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poynton 9th September 2007 12:46 PM

ECC83 - Long Plate vs Short Plate
 
I have not been able to find the answer to this with a search.

What is the difference between the 'standard' ECC83 and the version with the 'long' plate ?

Are there many variations in plate length and what are they ?

Are there variations in plate dimensions in other ECCxx tubes ?


Also, does the shape of the getter matter ? Does it affect the sound ?


Andy

SY 9th September 2007 01:00 PM

In order:

The length of the plate.

AFAIK, there is no "standard" ECC83 construction. I've got a few dozen different ones and the plates have all sorts of variation in length.

No doubt, but less so. However, some new production ECC81/82 have totally different looking plates, resembling ECC88.

Yes, if you're a collector. No, if all you care about is sound.

poynton 9th September 2007 01:24 PM

Quote:

Originally posted by SY
In order:

The length of the plate.

AFAIK, there is no "standard" ECC83 construction. I've got a few dozen different ones and the plates have all sorts of variation in length.

No doubt, but less so. However, some new production ECC81/82 have totally different looking plates, resembling ECC88.

Yes, if you're a collector. No, if all you care about is sound.

Doesn't the length of the plate (and other variations ) affect the electrical characteristic ?

Andy

Brett 9th September 2007 01:27 PM

Does it matter? They both sound like c*r*a*p.

Cycline3 9th September 2007 01:32 PM

Quote:

Originally posted by poynton
Doesn't the length of the plate (and other variations ) affect the electrical characteristic ?
I'm sure in some fashion it does. However, you must remember that a 12ax7 isn't a spec for how to MAKE a tube - it's a spec for how the tube should PERFORM in a circuit. As such there are more than one means to an end - short plate, long plate, med plate -- and the other changes as required to make each work similarly.

Robert McLean 9th September 2007 02:04 PM

Quote:

Doesn't the length of the plate (and other variations ) affect the electrical characteristic ?
The main factors are cathode material and temperature which determine the total emmission, and the ratio of grid to cathode distance vs plate to cathode distance ( or is it plate to grid ? ) which pretty well determines mu. Then the shape, ie cylindrical vs rectangular vs parallel plate. To a lesser extent the diameters of cathode and grid affect the equations involved. Length of plate doesnt really enter into it, except perhaps to increase heat dissipation. Also I suppose the longer the plate is, other things being equal, the less fringe effects come into play, and the closer the real tube should be to the text book equations.

jlsem 9th September 2007 02:37 PM

Quote:

Also, does the shape of the getter matter ? Does it affect the sound ?
The shape of the getter only tells you when the tube was manufactured or more precisely which production run. Some production runs were better than others in terms of quality, usually due to cost-cutting measures later on. What can really affect how a tube sounds compared to others of the same type is the purity of materials used in critical areas, mainly the cathode. Care in the precision of manufacture is also important. Different brands and different eras of manufacture can and do produce different sounding tubes within each type and physical characteristics such as plate size and shape, getter shape, third micas, etc. only serve as easy identifiers of preferred tubes.

John

JohnAtwood 9th September 2007 04:16 PM

12AX7/ECC83 shapes
 
The 12AX7 was introduced by RCA in late 1946 or early 1947, along with the 12AU7. The original construction was the "long plate" style, identical to the 12AU7 plates. As was common at the time, popular designs were cross-licensed between manufacturers, and since the design was specified to the mechanical level, all American 12AX7s through the 1950s had the identical long-plate construction.

In the early 1950s, various European manufacturers started making their own versions of the 12AX7 (some under license from RCA), and called it the ECC83. The early versions had the RCA-style long plates, but I've seen some some exceptions, such as the Mullard "box-plate" version. Some European tubes started to be imported to America, most notably tubes from Mullard, Telefunken, and Philips (labeled Amperex for the American tube company that Philips bought around 1945). These European tubes were generally made to better standards than the American tubes and typically had lower noise and microphonics. As a result, high-fi vendors started using these tubes. To fight back, RCA and GE came out in 1961 with a re-designed 12AX7 called the 12AX7A that had lower noise and microphonics. They did this by using a shorter plate structure (with the grid pitch and distances adjusted to keep the same electrical specs) which allowed stiffer cathodes and grids. This became the American "short-plate" structure. It seems to be based on the design of the 12AY7 by GE around 1949, which started with a box-plate but by the early 1950s had moved to a short-plate structure. It may also be related to the ruggedized 5751 which came out around the same time and had the short-plate structure.

Interestingly, Sylvania, the other first-tier American Tube manufacturer, made only a long-plate 12AX7A that had more ribbing and reinforcement than the old long-plate. Based on curve-tracer measurements, the Sylvania 12AX7As have somewhat higher bias currents than the standard 12AX7 (my observation), i.e. at a given negative bias, it draws more current.

As far as I can tell, the 7025 is identical to the RCA 12AX7A, but was made to cater to those who wanted an "industrial" type number. This was kind of a fad with hifi designers in the late 1950s. Maybe it was tested to tighter specs, but most 7025s are marked 7025/12AX7/12AX7A.

Philips (and hence Amperex and the other Philips companies such as Mullard and Valvo) adopted a short-plate design for the ECC83 around the same time as the American 12AX7A came out. With the exception of the box-plate Mullards, all the western European ECC83s were either long-plate or short-plate designs, that I am aware of.

On the other side of the iron curtain, the USSR came up with a completely different design for the ECC83/12AX7 that I think originated with the 6N2P. There was a shield between the two sections, the plates were even shorter than the American/European short-plates, and the plates were spaced rather far from the grids. Again, the grid and cathode were adjusted to give the standard ECC83 characteristics. This design also showed up in China, and the bulk of newer tubes from Russia or China are of this design.

So, with the exception of the later Sylvania 12AX7As, all these tubes have essentially the same characteristics - the i vs v curves. What, then makes them sound different? I don't really know for sure. I suspect the cathode material and processing makes the biggest difference. There may be secondary emission effects that depend on the plate material. As far as I know, the getter shape doesn't affect the sound, but if the getter flash got into the wrong places, it could cause leakage and noise. The type of glass used might even be a factor, since different glasses have different resistivities which affects how long electrons "stick" to the inside of the bulb.

I think the best we can do is note which tubes sound like what in which system, note it, and then use the knowledge to chose your next tubes. In this sense it is more of a taxonomy than a predictive science :-(.

- John Atwood

Cycline3 9th September 2007 04:32 PM

John,

Very nice read. Thanks.

poynton 9th September 2007 05:02 PM

Re: 12AX7/ECC83 shapes
 
Quote:

These European tubes were generally made to better standards than the American tubes and typically had lower noise and microphonics. As a result, high-fi vendors started using these tubes.
So, would I be correct in the assumption that long plate tubes could be considered inferior on the basis of noise and microphonics ??

Quote:

............... a long-plate 12AX7A that had more ribbing and reinforcement than the old long-plate. ..................

I have seen some long plates on EBAY listed as being smooth plate types as if it was better. From what you say above, the ribbed plates should have better performance in microphony ?

Andy

edit :- very well put, by the way!


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