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Bassman 70 vs 135 speaker taps final word (w/ mini rant)

Discussion in 'Amp Tech Center' started by andyfromdenver, Jun 13, 2016.

  1. andyfromdenver

    andyfromdenver Friend of Leo's

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    Hi internet users,

    There are a couple special breeds in the fender line with "hidden" output transformer (OT) secondary taps. The 135 is my bass amp and I love it and shared some mods here: http://www.tdpri.com/threads/my-bassman-135-last-mod-for-the-road.555712/

    People online have shared/recycled some info about it's little sibling the Bassman 70 which is incorrect.

    To wit, and sorry to call you out whomever you are.
    [​IMG][​IMG][​IMG]

    I am going to break it down and give a final word.
    The Bassman 135 is awesomely and secretly a dual tapped 4 ohm and 8 ohm amp. when plugging in two 4ohm cab loads (speakers) the jacks run in SERIES and access a "hidden" 8ohm tap. per the scheme (when I bought mine someone had rewired the taps to the more traditional config and isolated off the hidden tap) if you want to use an 8ohm cab in a stock amp, simply plug it in the extension jack and it will utilize the hidden tap. Don't put a dummy jack in the main speaker out as it will, again in a stock amp, lift the ground connection via the switch tab on the main:
    [​IMG] I have mine set up for either 4 or 8 on dedicated failproof switchcraft 11 jacks, the 8ohm tap to my 8ohm mesa 1x15 cab sounds big, fat, and what a 4 6L6 amp should put out, aka it is correct.

    jump to me on vacation and scouring craigslist. I see a Bassman 70 for a killer price and 100% stock untouched and pristine. ok, this could be my sweet 2x 6L6 two channel clean machine keyboard amp. I take a cursory quick glance at the scheme and do a search and get excited thinking there is also a hidden 8 ohm tap. WRONG! some, and the poster aforementioned have incorrectly assumed the 70 has the same SERIES switching arrangement. i mean on a quick glance one sees a complicated set up and moves on in their excitement to use a 1x12 8 ohm cab that any guitarist has lying around.
    i have since looked deeper and, again, per the schematic this complicated arrangement is infact PARALLEL: [​IMG]

    look at the two schematics i posted, the give away is the tip of the main going to the sleeve of the ext in the 135, and the tip going to tip on the 70. beyond that simply trace the pathways and the switching functions for further operation clarification.

    so, now i am a bit bummed that i "can't" use my 1x12 8ohm cab for my new Bassman 70 and deliver the PROPER reflected load to the OT primary.

    additionally, misinformed posters and recyclers of erroneous info have stated putting a dummy plug in the 70's main spkr out and running an 8ohm cab in the extension. with this error you will be running the 8ohm cab on the "hidden" 2ohm secondary tap. ummm, nobody likes that... should sound like crap comparatively.
    your best option when facing this hook up scenario is to run an 8ohm cab in the main "min load 4ohm" jack.

    my pledge: unlike most recyclers of bad info, when I get home, i'll send some voltage through the OT and get the turns ratio. armed with that an educated guess based on OT primary impedance best results will tell us 2,4,8 secondary. cause again, this webpage assumes it is a 8ohm secondary w/ a 4ohm like the 135, do they know something i don't? maybe.
    [​IMG]

    however, say what you will about Mercury Mag, I happen to think them very smart and passionate, and their replacement for the Bassman 70 is 2 and 4ohm.

    again, i'll do some measurements for my own satisfaction, and can post here for the curious.

    what i can tell you from playing the amp before buying, even the mismatch of running into an 8ohm cab sounds great and you likely would never know it's not a big fat clean 2x 6L6 machine. but, for those that desire perfection and optimal operation, sigh, ya need a stupidly rare 1x12" 4ohm speaker cab if ya don't want to break your back with a "big a" cab.

    oh yeah, almost forgot. pics or it didn't happen :)[​IMG]

    also, to state at the time of this posting, I have been working and studying my butt off re: tube amps and electronics for ~6 years, if someone who laps my knowledge level disagrees with any of the above information, please feel free to school me.

    and omg...should i implement my original andyfromdenver bassman mod idea to use the idle triode (1/2 12AX7) and wire the normal channel as a Vox Top Boost???? http://www.tdpri.com/threads/how-can-fender-out-british-marshall-answer-inside.616462/
    or keep it clean and OG... uggh, this is the amp everyone loves to hate (usually without actually playing/ owning) due to the ultra linear (UL) OT design. and they can be bought for 1/2 the price of parts...

    peace out,
    Andy
     
  2. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Nice find Andy. That amp looks very clean. I hope that you're enjoying the vacation. My vote would be to use the unused triode section as an added gain stage to the Normal Channel and reconfigure that channel as the Bright Channel of a mid 70's HiWatt preamp. The existing Fender tone stack is pretty close as is.

    http://hiwatt.org/Schematics/DR_Pre4Input_v2.pdf

    I would eliminate the negative feedback loop. There is plenty of negative feedback in the UL configuration. I would mod the PI by changing the 100R resistor between the 22K tail resistor and ground to a 2K2, and after the .1 uf capacitor add a 470R resistor and a .1 uf capacitor in series to ground. This mod will get very close to the sonics of the HiWatt balanced PI.

    Other than that, I would just perform necessary maintenance, clean up the wiring some and eliminate the .01 uf cap across the plate resistor on the Bass channel.
     
