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UL Bassman Mods

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

  1. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Wired in a filament transformer, have a 5U4 rectifier in there, B+ of 465 dissipating 19W per tube (I might put 5881 in there, yet). Rewired the doghouse, have room for 1 more node now, might have to ditch the UL because the lack of filtering (47uF is all I felt comfortable putting in as first filter stage being tube rectified and all). There's a bit of 120Hz hum that I don't care for, now.

    But the amp still sounds like absolute garbage. Through 1x15 with plenty of handling (200w Emi Legend) or a 4x10, Jensen Mod 35.

    I don't care for the volume and tone controls at all. Either channel gets into loose, flubby bass that overpowers everything, and does a pretty good job of sounding like germanium Fuzz if you crank the volume knob, even a little bit.

    This big, huge, UL OT...could it be the issue? Producing too many lows for the guitar speakers?

    I have Channel 1 somewhat marshallized at the moment with 1st gain stage (fully bypassed) into a cold-clipper (10k unbypassed) then into a warm stage (820 unbypassed). It might be cool if it weren't for the ovederpowering bass flubbage.

    Same with Channel 2, which I haven't really messed with from FM's HiWattish circuit there.

    The PI, is LTP pretty much. Is there anything there that could be causing the overall crappy tone? I believe the LTP was designed based on the following comment:
    Here is the circuit: You think that LTP with one size using .01uF and the other side using.1uF is a typo?
    [​IMG]

    I'm almost to the point of clipping out the whole circuit out from the PI back, and starting over. And if I do that, it's getting the 5F6A treatment in Channel 1, and EF86 in Channel 2.
     
    Last edited: Jul 3, 2017

  2. JD0x0

    JD0x0 Friend of Leo's

    Age:
    27
    Feb 22, 2009
    New York
    Hiwatts with UL Partridges that were apparently rated down to 10hz or something like that, and UL marshall Majors had no issues related to a large OT, AFAIK...


    The 10n(/15n/22n) Coupler and 100n coupler on the inputs of the PI look standard to me. I see that setup in a lot of amps, for whatever reason.

    Do you have grid stoppers on the stages? I can't tell if your flub is from oversized coupler/blocking distortion, or just the power section changes. Fenders seems to flub often, IME.
     

  3. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Blackface amps use a .001uF phase inverter coupling cap (on the upper PI triode) and a .1uF lower grid AC ground (on the lower PI triode grid) so your .01 and .1 values are pretty "normal".
     

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  5. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Went to all 0.001 coupling caps on the Channel I've been messing with. Now it just sounds anemic. Something ain't right. I'm starting over. Well 60% over. Keeping LTP, Power Section and UL for now.

    I don't like that the tone stack is right after the first triode on both of Channels as currently wired. I've neither played (at length) nor owned an amp with a cathode follower. I've used many MOSFET source followers to drive tone stacks, though. Time to have a cathode follower amp, and I got the Sino Chinese tubes last week just in case I went this way.

    So I'm gonna rewire it to JCM800 (ish) on Channel 1 and then do something with an EF86 into LTP for the 2nd channel.

    Also, Merlin says .01uF could be used on both sides of the LTP. And Dr Z used .047s in the Route 66...so I might try .022 as a starting point.
     

  6. JD0x0

    JD0x0 Friend of Leo's

    Age:
    27
    Feb 22, 2009
    New York
    1n couplers would definitely sound anemic, so that's no surprise. You may want to calculate the low pass in each stage, when selecting coupling caps. Bigger couplers tend to need bigger grid stopper resistors to prevent blocking distortion.

    If you calculate a low pass around -3db @ 75hz you can retain the guitar's full bandwidth, while trimming low frequencies that the speakers and guitar wont be putting out, for a tighter low end. Cathode bypass caps and couplers will both probably need to be tuned, since 10-25uF pass way more lows than needed for guitar.
     

  7. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Just took a first pass at a layout for the JCM800/EF86 mods. I used the template for the eyelet board in there now, so hopefully won't have to rebuild the whole thing, but some of the holes aren't ideal.

    PDF attached.
     

