VOX AC30/6TB Completely Dead

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by colejj, Jun 21, 2019.

  1. colejj

    colejj Member

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    I have a vox ac30/6tb that has gone completely dead on me. Traced no signal back to the amp. Noticed the standby light was dead. All tubes glowing except the rectifier. Open the chassis and immediately notice that a solder connection has come undone from what looks like is the rectifier tube spot (pardon my nomenclature). I’m assuming this is the culprit. All fuses look good. If it needs to be reconnected, which post do I put it in?

    Pic of Loose Solder Connetion:

    I’ve checked all fuses and they look fine. Anything else I could be missing in trying to diagnose this? If it’s simple I’d like to take care of myself before going to a tech.

    Link to Schematic: https://www.korguk.com/voxcircuits/circuits/ac301990.jpg

    TIA
     
  2. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Yes, it needs to be reconnected for the amp to work. It is odd that the apparent HV AC leads of this PT are yellow, that wire color is otherwise pretty much standard for the rectifier filament 5 VAC connections. I am assuming that the wire you are holding in the photo is the loose/disconnected one. If so it needs to go to pin 5 on that socket on your amp. Your amp has 'bulletproofing' diodes in series with the rectifier (tube) diodes, which is why you'd land that wire on pin 5 instead of pin 4.

    Don't know why the rectifier tube wasn't glowing, as the filaments are on pins 8 and 2 there. Is there a 5V fuse on the board somewhere? (the schematic you linked is not the right one for your amp, it shows SS rectification).
     
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  3. pdf64

    pdf64 Member

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    https://el34world.com/charts/Schematics/files/Vox/Vox_ac3093pa.pdf
    The TB schematic does indeed indicate that the yellow PT wires are for the HT winding, the black are the 5V rectifier heater.
    The red wire should be moved from rectifier socket terminal #2 to #8.
    Don't use standby on this amp model (hot switching arrangement damages the rectifier tube).
     
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  4. Tron Pesto

    Tron Pesto Member

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    I still mutter "wtf were they thinking when the wired a hot-switched standby on these amps? Who the hell was asleep at the wheel? Shame on them" whenever these amps pop up on the radar.
     
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  5. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    And it was a dopey move to pull the whole amp's DC current load through the 5V filament (connection to pin 2 instead of pin 8).
     
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  6. WillLane

    WillLane Member

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    The /6 TB RI's from Korg didn't start development until 1992. This should be the right schematic. I just got a /6 TB RI myself. https://www.korguk.com/voxcircuits/circuits/ac3093pa.jpg

    My /6 TB doesn't have those diodes. They don't show up in the schematic either it doesn't seem like. What are their purpose and why does TS' amp have them and mine doesnt?

    EDIT: Did some searching and found this, really interesting idea: https://www.premierguitar.com/articles/the-immortal-amplifier-mod-1

    [​IMG]
     
    Last edited: Jun 22, 2019
  7. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Yes it's just a simple mod, adding a preventive layer. Prevents unrectified AC from hitting the AC dead short of the filter caps in case of a rectifier tube plate-cathode short.
     
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  8. pdf64

    pdf64 Member

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    When Korg bought the brand in 93, Marshall got the design/build/production for Vox tube amps http://www.voxshowroom.com/uk/amp/ac30tbx.html
    Almost incredible that such errors were made by Marshall o_O

    With the red (HT dc rectifier output) wire connected to socket terminal #2, the rectifier heater is carrying all the HT current; that's bad, an unfortunate production error that, whilst not causing any obvious malfunction / immediate failure, puts significant stress on the rectifier heater. Especially so given the hot switching standby; I think it may also add the 5Vac line freq heater signal to the HT ripple.
    The red wire should be moved to terminal #8 (where the other black wire goes).
    The GZ34 schematic may help to show the issue https://bms.isjtr.ro/sheets/010/g/GZ34.pdf
     
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  9. Rob s

    Rob s Member

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    As an aside I really like how the Korg/Vox era AC-30s sound.
     
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  10. bulbasaur_85

    bulbasaur_85 Member

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    Agreed! Hope the owner doesnt give up on this one. Get it repaired and enjoy it...such great amps.
     
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  11. WillLane

    WillLane Member

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    Well it sounds like I get to have some fun. Would the 1N4007 diodes be the right choice for these AC30s?

    Also back to the OP: It is a little odd the connection just came undone. Maybe whoever did the mod above didn't make great solder connections. I just hope it wasn't excessive heat that loosened the solder. You are dealing with lethal voltages so drain the caps first before working on the amp.
     
    Last edited: Jun 23, 2019
  12. HotBluePlates

    HotBluePlates Member

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    Yes, they're fine. Your B+ will never reach 500vdc, even with all preamp & power tubes pulled, so the 1kV 1N4007 is plenty.

    The article didn't say it, but often when the rectifier fails as a short-circuit and puts a.c. on the filter caps, they explode.

    In my very first electronics class, the teacher showed us why we should only apply the correct-polarity voltage to electrolytic caps by connecting caps to wall outlet voltage. IIRC, the cap was clipped into a little circuit with a switch and a plug to a wall outlet. When flipped, the cap exploded like a little firecracker, leaving bits of metal and electrolyte-soaked paper everywhere.

    Some caps isolated by series dropping resistors may be saved, or only cause the resistor to burn open...
     
    Last edited: Jun 23, 2019
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