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Old 14th Jul 2011, 2:24 pm   #21
neon indicator
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Default Re: Triode vs Pentode

So why are Battery Radios all Pentodes? (Gain?)
DF96, DAF96, DL96?
Why is 6146 a Tetrode and not a Triode? It's designed as a VHF amp.
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Old 14th Jul 2011, 6:37 pm   #22
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Default Re: Triode vs Pentode

I did say broad brush!

My guess is that battery valves are mainly pentodes because they need all the gain they can get, and distortion is secondary. There is a battery triode (DCC90?) which was intended for VHF.

6146 is large signal, and I said that is complicated. Triodes are used for VHF (and up) receivers because of noise. Noise is not such an issue in transmitter PAs. Instead, you need to reduce anode-grid capacitance so use tetrodes/pentodes. Why these tend to be tetrodes rather than pentodes I don't know - someone will tell us! 6146 is really an HF valve which happens to extend up to 175MHz.
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Old 14th Jul 2011, 6:42 pm   #23
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Default Re: Triode vs Pentode

They're not. DAC32 (triode) - in 1.4V battery series valves - common enough.

In the 2V-heater battery valves there are oodles of triodes: HL23DD comes to mind as it's used in one of my battery radios. Triode detectors and twin-triode output valves were common.

I do agree that last-generation battery valves (Dx96) didn't include a triode - pentodes provided greater gain including for the AF voltage amplifier.

The 6146 is a tetrode, but it's a power amplifier not a small-signal amp. In this application, the benefits of a triode (no partition noise) are not needed. A triode would require neutralising (unless used in grounded-grid, which for a power amplifier would require LOADS of drive power). And as it's frequently used in Class C, the tetrode provides greater efficiency and requires less grid voltage swing.
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Old 14th Jul 2011, 8:32 pm   #24
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Default Re: Triode vs Pentode

I think 6146 is a VHF valve mostly used in HF radios. That's what I was told.

Don't 2V Domestic series mostly predate widespread use of Triodes?

A harder question seems to be difference between Beam Tetrode and Pentode
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Old 14th Jul 2011, 8:53 pm   #25
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Default Re: Triode vs Pentode

Quote:
Originally Posted by kalee20 View Post

The third harmonic is indeed an octave and a fifth (musically) and does sound 'interesting'. But add successive odd harmonics and you soon get unpleasant dissonance. (Even harmonics do the same, but by the time you get dissonance the frequencies are much higher and generally get lost, or are above upper hearing frequency range).

.
Music in it's raw state (not passed through anything electronic) is full of all sorts of harmonics, without harmonics everything would just be sine waves without the colouration that you get from instruments acoustics (halls, rooms) etc!
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Old 14th Jul 2011, 8:56 pm   #26
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Default Re: Triode vs Pentode

Quote:
Originally Posted by neon indicator View Post
I think 6146 is a VHF valve mostly used in HF radios. That's what I was told.

Don't 2V Domestic series mostly predate widespread use of Triodes?
Until the Pentode or Screened Grid (SG) was invented all we had was Diodes & Triodes!
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Old 14th Jul 2011, 9:12 pm   #27
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Default Re: Triode vs Pentode

Quote:
Originally Posted by neon indicator View Post
I think 6146 is a VHF valve mostly used in HF radios. That's what I was told.

Don't 2V Domestic series mostly predate widespread use of Triodes?

A harder question seems to be difference between Beam Tetrode and Pentode
I meant
Don't 2V Domestic series mostly predate widespread use of Pentodes?

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Old 14th Jul 2011, 11:15 pm   #28
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Default Re: Triode vs Pentode

As a general address to the subject of Triode Vs Pentode...well...they just evolved that way for a particular requirement, each has its merits..depends on the design requirements and functionality
perfomance etc.
I'm not a theoretical wizzard but I do remember the use of triodes as rf amps in the old TV VHF tuners, they did the job very well and neutralization as I remember was quite a simple and in general a reliable affair.
Audio P/A?...I am staying out of that one.
Pentodes in RF (HF) yep.
Pentodes above 35 mhz carefull layout other wise there might be a knock on the door.