  3. andyfromdenver

    andyfromdenver Friend of Leo's

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    Hi FM! having a great time, thank you :)
    The initial inspiration with my TB mod is to fit the faceplate, with an awesomely voiced norm channel. however, as nearly superfluous as the MV is, perhaps I can disconnect that from the circuit and use it as a Gain II for just the norm chnl with another gain stage.

    re: the nfb, i would hate to loose any stock measure that would take away from kybd/synth duties on the bass channel. i feel the flatter/cleaner the better, in that regard.

    i appreciate your input! i know we both got bit by the hiwatt bug recently.

    just to alleviate several readers fears, i am likely going to keep things stock.
    i have *starts counting in my mind* three 50 watters now... i need to keep an open mind about resale, esp. cause the price and condition was/is so excellent.
    if it was mega ragged out, then all bets would be off.
    however, my policy is that i have no qualms modding anything younger than me :).

    i got something else cool!
    [​IMG]
    it has transformer balanced ins and out.
    i've used those transformers in a tube mic pre. and for $30 i have 5 now. but i am going to recap it and replace the CC resistors w/ MF and shield the signals and use it as is. man, i remember when i just did one project at a time...
    http://cdn.shure.com/user_guide/upload/995/us_pro_m68rm_ug.pdf
     
    Last edited: Jun 15, 2016
  4. Commodore 64

    Commodore 64 Friend of Leo's

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    I've got a '79 Bassman 10 (UL). Wondering if Fred Mertz' suggestions will be the same.
     
  5. andyfromdenver

    andyfromdenver Friend of Leo's

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    @ Commodore,

    for some reason the above OT ref page lists a bassman 10 head and combo.
    i can only google one scheme, though the above lists two diff OTs.
    easy answer, if yours is the 3 9pin variety, then you don't have an extra triode to play with and there is no additional gainstage post channel mixing.
    if yours has 4 9pins (not sure such a bassman 10 exists per above statement), then it is ripe for added gainstage, or rewiring to unique hiwatt PI.
    http://schematicheaven.net/fenderamps/bassman_10_schem.pdf
     
  6. andyfromdenver

    andyfromdenver Friend of Leo's

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    but let us know and lets switch gears of the thread to "modding the bassman 10 brainstorm"
     
  7. Fred Mertz

    Fred Mertz Tele-Afflicted

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    As Andy indicated, the Bassman 10 lacks the third preamp triode to provide for an additional gain stage. But there are certainly some changes that could be made to voice at least one of the channels in the Hiwatt direction.
     
  8. Commodore 64

    Commodore 64 Friend of Leo's

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    I got this after some research, its hand annotated, but appears to be what I've got under the hood.

    Amp has a lot of oomph as a bass amp (feels like 70W claimed might be reasonable). As a guitar amp you have to dime everything to play with a drummer.

    It has 4 Carvin speakers in it (replaced by previous owner, I assume). They are bass speakers so maybe that's part of the perceived loudness issue.
     
  9. andyfromdenver

    andyfromdenver Friend of Leo's

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    i wasn't able to view the scheme. if your amp has the norm chnl mid pot, you could change that to 25k from 10k. plug that into the tonestack and see it pull out the deep scoop. that should help cut through.
    pic: top curve is 25k bottom curve is 10k.
    [​IMG]
     
    Last edited: Jun 16, 2016
  10. Fred Mertz

    Fred Mertz Tele-Afflicted

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    I played with the schematic some and came up with a circuit that I would like to try using the UL Bassman 10 platform. I haven't tried to transfer the schematic to a layout. But it should be doable.

    View attachment 361949

    The resistor values for the bias supply may require some tweaking.

    Schematic deleted from thread. Will be discussed in new UL Bassman Mod thread in Shock Brothers forum.
     
    Last edited: Jun 17, 2016
  11. andyfromdenver

    andyfromdenver Friend of Leo's

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    very cool FM :)

    the "Bassman 10k"

    *k for kewl of course*
     
  12. Commodore 64

    Commodore 64 Friend of Leo's

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    I'm willing to try those Fred. I'll be back in town this weekend, been vacationing all week at Oak island, NC.

    When I get back, I'll try to digest your changes. There's plenty of room inside the chassis. What were your main goals?

    Over at watkins, it was surmised that UL needs a bigger voltage swing, and that perhaps the guitar side of the circuit wasn't getting there.

    Edit to add: I'm also completely open to adding another 9-pin socket. Plenty of room for that.
    Edited again to add: Mine is beat to hell, and heavier than hell. I'm not averse to building a head cab, either.

    https://www.tubesandmore.com/products/R-VA2XXL -->Is that what you meant by ganged pot?
     
    Last edited: Jun 17, 2016
  13. Fred Mertz

    Fred Mertz Tele-Afflicted

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    I'm going to start a new thread to discuss UL Bassman mods in the Shock Brothers forum. I don't want to high jack Andy's thread anymore than I already have.
     
    Last edited: Jun 17, 2016
  14. andyfromdenver

    andyfromdenver Friend of Leo's

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    lol. i don't care. i got out my tiny buyers rem rant. but, only that it would be sad for internet users to not find your info easily.
     
  15. Fred Mertz

    Fred Mertz Tele-Afflicted

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    You had some very interesting points in your original post that merit discussion without me cluttering up the thread with my amp mod shenanigans.:)

    I'm pretty sure that you can use an 8 ohm speaker with your new Bassman 70. The extension jack does place the added speaker in parallel connection with the main speaker and shunts to a lower output winding, probably 4 ohms. It would make perfect sense for the primary speaker jack to connect to an 8 ohm tap. What confuses the matter is the language used in the labeling for the jacks, "4 ohm min. load". To me this implies that loads higher than 4 ohms are permissible.
     
  16. andyfromdenver

    andyfromdenver Friend of Leo's

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    hence we'll have to get some real numbers and see what's appropriate :)
     
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