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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Super Bassman troubleshooting help

Discussion in 'Glowing Bottle Tube Amp Forum' started by PMELIUS, Aug 7, 2017.

  1. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    More than once, I've been at the point where I'm just about to hit the amp with a sledgehammer. So then there's no question about whether it can be fixed or not, LOL.

    A while back in this thread, robrob made a comment regarding leaking coupling caps. I've only ever seen this once or twice with a new cap. But perhaps it's something to poke around for...I just go looking for DC after a cap. If there's a little, I note it. If there's a lot I note it.

    Then I go back and try to figure out if a little is OK or not (coming from some other part of the circuit). I'm not quite fluent with all the spots where a little DC offset can show up. So then I usually post here the results and wait for others to interpret.
     

  2. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    [​IMG]

    This has one more gain stage than the Super Bassman. And that looks wonky to me. That line goes to nowhere, WTH is that? Some kind of plate feedback scheme? If that were on my bench, I'd remove both the 220k and 470k. Connect that .01 coupler to the 100k plate resistor. Then clip the 25uF bypass cap off the cathode in V1B or V2A.

    See how it behaves then. You might have to dump some gain. But we'll see how it sounds first. If we do have to dump some gain, I think a split plate load would be in order in V1B or V2A (or both). You have some ~50k resistors?

    BTW, I had no luck getting tone I liked out of 3 gain stages configured somewhat like this amp. If all else fails, you've got another triode to work with and a Marshall JCM800 would be easy enough to do (see what I did there? trying to get robrob to reengage in this thread). ;0
     
    Last edited: Aug 22, 2017
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  3. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    That line in the schem does not exist in the circuit haha. Nothing comes off of the junction of that 220k and 470k between v1/v2 plates. Likely another oversight from Earth similar to the .02

    I will try your suggestion about the preamp modifications after I try a couple things in the power section first. I really wanna hear this thing the way it's supposed to sound before I go modifying it. I do have some 47k resistors if it comes to that - think that'll be close enough?

    I was told by someone to try 1k screen resistors to moderate the screen vs control grid, which seems logical but it will drive me nuts that the schem values aren't cutting it.

    Maybe after it's all said and done and this amp refuses to work - we could turn it into an 800 hahahahaha.

    Suspecting my grid stoppers just on a hunch - I just went and shotgunned them out and no change. I'm going to see where 1k screen grids get me and go from there.
     

  4. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    If power tube arcing is still the problem it's time to suspect the output transformer.

    It may have been damaged when the power tube socket was damaged.
     
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  5. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    I just swapped out the 470 ohm screen grids for 1k - no change. :confused:
     

  6. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    This is depressing. :(

    I'll start looking into techniques for testing OT
     

  7. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    Ah...I need a pair of VOMs or DMMs, and a 110/120 to 6.3VCT filament transformer, and either a variac or a light bulb socket. Variac is on my list right next oscilloscope.

    I did the "simple tests" last night actually from the geofex OT test tutorial and did not measure anything that showed concern. The "advanced test" i will need more tools than I have available to me.

    I'll entertain any other ideas to test in this thing before submitting it to a local, more qualified technician. I'm not going to replace the OT myself unless I'm positive that it's the issue.
     
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  8. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    I'm not 100% sure it's actual arcing. There's no pops and crackles associated with it. But the OT is something we need to eliminate from consideration. And then if the OT is hosed, we can spec a new one easily enough. Though, I'd be tempted to eliminate 2 of those power tubes...unless I was hell bent on using this as a Bass amp.

    I built a bench tester based on this: https://www.premierguitar.com/articles/the-super-secret-transformer-tester-1 I have a regulated power supply there, but you could use a simple wall wart 5-12V (or battery). If you have any inkling to build amps, especially from equipment you pillage from stuff on the side of the road, estate sales, or any other unknown source, this would be well worth investing. You need a normally open push button, a neon light, and a low voltage source to do it like I did.

    [​IMG]

    I have a variac, a good one and I've used it once. You don't need a variac. You don't need multiple DMMs. Do not waste your money. You need a simple DC power supply, that's it. This will test a transformer for shorted windings and will find shorts that other tests may not.

    If you plan to build/rebuild amps, then you also should make a dim bulb limiter.

    [​IMG]

    edit: If your OT is hosed I have one (actually 2) spec'd for a quad of EL34 that would probably work for you. That's your contingency if the OT is hosed. Wiring in an new OT is easier than the stuff you've already done, so never fear.

    Another edit: I was sure the OT in my Bassman 10 was the culprit of my issues, that's what finally led me to build that short tester. (It wasn't the issue, and once I could move on from that, I sorted it).
     
    Last edited: Aug 23, 2017

  9. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    I've been planning on building a light bulb limiter, this transformer tester looks super simple. Let me see what I can throw together with what I have at home!
     

  10. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    While I'm thinking about this at work - is there any way that the 15ohm safety resistor to ground could be the cause of flyback voltage?
     

  11. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    That's out of the circuit when your speaker is plugged in. Even if your shunt still touching, it would then be in parallel in which case which you end up with a 3.2-Ohm effective load. That would shift the operating point of your tubes slightly, but I don't think it would cause flyback voltages.

    A 470-ohm 2W resistor to ground off the tip would be a better solution than that 15 ohm safety resistor.
     
    Last edited: Aug 23, 2017

  12. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    Is there anything I can substitute for that neon lamp in the premier guitar tester build? I don't have one of those handy and can't find anywhere nearby that would have one!
     

  13. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    I was hoping to find something local so I wouldn't have to wait for shipping, but alas all of the radioshacks and electronics parts stores are obsolete.
     

  14. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    I found an electronics surplus place near me with none be-2's. Gone throw this together tomorrow or Tuesday and see what happens.

    Do I need to desolder my transformer leads before testing?

    Just saw your edits about having those OT's. Hopefully it doesn't come to that, but if it does that's good to know! Fingers crossed.
     

  15. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Yes. All leads have to be disconnected. That way the small voltage has nowhere to go so it keeps building by induction until the Ne bulb flashes over.
     

  16. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    Alright, so I built a crude little transformer tester hahaha.

    Assuming I assembled it right and applied the test correctly, I think I've got a bad transformer on my hands :(

    When I apply the 9v DC to the primary, the NE does not flash over when connected to the secondary.

    However, when I apply 9v to the secondary, the NE will flash over on the primary.

    I assume polarity of the NE-2 really doesn't matter in this test, or whether the 9v is + or -

    The NE does give a little flash when I cut the power, but I don't feel very strongly about that haha
     

  17. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    It supposed to flash when you open the loop. It's supposed to give a little flash when you cut the power. I don't think it's supposed to flash (or turn on) while the battery is still hooked up.

    It doesn't matter which windings you hook up to (as long as all other windings aren't connected).

    My only background is reading the RGkeen article, and making one, and testing 5 transformers or so.

    Also some resistance checks you can do with the DMM:http://www.geofex.com/ampdbug/outtrans.htm
     
    Last edited: Aug 30, 2017

  18. PMELIUS

    PMELIUS Tele-Meister

    120
    Nov 23, 2015
    Philadelphia
    So it doesn't flash when I turn on the power at all. When I turn it off I get a little tiny flash, that's it.

    The opposite way - 9v on the secondary/bulb on the primary. It flashes within a second of turning on the power and gives a little flash when power is removed.

    I left it at that last night but was going to check resistance today to see what turns up. I had already done those "simple tests" while it was in circuit but I want to see the results out of circuit now.
     

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