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Old 4th Nov 2015, 6:44 pm   #1
Phil G4SPZ
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Default ECL80 questions

I would appreciate any advice from the Forum valve experts on two questions that have been exercising my own and my friend Phil's minds for a week or so.

Q1 - why did the ECL80 have a common cathode? My understanding is that the ECL80 was developed primarily as a combined TV frame oscillator and output valve in a single envelope, and the established late-1940s TV frame circuit practice used a separate triode and tetrode with their cathodes coupled together to improve linearity. Hence it made sense to use a common cathode from the start, and may have saved heater power too.

Q2 - why did Mullard/Philips bring the suppressor grid out on its own separate pin? All the circuits I have seen show G3 wired to the cathode anyway. Were there any applications that involved using G3 in other ways?

All thoughts welcome!
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Old 4th Nov 2015, 8:56 pm   #2
ParcGwyn
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Default Re: ECL80 questions

The Mullard Pupil's Oscilloscope used the pentode section of a ECL80 as transitron oscillator for the timebase, this needed access to the suppressor grid.

Dave
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Old 4th Nov 2015, 9:18 pm   #3
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Default Re: ECL80 questions

It came as a surprise for me to discover that the maximum anode voltage for the pentode section is 550V. The max screen volts is 400. The absolute peak voltage is 1.2KV. I'd imagine this is a short duration pulse, let's say 10% of a duty cycle.
Max cathode current is 25mA.
So it turns out this is a tough little valve.
While servicing a Dynatron Fulmar TV set it came to my attention that the anode of the ECL80 frame output valve is supplied from the boosted HT supply which is 450 volts. A series 10Kohm drops the voltage down to 250 for the anode. the screen is supplied from the regular HT line of 220 volts.
After checking the ratings of the valve it became clear the valve is safely operating within it's ratings.

DFWB.
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Old 4th Nov 2015, 9:42 pm   #4
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Default Re: ECL80 questions

I'm more familiar with the ECL80 as a low-power audio preamp/output stage. I've also used a pair as a low-power 160/80M transmitter [xtal osc, pentode as PA, the other valve doing duty as mic amp/modulator] which would put out 4 or 5 Watts of RF. The ECL86 works well in this sort of thing too.
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Old 4th Nov 2015, 10:03 pm   #5
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Default Re: ECL80 questions

One of the original work-horses of television, used as line oscillator/sync separator, frame osc/output, sound output and....a frequency changer in at least one set......! Now what would you think if you saw an ECL80 fitted in a tuner unit......?
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Old 4th Nov 2015, 10:25 pm   #6
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Default Re: ECL80 questions

Quote:
Originally Posted by Sideband View Post
and....a frequency changer in at least one set......! Now what would you think if you saw an ECL80 fitted in a tuner unit......?
Employed as the mixer-oscillator in the Band 1 only 5 channel Pye FV1 and derivitives. FV2C, FV4C and the strange V54C.
Also used as the mixer-oscillator in Band 1 only Sobell TVs. 1953 models T143. Also models T224 thru to TRG174.

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Old 5th Nov 2015, 12:02 am   #7
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Default Re: ECL80 questions

Re Q1:

Evidently the ECL80 was designed from the start for a variety of applications. This is shown in the first two attachments, one being a page from Philips Book IIIC covering the initial range of TV valve and the other being the Wireless World release announcement.

Given that the noval base forced a choice between a common cathode with a separate pentode suppressor pinout and separate cathodes with an internal suppressor connection, one assumes that Philips saw the former as being more appropriate for the intended range of applications. But it was probably a marginal call.

Re Q2:

The usual reason for having a separate suppressor pinout for pentodes was to allow direct earthing of the suppressor and so minimize the risk of deleterious feedback that could cause bandpass response tilting and oscillation. The mechanism by which this could occur is well explained by Cocking in “Television Receiving Equipment”, pertinent excerpt therefrom as third attachment.

A second reason was that the suppressor grid could be used as second control grid, such as for agc, gating or mixing. For gating and mixing though, purpose-built dual-control pentodes, such as the 6AS6 and 6F33, were usually preferred.

