Question about twin reverb ultralinear MV sound quality

Discussion in 'Amp Tech Center' started by munkeyboy, Dec 29, 2020.

  1. munkeyboy

    munkeyboy TDPRI Member

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    I'm still struggling with this amp and wondering if it is by design (i.e. crappy master volume). I've replaced all filter/bypass caps. Also replaced coupling caps where they looked questionable (voltage on the wrong side). New tubes all the way, matched output tubes biased as ~60%. Plate voltages are correct within 10v, cathode correct all the way. Reflowed most solder points, but sure I've missed some.

    But I'm still having unwanted/flabby distortion with volume past 4-5 on the vibrato channel, with the master volume around 2-5. Haven't been able to go much past that due to my test speakers (2x 75w silver dome unknowns). Just wondering if this is expected or do I still have something going on. I can drop the bass to 1 to get rid of most of the flab, tho some crappy distortion does exist. Just can't tell if this is normal or not. I thought these Ultralinears are clean all the way.

    This is the hi input, low input is much better, but does still have some unwanted dirt (using jazzmaster pickups).
     
  2. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Umm, if you want cleans, why do you have the master turned way down?
     
  3. hepular

    hepular Tele-Afflicted

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    unknown speakers? hmmmm.

    but what TS is getting at might also be appropriate: if you did all that work you'll know where the MV is placed in the circuit--how about turning it up to 10, then trying the channel volumes at 1-3.

    but unknown speakers might well be the culprit ESPECIALLY if they're not really up to the demands of the gig.
     
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  4. munkeyboy

    munkeyboy TDPRI Member

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    Sorry, I didn't give more background. This is not my amp, I'm fixing it for someone. It came in pretty bad and not taken care of. It is just the chassis (I am making a head cab for it). I'm testing all the possible volume/tone positions before I give it back.

    Yes, this I already tried MV @10 and Channel Volume @4 and it sounds good. First amp I have touched with a MV and I don't know what to expect. I did not expect what I heard and fail to see the point if it sounds that bad. I do not get the same issues on the first channel.
     
  5. hepular

    hepular Tele-Afflicted

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    [Q I do not get the same issues on the first channel.[/QUOTE]

    Changes the situation. So, tubes not suspect, but sockets cleaned out? moving on from the easy stuff you've probably already done: does the statement i quoted mean that the MV works fine on the dry channel?
     
  6. munkeyboy

    munkeyboy TDPRI Member

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    Changes the situation. So, tubes not suspect, but sockets cleaned out? moving on from the easy stuff you've probably already done: does the statement i quoted mean that the MV works fine on the dry channel?[/QUOTE]


    Correct. Dry channel is good. A tiny bit flabby break up with Channel volume at 10, tone stack at 10, master 2, neck pickup... but that cleans up easy with either the tone stack, volume or any of the guitar controls.

    I did clean the sockets and retention, but now I second guessed my work there. Afraid of damaging to the tiny pin sockets, so I might not have re-tensioned well enough. V2 circuit is pretty much all new components. I also pulled v3 incase it was in the reverb.

    I'm not very skilled at scoping yet. I do have a scope and a function app, but still figuring this out. Maybe I just need to break out a listening amp and test cable.
     
  7. hepular

    hepular Tele-Afflicted

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    Correct. Dry channel is good. A tiny bit flabby break up with Channel volume at 10, tone stack at 10, master 2, neck pickup... but that cleans up easy with either the tone stack, volume or any of the guitar controls.

    I did clean the sockets and retention, but now I second guessed my work there. Afraid of damaging to the tiny pin sockets, so I might not have re-tensioned well enough. V2 circuit is pretty much all new components. I also pulled v3 incase it was in the reverb.

    I'm not very skilled at scoping yet. I do have a scope and a function app, but still figuring this out. Maybe I just need to break out a listening amp and test cable.[/QUOTE]

    in a fit of doing every mod i possibly could last year, i changed the 1st grid-leak resistor off of the input in my super-champ pretty from 1 meg to 19K. With results that were great if i wanted to either always play dirty or never turn guitar volume above 4 (master volume was on 3). Got a Warehouse veteran for christmas & figured it was less risk of hurting anything to pull the chassis to replace the speaker (instead of pulling reverb pan: space is tight). So, while i was in there, restored resistor to 1meg. That and the speaker . . . . oh wow.

    so, blah, blah, maybe it would be good to test some resistors, especially grid resistors while you're learning the scope stuff, then maybe dive into whether the tone caps might've drifted?
     
  8. Wally

    Wally Telefied Ad Free Member

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    Same old thing...I like to see a voltage chart? Or..at least a report that ALL voltages are correct...including cathodes.
    Have you tried pulling that tube from V1 and swapping it into V2 and/or V4? Review all work on the Vib channel since you say you did extensive work there.
     
