Advantages of ECC85 over ECC81?

[Tractorfan]

Hi all,
My old KB radiogram, (about which I'm constantly going on about & boring everyone witless), uses an ECC81 for the FM front end. Shortly afterwards the ECC85 was developed for this role and used from then on. This started me wondering why?
What's the advantage of an ECC85 over the ECC81 given that the '85 is said to have a short lifespan? Would it be worthwhile putting an '85 in the KB after altering the heater connections? I've already changed the 12AH8 for an ECH81 so originality isn't an issue, it's just nice to have it working on FM again.
Cheers de Pete

[G8HQP Dave]

There was some discussion about ECC81 vs. ECC85 in this thread. The outcome was unclear. They look very similar, except for the heater arrangements and the internal screen. The screen would help reduce oscillator radiation. My guess is that 85s have a short life because they are used at high anode current i.e. it is the circuit which is the problem rather than the valve.

[paulsherwin]

The ECC85 is a later valve developed by Philips/Mullard. The vast majority of American makers of FM radios carried on using the 12AT7/ECC81, and KB were heavily influenced by American practice. Most European makers adopted the ECC85.

[kalee20]

I have a Ferranti U1003 which uses a UCC85 in the FM front end - same one as when my mother bought the radio second hand in 1961. Still works fine, after many thousands of hours use (the volume control is wearing out though).

I also have a Grundig with ECC85 in the front end - may have changed it in the late 1970's, but definitely not since.

So it's a myth that the ECC85 is short-lived, unless abused in the application.

[Sideband]

Originally Posted by kalee20

So it's a myth that the ECC85 is short-lived, unless abused in the application.

I tend to agree with this as well. I have several sets with original ECC85's and they work well. Some years ago, I bought several ECC85's from eBay in order to stock up on what I thought were potentially unreliable valves (and it was also a valve that I didn't have at all). I used one, three years ago on a Fergy that really had been heavily used.

I fitted one (not from the above stock) to my Baird 301 some 20 years ago when I originally restored it and it is still working well (and it's had reasonable use during that time).

When I was a child, we had a Fergy set that was used extensively on VHF and it never had the ECC85 replaced in 15 years......

SB

[XTC]

I thought the problem of the perceived unreliability of the ECC85 was caused by AM/FM sets where the FM setting was hardly ever used and there was no anode voltage on the ECC85 on the AM setting. Running a valve for long periods with no cathode current causes poisoning. If you run an ECC85 under proper working conditions, it's no better or worse than any similar valve.

Pete.

[Synchrodyne]

I imagine that we’d need to find some original Philips/Mullard materials to ascertain just what were the key elements in the ECC85 design brief. The ECC81 seems to have been a general-purpose VHF double-triode, usable up to around 300 MHz (needs verification) and able to function as an RF amplifier, mixer, self-oscillating mixer, oscillator, shunt cascode amplifier, etc. It was also used as an AF amplifier, and advertised as such – see the attachment. Its split heater allowed it to be used in 6.3 and 12.6 V, and 150 and 300 mA heater systems. I don’t think that there was a UCC81 (100 mA), though.

The ECC85 was probably more narrowly focused on its FM front end role. Maybe – and this is purely speculative - frequency capability as an amplifier was traded for specific performance in say the self-oscillating frequency changer function, as it wouldn’t be expected to operate at much above 100 MHz. Given its heater parameters, both UCC85 and PCC85 variants were required, which added product line complexity. Thus one might reasonably conclude that Philips/Mullard had cogent reasons for adding three valves to its range – although adding a “UCC” variant, whether UCC81 or UCC85, appeared to be necessary anyway. Or perhaps it was simply that the interstage screen, mentioned by G8HQP Dave, was desperately needed to make it easier for set designers to reach oscillator radiation targets, and so provided the opportunity for a major redesign.

Cheers,

[paulsherwin]

European designers don't seem to have liked the ECC81 at all, and very early FM radios mostly used a pair of EF80s in the front end. I only have one radio using an ECC81, a KB MR10, and that doesn't perform very well, though this may be due to other factors than the front end valve type.

[G8HQP Dave]

Is it possible that the ECC85 was partly a 'spoiler', to prevent people from using cheap imported 12AT7 instead of nice shiny European ECC81? We must remember that in those days the rules about buying and selling were very different from today. Cartels such as BVA were encouraged, not banned. UK manufacturers were even forced to give identical valves (often from the same factory) different names to encourage brand loyalty. Did European manufacturers have to pay a royalty on each ECC81 they made, as it was a copy of an American design?

The reason why I suspect something like this is that in Philips own documentation it makes false claims about ECC85 - such as higher input impedance (only looks higher than ECC81 because measured at a lower frequency). The only really true claim is the presence of a screen.

Got to go, second half started!

[DangerMan]

I have just found my copy of "Mullard Radio Valve Manual (TV and FM)" dated September 1958, and it has a few comments of interest here as they probably show where Mullard were coming from. I can't scan it at the moment but here are the relevant pieces.
Describing a combined triode front-end, it states " .. double triodes such as the ECC81 have inadequate screening between the two sections for use in the front end. They have a high capacitance between the anodes (about 0.4pF) and therefore there can be high oscillator radiation."
It continues:
"The ECC85 has been designed specially for f.m. front-ends. Extensive screening between the two sections reduces the capacitance between its anodes to 40mpF without an external screening can, and to 0.8mpF when a screening can of 22.5mm diameter is used (1mpF is 0.001pF). The ECC85 also has a higher slope and a higher v.h.f. input resistance than the ECC81. The low coupling between sections of the ECC85 makes the problem of reducing oscillator radiation, particularly at the fundamental frequency, much more manageable."
i.e. The ECC85 inter-anode capacitance is a tenth of the ECC81's.
Later in the book, there are detailed data for both ECC81 and ECC85.
The input resistance of ECC81 is quoted at 50MHz, being between 14kohm and 25kohm depending on Va, Vg and Ia. Anode impedance varies between 10.5 and 16.5 kohm.
ECC85 is quoted at 100MHz, Va=230V, Ia=10mA, Vg=-2v, where input resistance =6kohm and Anode impedance is 9.7 kohm.

