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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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14th Jul 2011, 2:24 pm | #21 |
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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. |
14th Jul 2011, 6:37 pm | #22 |
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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. |
14th Jul 2011, 6:42 pm | #23 |
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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. |
14th Jul 2011, 8:32 pm | #24 |
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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 |
14th Jul 2011, 8:53 pm | #25 | |
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Re: Triode vs Pentode
Quote:
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14th Jul 2011, 8:56 pm | #26 |
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Re: Triode vs Pentode
Until the Pentode or Screened Grid (SG) was invented all we had was Diodes & Triodes!
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14th Jul 2011, 9:12 pm | #27 | |
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Re: Triode vs Pentode
Quote:
Don't 2V Domestic series mostly predate widespread use of Pentodes? |
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14th Jul 2011, 11:15 pm | #28 |
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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. |
15th Jul 2011, 12:19 pm | #29 | |
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Re: Triode vs Pentode
Quote:
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15th Jul 2011, 5:01 pm | #30 |
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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.
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15th Jul 2011, 8:49 pm | #31 |
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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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15th Jul 2011, 10:15 pm | #32 |
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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...
Phil
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15th Jul 2011, 10:49 pm | #33 |
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Re: Triode vs 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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16th Jul 2011, 12:18 pm | #34 | ||
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Re: Triode vs Pentode
Quote:
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:
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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16th Jul 2011, 1:13 pm | #35 |
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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 |
16th Jul 2011, 1:38 pm | #36 |
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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. |
16th Jul 2011, 1:48 pm | #37 |
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Re: Triode vs Pentode
Or just ignore the 1/2 ECC83 you don't need?
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16th Jul 2011, 1:59 pm | #38 | |
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Re: Triode vs Pentode
Quote:
John |
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20th Jul 2011, 11:13 am | #39 |
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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 |
23rd Jul 2011, 3:54 am | #40 | |
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Re: Triode vs Pentode
Quote:
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. Cheers, |
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