ASK AMP MAN
JEFF BOBER
Fender Amp Bias and Impedance
Jeff,
I tried to bias a Fender Deluxe AB764 using
the transformer shunt method. With the current setting on the bias pot, one tube measures 35mA while the other measures 19mA.
What are some possible reasons for the big
difference, and how close do the readings
need to be? Additionally, the plate voltages
are higher than the schematic shows.
Tom
true for the 470-ohm screen grid resistors. If a
substantially different voltage appears on the
screen grids of both tubes, an imbalance in
bias current could occur.
Hi Tom,
Thanks for your question, but before I answer
it I’d like to offer a clarification. The Fender
schematic designation AB764, as far as I know,
was only used for the Bronco and Vibro-Champ
models. The Deluxe Reverb in question here is
more than likely an AB763 version.
Regarding the plate voltages being higher
than the schematic shows, the voltages listed
on the schematic are merely an “
approximation” of the voltages that will appear in any
given production amp. Also, be sure that
your amp has the recommended 5AR4 tube
rectifier. A replacement Solid-State module,
unless it is manufactured to replicate the output of a 5AR4 tube, will produce a markedly
higher B+ voltage.
I hope that helps you achieve balance in
your bias.
Since you did not mention installing a new set
of matched output tubes prior to your biasing attempt, I’m going to assume that you are
trying to bias the existing set of 6V6 tubes in
your Fender Deluxe. In order for you to measure the same idle current on both tubes, the
tubes need to be matched. If you are measuring the idle current of two unmatched tubes,
it is not unusual for one tube to be drawing
16mA more than the other. Installing a new
matched pair of 6V6 tubes should alleviate
this situation. If in measuring the current of
the new pair of matched output tubes you still
find a substantial mismatch (more than a couple milliamps), the first troubleshooting procedure would be to note the current at each
socket and then reverse the location of the
tubes. If the current measurements follow the
tubes, then the tubes are not matched. (Hey,
it happens!) If the imbalanced measurements
remain with the sockets, then you should
check the value of the grid and screen grid
resistors. If the values of the 1.5K-ohm grid
resistors are substantially different (and they
really do need to be substantially different in
this instance) the amount of grid bias voltage
applied to the grid of each output tube would
be different, causing an imbalance in bias
current between the tubes. The same holds
Jeff,
First off, thanks for writing such a great col-
umn, and putting it all into layman’s terms,
it helps a lot!
Here is my question: I have a ‘ 67 Fender
Bassman head (blackface) which runs at 50
watts at 4 ohms. The only cabinet I have
right now is a Hartke 4x10, which is 8 ohms,
but wired in series/parallel. Currently what I
am doing is adding a second speaker cabinet to the Hartke that I have loaded with
one 12” JBL D41 (I think) rated at 8 ohms.
While I believe this is giving me the proper
resistance, it is a bit unwieldy. Can I build a
dummy speaker load to connect to either of
these cabinets to allow me to use just one
alone instead of both at once? Thanks again.
Shaun Walker
The Fat Cat Daddy
8-ohm 4x10 Hartke cabinet and the 8-ohm JBL
cabinet, both plugged into the two speaker
jacks of the Bassman head, will present the
head with the proper 4-ohm load for full power
transfer. Since the impedances of the cabinets
are the same, the output power of the Bassman
will be split equally between both cabinets. If
you run only one cabinet however, the output
power of the Bassman head is reduced due
to the impedance mismatch. Theoretically, it
would be reduced by half, but realistically it
turns out to be approximately a 40% reduction.
This means that any single 8-ohm cabinet used
will only receive approximately 30 watts from
a typical 50-watt Bassman. Now, let’s examine
the potential of using a load box and a single
cabinet with your Bassman head. You would
more than likely want to build the load box
to be an 8-ohm load so that the combination
of the load box and a single 8-ohm speaker
cabinet would present the correct 4 ohm load.
Now, if you refer to the previously stated information, you’re already seeing the potential
problem here. Since the output power of the
head would be equally split between the two
8-ohm loads, the speaker cabinet would be
receiving 25 watts and the load box would be
receiving the other 25 watts. Result: there will
be 25 watts of power that will be dissipated
across the load box and never be heard. What
a waste of 25 perfectly good tube watts. So
you see, while building a load box would certainly optimize the impedance load on your
Bassman head, the net result would be even
less desirable than what you currently achieve
by running only one 8-ohm cabinet with your
head, and I’m sure that’s not quite the result
you were looking for.
Hi Shaun,
Thanks for your question and thanks for reading and enjoying the column. I’m glad you find
it useful. To answer your question: Yes, but
why? You’ll understand once I explain.
Moral of the story: we should never waste 25
perfectly good tube watts!
The current configuration you are using with
your Bassman head seems to be correct. The
Jeff Bober
Jeff Bober, Godfather of the low wattage amp
revolution, co-founded and was the principal
designer for Budda Amplification. He can be
reached at pgampman@gmail.com.
