EL84/UL84 How do they differ?

[Tractorfan]

Hi All,
I've been following the thread here about substituting an EL84 for a UL84. I fully understand the different heater ratings and realise that they have different characteristics. But.
Apart from sticking in a different heater, how exactly do the designers achieve these differences in two valves which are both output pentodes and outwardly look the same? Is it by changing the electrode design or spacing? Or is it some other kind of magic that they work?
It's a bit like ECC81/2/3's - different impedances- all triodes. How
Cheers de Pete

[paulsherwin]

The differences are down to the design and spacing of the various elements.

The EL84 is a general purpose medium power output pentode. The UL84 is derived from it but was specifically designed for use in AC/DC class A audio amps where the HT is obtained by half wave rectifying the mains.

The HT in these designs is usually no more than 200V and often a lot less, so the UL84 operates with a lower anode voltage and higher anode current than the EL84 allowing it to deliver similar output power from the reduced voltage. Some American O/P valves are designed for use with HT as low as 100V at an even higher current.

Paul

[Sideband]

I don't have a characteristic book to hand but I think you will also find that the UL84 has a higher slope than the EL84. This is so that it can be used in things like single valve record players where the grid is driven directly from a high output cartridge. The HT on these is seldom more than 200v so the idea is to get as much gain as possible from the single valve.


Rich.

[WME_bill]

An interesting point. We usually assume that it is only the heater current /voltage rating which changes from european ELxx to ULxx to PLxx type numbers. It has intrigued me for many years that apparently the only case where this does not apply to Mullard/Philips is the EL86, which appears otherwise identical to the UL84 and PL84. Almost as if Mullard developed the EL84 output pentode (1956?) first. Perhaps it was Philips who later produced the UL84 and PL84, and then found that the EL84 was already used for an extremely popular output pentode, so they had to adopt EL86 for the 6.3v heater version. The EL86 has a higher maximum current rating than the EL84. This suggests a larger cathode area, so the rest of the construction will be different. As the power dissipation is the same, probably the anode area will be similar, so other internal differences like grid spacing etc will not be visible. The higher cathode current rating and lower impedance is why it is often used as a series regulator valve. The "push-pull" series output stage design was I suspect more popular in Europe than in UK. Bill

[Peter.N.]

The popular range of TV valves were also available with different heater voltages, there was an EL81 and an EY81 which seem to have had the same characteristics as the 'P' series. I once converted an old pye TV, LV20 I think, to run on 12 volts for a friend who had no mains electricity supply, using the 6.3 volt valves in a series parallel arangement and a goverment surplus rotary converter for the HT.

Peter.N.

[julie_m]

EL84 isn't exactly a low-gain device, though.

I have a Dansette record player which uses just an EL84 and an early semiconductor rectifier. HT and LT are obtained from a transformer, with the EL84 driven from the pick-up cartridge via volume and tone controls.

[Mike Phelan]

There was also an EL509 and EY500, used in French EMO colour sets.

I still maintain that for practical purposes, an EL84 can be used as a UL84 if you sort out a heater supply, despite the fact that there are some other differences.

The converse would not apply - the Hammond PR40 tone cabinet used 6 EL84s with the anodes normally glowing a dull red - cheap EL84s such as Zaerix did not last very long.

[Brian R Pateman]

Originally Posted by Mike Phelan

The Hammond PR40 tone cabinet used 6 EL84s with the anodes normally glowing a dull red - cheap EL84s such as Zaerix did not last very long.

And if I remember correctly, it was a false economy to change one on its own!

Regards,

[paulsherwin]

Originally Posted by Mike Phelan

I still maintain that for practical purposes, an EL84 can be used as a UL84 if you sort out a heater supply, despite the fact that there are some other differences.

This is true of most output pentodes in typical applications where they aren't being pushed to their limits. A socket rewire or adaptor will be needed in many cases of course, and the cathode resistor may need tweaking. Differences in slope will usually not produce audible effects in a typical domestic radio.

In fact there are lots of UL84s around, so it's not usually necessary to make a substitution.

Paul

[valveaudio]

Just as a matter of interest there was a E84L as well, this had lower capacitance. We used to use them in 5W 27MHz paging transmitters.

Trevor

[Tractorfan]

Thanks all for your interesting replies.
It's something I've wondered for years on & off. I sort of assume that if one ran an EL84 at a reduced HT voltage and increased HT current then it might mimic the characteristics of a UL84, or have got that wrong?
All clever stuff!
Cheers de Pete

[paulsherwin]

Originally Posted by Tractorfan

I sort of assume that if one ran an EL84 at a reduced HT voltage and increased HT current then it might mimic the characteristics of a UL84, or have got that wrong?

You can't exceed the rated current for the valve regardless of the HT voltage. The UL84 is specifically designed to operate with a higher current than an EL84 to compensate for the reduced HT.

Paul

[Tractorfan]

Of course not, silly me. Thanks Paul. It was a bit of an academic question anyway as I've got a couple of UL84's (and one or two PL84's) in the old junkbox but nowt to plug 'em into anymore.
Cheers de Pete