RCA stressed that a separate suppressor grid pinout, allowing direct earthing, was essential where the pentode was operated with part of its cathode resistor unbypassed, this usually being done to minimize detuning and passband tilting due to the agc bias. Thus for its 6X8 TV frequency changer it chose a separate suppressor pinout with combined cathode. It could not have been a major issue though, as Tung-Sol chose internally connected suppressor with separate cathodes for its contemporary 6U8 TV frequency changer. Philips did the same as Tung-Sol for its later ECF80/PCF80. And although the PCF80 was initially described as a frequency changer valve (e.g. Mullard Valve Tubes & Circuits #15 & #16), it soon segued into a multipurpose valve (e.g. Mullard Valves, Tubes & Circuits #26 & #27), in which role it covered most, if not all of the applications that had been listed for the ECL80. That, I think, demonstrates that the pinout configuration chosen for the ECL80 was a marginal call, as the evidence is that it would have worked well enough the other way around.

That the ECL80’s common cathode could at times be a bit awkward was acknowledged by Mullard in design for a push-pull amplifier using a pair of these; see Valves, Tubes & Circuits #2 & #3.

On balance, one might say that Philips made the wrong call with its ECL80 pinout choice, and that separate cathodes would have been preferable. Many ECL80 circuits appear to have been arranged with the cathode directly earthed, meaning that an internally connected suppressor grid would also have been directly earthed. But then it could have been that many circuit designers saw direct earthing of the cathode as sine qua non for the ECL80, to prevent any cathode coupling between triode and pentode, and so designed accordingly, even though they might have done differently had there been separate cathodes.

Still, what look to be wrong calls in hindsight are probably less the result of missing due diligence than simply the way a designer or a design group assessed the trades-off in the light of what was known at the time.

Cheers,
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Old 5th Nov 2015, 10:45 pm   #8
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Default Re: ECL80 questions

Many thanks for taking the trouble to post such a comprehensive response, particularly the attachments. I'm much the wiser. Thanks again,

Phil
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Old 5th Nov 2015, 11:44 pm   #9
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Default Re: ECL80 questions

Thank you Synchrodyne for such an in depth explanation. I have learned a lot from your post.
The one thought I have is that, as mentioned the suppressor grid may be biased to act as a second control grid, would biasing the suppressor grid so far negative as to cut off the anode current effectively cause the screen grid to "overload" by attracting the electrons that cannot pass by the suppressor?

Last edited by Andrew B; 5th Nov 2015 at 11:56 pm. Reason: ? question?
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Old 6th Nov 2015, 12:38 am   #10
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Default Re: ECL80 questions

Basically with a dual-control pentode, varying the potential on the suppressor grid has minimal effect on cathode current, which is determined largely by control grid bias, but rather it swings the cathode current between the anode and the screen grid. Thus mapping anode and screen currents vs. suppressor potential produces a set of “butterfly” curves.

The attachment shows the curves for the 6AS6, which was by design a dual-control pentode (in fact derived from the 6AK5). But the curves for a regular pentode would be of the same general form, perhaps with a lesser degree of control on a per volt basis.

As you said, when the suppressor grid swings sufficiently negative, anode current is cutoff and the screen current reaches a plateau. Conceivably the screen current in this situation could go beyond the design limit for a regular pentode not really intended for dual-control use. I hadn’t thought about it before, but dual-control pentodes might have been designed to handle relatively high screen currents.

Rmorg has a good article on dual-control pentodes, at: http://www.radiomuseum.org/forum/dua...pentodes.html; with sets of suppressor grid curves at: http://www.radiomuseum.org/forum/sup...surements.html.

Cheers,
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Old 6th Nov 2015, 1:07 am   #11
Synchrodyne
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Default Re: ECL80 questions

Quote:
Originally Posted by Phil G4SPZ View Post
Many thanks for taking the trouble to post such a comprehensive response, particularly the attachments. I'm much the wiser. Thanks again,
No problem! I forgot to include the link to the Mullard Valves, Tubes & Circuits Collection. They are in the following thread, first post courtesy of forum member Valvepower: https://www.vintage-radio.net/forum/...ad.php?t=62844. They are worth reading, as in effect they look behind the regular data sheets.

Philips Book IIIC, covering the initial TV valves and their applications, including the ECL80 is available here: http://frank.pocnet.net/other/Philip...tronTubes.html. Also there is Book IIIA, covering the Rimlock radio valves and fellow travellers. I don’t know if Book IIIB was ever issued – I have not seen any trace of it. I imagine it would have covered the initial release of the FM-AM novals.

Cheers,
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Old 13th Dec 2015, 5:40 pm   #12
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Default Re: ECL80 questions

Whilst this has gone quirkily off topic from the ECL80's use in audio - this has all made for some enlightening reading! Many thanks to all. Edward
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