    Attached Files:


  8. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Alrighty then. Got it all wired up and just had a 30 min test/play on it. I've got it loaded with:

    V1: EH 12AX7
    V2: Sino 12AX7
    V3: Winged C EF86
    V4: RCA 12AT7
    V5/V6: Tung Sol USA 5881
    V7: Hammond 5U4GB (Pull from a Hammond Organ)

    B+:442V
    A: 437
    B: 358
    C: 302
    D: 260

    V1
    1: 194
    3: 230
    6: 1.7V
    8: 2.6V

    V2
    1: 143
    3: 0.93
    6: 260V
    7: 143 (DC Coupled)
    8: 144 (cathode follower)

    V3 (EF86)
    1: 118V
    3: 2.5V
    6: 92V

    V5/6
    3: 437
    4: 435
    5: -51V

    Drawing 35 mA per tube, giving me 15W plate dissipation, or about 61% bias. Will bump that up some.

    Just realized I forgot to get the PI voltages.

    Anyhow, the amp sounds much, much better. All channels sound much, much better. No microphonics or squealies to speak of on any of the channels, even with everything dimed. The AA864/Hiwatt experiment was an abject failure. Does not appear that it was the UL OT making it sound bad, because I kept the UL topology intact.

    Channel 1: JCM 800 with lead, and low input (skips first gain stage). On the cathode follower, I put a split plate load (51k/51k) on the preceding gain stage, and a 0.1uF from the split load to the cathode (bootstrapping it...from merlin's book). I put it on a switch so I can switch it in and out. It does seem to behave just as merlin described, and a worthwhile use of the front panel switch [​IMG] .

    Channel 2 is Route 66 derived, EF86 into LTP.
    I don't think I'm getting full power tube drive from Channel 2. To utilize the exiting switch on that channel, I just made the cathode bypass cap switchable. Switching it out, there's really not enough volume to be useful. So I gotta think of a better use for that switch.

    Ruminations as of now:
    1. Considering a 12AX7 in the PI spot just to test-drive.
    2. I used 330k mixing resistors to bring Channel 1 and 2 into the PI. Can I mismatch them at all? --> I'd like to preserve as much signal from Channel 2 as possible. Could I do like 100k and 330k, for example?
     

  9. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    OK, some interesting discussion here, centered around using either side of the LTP for mixing. I like this idea! What say you?

    http://www.tone-snob.com/forum/topic/10095/mixing-resistors/

    I must be a nerd. I'm not using NFB, so this could work! I can't wait to go out and try it!

    Is it as simple as lifting the ground on the cap to the other PI grid, and connecting one channel there?
     

  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yes, just feed that grid a signal just like on the other LTP triode.
     

  11. j vegas

    j vegas TDPRI Member

    69
    Apr 26, 2003
    Fort Collins, CO
    I searched through a bunch of schematics looking for an example of your situation. I could not find an amp that used a pentode in the preamp with a mixing resistor before the PI.

    The Trinity TC 15 is one of the few that combine an EF86 channel with a 12ax7 channel, and it feeds either side of the LTP like you mentioned above.
     

    Attached Files:


  12. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Thanks guys...

    I rewired it so the EF 86 Channel goes to one grid of the PI, the JCM800 Channel goes to the other. I still needed to keep a attenuator of 220k on the JCM800 side, otherwise I get squealies when I crank the amp on the high gain input.

    To accommodate the PI as a mixer, I also balanced the plates at 100k.

    Also switched to 6L6GC since my Tung Sol 5881 were not matched very well. Got the JJ 6L6GC biased at 20W which brought my B+ down to ~430V.

    Pretty happy with the sound. Still not enamored with the Jensen Mod-35 quartet of 10s in my external cab. Not sure I want to put those back in the Fender carcass. The Bassman 10 came with a quartet of 8-ohm Carvin PS10 speakers in there. Not sure if he had them wired for 2 ohms or 8.

    Anyways, I've got the 2-ohm tap also wired up, so I'm gonna wire those PS10 in parallel and plug them into that and see how it sounds.
     
    Last edited: Jul 16, 2017 at 1:03 PM

  13. j vegas

    j vegas TDPRI Member

    69
    Apr 26, 2003
    Fort Collins, CO
    Glad it worked out.

    I would be interested to hear the difference between the ax7 and the at7 in the LTPPI.
     

