NOS EL84, etc. and newer Bad Cat models

cristo

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546
Some NOS EL84/6BQ5 and 7189 tubes have pins 1 and 2 internally connected.
7189A tubes also have pins 6 and 9 connected. Pin 8 isn't connected on any, AFAIK.
Some amps use pin 1 and/or pin 6 as a tie-down point for grid and screen resistors,
and previously, Bad Cat amps were among those said not to do this.

I have some GE EL84/6BQ5 grey oval plate matched pairs and a Mullard 7189 matched
pair (rebranded as IEC 10M 7189A) that have pins 1 and 2 connected. The stock JJ's and
most if not all other new production EL84 type tubes have pin 1 open, so the tie down
doesn't connect to anything with them.

I took a peek into my 2016 Black Cat 15R head (K-master model), and noted that the 1.5K
grid resistors bridge pins 1 and 2 of the output tube sockets, so if you use older tubes, they
can bypasses the grid resistor. This may make the amp prone to parasitic oscillations, take
away a measure of safety in case of a shorted tube, change the tone, and/or do other worse
things.

I thought of simply lifting the tiedown connection to pin 1, but figured it's tied down for
a (mechanical stability) reason, so I replaced that lead with a 220K resistor (nothing magic
about 220K, it just has to be a much larger value than the 1.5K one, so that it doesn't decrease
the 1.5K resistance significantly in parallel). The 220K is soldered to pin 1 below and to the
connection of the PI lead and the 1.5K resistor above. Only do this if you're comfortable and
knowlegeable about working inside amps because of the safety issue with high voltages.

One could also tie it down to pin 8 (unused) or pin 6 (unused, but then can't use 7189A tubes),
or saw or clip pin 1 or pin 2 on the tubes themselves (but then the tube can go in the socket two
different ways if you're not careful, you may damage the tube, and you also may not be able to
use that tube in other amps).

There's probably no problem with letting the end of the 1.5K and its connection from the PI float
above pin 1, but it's likely better in the long run if it's not free to move or vibrate.
 




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