EL84 electrode connection anomalies.

[Diabolical Artificer]

Split from this thread.

Originally Posted by Valvepower

Hi,

I encountered this exact same problem with a Verdik 10 amplifier, where pin1 was connected as the grid and caused problems with a pair of Russian EL84’s. I didn’t get as far as putting a signal into amplifier as the anode current climbed very steeply and I removed the power and investigated the problem further.

Regards
Terry.

I find this interesting. I have four Russian EL84,s but the pin outs are the same as a Mullard spec. IE Pin 1,NC, 2,G1, 3,K, 4+5,HTRS,6,NC, 7,a, 8,NC, 9,G2. Cathode and G3 internally connected. This is the first I,d heard of any differences in pinouts of different makes of valves. Andy.

Mods,forgive my illiteracy and inability to punctuate properly,I wanted to start a seperate thread,but was unable to,thanks.

[turretslug]

I encountered EL84 pin-out discrepancies years ago with a Sansui stereo receiver (p-p EL84 O/Ps).

The amp was wired to expect pin 6 as G2 (it was stamped "6BQ5" on the chassis, perhaps not always quite as equivalent as expected). It's the sort of thing that can be established quickly and easily with a bright light and a magnifying glass (getter permitting) but something to be wary of- especially when the pinout has the ominously ambiguous "IC" (internal connection).

Colin.

[jon_the_id]

The Mullard spec from the spec sheet here http://www.hifitubes.nl/weblog/wp-co...llard-el84.pdf
is:

1: internal connection
2: g1
3: g3
4: heater
5: heater
6: internal connection
7: anode
8: internal connection
9: g2

The Mullard EL84s I had had pins 1&2 shorted internally.
Not sure what pin 6 was connected to though.

[Leon Crampin]

The EL84 is a very marginal design and is prone to grid current because the control grid runs too hot.

Philips mitigated the effects of this by fitting grid rod radiating plates to the upper ends of the rods, by using copper rods and by connecting each grid rod to a pin on later valves. This is why pins 1 & 2 are each connected to a grid rod - improved heat conduction.

It's interesting to observe that the radiating plates were modified on the very last EL84s to be matt black on the outer (radiating) faces and mirrored on the inner faces. This was presumably to minimise the effects of one plate radiating energy to the other. The other change they made was to weld the plates obliquely to the plane in which the rods lie to minimise cross-radiation.

In a vacuum, the only ways of shifting heat energy are either by radiation or conduction - convection won't occur.

The EL84 is not a good design and suffers from excessive power density as a pentode. If Philips had not been so stuffy about their (own patented) pentode configuration and designed it as a beam tetrode, the power density problems could have been overcome.

The 6CH6 has practically the same ratings in a smaller envelope and does not suffer from grid current. Aligned grids allow the electrons to flow relatively unimpeded to the anode without uselessly heating the intervening electrodes.

Leon.

[turretslug]

'Tis a worthy and under-appreciated little bottle, the 6CH6 (EL821), I have a few stashed- at one point, they were thrown away by the bulk carton. Such is the fate of many "pro" valves. The low grid bias means maximum HT utilisation and low cathode resistor dissipation, there's not even much rewiring for applications that originally used EL84's.

I have a Pye CAT619 general coverage receiver with an EL821 O/P stage, I'm really struggling to think of "pro" applications that use EL84's. Perhaps it was only ever intended as a short-lived, high gain consumer device for single-stage amps (e.g. crystal pick-ups, radios with high-level detection).

Sooner or later, the audiophools will cotton on to the 6CH6...

[ppppenguin]

Would the EL822 also make a reasonable EL84 substitute? http://www.mif.pg.gda.pl/homepages/f...29/e/EL822.pdf

I happen to have a few NOS lurking

[Leon Crampin]

Does anyone know what the difference(s) are between the EL821 and the 822?

I originally thought that the 822 had a reduced screen voltage rating, but your data sheet does not bear this out.

The 6CH6 as an SQ type (CV4055) is a truly excellent device freely available at modest cost. This information is of course, confidential to this forum...

