Re: Type CA module in a 545


David Kuhn
 

Sorry, I accidentally hit enter and the message was sent before I was done.

The funny thing is it seems one tube being weak appears to drag the gain of
both channels down. Both channel one and channel two tubes sets are
basically in parallel except for the input grids. Any measured voltage
differences between a good gain module and lower gain module are very
subtle.

So at this point, I am not sure what to do, if I really want two good CA
modules (which I really do not, I just like the challenge, or it's my OCD
that drives me to have both the same). They are about $10 each on ebay. One
vendor is selling them "tested" for 99cents each. To purchase four "good"
boxed ones for $10 each is $40+shipping. I'm not sure my OCD is worth
that. lol

It's weird that the module could be that sensitive in design to a slightly
weak tube. Maybe all my 6AU6s are weak and I just do not know the
difference? With the recessed Gain Adjustment, how much overhead gain is
typical for a Type CA module? I would think, best case, I would be
adjusting the gain down a bit instead of having it "just" calibrate with
that pot fully CW.

On Sat, Jun 26, 2021 at 2:43 PM Daveyk021 <daveyk021@gmail.com> wrote:

Well, it's down to the 6AU6 tubes. I got them confused now between the
good module and the bad module. I can make the bad module the same as the
good module by swapping the 6AU6 tubes. I now have the eight 6AU6 tubes
all confused and intermixed - lol.

I have an eico-625 tube tester that I just fixed. It really can not see
the subtle differences between all eight tubes. Some read "630", some read
"640". Not much difference; all read good.

On Thu, Jun 24, 2021 at 12:04 PM David Kuhn via groups.io <Daveyk021=
gmail.com@groups.io> wrote:

"You should check the voltage divider from pin 15 of the plugin connector.
That pin needs to sink 150 mA at 75 volts in all plugins. The 75 volts is
derived from the +100 in the mainframe dropped through a couple of 12.6
volt tube heaters. In many plugins including the CA it is connected to a
combination of tube heaters and divider resistors with taps to provide
bias
for various parts of the circuit. In the CA there are also interconnection
networks with the +100 and +225 pins. One of those voltages from the
divider network is +37.5 that supplies the suppressors of the output
amplifier tubes. If that voltage is not right it could affect the gain of
those stages. There are also +69 and +12.5 volt taps that are used for DC
balance adjustment and to set operating conditions for other parts of the
circuit."

Hello Morris,

I should be checking those voltages and dividers this afternoon. The
filament resistance did go back to about 70 ohms once I wiggled most of
the
tubes and pulled them and re-seated them. The sockets do not look
corroded, but the tube legs themselves are. Getting the
filament resistance down, did nothing for loss of gain in the CA module.

Dave



On Tue, Jun 22, 2021 at 12:42 AM Morris Odell <vilgotch1@gmail.com>
wrote:

You should check the voltage divider from pin 15 of the plugin
connector.
That pin needs to sink 150 mA at 75 volts in all plugins. The 75 volts
is
derived from the +100 in the mainframe dropped through a couple of 12.6
volt tube heaters. In many plugins including the CA it is connected to a
combination of tube heaters and divider resistors with taps to provide
bias
for various parts of the circuit. In the CA there are also
interconnection
networks with the +100 and +225 pins. One of those voltages from the
divider network is +37.5 that supplies the suppressors of the output
amplifier tubes. If that voltage is not right it could affect the gain
of
those stages. There are also +69 and +12.5 volt taps that are used for
DC
balance adjustment and to set operating conditions for other parts of
the
circuit.

Let us know what you find.

Cheers,

Morris









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