Page 3 of 3 FirstFirst 123
Results 71 to 84 of 84
Like Tree29Likes

Thread: So I have been given free reign to gut a Blues Jr and do whatever I want with it

  1. #71
    Senior Member
    Join Date
    Sep 2010
    Location
    Florida
    Posts
    891
    Just to update this thread, the amp got built using the last drawing and has been in the backline at a local blues club getting hammered on several times a week for about the past 8 months and hasn't had any issues. The reverb did end up being kinda weak but gets reverby enough for most players. I think I'm gonna ask the owner if I can pick it up and mod the reverb for more output and start making that amp as an "original" design.
    ~Semi-No0b Hobbyist~

  2. #72
    Senior Member
    Join Date
    Oct 2010
    Posts
    513
    Thanks for the update.
    BTW, I see from the last schematic you posted that you chose 500pF for the coupling cap into the reverb circuit.
    I tried various values in that one spot and got a pretty big difference in tone, with some values considerably meatier.
    You might try testing various values before messing with the rest of the reverb circuit.

  3. #73
    Senior Member
    Join Date
    Sep 2010
    Location
    Florida
    Posts
    891
    Well here’s an interesting problem...

    I went to pick up the amp last night after the jam and the sound guy tells me that when you turn up the reverb it feeds back like hell. I get it home, dime the reverb and give it some volume and it plays fine.

    The tank is bolted to the bottom of the cab and not in a bag. Seems the obvious next step is to put it in a bag. Is there anything circuit-wise to consider or is it probably as simple as giving it some vibration isolation?
    ~Semi-No0b Hobbyist~

  4. #74
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    5,383
    Quote Originally Posted by mort View Post
    The tank is bolted to the bottom of the cab and not in a bag. Seems the obvious next step is to put it in a bag. Is there anything circuit-wise to consider or is it probably as simple as giving it some vibration isolation?
    Yes, a "tank sock" will help, but just a little. It also helps to cover the open side of the tank's metal box with a layer or 2 of corrugated cardboard you can razor out of a castoff shipping box. The rubber grommets and rivets supplied as tank mounts do nothing at this point - remove and discard them. Oh wait, keep the grommets if they're made of whitish translucent silastic, they may come in handy for some other thing later (says the guy who never throws anything away.) In extreme cases of tank howl, I've mummified the steel box with a layer of wide weatherstrip foam. Home Despot carries a weatherstrip that's intended for sealing the gap between pickup trucks and camper tops. It's about 1 1/4 inches wide & 1/8 inch thick, long enough to take care of several reverb tanks and/or other projects where it comes in handy. And under $10, what a deal. Cover as much of the metal reverb box as you can, also the cardboard cover, stick the whole shebang into a tank sock, then put a screw thru that assembly at each end. Now you've done as much as you can to mechanically decouple the reverb from the speaker cab.

    Also, there's no harm in reducing the maximum reverb signal by padding* it before it hits the reverb volume. I know, most folks are used to dialing the reverb up to 3 or 4, and there's no point in going further because that puts you at the far end of the tunnel, why go there. If you pad the reverb so that 10 is just a tad more than any sane person could possibly need, there's only one thing that can go wrong with that: complaints from the folks who are used to dialing it up to 3 and assume there's something wrong if you have to go beyond that.

    *Reduce the max signal available at the reverb pot by wiring a resistor in series with its signal terminal.

