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UL Bassman Mods

Discussion in 'Shock Brother's DIY Amps' started by Fred Mertz, Jun 17, 2016.

  1. Fred Mertz

    Fred Mertz Tele-Afflicted

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    The UL Bassman amps are not particularly revered and get an undeserved bad rap IMO. Most people point their finger at the UL output transformer. The UL OT is not the problem. The CBS engineers added the UL transformer to Fender circuits with existing negative feedback loops and also added implemented a voltage doubler power supply that pushed voltages to or passed the limits for 6L6GC tubes. In addition to the aforementioned changes various tone sucking shunt capacitors were implemented in the circuit as band aid fixes to address oscillations in the circuit caused by sloppy wiring. The purpose of this thread is to explore circuit mods to address some of these problems and explore amp voicing options.

    I have been bitten by the Hiwatt bug lately and was thinking of modding a UL Bassman circuit to provide one channel with Hiwatt voicing and the other channel with Fender voicing. The Bassman 70 platform with the third preamp tube offers the possibility of cloning amid 70s Hiwatt preamp in one channel and an AA864 Bassman preamp on the other.

    Modded Bassman 70.gif
     
  2. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Commodores I made a few changes to the Mods for the Bassman 10 to make them easier to implement. Attached is the revised schematic.

    bassman_10_UL-SCHEMATIC.jpg

    To answer your questions, the UL transformer doesn't a larger voltage swing IMO. The problem is with excess negative feedback. As noted above, the CBS engineers added the UL transformers to circuits with existing negative feedback loops with the intent of making the amps as clean as possible. Eliminating the negative feedback loop will go a long ways towards opening the amp up.

    An added preamp tube would allow for a third gain stage to bring more fun to the party. I initially was looking at the Bassman 70. With the third gain stage you can pretty much clone a Hiwatt preamp on one channel and maybe a Bassman AA864 on the other. You can achieve the same tone structure using the Bassman 10. You just don't have as much gain to play with, but a clean boost pedal could certainly more than compensate for the limited gain.

    The pot in the link that you provided is the correct type. I did make some adjustments to the previous schematic that provides for the "Lamar PPIMV" that should be easier to implement. A 250K ganged pot is used in the revised schematic.

    The Bassman 10 mods and my rationale are as follows:
    1. Remove negative feedback loop. With UL, it is redundant and results with too much negative feedback that IMO makes the amp sterile. The amp should open up noticeably with its removal.
    2. Replace PrePIMV with PostPIMV. The existing MV is rather limited in its effectiveness IMO. There isn't enough gain in the circuit. The PostPIMV should allow the preamp to overdrive drive the PI while maintaining reasonable volume levels.
    3. Reconfigure bias balance control to bias adjustment control so that one can make bias adjustments when tubes are replaced without changing out resistors. The resistor values on the schematic are a first guesstimate and will likely need tweaking.
    4. I adapted the Hiwatt PI feedback circuit to provide for dynamic balancing of the triode sections to address triode mismatches. I think that the PI balancing helps to provide for a clean tone when you ant to play clean and yet will not hamper the ability to overdrive the PI when one wants a dirty sound.
    5. Removed various shunting capacitors throughout the circuit. These capacitors were added by Fender as a means of preventing oscillations in the circuit induce by the sloppy wiring. To remove these items the amp wiring will need to be cleaned up.
    6. Implemented a mid 70's Hiwatt tone stack on the Studio channel. As of late that's the tone that I'm drawn towards.
    7. Implemented a tweaked tone stack from AA864 Bassman on the Normal channel. Tone stack is the AA864 Normal channel tone stack with Deep switch in lieu of Bright switch. I always thought that my Strat had a nice sound when played through the bass channel of a Bassman with the Deep switch engaged.
     
  3. andyfromdenver

    andyfromdenver Friend of Leo's

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    great work, and thank you for sharing!
     
  4. Commodore 64

    Commodore 64 Friend of Leo's

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    Incredible. Thank you Fred (and Andy). I'm trying to be on the same path as Andy, self learned, by consuming & building as much as I can, and I'm about 3 years in...

    I will make this a priority. Excited to try these mods, it is a rather lackluster amp as it stands. (for guitar).

