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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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Old 18th Jun 2019, 6:49 pm   #1
TrevorG3VLF
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Default EF91 to EF80

Why did the EF80 displace the EF91?
They seem to be similar.
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Old 18th Jun 2019, 7:23 pm   #2
HamishBoxer
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Default Re: EF91 to EF80

I would say the size, but feel that is not quite correct. Bush TV22's used both at different times.
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Old 18th Jun 2019, 7:28 pm   #3
paulsherwin
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Default Re: EF91 to EF80

I think it was just a case of adopting the B9A standard. As you say, they have very similar characteristics. Maybe the EF80 was a bit cheaper to manufacture.
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Old 18th Jun 2019, 7:33 pm   #4
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Default Re: EF91 to EF80

The EF80 has similar ma/v for a lower anode and screen supply, 170v as opposed to 250v, this would be useful in AC/DC sets were mains voltages could be as low as 200v.

That’s just a guess though.
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Old 18th Jun 2019, 7:33 pm   #5
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Default Re: EF91 to EF80

At some point the EF91 went from current to maintenance status, which is presumably when Bush changed, but that doesn't explain why unless they were phasing out the B7G range for standardisation purposes (EB91 excepting). All the other small bases went too.
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Old 18th Jun 2019, 7:34 pm   #6
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Default Re: EF91 to EF80

The EF80 has two cathode connections.

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Old 18th Jun 2019, 7:38 pm   #7
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Default Re: EF91 to EF80

One problem I found was the EF91 was not as reliable as the EF80.
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Old 18th Jun 2019, 7:47 pm   #8
M0FYA Andy
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Default Re: EF91 to EF80

EF91's were used extensively in military receivers and other equipment, whereas I don't think EF80's were!

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Old 18th Jun 2019, 7:51 pm   #9
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Default Re: EF91 to EF80

The EF80 is slightly later, and is fully screened internally, so doesn't need a can.
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Old 18th Jun 2019, 7:55 pm   #10
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Default Re: EF91 to EF80

Also the screen has it's own pin.

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Old 18th Jun 2019, 7:57 pm   #11
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Default Re: EF91 to EF80

The EF91 was not internally screened requiring fiddly external metal screens in TV RF/IF strips. Fantastic valve but they also ran very hot and in the confined space of receiver units, frequency drift could be a problem, the HMV 1807 is a case in mind. [Z77s]

The EF91 with it's blue internals was the most reliable of the equivalents. Mazda 6F12, MOV Z77. Brimar 6AM6/8D3 and the Cossor SP6. There are probably more! J.

Sorry, clashed with Lawrence.
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Old 18th Jun 2019, 8:06 pm   #12
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Default Re: EF91 to EF80

My dad kept a supply of Z77’s for the HMV we had in the early 50’s.
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Old 18th Jun 2019, 8:07 pm   #13
G8HQP Dave
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Default Re: EF91 to EF80

I read somewhere that the EF80 could provide performance with 170V supply rail which an EF91 needed 250V to deliver. The two cathode pins means that you can optimise either in-out isolation or input conductance, whichever is most important for your design.
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Old 18th Jun 2019, 8:15 pm   #14
ms660
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Default Re: EF91 to EF80

Some more info:

http://www.r-type.org/addtext/add113.htm

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Old 18th Jun 2019, 11:45 pm   #15
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Default Re: EF91 to EF80

Mullard released the EF91 and the EF42 at about the same time. For example both were mentioned in WW 1947 November.

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The EF42 was the high-slope pentode “TV valve” in Mullard’s then-new Rimlock domestic receiving series. As far as I can determine, the EF91 was intended more for industrial applications (which were essentially out-of-scope for the Rimlock series), along with other new B7G issues such as the EC91, ECC91 and EL91. Also, it would appear that the EF91 was more-or-less a clone of the Osram Z77, as were the Mazda 6F12, Cossor 6AM6 (Cossor did the American registration) and Brimar 8D3 (later redesignated as the 6AM6).

The Z77 was Osram’s high-slope pentode “TV valve” in its new miniature B7G “77” series, and the one valve served both domestic and industrial applications. Evidently Cossor and Brimar both adopted the same “one high-slope pentode for both domestic and industrial markets” approach. On the other hand, Mazda followed the Mullard pattern in offering the 6F12 primarily as an industrial valve and its Rimlock 6F13 as a domestic “TV valve”.

