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Using ECC88

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gingertube

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Joined 2004
#1
I am in the physical layout part of a build of Allen's SVP2 (Six Valve preamp) using 6 off ECC88. I paused in the layout last night as I seemed to recall that when using this tube as a cascode (or with one tube above the other like in Allens SLCF) then there was a preferred arrangement of which triode to use in the bottom section and which to use for the top section.

I eventually found a reference to that on The Amperex Data Sheet - Triode #1 (Pins 6,7 and 8) should be used for the lower input section and Triode #2 (Pins 1, 2 and 3) should be used for the top output section.

Now that is a little inconvenient for the layout I had proposed. For simplest physical layout I would have had them the other way round.

Is that recommendation specific to the Amperex Tubes or should it be applied to ALL ECC88 / 6DJ8?

Anyone have a view / opinion on this.

Thanks,
Ian
 
#2
AFAIK that applies to all brands/types of ECC88 however it mainly applies to the heater to cathode ratings being higher for that section. I would assume that if precautions are taken for that(i.e. raising heaters) maybe it is less important. Anyone else?
 
#3
gingertube said:
I eventually found a reference to that on The Amperex Data Sheet - Triode #1 (Pins 6,7 and 8) should be used for the lower input section and Triode #2 (Pins 1, 2 and 3) should be used for the top output section.

Now that is a little inconvenient for the layout I had proposed. For simplest physical layout I would have had them the other way round.

Is that recommendation specific to the Amperex Tubes or should it be applied to ALL ECC88 / 6DJ8?

Anyone have a view / opinion on this.
Click to expand...

Yes: ignore the recommend for most duals meant for VHF cascodes. With this type, you can't do that since the section they want you to use as the grounded cathode input has a Vhk= 50V (unusually low!) and the other half has a Vhk= 150V. I don't have any idea why they thought this was a good idea. 😕

It wasn't a factor when designing audio cascodes with 6BQ7s, as both sides have the same Vhk rating, and generous enough for audio cascoding.
 
#4
I've used these tubes in all kinds of situations where the heater-cathode voltage has been > ratings on the datasheets, and never had a problem.
According to 'Getting the Most Out of Vacuum Tubes, Robert Tomer' available at Pete Millet's wonderful site, a heater bias of +80volts has the best effect on noise and tube life. I've been using a heater bias in that range for years on 6922s (actually all my tubes) and all is well.
I don't think you'll have any problems connecting 1-2-3 as the lower section.
 
#6
Does that design still use an ECC88 as the line input valve. If so my experience is that gain is way to high for normal use. I would suggest substituting a nice triode with about half the gain for the input stage.

Just a thought.

Shoog
 
#7
Shoog said:
Does that design still use an ECC88 as the line input valve. If so my experience is that gain is way to high for normal use. I would suggest substituting a nice triode with about half the gain for the input stage.

Just a thought.

Shoog
Click to expand...

+1.
A cascoded E88CC has a lot of gain. Perhaps paralleling the two sections in a standard common cathode stage will be just as good?
I don't know the preamp you are trying to build, does it include a phonopre section?
 
#8
SemperFi said:
+1.
A cascoded E88CC has a lot of gain. Perhaps paralleling the two sections in a standard common cathode stage will be just as good?
I don't know the preamp you are trying to build, does it include a phonopre section?
Click to expand...

The cascade is configured as a super linear cathode follower with unity gain. However this is driven by a triode which is lightly loaded so will develop most of the mu of a ECC88.

I overcame this in my version of the FVP5 by using ECF80's whcih have about half the gain of an ECC88.

Shoog
 
#9
The Line amp is a ECC88 triode section driving a SLCF. The input triode has a "new" biasing arrangement which incorporates a 12/18 dB Line Section gain switch. One of the five inputs offers -12dB by a simple attenuator (intended for the CD input). That uses up 2 ECC88 per channel. Phono section is mostly the same as the FVP5A. JFet Cascode input plus CC output using 1 more ECC88. Hence the Six Valve Preamp (SVP2).
Cheers,
Ian
 
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