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Help diagnosing 5F1 issue

Discussion in 'Amp Tech Center' started by mattmayhem, Feb 14, 2018.

  1. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    So I just built a little mojotone 5F1 circuit kit and retrofitted it into my champion 600 reissue. I was waiting on an output transformer but was chomping at the bit so I went ahead and wired everything up with the old output transformer. Sounded amazing! No issues and the 5F2A tone circuit I added was working great.

    Well, the new output transformer showed up from triode a few days later so I wired it in and tried to fire the amp up and nothing... I looked and the rectifier and power tubes weren’t heating up. So I turned it off and checked around inside and discovered that the guide pin hadn’t done it’s job and the tubes were both seated incorrectly- rotated counterclockwise by one pin position!

    I reseated them correctly but now the amp was/is having an issue. Pretty weak output until mid way up the dial then awful kinda flabby squealing distortion and then seemingly back to normal at full volume.

    Exact same problem with brand new tubes, new pot, either output transformer, and all the resistors read at the correct values.

    So could I have damaged the power transformer in those 45 seconds that the amp was on with no load or is there a chance it is a capacitor? I didn’t smell any burning or anything which I know usually goes hand in hand with frying a transformer...

    One more bit of info, when I measure the voltage at all the reference points, i get the expected readings when the volume pot is all the way down. it goes up in the first position where the B+ voltage first enters the circuit from the rectifier tube as I dial up the volume pot, but decreases in all the other positions as I dial up the volume pot until I max it out at which point all readings returns to the expected values. I am pretty new to this so not really sure what to make of this.


    Any help would be greatly appreciated before I shell out for a new power transformer. Thanks!
     

  2. nasdak

    nasdak Tele-Meister

    171
    Mar 22, 2013
    france
    Wich tubes where badly seated exactly? Rectifier?
    If so you could have shorted the rectifier
    It's easy to check your pt for b+ and heating voltages...
    Fire the amp with the rectifier only and check for b+ after the rectifier
    Also check the rectifier heater supply (5v) and the other heater supply (6.3v)
     
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  3. Uncle Daddy

    Uncle Daddy Tele-Meister

    248
    Sep 26, 2015
    Maldon, England
    I did something similar in my Vibrolux 5f11, and fried one of the dropping resistors.
     
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  4. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    It was both the rectifier and the power tube that weren’t seated correctly. I still get the 5v and the 6.3v when I measure with my voltmeter. The tubes light up, the pilot light turns on and the fuse is good. Also, the B+ reads correctly at 375v except as I adjust the volume pot the reading goes up (maybe to ~415v) while all the other reference voltages drop (by maybe 15%) until the pot is wide open and then the B+ returns to 375v and all the other reference points return to the expected readings.
     

  5. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    Hmmm I measured all the resistors with my voltmeter and they all seemed to still be accurate...
     

  6. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    The rectifier one spot off may possibly have saved you some serious damage. That puts all 4 live pins in unwired slots, which kills the B+ supply at the source. Out of curiosity, was it a JJ 5Y3S? I did the same thing with that tube + a Mojotone supplied socket a few weeks ago. Tube slid right in.

    Mojotone had a run of Champ/Princeton OTs with the polarity reversed. If all component values check out, pull all tubes and treat it like a new build. If the squeal happens again, try disconnecting the negative feedback connection and see if it behaves.
     
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  7. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    It is a jj 5y3.

    I pulled the negative feedback resistor and no change although upon a second check, it looks like the negative feedback resistor is reading low. 1.9k instead of 22k...
     

  8. dan40

    dan40 Tele-Holic

    865
    Aug 19, 2015
    Richmond Va
    What is your voltage across the 6v6 cathode resistor with the amp idling?
     
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  9. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    ~19V except it slowly drops as I turn up the volume pot until the volume pot is all the way up at which point it returns to ~19 V.
     
    Last edited: Feb 14, 2018

  10. dan40

    dan40 Tele-Holic

    865
    Aug 19, 2015
    Richmond Va
    Have you taken voltage readings without the 6v6 installed? Does the voltage change as you adjust the volume without it?
     
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  11. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    Just tried this and no voltage change in the b+ or the other references as I dial up the volume pot. Just higher readings than with the tube in which I think is to be expected, right?
     

  12. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    And the voltages at the filter cap points - and all other test points.

    But voltages alone may not give you the whole picture when it comes to low signal. You really need to put it on a 'scope and check the signal at a few points in the circuit. It's very possible you have damaged a transformer - if lucky maybe just something minor - but you could replace various parts as recommended and solve nothing.