Cheers.
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Old 15th Jul 2011, 12:19 pm   #29
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Default Re: Triode vs Pentode

Quote:
Originally Posted by murphyv310
Music in it's raw state (not passed through anything electronic) is full of all sorts of harmonics, without harmonics everything would just be sine waves without the colouration that you get from instruments acoustics (halls, rooms) etc!
Mainly overtones, not harmonics. Halls/rooms mainly add reverberation and modify the frequency response, they don't add much in the way of harmonics. Unless the sound is really loud, air and building surfaces are reasonably linear.
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Old 15th Jul 2011, 5:01 pm   #30
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Default Re: Triode vs Pentode

Trevor's including instruments though. I suspect they add many more harmonics than the overtones contributed by the room. Otherwise a violin would sound like a violin in one room and a flute in another. Then again a lot of the 'sound' of an instrument comes from the transient at the start of each note. If the notes from a violin and a flute are recorded and faded up after the player has begun bowing/blowing they can be more difficult to distinguish than one might imagine.

Cheers,

GJ
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Old 15th Jul 2011, 8:49 pm   #31
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Default Re: Triode vs Pentode

Instruments have overtones (and transients). Rooms (possibly) and electronics (definitely) have harmonics and intermodulation. By reducing the latter as much as possible we hear the former.
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Old 15th Jul 2011, 10:15 pm   #32
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Default Re: Triode vs Pentode

Thanks for this really interesting thread and all the comments, chaps. Reading it has reminded me how much I still have to learn about valves...

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Old 15th Jul 2011, 10:49 pm   #33
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Default Re: Triode vs Pentode

Quote:
Originally Posted by neon indicator View Post

A harder question seems to be difference between Beam Tetrode and Pentode
One point worth remembering (if it hasnt been posted earlier) is that Philips had the patent on pentodes, and other manufacturers either had to pay royalties or seek / make do with the alternative.
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Old 16th Jul 2011, 12:18 pm   #34
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Default Re: Triode vs Pentode

Quote:
Originally Posted by GrimJosef View Post
Quote:
Originally Posted by neon indicator View Post
I've not taken much notice of valves for the last 40 years. So I'm a bit bemused reading about people "converting" or using EL34, KT66 or EL84 etc Audio amps in "triode" mode.
It's not a new idea. Williamson used triode-connected KT66s in the output stage of his amp in 1947.
Triode-connected KT66s (and other tetrode/pentode) output valves might have been reasonably common in the post-WWII era until the ultralinear configuration was well-enough established by 1955 give or take. At the time the perception was that triodes were better for “high quality” work than tetrodes/pentodes. It appears that Osram data for the KT66 envisaged triode as well as tetrode operation from the beginning. Larger output triodes, such as the PX25, were directly heated. Possibly that was either necessary or highly desirable – I have seen commentary to that effect, but do not have it to hand right now. So using an indirectly heated tetrode/pentode in triode configuration might have been easier for amplifier designers. From the valve manufacturer perspective, it allowed one series of output valves to fulfil both triode and tetrode/pentode functions.

Some of the early Leak “Point One” amplifiers used triode-connected KT66s, including the TL12. On the other hand, the TL25, aimed more at industrial than domestic applications, used tetrode-connected KT66s. Leak started its swing to the ultralinear output configuration with the KT61-based TL10 in 1954.

Quote:
Originally Posted by DangerMan View Post
You could say the EF86 is a special case.

Mullard's given reason for using a pentode in the first place was that they wanted the extra gain it gave as well as its inherent microphony and hum reduction. This gave them chance to use heavy overall negative feedback in order to reduce distortion but still end up with designs that gave full output from about 200mV input.

It is noticeable that Leak went another way with the Stereo20 and its sisters, which used triodes in the voltage amplifier and yet had much the same sensitivity and distortion, and no doubt if all EF86's other attributes had been available in a triode, or twin triode, it would probably have found use just for its screening.

It's pretty doubtful you could get enough gain from a triode of the era for use in a preamp with feedback-applied RIAA or tone controls set anywhere other than "flat". Not with decent distortion and hum/noise figures from AC heaters, anyway.
The Leak valve stereo power amplifiers all used a ˝(ECC83) input stage followed by an ECC83 phase splitter, whereas their mono counterparts of the same era stayed with the EF86 + ECC81 combination. Clearly the gain distributions were different between the mono and stereo models. Whether the stereo configuration of ˝(ECC83) + ECC83 was better (quieter) in practice as well as in theory than the EF86 + ECC81 mono combination I don’t know, but whatever the difference, Leak did not see fit to adopt it for the mono models. The fact that there was no such animal as a ˝(ECC83) would have made it difficult, although there was the possibility of using a triode-connected EF86 plus an ECC83. On the other hand the use of a ˝(ECC83) input stage in the stereo case allowed a single ECC83 to serve both channels, as compared with, for example, using an EF86 in each channel. Perhaps this was a consideration as much as using a triode input stage for its own sake. Then the higher gain ECC83 phase splitter became a necessity.

