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Why use a pentode?
Why use a pentode?
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Old 1st June 2013, 09:56 AM   #41
nigel pearson is offline nigel pearson  United Kingdom
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The question was approximately why pentodes ?

Please forgive anything unworkable in this diagram . I sketched it for fun to indicate the strengths of pentodes and pentode equivalents .

The idea is to use something like the 211 triode that is a pentode . Many will use an inter-stage transformer . These are seen as preferable to other ways of doing it if enough money is spent . An opportunity exists to make a considerable virtue out of that . UL or distributed load ( curve B ) .

The exact UL point for any pentode valve is not always easy to find out .

The 803 I choose for fun . No idea if it is a good choice ? It might need very special driving to work .

If anyone has had a bad experiences of UL try it without loop feedback .

The amp sketched I suspect will offend most lovers of valves ? It shouldn't . I suspect there is a very great danger that it might work !
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Old 1st June 2013, 12:44 PM   #42
DF96 is offline DF96  England
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Quote:
Originally Posted by MerlinB
But RCA made any number of small signal pentodes, so didn't they have to pay the rights after all? Why didn't they come up with teeny little small-signal beam tetrodes?
If you look inside some small-signal 'pentodes' they are actually small beam tetrodes. I remember first noticing this in the 6BZ6.

Maybe Philips would only license the pentode to others for small-signal use? I don't know.
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Old 1st June 2013, 12:53 PM   #43
nigel pearson is offline nigel pearson  United Kingdom
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Anyone like to list new production pentodes . Especially smaller ones . EL 84 seems the obvious one .

Here is one of the small beam tetrodes .
30FL1 @ The National Valve Museum
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Old 1st June 2013, 04:04 PM   #44
yagoolar is offline yagoolar  Poland
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Send a message via ICQ to yagoolar Why use a pentode?
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Originally Posted by Shoog View Post
Either import from USA or direct from China. there is no demand for Compactrons in Europe.

I recently got some Chinese Masgnoval sockets from Germany though.

Shoog
It is no problem to buy magnovals in Poland, i.e. at allegro.pl
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Old 2nd June 2013, 08:50 AM   #45
yagoolar is offline yagoolar  Poland
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Send a message via ICQ to yagoolar Why use a pentode?
http://allegro.pl/listing/listing.php?string=Magnoval
Less than 2€
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Old 2nd June 2013, 10:13 AM   #46
nigel pearson is offline nigel pearson  United Kingdom
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I have seen lots of words written about the RH series of amps , but no graphs . Strange people should fight out a long battle and not challenge the implied performance figures . I think the originator suggested 1.84 V sensitivity and 12 watt plus for the RH88 . Here are my tests . I abandoned it as a result . The circuit as posted previously and a 2K4 load at 450 V . If I misinterpreted the intentions I apologies in advance .

My amp has neither Schade feedback nor triode input . One of the other writers here pointed the way on it ( thanks ) .

The 2 watt test is the signal generator directly into the KT88 . There is much to commend in the design . I feel the ultimate distortion is too high and the sensitivity too low . The figures I show are just single harmonics , THD will be higher .

As you can see the resultant distortion is a composite of two stages . They interact and cancel . Transformer coupling allows some manipulation of that .
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Old 3rd June 2013, 12:22 PM   #47
ilimzn is offline ilimzn  Croatia
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Originally Posted by nigel pearson View Post
807 and LS50 true .

As I understand it EL 34 was a product of hurt pride . The other people had stolen the publicity war . EL 34 was to be almost as good for less money . It is my favorite by a long way . As far as I know all beam tetrodes are kink-less . KT is just a name . Perhaps MO had already registered the name before selling to RCA ? New stock EL34 are as little as $20 a piece even in the UK . I have some old ones marked Marshall that are very good . I wonder if they are Mullard ? I used to sell new production MO KT 66 and 88 in 1974 . I bought them from Lugtons I think ? At $12 a piece ( 88's ) I was embarrassed to say how much when people bought them . The book my friend had was new for 1972 promoting designs using KT88 .

Having read Mr Schade's paper he certainly understood the device . The curves to my eyes look like pentodes .

The TT21 is a nice device . The story I heard is some young technicians had some KT 88 modified to resemble 807 . It fitted maximum allowed transmitter power for certain licenses . They offered the production ladies some dance tickets to say thank you . They showed them to their boss on Monday morning who instantly put them into production .

I think even to this day beam tetrodes seem to suggest superiority . One has to argue the case for pentodes strongly as it always seems to be implied that it is the poor relation .

Beam Power Tetrode
Shunt feedback as a concept has been known a long time before Mr. Schade came along (actually, Dr. Schade), but he was the one thet shows it makes a pentode type device look like a triode.

Re 6L6 and 807, this is essentially the same tube packed into different envelopes and sockets for different requirements. In particular, with a high-voltage anode you are risking inter-electrode arcing if you route it to a standard socket, hence the top cap - far away from the other pins. TT21 ihas a top cap for this reason.