    Fred: From a watkins discussion:
    And here are the graphs we were looking at (6550):
    [​IMG]

    [​IMG]
     
    Last edited: Jun 17, 2016
  5. JD0x0

    JD0x0 Poster Extraordinaire

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    I would love to get my hands on one of these cheap. They seem like they would be great platforms for some mods
     
  6. Jeru

    Jeru Tele-Holic

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    Thanks folks..! I moved in March and have been doing construction-type improvements to the new place -- had to shelve my AA864 build project mid-winter.

    This thread REALLY makes me want to get back to my build. I was going to make it a single channel, but there's all that room in the chassis...
     
  7. Fred Mertz

    Fred Mertz Tele-Afflicted

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    The graphs are comparing apples and oranges IMO. On paper it does look like the UL circuit requires a larger input signal applied to the output tube grids than the AB1 pentode circuit. However, the UL transformer applies negative feedback via distributed load internally within the output transformer. To overcome these losses, the UL circuit requires a larger signal on the grids of the output tubes. The increased signal requirement is included in the peak to peak signal requirement per the datasheet. The AB1 pentode graph does not take into consideration any losses from negative feedback implemented between the output transformer and the input to the PI. To overcome the losses from the negative feedback loop, the signal applied to the PI must be increased for the AB1 pentode circuit. It's a matter of where the signal is increased, at the output tube grids for UL, or at the PI tube input grid for AB1 pentode with negative feedback.

    The graphs that you posted look strange. They do not agree with the operating points published on page 3 of the datasheet.http://www.drtube.com/datasheets/6550a-ge1972.pdf
     
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  8. elpico

    elpico Tele-Afflicted

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    The distortion character of a UL amp is quite different than a traditional pentode amp. May not be of interest here, how often are most people going to drive a 50w amp into power tube distortion really, but for people who do it might be worth mentioning. The difference is much less noticeable when playing clean.

    Removing the unnecesarry nfb loop is the way to go with UL but the distortion character of the output stage remains more like a triode amp than a pentode one and lacks the more aggressive upper mid tone of a pentode amp. That's not to say UL sounds "bad" by any stretch, it can sound very good, but I think it would be fair to say it doesn't sound "traditional". If the goal is to create your own tones this might be just the route you want to take. If the goal is to cop those familiar, classic tones then keeping the nfb and disconnecting the UL feedback is the route to take.
     
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  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Removing the NFB loop adds a lot of phase inverter distortion but the UL power tubes stay pretty clean.

    From my Overdrive webpage:

    "Screen grid current, much like control grid current, causes nonlinear distortion. Overdriving a tube with a screen grid results in screen grid conduction and screen voltage drop which leads to lower tube gain. In pentodes and beam tetrodes screen current has more affect on overdrive tone than control grid current. One of the reasons is that screen voltage drop not only shifts the tube's operating point along the transfer curve but it actually changes the shape and slope of the tube's transfer curve. This dynamic transfer curve morphing adds tons of nonlinear, harmonic and intermodulation distortion. This is the main reason most people prefer the sound of power tube distortion over preamp (triode) distortion."

    The UL screens prevent power tube distortion which is not a good thing unless you want a super clean pedal platform and then a much more inexpensive solid state amp is the way to go. The UL screens shift the amp's balance toward preamp distortion.

    At the very least put that UL tap on a switch. The increase in power tube distortion is worth the extra work.
     
    Last edited: Jun 17, 2016
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  10. Fred Mertz

    Fred Mertz Tele-Afflicted

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    It really depends upon the screen grid tap ratio as to how the much the pentode behaves like a triode. A 20% tap is going to behave more pentode like than a 40 or 50% tap. The goal of UL was to obtain the linear response characteristics of a triode with the efficiency of a pentode. The different screen grid tap ratios represent different compromises between pentode and triode operation. I agree that the difference in the circuits is subtle when played clean and that the overdrive characteristics are different. IMO the UL amp has a smoother transition to overdrive and has softer clipping. I've played UL amps with the feedback loop removed. They can sound very good with very pleasing distortion characteristics.

    One of the reasons for me starting the thread was to explore opportunities to maximize the potential of the amps using the UL transformers. Otherwise, we're back to clipping the UL leads and retrofiting the amp to a blackface circuit. Where's the fun in that?
     
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  11. elpico

    elpico Tele-Afflicted

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    Yep that's true, but I'd throw one caveat on that: a 20% feedback tap doesn't equate to sounding 80% like a pentode.

    The number refers to the amount of feedback used, not the character of the sound. It doesn't take 100% feedback to make it sound 100% like a triode, it takes much less. I imagine that's why a 40% tap became a common value - that's enough to eliminate the pentode distortion character and make it sound fully like a triode. No point in going further.

