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ECC86 vs. ECC88

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I've been thinking about a new LV design and have been comparing the ECC86 and ECC88, and I've begun to wonder if they are the same tube.

If you compare the plate curves, they are very similar (within 20%) over the range that the ECC86 is shown (especially if you consider the accuracy of the hand-plotted graphs at low voltages).

Externally the two types appear identical, at least the Amperex versions I have.

The capacitances are virtually identical (I was surprised at that... expected closer spacing on the LV ECC86).

The biggest difference seems to be heater current - 330mA (ECC86) vs. 365mA (ECC88). But only 10% different...

I have dropped the ECC88 into circuits designed for ECC86 and they work very well, almost identically in fact.

So, any theories? Was the ECC86 just a "selected" ECC88? Anybody know the background of the ECC86?

Pete
 
From what I remember, the ECC86 was specifically designed to operate on a very low anode voltage (12V), for car radios. The ECC88 is a TV VHF tube.

So says the datasheets.

But as somebody who (among other things) writes IC datasheets for a living, I can tell you not to believe everything you read! It is common to do "marketing spins" where one re-targets a device just by changing the description on the datasheet...
 
I have dropped the ECC88 into circuits designed for ECC86 and they work very well, almost identically in fact.

This claim is pointless without a schematic with voltages showing. Perhaps you put ECC88 in a circuit with Va of 80+V and it's understandable that ECC86 performs well under such conditions. Then again you may have dropped it into 12V circuit and actually discovered something useful. Ergo: show us schematics & voltages ;)
 
Pete,

What about the more than 2X difference in μ? A triode's amplification factor is directly related to its geometry.

But mu and Gm change with voltage and current - they are nowhere near constant, dropping with lower plate current. The 2x difference seems about right...

Take a look at the graphs of the 6BQ7 (somewhat similar tube, I don't see a similar graph for the ECC88) here:

http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6FQ7.pdf

See how mu drops off with current?
 

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I've always assumed that the basic geometries between the ECC88/6DJ8 and the ECC86/6GM8 were quite similar, but that the grid pitch and hence transconductance were quite different.

The mu's of both types are relatively constant over a wide range of currents as can be seen from the curves, but the mu of the ECC86 is about half that of an ECC88.

I have heard (reliable hearsay) from others that the ECC88 operates "surprisingly" well at voltages too low for most other types not deliberately designed for very low voltage operation, but have never verified this myself. I have a few ECC86 awaiting their day... :D

It seems a shame to potentially toast a perfectly good ECC86 by running it with voltages typical of an ECC88. Finite supply and they are known to work really well in low voltage headphone amp applications. (Demand sky rocketed a couple of years ago, just as I got interested in the type -- of course.)
 
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Demand sky rocketed when Broskie started using them in low voltage Akidoes and low voltage Super Linear Cathode Followers. For a while they became more expensive than ECC88's - don't know what they are at the moment.
Seems a shame to fry specialist tubes just to see.

The ECC88 does seem to work well at voltages down to 24V, but my experience was that they tend to draw a bit more grid current when made to do so. This is a bit of a problem when you couple a volume pot straight into their grids.

Shoog
 
I've xray'd Amperex 6DJ8 (Tektronix pulls, probably same as your stash).
Posted to this very forum, not too hard to dig back and find with search.

If I had a loaner of your ECC86, I'd be happy to snap a few comparison
shots of the internals. I work in Plano, where the machine is located...
 
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