LS50 -> GU50 is an entirely different thing, a 'proper' pentode, but with some tricks from a BPT that make it perform even better. With BPTs aligned G1 and G2 are a necessity to produce the virtual G3. Don't confuse G3 with the beam plates. Because the grids in a BPT were wound around two support rods just like in regular tubes (later on some got frame grids), the effective geometry of the grid changes a lot near the rods and in fact the rods themselves make for a large discontinuity in the shape of the electric fields. The beam plates keep a large majority of electrons emitted off the cathode on a path to the plate where they pass through where the grids have proper relative distances.
However in order for that to happen, G1 and G2 have to be wound with the same pitch and aligned so that G2 wires 'hide behind' G1. This has a very welcome side effect of keeping G2 current low. When expressed as a fraction of total cathode current, BPTs often offer several times lower G2 currents compared to straight pentodes, which potentially makes them more efficient in 'pentode mode'.
The LS50 in particular is one of the rare straight pentodes that has aligned grids (G1, G2, G3) and amongst other reasons this was done for efficiency, since it was used in portable equipment. Apparently the GU50 was initially made on original German equipment but it was refined somewhat (different socket, grid and anode doping for higher temperature operation). To put thing simply, the GU50 is unbeatable when it comes to performance/price and it's also EXTREMELY difficult to kill. THe LS50 also had civilian versions but these were not very popular due to a difficult and expensive socket, added to a rather expansive to make tube.

Regarding the RH series amps - the original ECC81 driver is not the best choice but I suspect it was used partially because it's easily available and also because it's relative non-linearity can be used to partially cancel out the nonlinearity of the output stage. Regarding the possibility of getting lower Rp than in a straight triode G2 to anode strap - IIRC Schade hinted at this but there is a caveat. With G2 strapped to anode, the feedback gives you atriode with basically muG1G2, whereas you can easily get other combinations from shunt feedback - BUT then your driver has to do a whole lot more (and in fact even impossibly more in real world situations). G2 also sits in a rather different place compared to G1, and has it's own mu, gm, rp parameters which are not linear and may not track too well with G1 parameters. So, YMMV.
That being said, ECC81, even wit an unbypassed cathode resistor is not the best input stage WRT gain but has it's merit depending on the output tube used. It's resulting Rp is quite low so the feedback signal is attenuated and - this is the important bit - because it itself sees a low load resistance, the distortion is high - but, in this case it's needed to cancel ot the output stage distortion. So, while a pentode is a better choice WRT high Rp and gain, making it very linear might not be the best strategy. The downside of using a less than linear device or operating point is the same as for the ECC81 - tube variability between instances of the same type. Even with the RH series, plug in a different ECC81 and you get a different situation - exact matching was not one of the design requirements of this tube.
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Old 3rd June 2013, 01:38 PM   #48
TheGimp is offline TheGimp  United States
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A caveat of harmonic cancellation is that you may end up with lower total harmonic content at the expense of listen-ability.

In my experience the rate at which each harmonic is cancelled is very bias point dependent.

For example, I have seen lower second and fourth harmonics at the expense of greater fifth harmonic and marginal lower third harmonic. Change bias by 5% and everything changes.
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Old 3rd June 2013, 02:08 PM   #49
DF96 is offline DF96  England
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Yes, harmonic cancellation is fine if you love tube rolling and are happy to indulge in it frequently as the valves wear out and change their harmonic pattern. Bad engineering, though.

One exception is LTP, which relies on it - but there the two valves carry the same signal (approximately) and are usually in the same envelope. Another exception is a genuine balanced SRPP, but very few people seem to build these. Also push-pull, of course. All are balanced stages, carrying the same signal in each half.

Last edited by DF96; 3rd June 2013 at 02:11 PM. Reason: extend
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Old 3rd June 2013, 11:14 PM   #50
ilimzn is offline ilimzn  Croatia
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Quote:
Originally Posted by DF96 View Post
Yes, harmonic cancellation is fine if you love tube rolling and are happy to indulge in it frequently as the valves wear out and change their harmonic pattern. Bad engineering, though.

One exception is LTP, which relies on it - but there the two valves carry the same signal (approximately) and are usually in the same envelope. Another exception is a genuine balanced SRPP, but very few people seem to build these. Also push-pull, of course. All are balanced stages, carrying the same signal in each half.
Agreed, that's why I specifically mention it in conjunction with the RH designs. One would need to tweak the operating points for every tube and also as they age. Add to that that ECC81/12AT7 was never intended to be tightly specified, and is nonlinear to begin with. Also good point re lower distortion but possibly a not so nice harmonic profile of the distortion. Some wrk is required already just to get two same type or dual tubes to do this right, much more difficult finding a pair of completely disparate ones that fit well, tolerances notwithstanding. One may end with good cancellation only to the first approximation, which is the second harmonic, resulting in an unfavorable distribution of the other harmonics.
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