writes:
>I am interested in trying cathode biasing on my AB763
>Bandmaster, and someone on the net was nice enough to
>give me a really good explanation of it (thanks, Dave!).
>However, I am not an EE, and before I try it, I would like
>to lay out the steps as I understand them, and hope that
>maybe someone like Mark, Dave, et. al., can take a quick
>look for me, and make sure I haven't missed something. I
>hate to blow things up 8-/.
Awww..you're no fun.
>Oh, this BM has been converted to EL34's, using the Fischer
>mod from the TAB4. Don't know if that matters.
Expect to develop a bit less bias voltage for the EL34's (as
compared to 6L6 family), but you can run the current a tad
hotter if you wish.
>1) Remove ground wire from cathodes (pin 8).
>2) Install resistors (10-watt or better)
> a) Tie both cathodes together, then run a 500 Ohm resistor
> to ground.
----- Supposed to be an OR here, right? -----
> b) Run one 1K resistor from each cathode to ground.
The separate resistors (b) will require more efficient bypassing, but
the single resistor method (a) will be more sensitive to current-hogging
if the output tubes are not matched or balanced.
>3) Bypass the caps with electolytics (may be optional).
Not optional unless you are both running method (a) above, and
are definitely staying in class A.
>4) Remove bias supply (- voltage) from grids (pin 5).
You could stay safe on the first run by converting your bias to
go from 0 to say -35 or so (make sure you compute all the resistor
wattages in the divider,etc). This will allow you to leave the
bias control in the circuit and slowly ramp the 'fixed bias' voltage
to 0, monitoring the voltage at the cathode resistor as you do this.
This is just a precaution to make sure you don't toast tubes if
something is not wired correctly.
>-->Here I get fuzzy<--
>??? 5) Hook bias supply to cathodes (doesn't seem right...)
Nope...the current pulled thru the output tubes develops a
voltage across the cathode resistor. The output tubes' grids
are at 0 volts, but the cathode now goes positive. The bias
voltage is actually the voltage from the control grid (signal
grid) to the cathode, so it will still 'look' like a negative
value. Follow? In other words, the cathode will still be
more positive than the grid.
>I'd appreciate any clarification. Posting is fine; If you
>can e-mail, too, that would be smashing, since my feed runs
>about 3 days behind 8-[.
Posted and emailed, but please direct replies to the news group.
>Oh, and if I forgot something _please_ let me know >8-0.
Well, if you use the single resistor method, then you will know
how much current BOTH output tubes are drawing, but you won't
know if they are balanced. So you might consider putting 1 ohm
resistors between the cathodes and the BFR (big resistor). You'll
be able to tell individual tube current by monitoring the voltage
across each 1-ohm resistor.
Also, make *sure* you get the bypass cap polarity straight. And
use very good quality bypass caps. If they short out, they will
effectively remove the bias voltage, which will red-plate your
tubes in a hurry.
MGarvin
PS: Be careful and have fun.
From cign--(at)--elios.phy.OhioU.Edu Mon Jun 17 00:46:21 CDT 1996
Article: 10882 of rec.audio.tubes
Newsgroups: rec.audio.tubes,alt.guitar.amps
From: cign--(at)--elios.phy.OhioU.Edu (Dave Cigna)
Subject: Re: Cathode Biasing - need a sanity check (please)
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In Mr. Snakey wrote:
>I am interested in trying cathode biasing on my AB763
>Bandmaster, and someone on the net was nice enough to
>give me a really good explanation of it (thanks, Dave!).
>
>However, I am not an EE, and before I try it, I would like
>to lay out the steps as I understand them, and hope that
>maybe someone like Mark, Dave, et. al., can take a quick
>look for me, and make sure I haven't missed something. I
>hate to blow things up 8-/.
>Oh, this BM has been converted to EL34's, using the Fischer
>mod from the TAB4. Don't know if that matters.
>
>1) Remove ground wire from cathodes (pin 8).
>2) Install resistors (10-watt or better)
> a) Tie both cathodes together, then run a 500 Ohm resistor
> to ground.
> b) Run one 1K resistor from each cathode to ground.
This should be (a) OR (b). You should probably choose (a)
in your case. Some amps that are running class A, or close
to it, sound good with separate cathode resistors.
>3) Bypass the caps with electolytics (may be optional).
Lately, I've come to the conclusion that bypass caps are
never optional in power amps. Preamp tubes are a different
situation.
You want the cap to be big enough to work at low frequencies.
There is no hard and fast rule, but if you choose one that is
30000/R (uF) or bigger, then that should be big enough.
>4) Remove bias supply (- voltage) from grids (pin 5).
>
>-->Here I get fuzzy<--
>
>5) Turn on the power, and enjoy cathode biasing
>
>-or-
>
>5) Hook bias supply to cathodes (doesn't seem right...)
>6) Turn on the power and ... as above.
Hmm... It's important that the grids are referenced to some
steady voltage through 'grid load' resistors. In a stock
Bandmaster each grid is connected to about -50 volts (bias
supply) through a 220k resistor (silverface amps may vary.)
With cathode biasing, you want to connect each grid to 0 volts
(ground) through a 220k resistor. Fortunately, this is very
easy to do on many Fender amps.
Near the end of the eyelet board, by the tremelo controls you'll
find a pair of 220k resistors that form a "V" shape (silverface
amps might be different.) Don't confuse these with the phase
inverter plate resistor that also form a "V" (probably 100k/82k.)
One end of each resistor is connected
to a cap and also to the grid of one power tube via a wire
under the eyelet board. The other ends of the resistors are
joined together and connected to the bias supply via a wire
that might start under the eyelet board. What you need to do
is disconnect this wire *from the bias supply* and reconnect
to ground. Make sure that the ground connection is good and
reliable. A flaky connection here could spell disaster for
your power tubes.
-- Dave Cigna
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