The Leak control units also provide an interesting illustration. Leak moved from a single-stage pentode (EF40) to a two-stage (ECC40, then ECC81, then 2 x EF86) configuration, the (2 x EF86) being definitive. In the two-stage models, both EF86s had shunt feedback, the input EF86 providing the initial gain and (disc) equalization, and the second EF86 providing tone controls and filter functions. But the second EF86 was either pentode or triode configured, according to the overall gain required for specific models.

On equalized input stages generally, it seems that the EF86 with shunt feedback was adequate (for the times) in a signal-to-noise ratio sense down to input sensitivities of around 4 mV. For greater sensitivity (say down to around 2 mV) the next step seemed to be an ECC83 configured as a series feedback pair and with DC heating. This was used by some of the tape recorder makers, Brenell and Revox amongst them as far as I know. In any event DC heating would have been very desirable because of the unbypassed cathode resistor of the first triode of the pair, but whether it was otherwise needed because the ECC83 had higher relative hum levels than the EF86 is not clear. Less commonly, the ECC83 was also used as a low level input amplifier in a shunt cascode configuration, a (perhaps surprising) example being the Murphy T1 tape recorder.

Mention of the cascode brings us back to Leak, its Troughline II FM tuner having a cascode (ECC84) RF amplifier, considered beneficial at Band II frequencies, but unusual in UK FM practice. Evidently the VHF cascode was developed as the optimum way to use two triode RF input stages ahead of a chain or pentode amplifiers when a single stage would not provide enough gain to more-or-less nullify the noise contribution of the first pentode. Whether it was developed with TV receivers in mind I don’t know, but its availability surely helped TV receiver design. At Band III frequencies the cascode could provide enough low-noise gain to allow the use of (very noisy) pentode mixers. Such may seem counter-intuitive at Band III frequencies, and for this band alone probably would not have been used. But the otherwise desirable upward migration of TV receiver IFs to the point where they were nudging the bottom end of Band I made the pentode mixer very attractive for that Band, avoiding the need for channel frequency-dependent neutralization of triode mixers, something not particularly wanted for mass-produced equipment.

Whilst the cascode RF amplifier was near-universal for TV receivers (until frame-grid beam triodes became available at least), there was evident ambivalence at FM Band II frequencies, where many makers chose pentode RF stages as being “good enough” from the noise viewpoint. Apparently there was a “rule of thumb” that if a single-stage and therefore low-gain triode RF amplifier were used, then a (relatively low noise) triode mixer was necessary, this being encapsulated in the ubiquitous ECC85. Pentode mixers required pentode (or cascode) RF stages to provide enough gain to nullify mixer noise contribution. High slope pentode RF stages seem to have been preferred over the vari-mu type because of their directionally lesser noise contribution.

Maybe the RF triode noise benefits extended downwards to 20 MHz or lower. Towards the end of the 1950s, cascode 1st RF stages made their appearance in some HF receivers. Previously, Dynatron had included a triode broadband “pre-RF stage” operative only on some of the SW bands in its T139 tuner/control unit. In fact this was a Z77 high-slope pentode configured as a grounded-grid triode. Use of pentodes strapped as triodes was not that rare in HF receivers anyway, such as for stand-alone oscillators, cathode followers, etc. Possibly the use of readily available standard valves was a factor here. In its early 1950s TA160 bandspread export receiver, Murphy used a high-slope pentode (6F1) tuned RF stage because of its lower noise contribution as compared to the vari-mu type.

So the empirical evidence is that the whole triode vs pentode issue (inclusive of pentodes strapped as triodes) is in parts rather fuzzy.

Cheers,
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Old 16th Jul 2011, 1:13 pm   #35
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Default Re: Triode vs Pentode

I had a look at the "sub-miniature" Russian "6" series Triodes. As suspected they are not really suitable for Battery operation as the indirectly heated cathode with 6.3V heater takes 400mA. HT 100V
http://www.radiomuseum.org/tubes/tube_6n16b.html

That's 2.5W compared with 27mW for TWO x 1j24b pentodes (HT 50V), or 130mW for two of the higher power 1j29b pentodes (HT 100V)

Today for semiconductors the Cascode is one of the "best" RF front ends. I'd guess adoption was slower for valve Radios due to extra cost and the fact also that the idea of the circuit was not widely known till well after Pentodes available.