  9. munkeyboy

    munkeyboy TDPRI Member

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    Thanks Guys,

    Here are the voltages:
    twinreverbvoltage.PNG

    Yes I've swapped tubes. When this amp first came in, I changed the filter caps first thing. After that, I turned it on and tested it with old tubes I had laying around as it didn't come in with tubes. The issue existed then (or one like it), but much worse. Since, I've replaced all tubes, replaced all electrolytic caps and a lot of other caps and resistors.

    As is, the amp is usable, it just doesn't sound right on the vib channel at higher volume and bass past 3-4 and master volume down low. Flabby/buzzy breakup, unlike the normal channel. I did test MV to 6 and vib vol up to 4-5 today and vise vera. Sounded ok, but at that volume I can't tell bad speakers vs other things in the house buzzing. Then MV to 6 and vib vol up to 10, sounds pretty flabby/buzzy. Plus my ears are now bleeding.

    But again, I don't know what to expect with this amp. I've owned a twin RI before (for 1 day before I traded it) and it didn't sound like this. But it didn't have a MV and was not an ultralinear and it had it's own speakers.

    The voltages are slightly concerning. 237v should be 250 according to the schematic, but not sure if that is within error margins. I have replaced out of spec dropping resistors. Also, not sure how to accurately test V5 (vibrato). The test voltage above are with vib turned off via footswitch. Foot switch on, the voltages go up and down.

    I am going to breakout my huge soldering iron and try to resolder the ground points on the chassis. They do look suspicious around the brass plate.
     
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  10. munkeyboy

    munkeyboy TDPRI Member

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    I guess if I can't get further, I can always mod the circuit to filter low freqs or to @hepular's point, alter the grid-leak to something over 1meg. I'll also try to get some speakers that I know can handle this for sure.
     
  11. sudogeek

    sudogeek Tele-Meister

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    So V5 is the tremolo. The symptoms and voltages of V5 could suggest the tremolo ‘roach’ is shot and the tremolo is stuck at the lowest point of the curve or oscillating about some low point. Consider replacing the optocoupler and carefully check the components of the tremolo circuit. The scratchy pots could just be dirt but also DC on the pots. Check the caps for leakage, especially in the tremolo circuit.
     
    Last edited: Dec 30, 2020
  12. Wally

    Wally Telefied Ad Free Member

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    Compare your voltages on V6 to the schematic. V5 is a little strange, too, but it has nothing to do with your problems with the Vib sound.
     
  13. munkeyboy

    munkeyboy TDPRI Member

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    Ok. the tremolo does have have a short sweep (if that's the word) on the intensity. Meaning, you don't really hear the oscillation until around halfway up the pot. But it is very strong at 10. I don't have scratchy pots.


    V6 voltage is close. I measure 270v on the plates versus +280v on the schematic. And 107v on the cathodes versus +110v on the schematic.

    The Owner is going to drop of his Eminence Legend 1518 15" in the next few days. That at least is a known speaker, though it is 8ohm.
     
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  14. schmee

    schmee Doctor of Teleocity Silver Supporter

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    My problem with most MV amps is they sound like crap unless the MV is wide open. That is likely what you are experiencing. They get distorted and nastier with the MV down. I have tried all 4 popular MV types in personal amp builds and didn't like any one of them.
     
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  15. Wally

    Wally Telefied Ad Free Member

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    I stand corrected and blame it on old eyes reading blurry numbers on the schematic
     
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  16. munkeyboy

    munkeyboy TDPRI Member

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    It's not just you. It's hard to read for sure.

    I take it 10v difference is not bad?
     
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  17. Zipslack

    Zipslack Tele-Meister

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    My gut says problem with vibrato/roach, but I'll also throw this out there...

    This model has the push-pull MV? Make sure it's NOT pulled out to the "boost/crap" position. If it's in, make sure the switch on the pot is making/breaking correctly,
     
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  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Just a hunch.
    Check the 1M resistor on the input jack.
     
  19. munkeyboy

    munkeyboy TDPRI Member

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    Already replaced that one. We found that a couple day's back due to low voltage on v2a.

    It can't hurt to order a new roach and recheck that circuit. MV is pushed in for sure, but not sure if the switch is working correctly or not yet.
     
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  20. BobbyZ

    BobbyZ Doctor of Teleocity

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    Spend a bit more time trying it with the MV on 10. Yes those really are horrible master volumes, the pull boost circuit is horrible too. Just put it on 10 and don't pull it out, if the amp puts out a lot of clean volume then it's fine.
    Two 75 watt speakers is good for 150 watts, it's a 135 watt amp, that's it's clean rating, you ain't gonna get it into distortion unless you completely dime it. The volume of it will keep you from doing that, if it's working right.
    Unless those speakers are adding distortion? Got a better idea what they are or if they've suffered the ravages of time?
     
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