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Ecc88 pin 9 connection??

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#3
Hello,
If you are going to roll other tubes that have a pinout the same as ECC88 except the 9 pin, you may want to AC ground the 9 pin with a 100 nF capacitor. Other tubes pinout the same except the 9 pin ties to the heater like a center tap between pins 4 and 5.
DT
All Just for Fun!
 
#4
waynzworld said:
Hello can anybody tell Me if the pin 9 connection on the ECC88 marked (s) is suppossed to be an earth connection??
Click to expand...

Yep, it's an internal shield between triode sections. If you're using the ECC88 at audio frequencies, it's presence isn't so important as it would be at VHF. You can just as easily leave it unconnected. If you do connect it, make certain that it will never become more positive than either cathode. If it does, it'll act like another plate, throwing off bias points, and possibly causing instability.

That was a problem once when I ordered 6FQ7s and got 6CG7s instead. The types are otherwise identical, except that the 6CG7s have that internal shield, and the 'FQ7s don't. It's especially bad since these run with the cathodes negative. So I didn't connect the shield to anything, and had no problems doing that.

Another potential problem concerned 6DR7s. One sample I have has an "unauthorized" modification: an internal shield connected to the cathode of the large signal triode. That could pose problems if running the opposite triode with a negative voltage on its cathode. You have to watch for stuff like that.
 
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#5
This is interesting. Some tubes which were designed specifically for cascode operation (ECC84 immediately springs to mind) have shield connected to one cathode and it is that cathode that is the upper one in the cascode, meaning that the shield is always at potential at least as high as anode of the bottom stage with regards to cathode of the bottom stage, yet this causes no problems and is so by design.

I hope this makes sense.
 
#6
On the ECC84 the shield connects to the upper grid, not the cathode. This is because that will be grounded for AC. I assume this won't upset bias as long as the electrons can't see the shield because the anodes are in the way. I guess there are no anode holes facing the shield?

The same ought to be true for an ECC88. As this has a frame grid almost all the electrons will stay in the narrow space where the anode bends in to be near the grid and cathode. However, to be on the safe side keep it negative - some data sheets insist on this.
 
#8
Miles Prower said:
If you do connect it, make certain that it will never become more positive than either cathode. If it does, it'll act like another plate, throwing off bias points, and possibly causing instability.
Click to expand...
Is that really likely? The shield sits between two anodes which are already at high voltage. Is a bit of positive potential on the shield really going affect the bias?

EDIT: Just answered my own question. I put an ECC88 on my curve tracer and tried hooking the shield up to the anode- it caused no observable change.
 
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#9
Hi,

Arnulf said:
This is interesting. Some tubes which were designed specifically for cascode operation (ECC84 immediately springs to mind) have shield connected to one cathode and it is that cathode that is the upper one in the cascode, meaning that the shield is always at potential at least as high as anode of the bottom stage with regards to cathode of the bottom stage, yet this causes no problems and is so by design.

I hope this makes sense.
Click to expand...

Let's keep in mind what tube types are designed for.

Back in 1980: Looking for valves designed for audio only.

Back in 1990: What else can we use?

Since 2000: Any valve can be used for audio....

Anno 2010: I think I'll use my shot 6080's and recycle them for rectifier use. ...

Yeah sure you can use some computer type valves for audio, just don't expect the world of it......

Cheers, 😉
 
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