  14. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Carvin PS10 sound fantastic. Not even going to bother with the Mod 10-35s. Once I play this in a bit, I'll try subbing the 12AX7 in the PI.
     

  15. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States

  16. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    I was at my wits end. I'll tell you what though, I won't be in a hurry to obtain (or recreate) a Blackface Bassman circuit after that debacle. Boy, that sounded nasty. All that tone plate driven shaping after the first triode, then out to 2 more triodes. That was just nasty tone. And that second channel, whatever it was, HiWatt inspired or whatever. Didn't sound any better.

    I've a big thumbs up for the late, cathode follower driven TMB and Marshall's cold clipper followed by a warm biased stage. Glad I added that extra 9-pin socket, after all. Was really starting to question why in the world I did that, heh.

    I suppose I ought to take a bang at a schematic for this, now that I'm a bit more experienced with the layouts, I guess I should tackle the schematic part. DIYLC does have some schematic blocks. Too bad it can't read a layout into schematic, heh. Final layout attached.
     

    Attached Files:

    robrob and JD0x0 like this.

  17. JD0x0

    JD0x0 Friend of Leo's

    Age:
    27
    Feb 22, 2009
    New York
    Did you play with lead dress on the Blackface attempt? I have a SF Twin on my Bench and the OD was pretty nasty until I moved the lead dress which got rid of some parasitic oscillation which was causing a nasty harsh fizz/crack on top of the notes. Went from sounding like crap to proper OD/distortion.

    Note, due to the vibrato channels long grid wire length, that channel tends to have more issues.

    Glad the new mods are working out better for you, though.
     

  18. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    I chopsticked and moved wires around quite a bit. I have had lead dress problems in some of my previous builds. I like to think I'm getting better at that with each build. But anyways, the symptoms of bad lead dress that I've chased is more like squealies and screeches on top of tone or flat-out blocking the signal. Loud and nasty, but not too reminiscent of the Blackface attempt.

    That said, I do believe that some of the nastiness was due to the huge bass response of them amp, and I attribute that to the UL topology (based on some reading). It would have behooved me to try the Carvin PS10 in the amp before I rewired it. I think perhaps I was hearing nasty speaker break-up from the Mod 10-35 and maybe even the Emi Legend 15 I tried. Certainly that wasn't all of the issue, but I do think it negatively contributed to the overall sound...

    Also the pots, I reused from original Bassman10 and I think that was a mistake. I know the mids was like 100k and bass was 250k. I ripped all those out on this rebuild, like I should have done in the first place.
     
    Last edited: Jul 17, 2017 at 7:16 PM

  19. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    SO, the Marshall Lead Channel is WAY louder than the EF86 Channel. The EF86 Channel is closer to the Marshall Low Channel in max volume. I understand I've got way more gain on the The Marshall Lead Channel which layers preamp distortion/clipping. But there's still a point of max drive, right?

    I guess I was hoping to be able to fully drive the power tubes with the EF86...I'm looking for similar volume, just with a different character. This is primarily academic, I suppose. I think both channels are plenty loud for bar gigging. But that said...what about adding a MOSFET source follower to drive the tone stack on that EF86 channel?

    Edit: to add a partial schematic I've been working on in DIYLC.
     

    Attached Files:


  20. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Just spent 2 hours playing it in, with my P-90 equipped Dano.

    There's no volume problem, lol, on the Rt. 66 channel. Dunno what I was thinking. Tho, the P-90s are pretty hot (Rose) compared to the BL Keystones. So that might have something to do with it.

    I'm pretty excited about this amp. I'm gigging it tomorrow. I'm especially amped, that I can walk over to the amp and swap channels for different songs. That Marshall Channel sure is something else. Something I'm not used to, dirty fun, that's what it is.

    That JCM 800 Channel sure is nice. (I'd never played a Marshall circuit before).
     
    robrob likes this.

  21. j vegas

    j vegas TDPRI Member

    69
    Apr 26, 2003
    Fort Collins, CO
    That sounds really cool.

    I can't tell from your layout, are your channels in phase? Have you considered jumpering? Maybe an A/B/Y pedal for gigging? I wonder what the Marshall channel + and Ef86 at the same time would sound like.
     

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