Re: No bias voltage on pin 5 regardless of bias pot setting
- From: "electron@xxxxxxxxxxxxx" <electron@xxxxxxxxxxxxx>
- Date: 10 May 2006 04:14:09 -0700
Brad McGowan wrote:
I recently built a Fender Deluxe clone amp using an Allen Amp kit. It
was working beautifully for a while and then all of a sudden died on
me. I've noticed that when the amp is powered on (still in standby)
that there is very little bias voltage on pin 5 of the power tubes
(6V6). At the max negative setting on the pot I should be getting
around -50 to -60 V. However I'm getting about -0.100 V. The
interesting thing is that when I turn the bias pot to the opposite end
of the range I get the same reading. If I then turn it back the other
way towards max negative and then quickly take a voltage reading I get
a transient reading of about -40 V, which then becomes more positive
until it hits just under zero. The time it takes for the reading to
settle at zero is probably about 5 or 6 seconds.
My question is this:
What is causing me to get no bias on pin 5?
My first guess, on an amp that previously had good negative fixed bias
voltage on the output tube (and no 'wire fell off') would be a leaky
('shorted') grid *coupling* capacitor (the non-polar capacitor coupling
signal from the previous stage to the output tube control grid).
This would be a relatively common failure mode for a much older piece
of equipment, where the coupling capacitor is DC leaking the plate
supply voltage from the prior stage ('+') and it 'cancels out' the
negative bias supply voltage on the output tube grid.
Should I suspect that the
bias cap is bad? Would an bad solder joint at the ground of the bias
cap cause this phenomenon?
Now that *is* possible, and it may very well 'read' that way,
*depending on your meter*. If the bias supply filter electrolytic
capacitor has gone open, try reading the bias voltage with the AC
voltage range on your meter; then reverse the leads and read it again
(!)
If you end up with what looks like a healthy voltage when reading with
your meter set on *AC volts* (and it may only read so with your meter
leads in one 'position', i.e. either red to chassis, *or* black lead to
chassis) *then* an open 'bias capacitor' might indeed be your problem.
Any tips on how to best troubleshoot this?
If the coupling capacitor is remiss, you can easily test this by
lifting one of it's legs (connections); if the negative grid bias
voltage reappears, bingo!
thanks,
Brad
HTH,
-Robert
QTS
http://www.Braught.com
real email addy : Robert@xxxxxxxxxxxxxxxxx (remove NoSpam to reply :
Duh!)
.
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