    I've built amps with pentode, UL, triode switches and adjustable/defeatable nfb in the past. I was using hammond transformers with 40% taps. I don't know what ratio the bassman transformer used, but my comments are based on experiences with 40% taps. To describe the "zero feedback UL character" compared to "normal pentode character" I'd make an analogy to singing. In UL mode I find the distortion shifts down into more of a chest and throat voice. The head voice is absent. You could maybe imagine this voice as a more of a deep growl and the pentode as more of a shout. Much more head voice to the second one.

    I could see someone finding the UL distortion character to be "mellower" or "smoother", but also "unfamiliar". Switching to pentode mode (with or without nfb) shifts the distortion back up to the more aggressive, upper midrange tones we're used to.

    It's fun to experiment with anyways, and if you're open to more unique tones rather than just the traditional ones then who knows what you'll find in there.
     
  12. Fred Mertz

    Fred Mertz Tele-Afflicted

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    I agree that the UL transformer does primarily address output tube distortion. And removing the feedback loop does prevent the circuit from removing PI distortion. However, the negative feedback loop added to most pentode circuits attempts to correct for distortion that is introduced in the PI as well as the output tubes.

    UL operation most certainly does not prevent power tube distortion. The screen grid voltage swings commensurate with the plate AC voltage swing, at a ratio consistent with the ratio of the transformer screen grid winding. The screen grid voltage swing does prevent hard clipping and has a more gradual transition to clipping than pentode operation. But, if the PI can swing a large enough AC signal, the output tubes will clip. I've played UL amps with no additional negative feedback. They are not the uber clean non-responsive amps that you imply. Ever try Dr Z Route 66?
     
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  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    When I said "prevent distortion" I should have said "reduce" as I realize UL power tubes will distort but the very purpose of the UL operation is to minimize distortion. That's why UL is so popular in audio tube amps. Removing the NFB loop will cause a disproportionate rise in phase inverter distortion since UL is minimizing the power tube distortion.

    If I was building an amp with an ultralinear output transformer and had to choose one way to hook it up I would definitely not use the UL taps but a UL switch is the way to go.
     
  14. Screamin Willie

    Screamin Willie TDPRI Member

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    I'll play.

    I have a Bassman 70 that I have reconfigured to pure AA864 from the Normal input to the output. UL taps have been disconnected. The Master volume has been moved and is now a LarMar PPIMV. It's been this way for a few years.

    I've played with a few different ideas/configs for the Bass channel, including a cathode follower utilizing the spare stages.

    The Bass channel is currently a Bandmaster Normal channel.

    Funny this should come up. I've been thinking about my Bassman 70 a lot lately and thinking about getting back into it.

    Willie
     
  15. Screamin Willie

    Screamin Willie TDPRI Member

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    I guess I should have read this a little more carefully before jumping in. LOL

    Willie
     
  16. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Over the week-end I transcribed the Modded UL Bassman 10 schematic to a layout using the existing circuit board. During the process, I made a few tweaks to the circuit to make the implementation a little easier and to correct a couple of errors. I could not find a UL Bassman 10 layout online, so I used the Bassman 10 layout as a starting point. I updated the original schematics that I posted and will post those subsequently.

    Modded Bassman 10 UL Layout.gif
     
  17. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Updated UL Bassman 10 schematic.

    Modded Bassman 10 UL Schematic.gif
     
  18. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Updated UL Bassman 70 schematic.

    Modded Bassman 70.gif
     
  19. Fred Mertz

    Fred Mertz Tele-Afflicted

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    It comes done to personal preference as to whether one uses global feedback or UL. Most people remove the UL connection without experimenting to see how the amp sounds without the global feedback attached. I would encourage anyone considering modding one of these amps to just lift the global feedback connection at the PI and play the amp awhile to see how they like the sound first. All of the mods can be used in a non-UL circuit. These amps do make excellent platforms for mods. Attached is the output section with global feedback rather than UL feedback.

    Modded Bassman 10 Non UL Output Section Options.gif
     
  20. Commodore 64

    Commodore 64 Friend of Leo's

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    Fred,
    If you were me, would you add another 12AX7?

    Or are you curious how the Bassman 10 mods sound without it? You don't have a Bassman 10 to experiment with, am I correct? Maybe I'll save the extra triodes for later and just see what the mods do without 'em. At least at first, heh.
     
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