Leon.

[ppppenguin]

Here's the EL821 data, also from Mullard. http://www.mif.pg.gda.pl/homepages/f...29/e/EL821.pdf

And for completeness the Mullard EL84 data: http://www.mif.pg.gda.pl/homepages/f...010/e/EL84.pdf

The EL822 presumably has better high frequency properties than the EL84 or EL821 since its intended application is professional video amplifiers. This may mean greater care needed with grid stoppers and/or ferrite beads.

[Leon Crampin]

To answer Jeffrey's question above - the 6CH6 is an excellent substitute for the EL84 in most applications.

I have two Bush VHF61 radios, one fitted with a 6CH6 (suitably biased) and the other as specified with a good EL84. The two sets are practically indistinguishable in performance - although if I were deluding myself I might favour the 6CH6. In truth, given the size of the loudspeaker in this set, there's no difference.

I know which one would be better after about 1000 hours' use though.

Leon.

[cmjones01]

At the risk of thread drift, I'm curious about other power valves in small packages and their shortcomings. I have a handful of N78 valves which have ratings almost as high as the EL84 but in an even slimmer B7G bottle. Does anyone know if they start to suffer from grid current, for example?

[Valvepower]

Hi All,

Regarding the EL821 It does look an interesting valve – I’ll look out for a few at swapmeets to try out.

Further to my original post I’ve looked at an earlier Verdik amplifier (5-10 versions) and this was wired correctly, with pin2 as the g1 connection.

I’ve seen another 6BR8 based Verdik amplifier and this was wired with pin1 as the g1 connection. There would have been a good period of time between the manufacture of these two amplifiers so it was not a one off wiring fault but existed in the 6BR8 variant of this amplifier. Maybe the original engineering unit was wired incorrectly but wasn’t picked up because of the internal connection between pin1 and 2.

I’ve had a look at some EL84’s and I noted on a few them one of the grid supports was connected to pin1 and the other grid support was connected to pin2 whereas the rest just had a direct connection between pin1 and 2 on the base as described by Leon above.

I remember reading Leak had thermal runaway problems with the EL84 in early amplifiers. Leak did modify the design as the EL84’s were being by overrun with high g1 resistors. The problems discussed by Leon wouldn’t have helped with thermal run way problems in these designs.

Regards
Terry.

[murphyv310]

Hi.
N78's in my opinion is a very poor valve and has a limited life. I am more of a beam tetrode & KT person.

[Chris Parry]

All agreed what's been said here about types EL84, 6CH6 and N78. All of them run too hot in service. The difference is that the 6CH6 can take it in the longer term, and the others can't. Valvebases can often be found badly carbonised, unless PTFE or ceramic. Beam tetrodes? My opinion: good in push-pull, but inclined to distortion when used single-ended. Having said that though, the 6V6 has to be the yardstick for reliability and longevity. Chris.

[Leon Crampin]

The 6V6, 6BW6 and 6AQ5 are all the same device in octal, B9A and B7G forms respectively. The 6AQ5 is slightly derated.

The 6V6 is indeed a yardstick for reliability but of course has the lowest power density. The 6BW6 seems to survive well but I have no first hand experience of the 6AQ5.

They are all efficient beam tetrodes - with a modest heater power requirement for their anode dissipation rating. The problem with all of them is poor slope.

In a radio with a triode pre-amplifier with a mu of about 40 there is insufficient gain to fully drive the output stage let alone provide a margin for the application of feedback. FM radios need more gain - which is also handy when a low output cartridge is connected to the gram input.

These valves therefore never found much favour in the UK. MOV, who blatantly copied the American octal range had to design their own output valve (the KT61) to provide adequate UK performance.

Nearly all of the large envelope output devices are reliable - an EL33 is just as good as a 6V6 and has the higher slope.

Leon.

