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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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    I picked up a Bassman 70 head in very good condition. It's clearly been worked on by several POs. But all tidy enough. No issues. No mods that I know of. Sounds big and clean as to be expected. Very nice with pedals. I have it with my homemade 1x12 and a new Jensen C12K. Not a bad match. A little dark, but still very Jensen-y. I'm sure it will sweeten as it breaks in.

    EDIT: pretty sure it has bias level pot added by the last owner.

    I modded my Boosta Grande a bit to decrease the high roll off, and it sounds fantastic with my Bassman 70. I guess I just really like the pre-amp distortion/crunch it helps push. But of course, it gets a little loud with the boost. No biggie, as I can use drive pedals. But it just sounds soooooo much better with just the boost.

    So my questions....

    Would modding the MV or adding a PPIMV help with getting some preamp crunch at lower volumes? Googling and prior TDPRI threads suggest a dual ganged pot after the PI is an easy MV install. Also, would taking or switching out the global feedback help? I'm more inclined to think it will make it louder, as opposed to help with preamp crunch. Is it worth trying? Unless someone could make a really good case for it, I see no reason to mess with the UL taps.

    Any ideas or suggestions would be helpful. Thanks!
     
    Last edited: Oct 22, 2020
  2. Wally

    Wally Telefied Ad Free Member

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    Does the stock master volume not do it for you? Got pics of the circuit? Have you considered modding the Bass channel for a hotter, less-scooped guitar-oriented circuit instead of leaving it for bass. Or...if you want that channel for bass, you could mod the Normal channel, which is basically a straight Normal channel out of the AB763 guitar amp world.
     
  3. Whatizitman

    Whatizitman Friend of Leo's

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    I got it with some intention to mod if needed. But now I'm not so sure I want to do anything drastic. It's in pretty good shape, and sounds great with pedals as is. The PO put in new tubes, biased, caps, etc... He's a bassist, so I think he left it pretty stock specs, except for adding a bias level pot.

    The master doesn't really do much. But I wasn't expecting it to. That's been my experience with UL Fenders. I was ok to just use pedals for drive. It's just that the boost made me want more of that sound from it, if possible. I've bridged the channels, and it sounded big and fat. But way too much low end for guitar. I like typical fender sound, so I don't really want to marshallize it. I just would like to get the pre-tubes cranking on the normal channel a bit more on their own, if possible. Is pulling V1 safe enough to do? And will it make much difference?

    I'll upload some pics in sec...
     
  4. Whatizitman

    Whatizitman Friend of Leo's

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    I haven't measured or touched anything on it. Just opened it up to take a look and pics, and put it back together. I did do some tube swapping - put in 12ax7 on PI and V1 and V3, I think. I've never seen a jack switch like that on the EXT. I looked for the NFB wire, and got a little confused with it all. Figured I would ask many questions before going further on anything else.

    3398BAA2-7ACB-442B-A92C-B3B3D0420FD4.jpeg 65B45D08-3B1A-443C-B21A-BF9243D99134.jpeg A618A0BD-5AD1-477C-BA0D-1CCD59875F18.jpeg
     
  5. Wally

    Wally Telefied Ad Free Member

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    A bias trim pot has been added to the bias balancing pot so that one can adjust the bias voltage as well as balance the outputs....or unbalance if one cares for that. As for the Master Volume, I acknowledge that most of us consider these circuits to be less than desirable for getting preamp distortion at low volume, but that has more to do with the preamp than the MV. I suggest you run that boost pedal at the preamp to get thenpreamp drive you seemingly like and then adjust the MV downward to see if you can find a usable preamp sound at a lower output volume. If you can’t then you are
    I king what happens when a hotmpreamp is hitting an output section that is also going to work. That results in volume. A post-PI MV will not cure that. A good attenuator might. what you have there is a big, clean amp...two gain stages in each channel. It is NOT a circuit that is based on the AB165 Bassman, which has a hotter preamp. That amp can be changed into something that might surprise you, but it will still be a loud amp without some help.....low efficiency speakers or an attenuator.
     
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  6. loopfinding

    loopfinding Tele-Holic

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    Pull the common gain stage tube (v2 right?). Because of that local NFB resistor, the signal should still bleed through to the PI. Slam the preamp with some OD and report back. More or less at small room recording level at full volume. Except you also have a master, so adjust volume to taste.
     
  7. Whatizitman

    Whatizitman Friend of Leo's

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    I thought the preamps on these (AA371?) were similar to the AB165? It's the earlier ones that have two gain stages. Later ones have three, right? I know the Bassman 10 is just one pre tube, though.

    Either way, I'm good as it is.

    Still curious about NFB loop, though. I guess the internet consensus is that since the UL has feedback, that global feedback is redundant. Would cutting or switching out the global loop make a difference in pre-amp gain and distortion?
     
