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63.16.30.7
In Reply to: RE: ECC83............. posted by . on March 26, 2008 at 15:59:02
On my mc1's the only difference seems to be the absence of the two long slots separating the two triode sections. The slots are there to prevent possible B+ bleeding from one plate to the other, and is seen on many power tube construction. mica is, after all a naturally occurring substance and can be contaminated, even though manufacturers go to great lengths to chemically treat and clean the mica sheets.The number of mica fingers on the top and bottom mica sheets are identical as well as the thickness of those sheets, although I have not dissected these tube because they are still in good working condition. In both tube types, my examination shows that the cathode, grid rods and the plates are rigidly anchored in the mica sheets with a secure friction fit, as evidenced by some very minor flaking on the bottoms of the mica.
The f92 reveal that the mica sheets used for anchoring the internal structure is identical to the f-91. The primary difference is the addition of a third sheet sandwiched against the top of the top mica which has the characteristic mica topping to the cathodes: made to avoid cathode contamination from the getter flash. This is a feature of many European tubes following the f92 design, although I would not say that the construction begins with the f92's. American tubes, in order to combat the same issue, add a floating third mica sheet located well above the normal top mica.
The little wire is located above the top mica, where theoretically the top mica will have already dampened the grid support rods. It does not and will not affect the grid rod structure between the anchoring mica sheets which is primarily the cause of any microphonics.
If indeed the wire is a piece of filament wire, then the heat from the grid rod can activate a slight amount of electron flow which may affect grid performance. I hesitate to make such speculation as I have not dissected any f91's and the test would be to see if indeed the little wire has a cathode coating upon it.
Stu
Edits: 03/26/08Follow Ups:
'The little wire is located above the top mica, where theoretically the top mica will have already dampened the grid support rods. It does not and will not affect the grid rod structure between the anchoring mica sheets which is primarily the cause of any microphonics.
If indeed the wire is a piece of filament wire, then the heat from the grid rod can activate a slight amount of electron flow which may affect grid performance. I hesitate to make such speculation as I have not dissected any f91's and the test would be to see if indeed the little wire has a cathode coating upon it.'
You are right about the wire , it does appear to have some form of insulating coating . For a valve with dissipation as low and used at low currents such as an ECC83 , the copper grid posts alone would easily dissipate enough heat . I've attached a picture for those following the thread
I've never seen any other tube manufactured with that feature other than the f91's and they were in production for only two years.
Sure wish someone knew more about that wire!
Stu
Well it may be cathode coating although I suspect if it is , it is only there for insulation . All the others including double crossbar getter mC1 all had the long grid rods without the top mica retaining tab . If I ever come across a low emission f91 , I'll tear it apart and post pictures
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