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Old 19th June 2013, 07:40 AM   #71
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Glad to say there is a plaque .
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I think PCL 84 drives 211 well ? My experiments seem to favour a pentode over a triode on all count if driving a triode . My next step is to have a UL driver transformer , cost is about $50 . Loosely speaking still why use an input pentode . MY3-275 , wow !!

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Old 19th June 2013, 07:46 PM   #72
MikeVou is offline MikeVou  United Kingdom
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@nigelpearson - thanks for taking the trouble to put up those fascinating glimpses of Blumlein's role in WWII !
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Old 20th June 2013, 10:05 AM   #73
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I am if no one has guessed dyslexic . Alan Blumlein so badly so he would have failed to advance at school . His mother paid a man who usually got people into university to get her son to read and write . It was this man who first identified his talent .

I employed a man called Bob who appeared in the film The Leather Boys on his BSA Gold Star . Many years later he asked me how he passed his interview . Simple I said , you told me you maintained that Gold Star . Bob could co anything I asked . Often things beyond the possible if reading a book about it ( bearings ) . To top that he could do it with tools barely able to do it . It was years later I found out that he had the writing age of an 8 year old . Later he told me that he had wired most of the flight decks of the Concorde's . He had a friend do all the paper work ! He told me the French twigged it and didn't care . The Brits never did . Not a boastful man , took him about 9 years to say it .

Like the various people in history I never met Alan Blumlein except in his ideas . The story I liked is if he found a young engineer struggling with a problem he would say he was bit bored and might he help ! I wish he would in spirit form do that for me .

Sorry we got so far away from why input pentodes were popular . For what it is worth I guess transistors did it better ? Very rare one saw a hybrid so even that theory holds no water .
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Old 4th July 2013, 02:29 AM   #74
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Quote:
Originally Posted by Rundmaus View Post
How much of the compactron is really usable/used then, do you leave many sections 'idle'? I think I remember reading something about the limited possibilities with compactrons due to the often shared cathode...

Rundmaus
With the 6M11, one thing you could do is use the two triodes as an LTP splitter, with the pent in service as an active tail load. That just might save you an extra hole in the chassis if you didn't want to solid state the tail load.

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Originally Posted by tubelab.com View Post
There are probably just as many, if not more, ham radio transceivers out there eating up 12BY7's than there are Citations. The 12BY7 is the RF driver tube that feeds a 6146A or a sweep tube final RF power amp in dozens of tube and hybrid radios.
Let's not forget color TV vid amps. That's what the 12BY7A was originally designed to do.
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Old 4th July 2013, 12:15 PM   #75
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Apart from EL 84, 34 Gu 50 what plentiful pentodes are worth looking at ?

I suspect some have the beating of 300B and 211 if triode strapped ? Price would be important as would the tube base . Gu 50 is not a standard type which is a pain . Ones that resemble a big EL 34 would be good . $50 is about as high as I want to go . In current production would be even better .

In transistors the video type devices are well liked . MPSA 42/92 are similar survivors . BF720/721 resemble video devices .
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Old 4th July 2013, 03:00 PM   #76
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Apart from EL 84, 34 Gu 50 what plentiful pentodes are worth looking at ?
I like TV horizontal and vertical deflection types: 6BQ6GA/GTB, 6CB5, 6AV5, 6DQ5, 6LW6 (horizontal deflection types -- some of these are real beasts that can put out north of 100W at reasonable rail voltages).

Some vertical deflection types: 6LR8 (small signal triode + beam former) this one can use the triodes as either an LTP driver or cathode follower grid drivers) The 10JA5 also looks good. These vertical deflection types can also use stock OPTs, as their loads come close to what's available "off the shelf", and the distortion performance looks pretty good if you don't need more than ~30W.

As for availability, it's pretty good if you go for the odd heater voltages as lots of these were made in many different heater voltages to accommodate series heater strings across the AC mains in TV sets that didn't use a PTX.

Some of these will work as pseudotriodes, and can tolerate considerable V22 spec busting if used in this manner (6AV5s in particular). Others, not so much and/or it's brand specific. YMMV if you try it.

The main problem with these, and probably the spec sheets make no mention of audio final use for this reason, is that they don't work so good as Class A amps, as the most linear part of the characteristic is well within red plate territory. As Class A, SE amps, they do sound good until the glass melts. Otherwise, you have to use 'em as Class AB, PP finals to hit the most linear part of the characteristic.

In that regard, the high current capability makes for easier to design and build OPTs since the Rl / phase is so much lower. (4K4 (P-2-P) works just fine with 6BQ6s, and that, too, comes close to a stock OPT that was intended for Class A, PP 6L6s -- though you do sacrifice some power output capability if you use such an OPT -- which I did as I already had one in the junk box.)
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Old 4th July 2013, 05:52 PM   #77
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Thanks for that . I wish my late brother was around to join in .

Sort of on my list although a museum device .

813 @ The National Valve Museum
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Old 4th July 2013, 06:56 PM   #78
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Miles,

I understood that deflection tubes were notorious for varying gm, that not being important when used as deflection devices. Is that your experience?
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Old 4th July 2013, 07:22 PM   #79
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^^

No worse than the tolerances for most other types. (The only type that's notorious for this that I heard of is the 6SJ7, but that's a small signal pent.) I found that in circuit performance comes to within 5% of design nominal predictions, and that the H3 predictions from loadlines are "high side" errors. First I ever heard of this claim.
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Old 4th July 2013, 10:57 PM   #80
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Thanks Miles!
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