Fender 6G3 - problem diagnosis

Discussion in 'Amp Tech Center' started by Steve-0-, Nov 2, 2019.

  1. dan40

    dan40 Tele-Afflicted Silver Supporter

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    You could measure the negative DC bias voltage on pin 5 of each power tube. It should stay steady on each socket with the trem function turned off. Be sure to check each socket also. If your amp has a bias pot, be sure to sweep through it's range checking for a steady voltage at all points on the pot. With the trem engaged, the bias voltage will vary up and down as this is how the tremolo effect is produced.
     
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  2. BigDaddy23

    BigDaddy23 Tele-Meister

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    I would be checking bias voltages and all components within that section of the circuit. Post a pic of the guts too, mate.
     
  3. Steve-0-

    Steve-0- TDPRI Member

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    Ok - so negative bias voltage is steady across the sweep of the bias pot. Measured both at pin 5 but also at the bias board. Checked the bias resistor/trim-pot combination and both parts are within spec. I was a bit suspicious of these but all seems fine.

    With the trem turned down (no pedal to turn off) the voltage at pin 6 of V2 varies quite significantly. Should be 205 but varies from about 130 to 220 ish. Hard to give an exact range.

    Apologies, I should have taken some gut-shots yesterday but forgot. Will be a few days now as I'm away with work for a bit. Circuit is as per the schematic and layout drawings and all connections appear fine with no cold joints. All new caps are firmly in place.

    Cheers,

    Steve
     
  4. King Fan

    King Fan Friend of Leo's

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    I may be missing something. Do you mean the negative bias 'is steady' -- ie doesn't change as you sweep the bias pot?

    Before you get a scope, were we working on a complete voltage chart and chassis pics?

    Also I know it's not nice to say go spend money, but a different, known-good set of tubes (at least one preamp tube and two power tubes) are really needed for troubleshooting.
     
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  5. Wally

    Wally Telefied Ad Free Member

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    I was confused by the ‘steady voltage’ during the sweep of the bias pot, as well. ???
    That plate voltage at 280 volts is way low....something was drawing a lot of current at that time.
     
  6. Steve-0-

    Steve-0- TDPRI Member

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    Sorry guys, wasn't dead clear.
    When the bias pot is swept, all voltages vary in the manner in which you would expect them to..... then they stay steady as opposed to fluctuating. This is with the trem turned right down.

    I have been working from the schematic and layout drawing and all voltages are ok. Admittedly, this isn't a full voltage chart.

    I have a number of known good pre-amp tubes that in have been using. They only have a few hours on them before I found some decent NOS Blackburn Mullards to use. I am sure these are ok but they have been taken out while I sort this.

    Agreed, need a set of know good 6V6s, but as the JJs are supposed to be indestructible, I figured the chances that they have both dies and are exhibiting the same symptoms at the same time is small.

    Yeah, when the plate voltage dropped and the current shot up I got a surprise. Can't think I was do I g anything stupid but haven't been able to replicate it. But Yeah, something was up that was drawing a bucketload of current.

    Cheers,

    Steve
     
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  7. Wally

    Wally Telefied Ad Free Member

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    Steve-O, I would have to make that voltage drop/current draw thing my priority before I wondered about the trem. Ommv. It could be..but I doubt it...that the two are related; but the current draw situation is more of a problem, imho. And....if the two things are related, you fix both problems when you fix the problem that would take out transformers. you say you have replaced both transformers? If so, why? Was the amp dead when you got it? If so, this current draw problem may be have been a problem for a long time.
     
  8. Steve-0-

    Steve-0- TDPRI Member

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    Wally, when I got the amp it was in a fairly bad way. There was a vintage era RS power trannie in there but it wasn't putting the right voltages out. When later looked over by a tech, the OT was diagnosed as duff as it killed a tube.
    Both replaced with classictone.

    Good point on the current draw..... it seems possible that there is an intermittent that could have killed the previous OT but given the state of the circuit when I got it and knowing what the previous owners were like, I assumed it was run without a speaker connected for some time this killing the OT . Speculation though.
    Only saw the high current once.. .. went back to not so and didn't return the rest of the evening..

    Cheers,

    Steve
     
  9. Viejo

    Viejo Tele-Afflicted

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    I would bet that since the tremelo is power tube bias vary on that amp that the problem is in the adjustable bias circuit that was just added.
     