Circa 1949 Philips adopted the B9A base for its definitive range of TV valves, and also for new radio receiving valve issues. Thus its enneode FM demodulator, which started life as the EQ40, quickly became the EQ80. The EBF80, I think introduced when it was found that the EAF42 did not work as well as expected, out of necessity used the B9A base. At the same time, Philips also strongly embraced the power-transformerless, live chassis form for TV receivers, which necessitated series-string heaters, with 300 mA being the chosen heater parameter, and which implied a lowish upper limit to the available HT, as mentioned by G8HQP Dave. This outruled the EF42 (or a B9A copy thereof), which had a 330 mA heater and worked best with 250 V HT. Thus was developed the EF80, first mentioned in 1949, in which some slope was given up to allow the use of a 300 mA heater (whilst staying at 6.3 volts) and 170 V HT. The B9A base allowed two cathode pinouts, considered to be beneficial at VHF, whilst retaining separate pinouts for the suppressor and internal screen.

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At about this time Mazda released the 6F1 as an improved 6F13, with two cathode pinouts. However, as Mazda had retained the Rimlock base, this required that the suppressor and internal screen shared a pinout.

Osram simply cloned the EF80 as the Z719, in its B9A-based “7n9” series (which had started with the X79).

Through all this the Z77 and its various clones such as the EF91 continued unchanged, and were widely used in industrial equipment – and some domestic equipment – through to the end of the valve era.

Thus I think that one could say:

1. The EF80 was the direct replacement for the EF42, which had been a contemporary of the EF91.

2. The reasons for that change were fourfold: to conform to 300 mA heater current; to work with 170 V HT; to provide improved performance by having two cathode pinouts; to use the B9A base.

3. Like the EF42 before it, the EF80 was intended to run alongside the EF91, and not to replace it. Whilst they overlapped, there were differences, and the EF91 evidently was a good fit for many industrial applications.


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Old 2nd Jul 2019, 3:26 am   #16
Synchrodyne
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Default Re: EF91 to EF80

This pair of Mullard advertisements from WW 1948 illustrate both the overlap of and the differences between the EF42 and EF91.

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The EF42 was presented as a TV frequency changer. The EF91 was presented as being suitable for several applications in TV, FM and VHF communications receivers, including RF amplifier, mixer, oscillator and IF amplifier. The mixer and oscillator example circuit shown was very similar to that shown for the EF42.

These 1954 Mullard advertisements for the EF80 and EF91 show their similarities more than their differences.

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The EF80 was described as being primarily a RF or IF amplifier for TV receivers, but also suitable for use as a mixer, video amplifier or sync separator.

The EF91 was described as being primarily a RF amplifier or mixer for TV receivers. This was rather a narrow definition of its application range, but then Mullard did sometimes tailor its advertising to show specific rather than all applications of a given valve. (The ECC81 provides a good example of this.)

Be that as it may, advertising a high-slope pentode as a TV RF amplifier in late 1954 was rather anachronistic, as by then rest-of-world best practice had moved to the cascode as the choice for this job, and the UK was on the cusp of catching up with change.

The EF80 was supplemented, but not supplanted by the frame-grid EF184 in 1959. The EF184 also appears to have been the industrial offering in this class, so it also could be seen as supplementing the EF91.


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Old 2nd Jul 2019, 10:54 am   #17
short wave
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Default Re: EF91 to EF80

first 2 lines here may give a clue

http://www.r-type.org/exhib/aaa0008.htm
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Old 2nd Jul 2019, 12:51 pm   #18
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Default Re: EF91 to EF80

Usual definitive post from Synchrodyne! Superb, and thanks - I've wondered the same myself, about the EF80 and EF91.

Following on from the link by shortwave, why is the vari-μ counterpart to the EF80 not the EF81, as the EF91 and EF92 numbers are paired?

And yes, the internal screen, twin cathode leads, and suppressor grid pin for the EF80 does make it highly versatile.
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Old 2nd Jul 2019, 2:39 pm   #19
G8HQP Dave
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Default Re: EF91 to EF80

The EF92 is not a vari-mu version of EF91; it has much lower transconductance for a start. The two valves are not really related at all. The EF92 could be considered as being either
1. a miniature version of some octal RF pentodes such as 6K7
2. an EBF80 with no diodes

I suspect that the EF92 was developed when someone realised that there is more to valve and circuit design than raw gain, and that the requirements of an RF/IF pentode for AM and other narrow-band systems can be quite different from wideband systems like TV, FM and radar.
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Old 2nd Jul 2019, 7:43 pm   #20
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Default Re: EF91 to EF80

Hi Synchrodyne, thanks for the posting of the WW adverts.
I have several of these valve application sheets and they are very useful for reference data. Do you know how many/ which valves were covered and if they are available on line.

Thanks, Ed
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