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Old 25th Feb 2017, 11:48 am   #1
Edward Huggins
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Default The Mullard EL42

The recent "Preferred Black Box" thread made me think about this little valve.

I built an amplifier in the mid 1950s using an EF40 (low-noise in its day) EL42 and EZ40 - or sort of pre-Mullard "3-3".

It occupied a strange place in the then Mullard range as it was lower powered than the more usually found EL41, but nice in Push Pull and with just enough power to allow UL configuration. But they were not used much elsewhere.

Too low powered (2.2watts) for a Radiogram or a large Radio, one wonders in some ways, and however sweet a little valve it is, why Mullard developed it and for what applications? I wonder how other learned Members regard the EL42?
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Old 25th Feb 2017, 11:53 am   #2
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Default Re: The Mullard EL42

I believe it was originally developed for use in car radios.
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Old 25th Feb 2017, 12:12 pm   #3
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Default Re: The Mullard EL42

The B8A version of the earlier EL32 often employed in low power transmitters. It's low heater power, 200m/a was a great saving in car radio as Paul has mentioned. A good little valve. John.
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Old 25th Feb 2017, 2:14 pm   #4
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Default Re: The Mullard EL42

I wondered if Pye got a good deal on them as they weren't so popular? Ditto the PCL83, why not the more obvious ECL83?
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Old 25th Feb 2017, 5:01 pm   #5
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Default Re: The Mullard EL42

As noted, I too believe it was intended for car radios.

Think of it as being the B8-based option for those who refused to embrace that other diddy little output valve the EL91.
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Old 25th Feb 2017, 6:53 pm   #6
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Default Re: The Mullard EL42

Quote:
Originally Posted by wd40addict View Post
I wondered if Pye got a good deal on them as they weren't so popular? Ditto the PCL83, why not the more obvious ECL83?
Pye used the PCL83 in a lot of their 1950s TVs for sound output, the VT17 for example. If they were buying a lot of PCL83s they could probably get them cheaper than ECL83s.
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Old 25th Feb 2017, 10:44 pm   #7
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Default Re: The Mullard EL42

The EL42 was part of the original Philips Rimlock series, and it was designed as a low-power consumption output valve for use in car radios and other vibrator-powered receivers. It was specified for push-pull as well as single-ended operation. This was covered in Philips Book IIIA, page 106ff; see: https://frank.pocnet.net/sheets/046/...kIIIa_1952.pdf.

I imagine that its heater current, 200 mA, was chosen to align with that of the EF41 and EAF41/EAF42, to facilitate the use of series pairs in 12-volt car radios. The EBC41 and ECH41/ECH42 both had 230 mA heaters, and the EZ41, also designed for car radio applications, had a 400 mA heater.

One may see that an EL42 would be attractive for a domestic appliance where a push-pull output was desired, but where the output power requirement was modest such that an EL41 pair would be “overkill”, and at a time when apart from the sometimes awkward-to-use ECL80, suitable audio triode-pentodes had yet to appear.

The successor to the EL42 was the EL85, which retained the same audio characteristics, but which had a separate suppressor grid pinout rather than an internal connection to the cathode. Evidently this was because as well as its AF role, it was also specified for use as a mobile equipment RF amplifier at frequencies of up to 120 MHz. I am not sure exactly when the EL85 was released, but suspect it was, in a proximate sense at least, in the final batch of Philips’ noval-based radio issues, along with the EBC81, EF89 and EBF89.

The ECL83 and PCL83 had quite different triode sections, although their pentode sections were similar. The ECL83 had an 85 mu triode, whereas the PCL83 had a 17 mu triode. Evidently some amplifier designers preferred the lower-mu triode of the PCL83. For example, the GEC “Junior” amplifier used a pair of LN309 (PCL83) in an ultra-linear circuit; there does not appear to have been a non-technical reason for not using the HN309 (ECL83), so one assumes that the LN309 was a better fit. (This circuit was described in the GEC amplifier book, as well as in “Radio Constructor” for 1956 November, page 252ff.)

Although the PCL83 was a member of the 300 mA series-string heater group, its heater voltage of 12.6 volts meant that it was not so difficult to use in conventional AC equipment, and it was also easily used in 12-volt car radios.


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Old 26th Feb 2017, 12:21 am   #8
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Default Re: The Mullard EL42

The common cathode of the ECL80 is not such a problem in a battery-powered set. You can tie the cathode to the positive terminal of the battery; and then you have a negative supply from which you can derive independent bias voltages for the triode and pentode grids. You also waste less of your hard-won HT.

