New forum member - Bassman 135 speaker impedance question:

Discussion in 'Amps and Cabs [BG]' started by KenB, Jul 16, 2008.

  1. KenB


    Jul 15, 2008
    Yellow Springs, OH
    First of all, hello to everyone from a new forum member!

    I have a 1978 Fender Bassman 135 that I have owned since 1984. I haven't used the amp since 1986, so I am considering selling it.

    The still amp works perfectly - I recently turned it on to test it (expecting a flash, explosion, smoke or other sign of catastrophic failure due to old caps or a suicidal original power tube), and no noise, his or other unpleasant stuff! Still, I am considering replacing the power supply caps, just because...

    I have been reading a bunch of info on the 'net about amp recaps and other amp topics, and I have been examining the amp schematic. By the way, I am an experienced electronics and audio hobbyist, having built all of my own effects boxes in the "old days", as well as building a bunch of stereo components from scratch over the years.

    So I can read a schematic. But, in looking over the 135's schematic, and reading the advice/coments of various folks on this and other internet forums, I have run into a question about connecting speaker cabs of various impedances to the Bassman 135. I am hoping some of you tube amp gurus out there will be able to enlighten me...

    Here is what I know for sure:

    1) The Bassman 135 is designed for speaker cabs of 4 ohm minimum impedance, for both the main cab and any extension cab.

    2) The Bassman 135 and the similar 135 watt Twin Reverb amps are unique among Fender amps, in that the jacks for the main and extension speakers are wired so that when both jacks are in use the cabs are connected in series rather than parallel, and plugging in an extension cab also switches the output transformer to a different tap (switched from a 4 ohm "center" tap to the "whole" 8 ohm secondary winding).

    This arrangement makes perfect sense to me, and reading the schematic confirms the wiring and switching arrangement.

    Now... here is the question that is bothering me:

    When you trace out the wiring and switching of the output connections, it is obvious that when one cab is connected to the main out jack, that cab is connected to the "center tap" or 4 ohm tap of the output transformer. Also, when you connect cabs to both the main and external jacks they are connected in series, and use the "whole", 8 ohm secondary winding of the output transformer.

    But, if you connect a cab to ONLY the external jack, that cab is then connected across the "whole" 8 ohm secondary (The 4 ohm center tap is left floating and the main output jack is shorted out of the circuit).

    So to me, logically, this means that since the output transformer doesn't "know" whether one or two cabs are connected, only what impedance its 8 ohm winding sees, that it would be perfectly OK to connect a single 8 ohm cabinet to the external speaker jack. (8 ohm winding, 8 ohm load, perfect!)

    I have seen references to this kind of connection scheme in various forum posts, both pro and con (and some of which were just plain silly and wrong, making it painfully obvious that some self-professed "techs" can't read a schematic, or aren't aware that the output arrangement of the 135 watt amps is different from other Fenders). One post stated that I should "never" connect a single cab to only the external speaker jack.

    So, my question is;

    IS it OK to connect a single 8 ohm cabinet to the external speaker jack of the Bassman 135, and if not, WHY not? (The more technical and theoretical the explanation, the better!)

    Thanks a bunch for any comments, advice or wisdom, and again, Hi to all!

  2. Cant answer your question Ken, but welcome to the forums !
  3. okabass


    Mar 19, 2005

    If the schmatic is right, and nobody has changed the output wiring, yes. You are right.

    You better check the amps actual wiring. Check and clean the jacks switches too; they may have some oxide sh*t. When you switch the amp on, observe the powertubes plates. If they are becoming red, shut it off, and check the wiring again. As you allready may know, open or no load can kill the power tubes easily.
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