Ampeg V4 Power Tube Meltdown

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Ampeg V4 Power Tube Meltdown

Post by Recycled_Brains » Fri Jun 17, 2011 8:46 am

Recording this past Tuesday, and upon finishing a take, I smelled something reminiscent of bacon frying, and saw the faintest amount of smoke in the booth where my Ampeg V4 and Marshall JMP were residing.

I looked in back of the V4, and saw that one of my power tubes (2nd from left) was lit up like a big orange light bulb. Imagine the brightness of a jewel light on a Fender amp, or Hamptone pre, but the size of a 7027. I could feel the incredible heat radiating off of the thing. Looked like it was about to explode.

So, I shut her down. Smelled like some wires maybe got burned in there, or at the very least, I imagine the plastic in the tube socket melted a bit.

What could cause something like this? I had the 2 amps going simultaneously with a Radial ABY. Marshall showed no signs of problems.
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Post by Marc Alan Goodman » Fri Jun 17, 2011 4:16 pm

When I used to tour with my V4 it would lose tubes so often I started taking them out during transport. The problem I was having is that the power tubes are recessed slightly into the chassis. In transport they would get banged gently into the edges of the chassis until tiny cracks would form along the base. Then when they got hot and the glass expanded all the sudden one would light up like a flash bulb. It took me forever to figure out what was causing it...

Anyway those are pretty touchy amps so it could be a lot of things, but if you're moving it around a lot you should check the base of the blown power tube.

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Post by Mach Schnell » Fri Jun 17, 2011 5:07 pm

That condition is more commonly called "redplating" that could be due to a loss of bias voltage which regulates the plate wattage. It could also be loose contacts in the socket, failing bias power supply or just a power tube exiting this plane of existence. :wink:
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Post by anticpunk » Sun Dec 04, 2011 12:58 am

I get the V2/V4/V4B breed across the bench from time to time.

IF it is running the original tubes (probably Magnavox 7027s), they are due for a retube as the factory bias in the 70's was warm and with component drift, they probably have been slowly burning to death all of these years.

If it hasn't been done already, it probably needs a recap as well. 2x 50x50F&T caps, a 3x 40uf can, a 47uf atom and which ever bias cap your particular amp was fitted with.
These amps blow bias caps which takes out tubes, screen resistors, flyback diodes and sometimes the F10 rectifier diodes or even one or both of the transformers.

The voltages inside these amps are on the higher side of common where vintage amps are concerned. These also run the compactron triple-triode which is no longer made anywhere and is only a NOS part so becareful with it as they can be pricey.

The amps are fixed bias, and no trimpot provided. They are easily converted to adjustable bias and will run 6L6GCs no problem given you bias them correctly. JJ does make a 7027A and some people suggest upping to EL34, 6550, KT88 etc. all of which will require the bias adjust mod.

These amps stink when the diodes fry, and if the tube went south and took out the flyback diodes on that leg of the P-P circuit you would almost certainly get a good whiff. Also look on the bottom of the caps where the solder tabs come out. There is a vent there and if you see liquid/bulging/white crap,etc... you need a recap.

If you are willing to dig into it, The parts for all of it will run you around $200 including tubes.

I know anyone reading probably knows but - The voltages in a tube amplifiercan kill you!!! Drain the caps using a decent sized resistor some wire and alligator clips and always test around at all of the caps before you stick your hand in there for any reason.

hope this helps,


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Post by themagicmanmdt » Mon Dec 05, 2011 2:12 pm

i had one similar to yours - yes, those diodes could be the things that fried/failed... bias resistors went out, too... including a wirewound that charred!!

pretty much all anticpunk said.

small bits of the power section might need to be rebuilt, unless is the PCB issue also mentioned.

i guess i'm not adding anything else useful to this conversation. just nodding my head.
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Post by kevin206 » Sun Dec 11, 2011 4:36 pm

I had a friend with this amp. I'm not sure if it went out completely or just started acting up. His amp tech in Nashville suggested some of the upgrades mentioned. He went with a more common tube and we were all very pleased with the results. Sorry, I don't remember if they went with with 34s or 6L6s.

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