimized for DC and low-frequency operation, not audio. As you say, they were for analog computing, and that usually didn't involve much in the way of high frequencies. The thing is, that speed was often of the essence in analog computing. Many problems they were used for involve optimization of parameters using large sets of systematically-changed variables, intelligent searching techniques, or Monte Carlo techniques. I recall that the op amps in the EIA 680 that I worked with later on, was speced to be something like 1% accurate while reproducing a 10 KHz sine wave at 10 volt p-p. IOW, it was 0.1 dB down at 10 KHz while putting out maximum rated output, so it had something like 50 KHz or better bandwidth. |
#44
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"Mike Rivers" wrote in message
news:znr1120170690k@trad In article writes: There's nothing wacky about local feedback at all. It is relatively simple to implement, and it pretty much works as designed. Oh, OK. I thought you meant the "internal feedback" concept that's going around on rec.audio.tubes. So, why must a triode design inherently have more local feedback than any other? I was wondering about that term "local feedback" too. It sounds like one of those terms coined by the same folks who talk about LDC and SDC. Is it some hip name for not bypassing the cathode resistor? Not bypassing the cathode resistor is one way to get local feedback. Other techniques include a resistor from the plate to the grid usually with a blocking capacitor, and cathode windings on the output transformer. |
#46
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Arny Krueger wrote:
The sort of vacuum tube op amps I'm thinking of were more on the order of this one: http://ed-thelen.org/dc_amp.gif Man, that thing has a HUGE number of tubes in it! There's really no excuse for vacuum tube amplifiers to be terribly slow, after all Tektronics made some pretty fast 'scopes out of bottles.... There is indeed: remember that you can either have gain or you can have bandwidth, and the whole point of the op-amp is that you can trade one for the other. If you want wide bandwidth at high gains, you need a whole lot of open loop gain inside the box, and that means a big box with a lot of tubes in it. And it means some drift issues. For most of the things op-amps got used for back then, drift was critical. If you didn't want response down to DC, there were plenty of other solutions that were easier or cheaper (and often involved transformers). Look at those 1.8M plate resistors! Yow! Gain at all cost, indeed. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#47
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"Scott Dorsey" wrote in message
Arny Krueger wrote: The sort of vacuum tube op amps I'm thinking of were more on the order of this one: http://ed-thelen.org/dc_amp.gif Man, that thing has a HUGE number of tubes in it! There's really no excuse for vacuum tube amplifiers to be terribly slow, after all Tektronics made some pretty fast 'scopes out of bottles.... There is indeed: remember that you can either have gain or you can have bandwidth, and the whole point of the op-amp is that you can trade one for the other. If you want wide bandwidth at high gains, you need a whole lot of open loop gain inside the box, and that means a big box with a lot of tubes in it. And it means some drift issues. For most of the things op-amps got used for back then, drift was critical. If you didn't want response down to DC, there were plenty of other solutions that were easier or cheaper (and often involved transformers). Look at those 1.8M plate resistors! Yow! Gain at all cost, indeed. --scott Look again - they 1.8 meggers are in series with the grids and are bypassed. The plate resistors are 620K, 180K, and 240K. The 620K plate resistors for the input stage are a bit misleading because there's 24K worth of local feedback. |
#48
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In article ,
Pooh Bear wrote: Scott Dorsey wrote: Pooh Bear wrote: Ohhh - talking of which you'll easily get an open loop gain of around 4000 with a transistor operating off 250V. It's kinda necessary to use feedback to tame the gain. This is a major irritation to my mind, and you can hardly even _get_ small signal semis at that voltage. MPSA 42 and 92 ( 43 / 93 ) to name a couple ( of complementary pairs ) ! Cheap and wickedly good. Low Cob etc etc .... etc ! Blah blah blah. And they're 'mercun and yet you don't know them !!!! I've used them *everywhere* ! You find a gem - you use it ! This is the kind of thing I was talking about... how much does the hFE vary? There is a minimum rating of 40 on the data sheet, but no maximum rating at all. But the series I was specifically talking about as being designed for CRT stuff was the MPSW42, which is actually a great device if you can live with the wide hFE range. I think it's pretty much the same as the MPSA42, just with higher dissipation. For all I know, it could be the same die in a bigger package. I think the Motorola guys turned me onto these things when I asked them for a higher voltage MPS L01. I saw some folks who made a preamp for moving coil phono cartridges, who used the 2N3055 as a front end, because the large area allowed them to better match the very low output impedance of the cartridge. Noooooo ! Not 'match' ! They were purely looking for low Rbb and Ree. Another story entirely and one that a 2N3055 might do ok at, but that line of approach has been *very* discredited ! There are far better devices that fill that bill without being power semis ! The 'nice devices' even have an hfe worth talking about ! The THAT large area array seems like a good idea. (And I confess, I think of the low base resistance as being an impedance matching sort of thing, which is probably not so good.) Well..... I'd actually be quite interested in pursuing the discussion about thermionics but I reckon only a few ppl frankly care ! More are simply into the 'toobs are best' ****wit mentality and I'd hate to be associated with them ! I think that's kind of depressing. Because I like tubes, and I like designing with tubes and listening to tubes. And it's a shame that crowd has given tubes something of a bad name. What is worse is that some of the finest tubes made are TV and military designs that you can't give away today. Those Raytheon subminis have some of the best high-gain low-microphonic performance of anything around, and people are using them as jewelry because there's no demand for them for anything else. Don't even get me going on compactrons, which are the height of unfashionability. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#49
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"Scott Dorsey" wrote in message
What is worse is that some of the finest tubes made are TV and military designs that you can't give away today. Those Raytheon subminis have some of the best high-gain low-microphonic performance of anything around, and people are using them as jewelry because there's no demand for them for anything else. My radars had literally 100's of 'em, all neatly wired to carefully swaged silver-plated posts on FRP circuit boards, clipped to heat sinks that were mounted on the back of the cakepan-like chassis. I believe they were rated for 10,000 hour MTBF. My largest radar had over 400 of them, and it dutifully went down about once every 24 hours. Do the math! They subminis pretty well duplicated the common signal-handling tubes of the day. There was one that was 12AX7-like, another that was 12AU7-like, others that were like 6AU6s, and even a 6AL5 work-alike. The biggest concentration of them in one chassis was a triple IF strip with about 10 transformer IF stages per channel. Perhaps the most impressive item in teh system was the spectrum analyzer. Balanced mixers converted three adjoining frequency ranges of the inbound signal to the same range of intermediate frequencies which passed through three identical sets of additional tuned circuits and rectifiers to provide a fast-response spectrum analysis of the inbound signal as the antenna rotated. |
#50
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Arny Krueger wrote:
Perhaps the most impressive item in teh system was the spectrum analyzer. Balanced mixers converted three adjoining frequency ranges of the inbound signal to the same range of intermediate frequencies which passed through three identical sets of additional tuned circuits and rectifiers to provide a fast-response spectrum analysis of the inbound signal as the antenna rotated. AN/APR-6? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#51
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"Scott Dorsey" wrote in message
Arny Krueger wrote: Perhaps the most impressive item in teh system was the spectrum analyzer. Balanced mixers converted three adjoining frequency ranges of the inbound signal to the same range of intermediate frequencies which passed through three identical sets of additional tuned circuits and rectifiers to provide a fast-response spectrum analysis of the inbound signal as the antenna rotated. AN/APR-6? The AN/MPQ-34 had the spectrum analyzer, the AN/MPQ-39 had over 400 subminiature tubes. |
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