  5. #75
    Senior Member
    Join Date
    Sep 2010
    Location
    Florida
    Posts
    891
    Quote Originally Posted by Leo_Gnardo View Post
    Yes, a "tank sock" will help, but just a little. It also helps to cover the open side of the tank's metal box with a layer or 2 of corrugated cardboard you can razor out of a castoff shipping box. The rubber grommets and rivets supplied as tank mounts do nothing at this point - remove and discard them. Oh wait, keep the grommets if they're made of whitish translucent silastic, they may come in handy for some other thing later (says the guy who never throws anything away.) In extreme cases of tank howl, I've mummified the steel box with a layer of wide weatherstrip foam. Home Despot carries a weatherstrip that's intended for sealing the gap between pickup trucks and camper tops. It's about 1 1/4 inches wide & 1/8 inch thick, long enough to take care of several reverb tanks and/or other projects where it comes in handy. And under $10, what a deal. Cover as much of the metal reverb box as you can, also the cardboard cover, stick the whole shebang into a tank sock, then put a screw thru that assembly at each end. Now you've done as much as you can to mechanically decouple the reverb from the speaker cab.

    Also, there's no harm in reducing the maximum reverb signal by padding* it before it hits the reverb volume. I know, most folks are used to dialing the reverb up to 3 or 4, and there's no point in going further because that puts you at the far end of the tunnel, why go there. If you pad the reverb so that 10 is just a tad more than any sane person could possibly need, there's only one thing that can go wrong with that: complaints from the folks who are used to dialing it up to 3 and assume there's something wrong if you have to go beyond that.

    *Reduce the max signal available at the reverb pot by wiring a resistor in series with its signal terminal.
    It does have a nice thick piece of corrugated cardboard between the tank and cab floor. And as for the reverb level, the effect at 10 sounds about like the average reverb level at about 3. Was actually considering increasing the reverb level(scroll up in the thread) for those that like it at 4 or 5 so now I have two objectives that are competing with each other lol.

    What I may do is decouple as much as possible and then see if I can get the sound guy to meet me up there a little early before a jam with everything set up on stage and see if that helps the feedback situation. If so, then from there I can tweak to get a little more reverb effect and then meet with him a second time before a jam to test again. They mic the amps so the mic may have something to do with it... in fact I need to ask him if the feedback is in the amp or in the PA.
    ~Semi-No0b Hobbyist~

  6. #76
    Senior Member
    Join Date
    Aug 2017
    Location
    Portland, OR
    Posts
    238
    Quote Originally Posted by mort View Post
    It does have a nice thick piece of corrugated cardboard between the tank and cab floor. And as for the reverb level, the effect at 10 sounds about like the average reverb level at about 3. Was actually considering increasing the reverb level(scroll up in the thread) for those that like it at 4 or 5 so now I have two objectives that are competing with each other lol.
    Just a thought, maybe the objectives aren't competing but actually completely aligned. If you have to turn the reverb up to 10 to get "normal" reverb levels then presumably that is where people are using it. Maybe it starts feeding back at band volumes because you are trying to amplify too small a signal from the tank and have essentially a high gain path that is underdriven by the intentional signal and then gets hijacked by the feedback signal.

  7. #77
    Senior Member
    Join Date
    Sep 2010
    Location
    Florida
    Posts
    891
    ^^^^ interesting. I'll keep that thought in mind.
    ~Semi-No0b Hobbyist~

  8. #78
    Senior Member
    Join Date
    Oct 2010
    Posts
    513
    Ted Weber came up with an interesting way to vibration-isolate the tank while bolting it to the bottom of his Princeton Reverb (6A14) kit: he uses two short steel springs, one drywall bolt stud, and a long wood screw for each of the four holes in the tank. The two springs are essentially used to suspend the tank and dampen vibrations in both directions, much more so than the included white translucent grommets that Leo mentions.

    Still, the tank is open to the air if suspended in this manner. It would probably benefit from the other damping treatments that Leo mentions.

    Definitely consider trying other input cap values than 500pF. glebert may be right--you might simply be sending too little signal to the reverb. When I changed my 500pF, I got a ton more signal and midrange into the reverb, resulting in overdriving the tank (but no feedback). Essentially, it meant that to keep the cap change, I'd have to reduce the gain on the driver side. I was too lazy to do so and just put the 500pF cap back.

  9. #79
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    170
    You said this is a back line amp.