  10. Steve-0-

    Steve-0- TDPRI Member

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    I spoke with a tech yesterday and although the adjustable bias circuit does what it is supposed to do and the tremolo has worked fine since it was installed (several years back), this is where I am going to start looking next. I am also going to check a number of the resistors in the trem circuit and check the intensity pot as well. After that, it will be into the caps.

    Cheers,

    Steve
     
  11. BigDaddy23

    BigDaddy23 Tele-Meister

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    Don't forget the gutshot!
     
  12. Steve-0-

    Steve-0- TDPRI Member

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    Haha - don't worry, I won't forget them this time, assuming I can work out how to upload them! It'll be a few days until I get the chance to work on it again so I might be a bit quiet until then but I'll be back.

    As a heads up - it ain't pretty......

    Cheers,

    Steve
     
  13. King Fan

    King Fan Friend of Leo's

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    Easiest / best is drag-drop your jpg file into the box you're writing in, or use the Upload a File button. On mobile, just click the 'pic' icon...
     
  14. dan40

    dan40 Tele-Afflicted Silver Supporter

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    With the trem speed and intensity set to maximum, are you seeing any modulation in the bias voltage. IIRC, it's hard for a digital meter to track the fluctuating voltage perfectly but you should see some change if the circuit is functioning. If the bias stays steady with the trem set to max, I would look at the LFO circuit and it's associated capacitors. You may have a bad cap that is preventing the oscillator from firing up and developing the trem signal. Here is a great primer on the LFO circuit...

    http://www.valvewizard.co.uk/trem1.html

    Also, as Wally mentioned above, be sure to try and find the issue that caused that low voltage reading that you spotted earlier. May be related to the LFO circuit and it's triode.
     
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  15. BobbyZ

    BobbyZ Doctor of Teleocity

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    I'd be surprised if the adjustable bias mod is the issue. Unless it's way off but that can happen with a fixed resister or a pot.
     
  16. Wally

    Wally Telefied Ad Free Member

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    Thanks for the link, dan40. I have a Md. 6 Alamo Paragon trem has a similar problem with the trem....the pulse is there but the effect is not. The circuit is rather inaccessible on this Alamo amp, so when I gather the initiative to dig deep, I will refer to the Wizard. I wish the circuit were not so much of a ‘beyond a rat’s nest’ thing.
     
    Last edited: Nov 8, 2019 at 12:30 PM
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  17. Steve-0-

    Steve-0- TDPRI Member

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    Right then..... gut shots. It's old and cranky. 20191108_141119.jpg 20191108_141132.jpg

    So..... checked all resistor values. Everything in spec except the 56k fella in line with the extension speaker Jack. It measures at 1.5k. Needs changing but looks like a pig to do. Also not sure if it would have any impact.

    Thoughts on that please?

    With trem intensity down, I am seeing a variation in the voltage at pin 6 on v2. Should be 205 but ranges from about 150 to 230 ish.

    Need to check how the bias voltage varies with the trem on. Will get back to you on that. Only thing is..... you can here the variation in speed and intensity of the pulse, it just doesn't affect the signal.
     
  18. Wally

    Wally Telefied Ad Free Member

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    That 56K resistor is the ‘top’ resistor in the negative feedback loop. Steve-o, you may be aware of this, but the trem circuit is everything that is connected to pins 6,7,&8 of V2. You can see thAt the bias circuit is connected, and there is a feed to the control grids of the 6V6’s.
     
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  19. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Normal Fender global negative feedback comes off the OT secondary at whatever voltage (I imagine it varies some from amp to amp) and then is knocked down by a voltage divider before being inserted back into the signal chain. That divider in many amp models calculates out to about 2.5%.

    If that reading of the 56k is really true and the resistor is shot and only giving 1500 ohms, then the voltage divider is only knocking signal down 50%. That would be really powerful negative feedback and would quiet the amp substantially, maybe have other consequences.

    But that reading may be inaccurate if you didn't lift one leg of the resistor out of the circuit so it's no longer in parallel w/ any other resistances.
     
    Last edited: Nov 8, 2019 at 12:55 PM
  20. Wally

    Wally Telefied Ad Free Member

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    I just took some measurements on a NFB circuit with the resistors in place. There was not one way of measuring that gave anything close to an accurate reading either the top or the tail resistor.
     
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