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Egnater Tweaker 15 Low Power & Sounds Muffled

Discussion in 'Amp Tech Center' started by Kraftybob, Sep 29, 2020.

  1. Kraftybob

    Kraftybob TDPRI Member

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    Hello everyone. Long time lurker here.

    I'm just getting into amp repair strictly as a hobby. Kind of a Flip or Flop thing but with amps instead of houses. My first attempt is this Tweaker 15 that I bought with the description of "light on, no sound". Some of you may be wondering WTH is he thinking? The reality is even if I lose money on this one it's been well worth it as I've learned a lot already. I'd just chalk it off to start-up expenses :)

    When I first tried the amp there was nothing - no hum, buzz, static. No sound out of the loop into another amp and nothing from another amp back into this. I opened the chassis and the OT was completely disconnected with a piece of tape on it labeled "Bad". Alrighty then - there's my first clue.

    I tested the PT and that's good. I then tested what I could on the OT (resistance, continuity, shorts, etc.). The only thing that stood out to me was the resistance between the primary leads (below). I don't know if this is ok or not, but later when I checked voltages at the power tubes they look good - so maybe this is fine?

    Red (CT) to Blue: 164 Ohms
    Red (CT) to Brown: 61 Ohms
    Blue to Brown: 120 Ohms

    I reconnected the OT to start testing voltages and noticed the Preamp tube was dead. I replaced that and now I have sound. However, it's not loud and it sounds really muffled. I then re-tested the loop and connecting another amp's send to this return yields no sound at all. I then connected the Tweaker send to my other amp return and it sounds really good. Loud too! Now I'm getting somewhere. I then cleaned the send/return jacks and swapped the 6V6's for EL34's that I know are good. Back to low, muffled output.

    Now I'm concentrating my efforts on everything past the loop. I checked every single resistor between that and the power tubes and for the most part all are perfect. I have a few resistors that measure lower than their value, but I did not take them out of circuit so I wondering if that has something to do with it. They don't look burnt or look like they were ever replaced. Also, I'm pretty sure that when a resistor goes bad it's either open, or a higher value, not lower.

    I then measured the voltages at the power tubes and here's what I have (6V6's back in). I'll list one because they both measure pretty close to each other (DC volts of course):

    Pin 1 - 21v (Tied to cathode)
    Pin 2 - 44.6v (Heater)
    Pin 3 - 362v (Plate)
    Pin 4 - 355v (Screen Grid)
    Pin 5 - 0v (Control Grid)
    Pin 6 - 0v (No Connection in tube)
    Pin 7 - 45v (Heater)
    Pin 8 - 21.4v (Cathode tied to Suppressor Grid)

    Pins 2 to 7 - 6.7 vac

    I then powered it back up, and with a wooden chop stick, pushed on tube pins, wires, connectors, components, etc. looking for loose connections etc., but nothing stuck out. All the solder connections on the board look good too.

    So now I'm back to wondering if the OT is bad after all, and if so, how do I confirm it? I'd hate to buy another one at $150 only to find out that wasn't the problem. Anything else I should be checking?

    Thanks all!
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    As I have explained to many, the winding of a transformer will have different DC resistance than AC resistance or impedance. It is not easy to measure the impedance but suffices to explain that the windings are interwoven or flat wound. One tap on top of the other and if you think about how the first turn uses less length of wire than the last turn, which has to go further because the former is now enlarged with the previous windings, it becomes easy to understand that there may, in some cases, be almost twice the length of wire to equal almost twice the DC resistance but the inductance is still the same because the number of turns is equal.
    When an output transformer fails it normally fails in one of two ways.
    Most common is open circuit, maybe not in your case and shorted turns that get hotter and hotter, caused by Eddie currents increasing due to the increase in the amount of shorted turns that in turn reduce the inductance and ability to move power from the primaries to the secondaries. The tell tale clue is poor/no bass and low power to start with, not muffled.

    Watch you don't damage the mains transformer using EL34s as the heater current is far greater than 6V6s!

    Check the anode voltages of the pre amp and driver valves for low/ wrong voltage.

    Please add schematic as I cannot find Tweaker 15 in my collection and not sure without a diagramme or post the model number.
    Engle, like many other amplifiers have many relays and switches to consider but I don't know an Engle with 6V6s.
     
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  3. Burning Fingers

    Burning Fingers TDPRI Member

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    Please add schematic as I cannot find Tweaker 15 in my collection and not sure without a diagramme or post the model number.
    Engle, like many other amplifiers have many relays and switches to consider but I don't know an Engle with 6V6s.[/QUOTE]

    Jon..here is a schematic for the Tweaker 15
     

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  4. Jakedog

    Jakedog Doctor of Teleocity Ad Free Member

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    Egnater, not Engle. Also, part of the magic of the Tweaker is that you can plug and play virtually ANY standard power tubes without fear of damage or need to bias. 6L6, 6V6, EL34, 6550, 5881, KT’s, you name it. It’s a seriously fun amp.
     
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  5. Jon Snell

    Jon Snell Tele-Holic

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    Ah, that makes all the difference.
    I would connect the send and return together with a guitar lead and test it again.
    There is not much to fail, maybe the anode load resistors will cause distortion and lack of power.
    I would insert a signal and trace it through with my oscilloscope to find the issue. Don't know what you have test gear wise.
     
