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#1 2014-05-18 04:31:47

Ian A.
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Registered: 2014-05-17
Posts: 4

V4 continually dies...

Ok, so I have a V4 distortion model that continually blows up after about 3-4 minutes of use. I do not work on my own amp. The problem started when one of the original magnavox 7027a finally gave up the ghost. I took it to a local tech who added a bias mod and re-tubed it with EHX 6l6gc.
  The first time I brought it to the practice space it worked ok, then band practice came and my bass player took it off standby and had forgotten to hook it up to a speaker cabinet, it predictably was not happy with this. Embarrassed, I took it back to the tech and had him fix it. However, since then every time I have gotten it back from him it blows up when we try to use it. I have made sure every time that the amp was properly hooked up to the speaker cabinet, basically double checking my bass player who is using it.
   Is failed the past 2 times. First time a tube failed taking a resister with it (this smoked liked crazy). The most recent time it just powered off when using it, lights went dead and amp died. There was a small scent of electronic death, but it wasn't billowing smoke this time. I am at my wits end. Any ideas?

EDIT: Last failure tripped the circuit breaker, hit the button hooked it up to a cab to test it, turned it on and one of the 6l6 is failing to light up, also there is a hum through the cab it was hooked up to even with the standby on.

EDIT v.2: Hum was caused by a heater filament, it killed another tube. Tech suggested not using the Standby switch, or in the future modifying it to have less internal voltage, I am now definitely switching to a 6550 or kt88 type tube.

Last edited by Ian A. (2014-05-20 21:33:16)

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#2 2014-05-20 22:26:17

ROBO
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From: na
Registered: 2010-01-26
Posts: 134
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Re: V4 continually dies...

Did you tech modify it correctly for the 6L6 tubes or just put them in there and bias the amp. You need to change out certain resistors if you swap to 6L6's or EL34's. Its not uncommon to see these amps modded for those tubes but it needs to do done properly. I forget exactly what has to be changed but I am pretty sure its the screen and plate resistors.

You might ask your tech to go through and check all the voltage points though out the amp to make sure you are getting the proper voltage where you need it next time you are there.

Replacing the filter caps would be a good idea too if they are original, they tend to dry out as they get older. Any original filter cap in a V4 has to be 30-40 years old at this point and they dont last forever, especially if the amp sat un powered for an extended period of time.


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#3 2014-05-26 16:58:43

Ian A.
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Registered: 2014-05-17
Posts: 4

Re: V4 continually dies...

Yeah the amp has been completely recapped in the past few years. In the past few days I moved the bias trim pot that the tech installed to the back of the chassis where it can be accessed without popping the amp open. They had glued it to the power transformer before. I also put a quad of Tung Sol 6550's in it and biased it to a reasonable point and so far so good.
I have read in a few places that the procedure for swapping the tubes on a later era v4 is different than the earlier ones, however I think that without the bias mod those 6550's would have been biased super hot in the amp, at least it was before I adjusted it. Either way I am not sure a modern production 6l6gc is the way to go in a distortion knob v4  without lowering the internal voltages of this thing. It ate so many power tubes.

EDIT: Update - Killed a 6550 in the same spot that it just killed the last 6l6gc.

Last edited by Ian A. (2014-05-26 21:30:21)

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#4 2014-05-26 23:45:54

ROBO
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Registered: 2010-01-26
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Re: V4 continually dies...

Do you know if the power section of the amp has been modded to accept the 6550's?

I was wrong about the 6L6's they can go in w/o modding the power section from what I read in another one of Steve's post.
They will plug right in place of the stock 7027's

The 6550's however I know for a fact have to have changes made to the amp in order to work correctly. Ampeg has a service release many years ago documenting the procedure to convert the V series amps to 6550's when the 7027's were out of production and hard to find.

If you keep killing tubes in that one spot I would look at that socket and see what is up.

Measure all the screen, plate and grid resistors to make sure they are in order.
Clean and re-tension the sockets to make sure its getting a good connection.
Measure all the voltages in the power section and see if everything is in spec, a 10%-20% variance is normal.

When it failed how did it fail?
Redplate? Short out? D
id you see any flashes or anything like that in the tube?

It might be a good idea to rebuild part of the power section
Plate and screen resistors are cheep and easy to replace.
So are the grid and bias feed resistors.
So is everything in the bias supply.

It might sound like over kill but if your planning on playing this amp a lot it would be good preventative maintenance.


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#5 2014-05-27 06:12:18

Ian A.
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Registered: 2014-05-17
Posts: 4

Re: V4 continually dies...

It doesn't sound like overkill. I am not sure about the service release, I would love to read it if you know of where to find it. For what it is worth it does say on the schematic on the chassis that 6550's can be subbed.

It doesn't sound like overkill to me to make it work, I did use this amp extensively for several years before I replaced it with a Science Amplification head. It worked rather well on the stock tubes in that time. I've owned it about 5 years now.

At first the tubes were too cold and there was an odd output sound, adjusted the bias (very unscientific and part of the problem I'm sure) until it sounded like an amp again, played on it for a bout a minute or two, then turned up the bass eq knob. that is when it shorted. I am fairly sure that it was a short in the tube same as the 6l6 it was eating, 3 tubes fire one tube had a blue light in it but otherwise was not lighting up. Hoping that the other tubes are still usable though.

I am not sure that it matters but I have been using this head as a bass amp most recently since I have not been using it for guitar I let my bassist use it until it started eating tubes.

Thanks for the input ROBO.

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#6 2014-05-28 03:00:52

ROBO
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From: na
Registered: 2010-01-26
Posts: 134
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Re: V4 continually dies...

the conversion is the "the tube amp book" its written by that Groove Tubes guy Aspen Pittman, I know Steve isn't reap him on the bozo haha but its a pretty good book with lots of information in it especially for those who are interested in some the inner workings of tube amps.
I just got a scanner the other day, If it works I will try to scan a copy of that page and post it up if you want.

Has the amp been modded for an adjustable bias?
I see that you mentioned adjusting it, which would normally require one to remove certain resistors in the bias supply to raise/lower the bias voltage.

If it does have an adjustable bias mod I would have sure its done correctly and secure.

You should also measure the current draw of the tubes if you are going to bias them, OR at the very least measure the bias voltage to make sure its within a reasonable level.

Biasing the tubes by ear is pretty much doing it blind which can be expensive and dangerous.

If 3 of your tubes are burning but one is not I would try to switch that tube to a different socket, Turn the amp on but leave the standby off and see if the tube burns in the different socket.

See if the problem follows the suspect tube or if it is isolated to that socket.

If its isolated to that socket I would look to it to see what is going on, could be loose connection for the heaters or a loose socket pin keeping it from burning.


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#7 2014-05-28 04:34:28

Ian A.
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Registered: 2014-05-17
Posts: 4

Re: V4 continually dies...

It has been modded with a bias pot, this has been beguiling my tech for weeks now. I biased it using a probe into the range it should have been in and then I did the stupid thing of doing it by ear afterwords since the probe was borrowed and no longer in my possession, but it wasn't even close to right before the output sound was a rather strange almost chorus sounding type thing. It is a 76 distortion knob model, it seems as though the information out there on these is rather mixed.

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