ASK AMP MAN
JeFF BoBer
Two Letters about … Well, Twins!
Jeff,
I took out one of the 12" JBL speakers
in my 1970 Fender Twin Reverb and put
it in an old vintage Band-Master cabinet
to spread out my sound when we play
outside. I then plugged the cabinet into
the external speaker output in the back of
the Twin. Is playing with only one 8 ohm
12" speaker hurting the amp? Personally,
I didn’t think so since they’re supposed
to be in parallel. But is playing with both
speakers that way affecting anything?
Thanks.
Mark Roberts
Santa Barbara, CA
Hi Mark,
Thanks for reading the column. From everyone here at Premier Guitar, we appreciate
it. Let me just start by saying that I like the
idea of removing one speaker from your
Twin and placing it in an external cabinet
for occasional use. Removing one speaker,
as you mentioned, reduces the load on the
output stage to only 8 ohms, which reduces
the available clean output power in realistic
terms by 30 to 40 percent. For playing smaller
indoor gigs, a 60–70-watt Twin should be
just fine—I know I’d prefer it. It allows the
amp to soften up slightly by compressing a
bit more due to the reduced output power.
Granted, you won’t move as much air as you
would with two speakers but that could be a
good thing too, especially for people in the
first row! Plugging the additional speaker into
the extension speaker jack, since they’re in
parallel in this amp (not so with the 135-watt
version), returns the load to 4 ohms and the
power back to its original 100 watts, so you’ll
have a bit more clean headroom for larger
outdoor gigs. Also, the additional cabinet can
help make the sound more spatial. As far as
playing the amp with only an 8-ohm load, I
personally don’t feel that it causes any harm,
nor can I say that over the years I’ve seen any
damage that I could directly attribute to this.
Over the years, players have mismatched the
heck out of those amps and that output transformer (p/n 022889) and the amps still keep
kickin’, so I don’t think I’d be concerned.
Hey Amp Man,
Can I have two Celestion Vintage 30
16-ohm speakers in my 135-watt Silverface
Twin Reverb? The amp has two speaker
inputs and they each have “ 4 ohms” labels.
Years ago I had the amp in the shop and I
asked them to put the Vintage 30s in there.
Well, I pulled the amp out recently and
noticed there were two 16-ohm speakers in
there (makes 8 ohms) so I pulled them out.
But I really liked the sound better. I’ve read
lots of confusing stuff about this 135-watt
version of the Twin, specifically about how
it can handle the different ohms. But I don’t
want to mess it up and I hope you can set
me straight. Thanks.
Mark
Hi Mark,
The short answer to your question is yes, but
there may actually be two different configurations possible here, one being more optimal
than the other. I’m assuming your statement,
“But I really liked the sound better,” means that
you preferred the sound of the Vintage 30s in
your Twin over some other speaker options.
Since you stated that you had the Celestions
installed “years ago,” I have to assume that
you’ve been playing the amp for years with no
issues (otherwise you’d be writing me about
some catastrophic meltdown of your amp!).
Since it’s been fine up until now, I wouldn’t
expect, nor would I normally expect, that you’ll
be having any problems due to this setup. That
said, let’s explore another possibility.
In looking at the schematic for the 135-watt
Twin (which, by the way, was the same chassis
used for the Showman Reverb, Quad Reverb
and Super Six Reverb of that era), one of
the most notable differences is that the two-speaker output jacks are not in parallel, and
that there is an additional output tap on the output transformer. Very interesting! In following the output wiring I see that when a
plug is inserted into the main speaker jack a
connection is made to the 4-ohm transformer
tap through the switching contacts of the
extension speaker jack. This would be perfect
for the stock 4-ohm load of the standard
speakers. When a plug is then inserted into
the extension speaker jack the extension
speaker is connected in series with the main
speaker load and they are connected to the
8-ohm tap on the transformer. Perfect scenario, and optimal if you’re using a 4-ohm
extension cabinet. But here’s where it gets
interesting: if no plug is inserted in the main
speaker jack and only the extension speaker
jack is used, a connection is made only to the
8-ohm tap of the output transformer with the
ground connection being made thru switching contacts of the main speaker output jack:
optimal if someone had a pair of 16-ohm
Vintage 30s in their amp. Using this jack, you
would have full power transfer to the speakers and a proper impedance match, but if
you prefer the sound of the amp with the
speakers connected to the main speaker output—possibly due to the 30- to 40-percent
reduction of output power—I’d say continue
to run the amp that way and enjoy.
I hope that helps the twin Marks with their
twin questions about the Twin Reverb.
‘Til next time…
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.