  6. Kraftybob

    Kraftybob TDPRI Member

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    Thanks Jon, et al. I failed to mention that I did run an instrument cable from the send to the return and there's no difference. Although when I connected the send from the Egnater to the return of my Marshall DSL I really liked what I hearing. Because that worked expected it's leading me down the path of the power amp section being the problem. I can't wait to get this thing running and play with it a while before I sell it.

    I don't have an oscilloscope yet, but am looking to order the Hantek 2D42. It's a 2 Channel, 40MHz, 250 MSa/s handheld scope. It has a signal generator built-in as well. I looked at some older bench scopes online but for my purposes I think the Hantek will work just fine. I'm also ordering (4) 100w, 8 ohm resistors and a heat sink to mount them on so I can make an 8 ohm dummy load.

    Tonight I'm going to pull the board with the power tubes to check the sockets and pins. it looks like one of them might have been replaced in the past as a couple wires have evidence of being bumped with a soldering iron. There's nothing within reach of the wires to cause it, nor is there anything on the board that looks burnt to have caused this. Plus, I'm guilty of doing this a time or two and it looks exactly like that. The wires are not burned through, just a little scarring on the side. I rang them out to confirm.
     
  7. Jon Snell

    Jon Snell Tele-Holic

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    Well, you have discounted all in the pre amp section do there's not much left to go wrong.
    Check Cn2 is good connection and check the voltages on V2 pins 1 & 5.
    R27 and R32 are a possibility as is V1 pin 6 R15.
     
  8. Kraftybob

    Kraftybob TDPRI Member

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    Thanks Jon. Are you saying that I've prematurely ruled out the preamp section, or do you also think that's ok? Later today when I'm off work I'll check the items you mentioned.

    Just an FYI on the schematic there are to V1's - the preamp tube in the top board and the effects loop in the bottom board. This doesn't match the actual layout. I know what you mean above with V2 being the PI and I'll check that. Just wanted to point that out and I initially missed that. I've attached the tube layout per the manual for reference.

    Thanks again.
     

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  9. Jon Snell

    Jon Snell Tele-Holic

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    If the sound through the pre amp into your other amp is OK, yes. The preamp is OK.
    Screenshot 2020-09-29 at 15.39.42.png V2 is used to amplify the input from the Return signal so check the anode load resistors.
    V3 is the phase splitter/driver, check the anode load resistors.
    Doesn't inspire much confidence if they can't even get the valve ID right!
     
  10. Kraftybob

    Kraftybob TDPRI Member

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    I checked all the resistors and noted their values in red on the schematic. They all match the schematic except for R13 which shows a reading of 487K vs 680K. I'm thinking this is another later year change like R29 because the resistor colors are Grey/Red/Yellow which is an 820K resistor. I doesn't appear to be in parallel with another resistor so I'm not sure what's causing the lower reading. But as I mentioned before, I believe bad resistors are either open or measure higher than their rating so I don't believe that this is the problem. I could be wrong.

    I'm going to start taking disconnecting the wires and pulling the board to check the tube sockets. not sure how far I'll get tonight as it's getting late.

    V2 V3 Resistors.jpeg
     
  11. Kraftybob

    Kraftybob TDPRI Member

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    I pulled the board with the power tube sockets and everything looks good. No dark spots on the board from overheating, traces look good, I don't see any bad solder joints and with the tube springs/clamps removed the tubes are still tight going in so it appears the sockets are good. I may push the pins in a little just because I have the board out and I'll also spray a little contact cleaner in there before I re-install it.

    Tomorrow I'm going to test the OT again using the method below in the video - which is essentially putting a low voltage into the secondary's and measuring the primaries, then calculating ohms. I have a transformer that puts out 8 or 20 vac so I'll use the 8vac tap.



    One other point on the preamp test I did. I mentioned before that the amp worked when I connected the Egnater Send to the Return of my DSL. The only volume control was the Gain knob on the Engager - which makes sense because the Volume control on the Egnater comes after the loop and the volume controls on my DSL are before the loop. Works the same way with my Helix. However, the gain control went from 0 to really loud with the slightest rotation of the pot - not even to 1. It was really hard to control. Yet, when I'm running just the Egnater the gain and volume controls work as expected (it's just quiet and muffled). Don't know if this is a clue or not but thought I should mention it.
     
  12. Kraftybob

    Kraftybob TDPRI Member

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    I think I confirmed my OT is bad. Following the video above when I put 8 vac on the secondary, I get the following voltages, winding ratios and speaker impedance:
    • Orange: 25.52 vac, 3.06, 856 ohms
    • Green: 28.7 vac, 3.44, 677 ohms
    • Yellow: 31 vac, 3.71, 580 ohms
    I checked the measurements twice and confirmed the scale on my meter twice. Bottom is my winding ratios range from 3.06 to 3.71 - which is not good.

    Time to start looking for a replacement :(
     
  13. Kraftybob

    Kraftybob TDPRI Member

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    I tested and installed the new OT and installed all new tubes. Now I have a different problem :( Below is a recording but if you can't listen to it the best way to describe it is it sounds like when you play a chord as the amp is powering off. The chord is distorted and tails off. I'm wondering if there's a bad capacitor? None of the filter caps are bulging or leaking, but I know that doesn't always mean there isn't a problem. Also, I'm still lacking volume.

    I checked all my voltages and all the screen, grid and cathode resistors. The amp still plays normal when I run out of the loop into another amp so it appears the problem is still after the loop return. The speaker and cable are good as I've tested them with another amp. I'm thinking I'm at the point of getting an oscilloscope and signal generator to trace this out.

    Tweaker Tube Voltages.jpeg

     
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