TTFN
Pete

[Michael Maurice]

Korting used the ECC85 in the amplifier stages of their tape recorders. There's no FM just good old audio. Screening between the sections was not going to be a problem because it used 1 ECC85 per channel

[G8HQP Dave]

DangerMan is quoting from the same book as I have. It states that the input impedance is higher, then gives figures which show that it isn't! Its scales like frequency^2, so at 100MHz an ECC81 would be 3.5-6.25k, compared with ECC85 at 6k. "Higher slope" is true, but only just: 10% better? So the screen is the only significant improvement.

[kalee20]

I'm wondering at this point, why does the ECC85 need 0.435A to heat it whereas the ECC81 needs only 0.3A? Given that an intersection screen is necessary, why didn't Mullard just copy over the rest of the electrode structure from the ECC81 (which obviously worked OK)?

[f.cooper]

Hi,
One of earliest FM receivers released in this country were the Cossor Melody Maker 523(Sep1954)and 522(Oct 1954).Both of these receivers used the Cossor 6AQ8 valve which is listed to be equivalent to the ECC85.Which valve was actually the first to be introduced ?
The main reasons for using the ECC85/6AQ8 was the internal screen to help in the prevention of radiation by the oscillator which could interfere with other receivers on VHF and also TV as the second harmonic occurs within band III.Also good stability at 100MHZ is a requirement which means the reduction of inter electrode capacitance and variation of this capacitance which occurs with changes in temperature.
The ECC81/2/3 were just ordinary triodes with low,medium and high amplification factors for general purpose use in lower frequency applications and were not optimised for use at VHF.

[Tractorfan]

Thanks for all your replies.
I just thought that it was a technical advancement that the ECC85 superseded the ECC81.
So, to come to the final point in my OP. Is it worth shoving an '85 in instead of the '81 after swapping the heater connections & earthing the screen? Would it make any worthwhile improvement? There should be enough "headroom" in the heater supply to cope with the extra current.
Cheers de Pete

[f.cooper]

Hi,
Bearing in mind that the KB was designed to use the ECC81 then I would stay with that although the ECC85 may be better for this application.The component values were obviously chosen for using the ECC81. The down side of changing to the ECC85 may involve or necessitate the complete realignment of the front end of the receiver,
indeed in some cases just changing to another valve of the same type can upset the alignment.

[G8HQP Dave]

The ECC81/2/3 were just ordinary triodes with low,medium and high amplification factors for general purpose use in lower frequency applications and were not optimised for use at VHF.

The ECC81 was intended for VHF and half an '81 is an EC92/6AB4 - an RF triode (for FM and TV applications). I believe the '81 came first (1947 vs. 1952).

Half an '82 is EC90/6C4 - an RF oscillator triode (up to 150MHz). In this case the single triode came first (1942 vs. 1946). So '81 and '82 were at least partly intended for VHF use, but ended up being mainly used for other purposes.

The ECC83 is different. It is a low-hum improvement of the 12AX7 for audio applications.

[kalee20]

Well I'll be jiggered! Thanks Dave for that bit of history.

So: ECC83 is better than 12AX7 as far as hum is concerned? (Although, maybe later 12AX7 heaters copied the ECC83).

However, if the ECC81 was 1947, and ECC82 1946, why weren't the numbers the other way round?

I'm also a bit surprised Mullard didn't push the RF capability of these valves a bit harder.

[G8HQP Dave]

Sorry, I have sowed a minor confusion. The dates I gave were for US introduction. Still leaves the mystery that T comes before U in the alphabet. Maybe order of introduction does not quite correspond to order of registration.

Any ECC83 ought to be better than early 12AX7 for hum. The US equivalent for ECC83 is 7025 - they quickly caught up with Philips! However, it is likely that many later 12AX7 had a low-hum heater. These days many audio amps use DC heaters for the input stage. Just as well, as I understand that some modern versions have a normal heater even when labelled ECC83. So it is probably safe to regard 12AX7/ECC83/7025 as equivalents, although as always equivalent does not necessarily mean identical.

The ECC81 data sheet (1969 version) specifically describes it as an RF triode for use as oscillator, mixer or amplifier in TV receivers. This suggests at least Band III operation. The RSGB Handbook (4th ed, 1968) mentions it in a table of valves for grounded-grid at 432MHz (10dB gain, 10dB noise figure - it was the worst one in the table!).

[f.cooper]

Hi,
It appears that the American version 12AT7 was not popular for the european market, I believe this was introduced before the ECC81.
Just as a matter of interest looking through early TV service sheets the English Electric 1550 (1949-51)was amongst the first to make provision for FM radio, it was a combined radio/tv and used the ECC91 for frequency changer(this had a common cathode). The later model 1650 in 1951-3 used the 12AT7 as frequency changer.Note that both these used an EF91 as an rf amp.
Both the above were introduced long before FM transmissions in this country started.
I believe that the Baird TV P2014 (Sept 1953?)was amongst the first to use the ECC81 as mixer and oscillator, a EF80 used a rf amp.
Although listed as equivalent was the ECC81 actually identical to the 12AT7? Looking at the various manufacturers data sheets there were different claims for a working frequency but nearly all specified grounded grid.