    A tech with electronics knowledge and the right test equipment can figure it out fairly quickly - that's your best move IMO.

    I'm a bit concerned about the "guide pins" comment. It's not real easy to insert ONE tube wrong without hearing the guide pin snap off - I can't image doing it with two. But aren't you noting the direction tubes need to be installed before inserting them? How did they end up in the wrong position at all?
     

  13. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    Apparently from what I’ve read and experienced, the jj tubes slide very easily into the mojotone tube sockets in any direction. I am certainly not the only one who has made this mistake.

    The champion 600 is a pretty tight fit so you can’t really get a good view of the sockets from beneath. Having never had this problem before with the original sockets, I aimed the best I could and then assumed the guide pins were gonna do their job. Lesson learned.

    Not sure what your question is on the voltages. With all the tubes in, The b+ straight out of rectifier (let’s call it b+1) is 375v as the mojotone literature suggests it should be. As I bring the volume pot up, this voltage begins to rise until it peaks around 410v before dropping back down to 375v when the volume knob reaches its max. This drop to the correct voltage is sudden, not gradual as was the rise. All the other reference voltages indicated on the schematic (b+2 after the first dropping resistor, b+3, etc...) all do the opposite in that they read correctly with volume at 0, and slowly drop as I bring the volume pot up before suddenly returning to the correct readings when the volume is maxed.

    With the power tube removed, all the reference voltage readings are stable regardless of the volume pot position. So I wonder what insight this may provide.

    I was just hoping to see if anyone had a similar experience or had any suggestions to troubleshoot this myself, but I may take it in to a tech soon. In the meantime, I’d love to hear any other tips that I haven’t tried so far. I have access to a scope so might try that tomorrow. Trying to learn as much as I can from this mistake...
     

  14. dan40

    dan40 Tele-Holic

    865
    Aug 19, 2015
    Richmond Va
    Are you absolutely positive that you wired the new OT properly. It may be that no damage was caused by the incorrect tube installation and this issue is being caused by a wiring mistake on the new OT. It's hard to say since you never got a chance to hear the new OT after the installation. Another possibility is that the coupling cap between the power tube and second gain stage was damaged during the power up. Also double check the 6v6's 220k grid leak resistor to be sure it was not damaged. You may need to unsolder one end to get a proper reading on it.
     
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  15. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    ??

    There are no voltages on the schematic. What I meant was have you charted all voltages at tube socket pics for plates/cathodes/grids/filaments; at each filter cap (and so on), and are they within tolerance based on the power transformer specs, rectifier installed, dropping resistors and other design-specific (and installed) parts? With no signal applied why you were turning the volume control?

    FWIW - as far as the guide pin issue I don't know anything about parts Mojotone sells, but even after decades of amp work I turn small amps over and visually confirm tube alignment if I'm not absolutely sure of orientation; then I immediately mark the chassis or socket or whatever is visible with the guide pin position or a tube pin number (for preamp tubes or if the guide pin is off to the side or back). If you do that even good tubes with broken guide pins can easily be used.
     

  16. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    Solved! It was a coupling cap! Tested ‘em today and found the culprit. Back up and running with the classic tone ot and things are sounding sweet! IMG_2740.JPG
     
    Last edited: Feb 16, 2018
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  17. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    Thanks everyone for your suggestions!
     

  18. mattmayhem

    mattmayhem TDPRI Member

    Age:
    31
    36
    Jan 14, 2017
    Portland, OR
    The mojotone layout has those reference voltages on it that I was referring to.

    I originally noticed the volume pot issue when I did have a signal in.

    Trust me, after this, I will never make the mistake of putting a tube in wrong again.
     

  19. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Mojotone uses the brown Belton type sockets (they don't seem to be actual Beltons), and after I did it wrong once I asked around in an amp builders Facebook group if anyone else had. A couple of other folks had experienced it as well. I inspected the JJ 5Y3 after doing it, and there wasn't even a scratch on the guide pin to say I'd done it wrong, or any resistance to inserting it in any orientation.

    I'm a stickler for things, and I got bit by this while inserting it in a tweed amp with the back cover on. It looked just close enough to the right position from the angle I was at to hoodwink me.

    Yeah, the octal sockets in that amp now have Sharpie marks on them as a backup.
     
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  20. alnicopu

    alnicopu Friend of Leo's

    Oct 3, 2009
    georgia
    Dont feel bad. I had a JJ rect in one of my amps and had I not noticed it was going in a little tight, could have installed it wrong. The center part with the guide pin was smaller than normal. I pulled it back out and marked the guide pin and socket with some liquid paper so I had a visual when re-installing it after that.
     
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