(That's how I did it, anyway, when I built a battery-powered ECL80 amplifier -- and I can't believe I invented it .....)
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Old 26th Feb 2017, 9:52 am   #9
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Default Re: The Mullard EL42

I think in the Pye case the reason that they used the PCL83 in the Black Box was simply because they had thousands of them left over from television production. They were a highly strung valve that could give instability problems when used in sound and frame output stages.
Television receivers tended to employ the very latest Mullard valves and by 1957 this would have been the PCL82. I suspect that someone discovered thousands of redundant PCL83's in the stores and thought, 'what are we going to do with these?'
Use them in the Black Box was the probable answer.
The mains transformer also required modification due to the PCL83's 12.6v heater. I can't believe Pye would have gone to all this trouble when they could have purchased the more sensible ECL83. In my opinion the PCL83 version of the Black Box is nasty.
Cramped chassis that tended to get rather warm with the contact cooled rectifier squeezed up on the chassis.
I think the EL42 version is without doubt the best of a mediocre lot. John.
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Old 26th Feb 2017, 11:30 am   #10
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Default Re: The Mullard EL42

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Originally Posted by Synchrodyne View Post
The ECL83 and PCL83 had quite different triode sections, although their pentode sections were similar. The ECL83 had an 85 mu triode, whereas the PCL83 had a 17 mu triode.
Just for completeness it seems the UCL83 is the virtually same valve as the ECL83 (with the exception of the heater) sharing the higher mu characteristic of the triode section.
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Old 26th Feb 2017, 11:04 pm   #11
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Default Re: The Mullard EL42

for further completeness, the EF40 has identical characteristics apart from heater draw to the earlier EF37 and later EF86 the difference mainly being the base........................

Which suggests that Mullard/phillips got it right before WW2.

A.
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Old 26th Feb 2017, 11:06 pm   #12
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Default Re: The Mullard EL42

There may also have been an element of chronology in Pye’s use of the PCL83 in its Black Box series. As best I can determine, the PCL83 preceded the ECL83 and UCL83. So perhaps the ECL83 was not yet available when Pye did the design work for the PCL83-based circuit.

The PCL83 was evidently designed with TV field oscillator and output as its primary application, which might explain the choice of a medium-mu triode, roughly comparable to that in the ECC82. But there was quite a bit of overlap between TV field and audio applications, so the use of the PCL83 as an AF amplifier and output combination was not surprising. Also, the use of medium-mu rather than high-mu triodes for AF amplification was, if not common, not unknown. For example, in the late 1940s the 6BF6 double-diode triode, with a 17 mu triode, was introduced as an alternative to the more familiar 6AT6 (70 mu triode) and 6AV6 (100 mu triode). The 6BF6 triode was said to be particularly suitable for use as an output driver in car radios with push-pull outputs, as evidenced by this RCA advertisement from 1947: In that context, the use of a PCL83 pair for a push-pull audio amplifier does not appear to be too unreasonable.

Click image for larger version

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Speaking of car radios, Pye used the PCL83 in its 1956 TCR16 hybrid chassis. Here It was employed as an AF amplifier and driver for the push-pull transistor output pair.

Returning to the EL42, there was also a B7G-base derivative, the EL95, which looks to have been a contemporary of the EL85. Unlike the latter, it retained the internal suppressor grid connection. But both the EL85 and EL95 almost certainly post-dated the original Pye Black Box design.


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Old 27th Feb 2017, 10:32 pm   #13
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Default Re: The Mullard EL42

Rather belatedly, I thought to actually look at the circuit for the PCL83 version of the Black Box (BBA et al), and that did provide further insights.

It was actually somewhat unusual, consisting of half an ECC82 as an input stage, followed by the other half of same as phase splitter, with each PCL83 triode section acting as driver for its corresponding ultra-linear connected pentode output section. The main feedback loop went to the cathode of the input triode, but there were subsidiary loops, one to the grid of the phase splitter as part of the treble control, and another, which I haven’t figured out, but which looks as if it had both positive and negative elements, rather like the damping control loop on the Pye Mozart amplifier, but without the variable control.

That made it look somewhat like the Williamson amplifier, whose input, phase splitter and driver stages were based upon the 6SN7GT medium-mu double triode. And the ECC82 was, in a proximate sense, the miniature successor to the 6SN7GT. Not only that, but the PCL83 triode was very similar to that in the ECC82, same mu and slope, but having a higher allowable anode dissipation, 3.5 W as compared with 2.75 W.

So, even if the use of the PCL83 were effectively a forced choice for availability, inventory and/or commonality reasons, Pye evidently put some thought and effort into the design of the circuit that used it, such that it was not simply a misfit in a circuit that would have done better with a different valve.