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  8. Whatizitman

    Whatizitman Friend of Leo's

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    I think it's V1. I was gonna try that next. I don't know if that will cause any undue stress on any of the tubes?
     
  9. loopfinding

    loopfinding Tele-Holic

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    this one. it'll just reduce current draw on the power supply.
     

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  10. Wally

    Wally Telefied Ad Free Member

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    I see what you are saying, whatzitman...and I stand corrected. These Bassman 50 amps still do not have the heat that the AB165 through the AA370 Bassman amp’s have. I was confusing this amp with the Bassman 10 and thinking that the Bassman 70 was a continuation of that Bassman 10 rather than a continuation of the Bassman 50, which is fr9: the AB165 lineage. Hmmmm.....could it be the difference that local feedback resistance that makes some difference? I notice a difference in the resistance there. The global NFB does not act on the preamp but rather on the output section.
     
  11. Ten Over

    Ten Over Tele-Holic

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    Why would you remove one of the gain stages if you want more preamp distortion/crunch? How about removing the 220K feedback resistor on V2B?

    Bassman 70.png
     
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  12. loopfinding

    loopfinding Tele-Holic

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    you could really slam the input stage with an overdrive or boost without having that additional stage at a fixed gain spit it out to the PI, and i reckon it would make the pre PI MV less touchy than with the extra stage in, bringing its travel down to small room range. it isn't a great solution for live use, as it's a little too quiet, but i've done this on the ab165 (sans MV) to good effect for recording (though the gain on the third stage is different in that one).
     
    Last edited: Oct 22, 2020
  13. Wally

    Wally Telefied Ad Free Member

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    Hey not slam the preamp with overdrive and turn the existing MV down?
     
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  14. loopfinding

    loopfinding Tele-Holic

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    maybe i should look at the values of the mixing resistors in relation to nfb before offering advice :oops:
     
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  15. Wally

    Wally Telefied Ad Free Member

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    And maybe I should have realized that this amp is NOT a Bassman 10 because of that third gain/summing stage there, right? Living and learning is for me a daily thing, still....
     
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  16. Whatizitman

    Whatizitman Friend of Leo's

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    Fenderguru says you can pull V1 for more preamp gain on the normal channel. V1 is on the bass channel. I’m gonna try that now and report.

    http://fenderguru.com/amps/bassman/
     
  17. Whatizitman

    Whatizitman Friend of Leo's

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    OK, pulling V1 does help a bit. I still am cranking the booster all the way for the toanz. Not bedroom volume by any means. But not horribly loud, either. It’s a 70 watt amp through an efficient speaker, so it’s never gonna be bedroom viable. And that’s ok. Still sounds way better than A cranked tiny low watt combo, AFAIC.

    Wow. The difference between the clean boost and a drive pedal is just night and day. Granted, my drive pedals aren’t exactly “transparent”, either, whatever that means. But the boost just sounds soooo much better. The problem is it’s 20db of boost. And the preamp tubes aren’t saturated enough at low volume. So they get quite a bit louder with the boost. Oh but it’s a good loud LOL. :cool:
     
  18. elpico

    elpico Tele-Afflicted

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    I don't understand the comment about the master volume "not doing much". If you turn the mv to zero is the amp not silenced? It should be.
     
  19. Whatizitman

    Whatizitman Friend of Leo's

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    Right. I mean that the regular channel volume knob on UL Fenders don’t make the pre tubes crunch when cranked like a modern high gain amp would. Crank the channel volume, and the amp gets mighty darn loud at 3 on the MV, and it’s still clean LOL. That’s the nature of these beasts.

    But my experience with UL fenders (confirmed with this one) is that the preamp crunch is good, if you can find a way to get it. Years ago I had a UL Pro Reverb that I found had a good distortion on the reverb/trem channel if the MV boost was off (knob pushed in), channel volume cranked, and goosed with a clean boost pedal. Didn’t sound at all choked like the horrible pull-boost distortion. Night and day. The bassman doesn’t have that silly reverb distortion tap design (no pull boost. And no reverb to siphon from). And it seems to work just as well. Maybe it’s just me. But I guess I just really dig the SF Fender pre-tube crunch. Even better to not have to turn up to ungawdly volumes to get it. My old JMP 1987x couldn’t do that.
     
  20. elpico

    elpico Tele-Afflicted

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    Okay good. Well then it's easy enough to make the preamp crunch if you're willing to change a couple resistors and you'll have your crunch at whatever volume you want. Each of these will increase gain:

    - put the 470k channel mixing resistor back to the 220k value from the blackface/silverface circuits (you can tack another 470k across it if you prefer)
    - disconnect one end of the 220k local feedback resistor
    - change the 3rd stage grid leak from 220k to a more normal 1meg
     
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