HF is noisy anyway. Noise level received by aerial drops with rising frequency so that at Band I, a NF of 5dB can be fine and 1dB at UHF. At SHF / Microwave the received noise is so low that 0.5dB is worth while. LNBFs with 0.2dB or 0.1dB NF are probably marketing fiction.

So there is little value really on LW/MW/HF of Cascode Triodes compared to Pentode. It's for Instrumentation Amplifiers and low o/p Magnetic cartridges that a Triode should be lower noise than Pentode (the Pentode 1/f partition noise), not at 2MHz to 30MHz.

The cascode circuit reduces the effect of the "Miller" capacitance (grid/Anode) as the upper stage is essentially grounded Grid (makes capacitance be on o/p instead of to input) and makes the 1st grounded cathode stage have a load impedance that migates the Cag. At least I think that's the theory. This is why a Pentode with G2 suitably biased and loaded and not decoupled and G3 decoupled at RF is regarded Analogous to a Cascode (the G2 is equivalent to V1Anode / V2Cathode and G3 to grounded V2grid). At least that's what I was told many years ago.

Certainly I found in testing the Sub-miniature Russian Pentodes that the "right" value of G2 load (with no decoupling), while reducing LF gain by 20% to 30% resulted in "flat" response and dramatically improved RF response. Also of course for such operation a much higher HT voltage than for "strapped" triode operation was required
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Old 16th Jul 2011, 1:38 pm   #36
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Default Re: Triode vs Pentode

Half an ECC83 is available, in the form of EBC91/6AV6. Just ignore the diodes.

Pentode partition noise is mainly white, with only a small 1/f part. As a result, the noise advantage of triodes at RF is not so obvious for audio. That means that an audio cascode is little better than a pentode. Most valve 1/f noise comes from the cathode, so affects triodes and pentodes in the same way.

A pentode should normally be used with g2 decoupled, otherwise it is not acting as a pentode. Comparing a pentode to a cascode, the V1 anode/V2 cathode corresponds to somewhere in the electron stream, and V2 grid corresponds to pentode g2. Pentode g3 does not have a cascode analogue. Most pentode g3 provide little screening as the pitch is too coarse.
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Old 16th Jul 2011, 1:48 pm   #37
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Default Re: Triode vs Pentode

Or just ignore the 1/2 ECC83 you don't need?
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Old 16th Jul 2011, 1:59 pm   #38
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Default Re: Triode vs Pentode

Quote:
Originally Posted by Synchrodyne View Post
Larger output triodes, such as the PX25, were directly heated. Possibly that was either necessary or highly desirable – I have seen commentary to that effect, but do not have it to hand right now.
Perhaps, but it should also be borne in mind that the PX4, PX25 etc. were 1920s designs i.e. they were designed and released when oxide coated filaments were recent technology and indirectly heated cathodes were absolutely cutting edge. No doubt, valve manufacturers could have designed and marketed indirectly heated power triodes later on, but they chose not to, probably because indirectly heated power tetrodes and pentodes had a much larger market and these could be triode connected if desired.

John
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Old 20th Jul 2011, 11:13 am   #39
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Default Re: Triode vs Pentode

This is rather good on Pentodes, Triode, Tetrode and Beam Tetrode
http://www.tubebooks.org/Books/Happell_engineering.pdf

Found thanks to MikeyPP
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Old 23rd Jul 2011, 3:54 am   #40
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Perhaps, but it should also be borne in mind that the PX4, PX25 etc. were 1920s designs i.e. they were designed and released when oxide coated filaments were recent technology and indirectly heated cathodes were absolutely cutting edge. No doubt, valve manufacturers could have designed and marketed indirectly heated power triodes later on, but they chose not to, probably because indirectly heated power tetrodes and pentodes had a much larger market and these could be triode connected if desired.
Linsley Hood provides a few interesting comments upon this aspect, attached, that point to difficulties with indirectly heated output triodes.

But as you say, even could it have been done, there was no real need. I understand though that the PX25 (and maybe the PX4) underwent an upgrade or two in their lives, the last for the PX25 possibly being after WWII. Osram evidently opted for improving its existing output triodes rather than designing new ones. Thus amplifier builders wanting 25 W (anode dissipation) triodes had a choice of the improved PX25 or the KT66, triode-connected.

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