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questions about UL and global feedback, MV mods for Bassman 70

Discussion in 'Amp Tech Center' started by Whatizitman, Oct 22, 2020.

  1. Whatizitman

    Whatizitman Friend of Leo's

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    Nice! That gives me plenty to chew on. No hurry to do any work on it. But I will post back as soon as I dive in. All easy and reversible, too. My kinda project! :D

    Thanks, everyone!
     
  2. Wally

    Wally Telefied Ad Free Member

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    This is not so for these Bassman amps. In the two channel BF/SF guitar amps, pins 8 for the two input preamp tubes‘ cathodes are tied together. That is why removing one or the other of those tubes affects the gain in the remaining channel because the shared resistor now shows a lower resistance to the remaining cathode and that tube is biased hotter. In these Bassman amps, these two input preamp tubes do NOT share a cathode bypass circuit so pulling one input preamp stage tube has no effect on the other tube.
    https://schematicheaven.net/fenderamps/bassman_ab165_schem.pdf


    https://schematicheaven.net/fenderamps/fender_bassman70.pdf
     
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  3. Whatizitman

    Whatizitman Friend of Leo's

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    OK. Sooooooo......

    What the hell did I just do to my amp?!!! It’s breathing fire now!!! OMG that did exactly what @elpico said it would do. And then some! Almost no clean at all now LOL. Anything past 5 on the channel volume is Mesa territory. Certainly no need for a booster now. I hooked the local feedback loop back up to tame it a bit. I’m gonna rock my @r$ off with it now. In the meantime, can someone explain to me in layman’s terms how that little bit of resistor swapping brought out Mr. Hyde? Wow! :lol::lol::cool:
     
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  4. elpico

    elpico Tele-Afflicted

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    Haha I considered mentioning that if you did all of those you'd be past brown deluxe levels of gain. Figured you'd find out soon enough :D

    Such a big change is possible in this amp because it has a higher than average number of 12AX7/7025 stages. You wouldn't get that much gain out of every amp with just a few resistor changes.

    Because fender wanted the amp to be a clean machine they've deliberately cut the signal level coming out of the preamp. V2 and the four resistors that connect to it's grid form both a channel mixer and an attenuator. It combines the signals from the bass and guitar channels, but it also cuts their level down. The guitar channel even more so than the bass.

    The resistor with the biggest impact is that local negative feedback loop. With that loop still connected changing the 470k mix resistor to 220k reduces the attenuation by 6dB, basically you get twice as large a signal out of the preamp.

    But if you open the loop you add the gain of V2 now too, so it's like another 30dB of gain.

    Changing the grid leak to 1meg only has an effect if the loop is opened, but in that arrangement it should increase gain another 3 or 4 db.


    You can change the value of the feedback loop resistor to tweak how much gain you want. Alternatively you can leave it disconnected and tap the plate load like the brown deluxe does to limit much gain you want. The two methods might have different tones, even at the same gain levels.
     
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  5. Whatizitman

    Whatizitman Friend of Leo's

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    Man, I’m just digging it. Thanks!
     
  6. Riffraff12571

    Riffraff12571 Tele-Holic

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    This thread has me considering options. Back in February while we were all blissfully unaware of how the corona virus was going to crap on our lives I met up with a local Craigslist seller who was selling a pair of empty 112 cabs I wanted for my rack rig. While there we got to talking gear and hit it off. He ended up selling me a fixer upper Rockmaster preamp for $25 and flat out gave me a non-working 1980 Bassman 70. It was out of the head case & missing the power tubes and phase inverter but had RCA labeled Ei ECC83's in the preamp and all of the parts seemed to be there. I bought JJ 6L6GC's, a 12AT7 PI as well as new caps for it months ago. A previous owner replaced the power & standby switches and the convenience outlet with Home Depot type leaded replacements and keeping with the Home Depot theme used wirenuts to make all of the connections. He got lost along the way and screwed it up so it wasn't working. I finally got around to fixing it a couple of days ago. First I cut out the wirenut rats nest and put that right. I figured I should try fixing it first to see how it was before diving into the cap job. I put in the tubes, plugged it into the 112 cabs I'm using with my rack rig because they have 8 ohm speakers and the pair connected parallel gave me the 4 ohm load I needed and it powered up. It was dead quiet so I decided to hold off on the cap job for now. Naturally I cranked it and tried to get it to break up which it really doesn't do. Next I hit it with a few OD pedals which did the trick. I tried a DLS, Soul Food, Dano Pride of Texas (not bad), SD-1 then remembered I had an old Visual Sound Route 66 in a box in my car with a few other pedals I tried to sell early in the year when nobody was buying. The OD side of Route 66 is Tube Screamer like only with more gain and a bass boost switch, the other side is a compressor. Tube Screamers are always great with a Fender and that definitely woke up the beast in this amp. It rips with that. :D

    Yesterday morning I took a different approach and hooked it up to my 412 cab set up at 4 ohms and ran it clean with my Suhr Eclipse distortion. The bigger cab really opened up the tone with that and I was really digging the tone I was getting. This is exactly what I was hoping for with this amp. I already have crunchy tube amps with my 6505+ for heavy tones, my Plexi modded YBA-1 for Marshall tones, my tube preamps in my rack for getting my '80s on, my Masco conversion for tweed tones and a few small vintage combos for quirky, swampy tremolo and verge of meltdown tones. The one thing I didn't have was a dedicated tube pedal platform. This amp was made for the job. BUT...............now I'm thinking about how I could incorporate a few of these mods and make them switchable to have both the stock clean pedal platform and with the mod engaged an amp that's more responsive to boosts. I could replace the unused ground switch with a DPDT version and use that to parallel a 470k resistor with the mixing resistor and another resistor with the feedback resistor to drop the value of that. Simple switchable changes for a gain bump. I need to experiment with this.

    [​IMG]
     
    Last edited: Nov 18, 2020
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