    No doubt you tried switching the pan end for end b4 mounting it to keep the most sensitive end away from the PT.

    I'm wondering if it's sitting next to another amp on stage that has it's PT near that sensitive pan end.

    Would explain why it's fine on your bench, but has trouble on stage.

    Just a thought.

  10. #80
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,400
    The reverb pan is NOT to be screwed down tight, screws only in enough to let it float. That is the point of the spongey grommets.

    I also wondered if the weak reverb might be from the cables being reversed.

    My list:
    bag
    cardboard cover
    check all four corner springs for being connected.
    Wiring
    Strip of foam weatherstripping down the middle of the flat top surface to reduce resonance.
    Education is what you're left with after you have forgotten what you have learned.

  11. #81
    Senior Member
    Join Date
    Sep 2010
    Location
    Florida
    Posts
    891
    Doing some play testing at higher volume than I could last night. A couple of things...

    Note: the amp had a 12at7 for the reverb, NOT a 12ax7 like the last drawing shows. I did sub in a 12ax7 and also tried out a 250pF and 100pF with both reverb tube selections. The 12ax7 helped considerably with the reverb level and the pF cap swaps may have increased the reverb effect just a smidge, but for now I just went back to the 500pF.

    I can get the amp to feed back but it's independent of where the reverb level is set. Either volume control can be cranked and it's fine so long as the other volume isn't up too much. If the master is dimed, it plays fine until the preamp volume gets up to about noon or so. If the preamp volume is dimed it will start to feed back when the master gets up to about 10 o'clock.

    Just from general observation it seems like the preamp gain is just too strong. What's the most efficient way to calm down the EF86 gain?
    Last edited by mort; 01-22-2018 at 07:03 PM.
    ~Semi-No0b Hobbyist~

  12. #82
    Senior Member
    Join Date
    Sep 2010
    Location
    Florida
    Posts
    891
    Ok I experimented with the plate and screen resistor values for the ef86. The amp has calmed down a bit but could still stand to be calmed down a little more. Should I raise the 33k input resistor value a little for a little less input signal or raise the plate resistor value? To be honest I don't fully understand the theory behind what I'm doing, just doing some semi-educated experimentation...

    This drawing has all voltages measured and edited today.
    Attached Images Attached Images
    ~Semi-No0b Hobbyist~

  13. #83
    Old Timer
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    1,796
    Quote Originally Posted by mort View Post
    Just from general observation it seems like the preamp gain is just too strong. What's the most efficient way to calm down the EF86 gain?
    You could try removing the EF86 cathode resistor bypass capacitor.
    mort likes this.

  14. #84
    Member ChopSauce's Avatar
    Join Date
    Jan 2018
    Location
    So Paris, France
    Posts
    35
    I'd like to gut a Blues Junior, too. Not sure the outcome would be that great, though

    Quote Originally Posted by mort View Post
    What's the most efficient way to calm down the EF86 gain?
    I've been recommended the read of The Valve Wizard -Small Signal Pentode about this subject & sorry: I am not clever enough to explain it myself.

Page 3 of 3 FirstFirst 123

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. blues Jr Problem
    By Snivly in forum Maintenance, Troubleshooting & Repair
    Replies: 8
    Last Post: 05-07-2013, 03:31 PM
  2. Blues Jr oscillation-
    By drewl in forum Maintenance, Troubleshooting & Repair
    Replies: 11
    Last Post: 04-23-2013, 11:10 PM
  3. Blues Jr issue
    By daz in forum Maintenance, Troubleshooting & Repair
    Replies: 15
    Last Post: 11-10-2012, 07:46 PM
  4. Blues Jr noise
    By buddha guitar in forum Repair and Restoration
    Replies: 3
    Last Post: 03-31-2010, 06:31 PM
  5. f^*%ing Blues Jr
    By sbluestubes in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 03-23-2010, 03:50 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •