Re: Tek 454 channel B drift/ nuvistor characteristics

Stan or Patricia Griffiths <w7ni@...>

Hi Paul,

I have a real problem with the way Bill's and my forum works with my
computer. It takes forever to load and I just don't have the time to wait.
Bill says he can fix it as soon as he gets time to do it. In the mean time,
I have some info for you regarding your 454 nuvistors. See below:

msfix56@... wrote:

Hello All,

Please note that this is a copy of my post in the Tektronix Reference
materials forum with some comments removed as they would not make
sense here. If the bookmark is not in the bookmarks section here it
really should be added as they, (Stan Griffiths and Bill Den Beste)
have a wealth of good information available.

So here goes -

It was suggested that perhaps the nuvistors in the B channel were the
cause of the drift. This does seem to be the case. In an attempt to
prove that case I swapped the tubes out and the drift changed
channels. I have tested the nuvistors in the scope and several of
them test weak including the front end of the channel B preamp which
if you watch the tube test meter drifts up from about 3000 uMhos to
about 7600 uMhos before dropping off sporadically. The other tubes in
the scope vary from 10000 uMhos to 11500 uMhos. My question is that I
seem to be having a hard time finding the 154-0461-00 part number
tubes. According to the manual they cross over to an 8393
This is correct. 154-0461-00's are also known as 8393's.

and I have
found Tek part #157-0107-00 which also crosses over to the 8393. Is
there a significant difference in these two Tek part numbers?
Yes. The 157-0107-00 is an aged and checked 154-0461-00 (8393). It has been
aged for 100 hours, checked for microphonics and grid current.

someone tell me what the uMho readout for the 8393�s should be for
this application.
I don't think the transconductance of the nuvistor is very critical in this
application. I notice 8393's are not checked for transconductance when they
are selected to become 157-0107-00's. I believe the randomly varying
transconductance you see is related to the drift you see. I think the aging
process causes the nuvistors to eventually stabilize.

I notice that Tek uses two part numbers for the
8393�s in the 454 two of them are "aged" while the others are not.
Will this make a significant difference in the scopes performance or
will it just drift for a while until the tubes get "set in their
ways" :^) Thanks for reading all this and for all your help.

Paul, I would just use some plain 8393's in the input circuit of the 454 and
check them for microphonics by jarring the instrument slightly and watching
the trace for noise. I think the cal procedure calls out a test for grid
current. Grid current shows up as a vertical trace shift while grounding and
ungrounding the vertical input at the most sensitive position of the vertical
attenuator switch.

Hope this helps.


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