Re the initial Black Box BBH series, I suspect that the use of a triode-connected EL42 pair was quite unusual; most car radio applications probably had them pentode-connected. Similarly their use with an ultralinear connection in the BBH Mk II series was probably equally unusual. In unusuality stakes, Pye must have been a rare case of an equipment maker who used both the conventional (Blumlein; Hafler & Keroes) ultra-linear circuit, as in the BBH Mk II and BBA, and the Walker partial cathode loading circuit, as in the Mozart HFA10 amplifier.


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Old 28th Feb 2017, 1:42 am   #14
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Default Re: The Mullard EL42

Quote:
Originally Posted by bikerhifinut View Post
for further completeness, the EF40 has identical characteristics apart from heater draw to the earlier EF37 and later EF86 the difference mainly being the base........................

Which suggests that Mullard/phillips got it right before WW2.
As I understand the chronology, the EF40 was part of the initial Philips Rimlock valve range and introduced the bifilar heater for lower hum transfer than was the case with the EF37, which originated as a lower microphony version of the EF36. Then somewhat later, circa 1950, the EF37A, with the bifilar heater, was introduced, initially supplementing but later supplanting the EF37. It was described in Mullard Outlook Volume 1, Number 3, 1950 November. The EF86, as said essentially a rebased EF40, followed circa 1952. Data on hand suggests that the EF37, EF37A, EF40 and EF86 all had 6.3 volt, 200 mA heaters. So they may have been suitable for 200 mA series strings as well as for 6.3-volt parallel circuits. The 200 mA concept was pretty much dead (except perhaps for Mazda) by the time the EF86 arrived, so its 200 mA heater was more happenstance carryover. But both 100 mA (UF86) and 300 mA (PF86) versions followed, although possibly their very low hum attribute were somewhat wasted in series-string circuits.


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Old 9th Mar 2017, 12:17 am   #15
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Default Re: The Mullard EL42

Quote:
Originally Posted by Heatercathodeshort View Post
The B8A version of the earlier EL32 often employed in low power transmitters. It's low heater power, 200m/a was a great saving in car radio as Paul has mentioned. A good little valve. John.
A curious aspect of the transition from the EL32 to the EL42 was a reduction in maximum anode dissipation from 8 W to 6 W. In general this would not have been an automatic by-product of miniaturization, in that the Philips Rimlock valve bulb diameter was chosen with 9 W output pentodes in mind, namely the EL41 and UL41.

But a clue is found in this comment from the EL42 description in Philips Book IIIA: “As the amount of space in car-radio sets is usually restricted, the dimensions of the EL42 have been kept at minimum; this valve is no larger than the pre-amplifier valves." Quite possibly then it was the design target maximum length for the EL42, and its resultant restriction on anode length and hence anode area, that determined the 6 W maximum anode dissipation number, which was nevertheless deemed still to be adequate for its intended applications.

The EL40 was 60 mm over tip and pins, as were the EF41, EAF41/42, ECH41/42, EBC41, EF40 and EZ41. The ECC40, which might have found application as amplifier and phase splitter ahead of an EL42 push-pull pair, was a bit longer at 67 mm. In contrast, the EL41 and UL41 were both 76 mm overall.

The EL32 was described by Mullard as an “Output pentode with 200 mA heater for use in A.C., D.C./A.C. or car radio receivers.” So the 200 mA heater, which the EL42 inherited, was a deliberate choice to fit 200 mA series-strings as well as 6.3-volt parallel arrays.

The EL85, successor to the EL42, was a bit longer, at 67 mm overall. As mentioned upthread, the EL85 was recast as being a mobile equipment RF amplifier as well as an AF amplifier, the latter presented as a subsidiary role. That no doubt reflected the then-growing market for mobile VHF RT equipment and perhaps also it anticipated that car radios would soon move to the hybrid form, with an output transistor rather than an output valve.


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Old 16th Mar 2017, 5:29 pm   #16
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Default Re: The Mullard EL42

Grundig used the EL42 as an erase/bias oscillator in tape recorders in the 1950s. Its mediocre performance compared to the EL84 used för AF output was probably sufficient för this application.
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Old 16th Mar 2017, 6:17 pm   #17
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Default Re: The Mullard EL42

What are people's experiences of EL42 reliability?

I ask because every time I've encountered them in TV sets I've had problems.
Either they have been downright low emission, or show symptoms of being slightly gassy and becoming unusable when hot.
This has been most evident when used as frame output valves in sets where all the usual causes have already been eliminated, leading to much gnashing of teeth and pulling out of hair.

It has been compounded with NOS Mullard replacements showing the exact same symptoms, and only careful selection of old stock items... even such as those removed from car radios over 45 years ago... has saved the day and got the sets going and drift-free.

Perhaps it's a side effect of the rigid pins and button base which has permitted a few molecules of air to get in (?) but if so, no amount of running has ever got the getters to soak up this gas.

Or have I just been unlucky?

Pete
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