[G8HQP Dave]

In a radio with a triode pre-amplifier with a mu of about 40 there is insufficient gain to fully drive the output stage

There are short superhets (e.g. Pye P45) which have no audio triode, just a high slope output valve (EL41). Seem to work fine. An output valve with gain about 3 or 4 times less is quite happy with an audio stage giving a gain of 10 or more. The 'standard' triode+dd (e.g. 6Q7, EBC41, EBC90) has a mu of 70 so plenty of gain.

[Leon Crampin]

Fair point of course, but you would be hard pressed to make a decent short superhet with a 6V6 output stage.

As it is, European short superhets are generally bargain basement items designed primarily for local station use. The low audio gain usually precludes the use of feedback (discounting the Philips system as used on the 170A where the feedback reduces as the volume control is advanced and actually reverses in phase) and it also necessitates the use of a loudspeaker designed for efficiency rather than uniform frequency response.

European sets had high slope output valves since the mid '30s and for most sets, a low mu triode preamplifier was all that was needed. Later triodes had higher values of mu as pointed out, but then FM and more usable pickup gain were requirements and feedback was nearly always used on the better sets.

The Americans did go to some lengths to offset the poor gms of their beam tetrodes - the 6AV6 double diode triode has a mu of 100.


Reverting to an earlier topic, the EL821 and 822 are indeed different - apart from anything else they're different sizes. The EL822 is the same length as an EL84 which is handy for Bush sets which use a retainer.

The EL822 will also operate at EL84 bias levels - so the valves should be a straight swap with a minor re-wire. I have a couple of new EL822s so I'll try this out in a VHF64 and report back.

Leon.

[Chris Parry]

Surely 6V6 is not efficient! Uses several % of available HT just to set up its bias, and then when presented with an AC signal, saturates poorly. Later USA types such as 6DS5 saturated far better, and used less bias. The distortion is noticable with this type but there again, it is a beam tetrode....

I look for EL95 and its predesessor the EL42 for efficiency. These things need little bias and "sound nice" in class A. Lead short lives though, if thrashed.

Discussing EL821 & 822: Type 6CL6 is a miniaturised 6AG7, available as the even smaller type 6AH6. Good driver valves these are. Not very efficient, though.

As for MOV copying the USA standard range, worth pointing out that KTW61/2 is a BT version of 6U7G, and no USA equivalent exists. Similarly, the X65/66 is not a straight copy of type 6J8G. The MHLD6 is a wholly British valve, with no equivalent in the USA standard range. MOV deserve more credit, I reckon.

Chris

[Neil Purling]

If you were utilising a 6CH6 in place of a EL84 you have to more one connection & use a link wire as g3 & the cathode aren't internally connected in the 6CH6.
That much I can deduce. What about changing the grid bias? A smaller resistor?
But other than that the gain is the same?

[Leon Crampin]

The 6CH6 does indeed need less bias than the EL84/822 and most other output valves - with the notable exception of the KT61.

This is readily achieved by shunting the existing bias resistor to give a total value of 100 Ohms. It's dead easy to do, takes out any inaccuracies in the existing bias resistor and reduces the dissipation in it. The existing bias can easily be restored in the event of reversion to the original valve type.

There are several measures of "efficiency" relating to output devices and a low "knee" is certainly a valuable attribute - most often found in beam tetrodes. Valves needing a high bias do indeed shoot themselves in the foot in terms of wasted energy - again the 6CH6 scores well in this respect.

I'm afraid I can't share Chris's enthusiasm for the Philips 200mA heater range of "output" valves (EL2/32, EL42, EL95 etc.). Starved of heater power they have a short life and indifferent performance. OK as a tape bias oscillator perhaps. Why ever did the EL2/32 have a top cap grid? Was it really an RF pentode originally?....

Leon.

[turretslug]

Now nicely off-topic, the 6CH6 can be had in wire-ended CV4056 guise, where it offers the possibility of fitting to the base of a worn-out and difficult to find valve without chassis surgery. It'll need re-biasing- but at least if you don't bother, it's much more likely to be under-, rather than over-, run!

I've a few of these, again rescue items, but they sometimes appear for very little outlay.

Colin.