Problems with my ARC VT 130SE

[Serratoga]

Hi Everyone,
My Audio Research VT 130SE is giving me problems it started out with a problem in the V4
position. I installed a set of Sovtek 6550WE tubes after a week listening to the tubes I heard a
static thump in one of the speakers, I immediately went over to the amp and noticed that the
left channel would not Bias. I had to turn the knob almost to 5:00 position, while the right channel was sitting at 11:00. I called Audio Research and they told me that when a tube goes it usually takes the 100 Ohm resistor with it. I tested the resistor for that tube and in fact it was dead. I ordered the OEM resistors from ARC and installed a new one in place and tried another set of tubes and the same thing happened but this time it was a lot more violent, the resistor blew up , a chunk was missing from it, it smoked as well and the main fuse blew. I decided to test the the tubes on my tube tester and both tubes that were in that V4 position are
pretty well done, they don't have any shorts but the life has been completely sucked out of them. I called Audio Research and the told me that it could be a couple of things first check the
12BH7A driver tube in which I did and it's fine and then check the tube socket for the 6550 V4 position. I was told that if the pins don't make full contact the tube will run away and it can't bias? So I should try to close the pins and try again...... Does anyone have any other idea of what might be causing this problem???

Thanks

[kevinkr]

I've had similar problems with several pairs of M100 monoblocks I used to service - I finally got so exasperated I refused to service any ARC gear.

First make sure all of the voltage regulators are still alive, and check all of the socket connections and passive components associated with that socket.

I don't have the schematic so I cannot give specific advice.

In many cases it is the quality of modern output tubes that can be blamed squarely for these failures. My previous clients were inveterate "tube rollers" and this inevitably lead to catastrophic amplifier failures as the tubes they installed were not reliable at the voltages and currents used.

Quality issues seem to be a big problem with certain brands of 6550 and KT88 lately. (I'm having a lot of problems with KT88 from various vendors.) Buy your tubes from someone like Jim McShane who tests and burns in power tubes under real world conditions for long enough to weed out the bad ones - this should help quite a lot.

[Serratoga]

Thanks Kevin, if you have a moment have a look at the schematics,
here is the URL:

http://www.arcdb.ws/VT130/VT130.html

Thanks

[llwhtt]

If V4 is the only tube position acting up make sure the 1K resistor on pin 5 is OK. If it is bad (open) there will be no bias voltage and the tube will run away as it seems two have already.
If the 12BH7 was to blame then V6 should be acting up also.

Craig

[Serratoga]

Well , I checked the 1K resistor it seems fine, I also took some readings on pin 4 and pin 5.
V4 and V6 are reading -46.9v for pin 4 and 430v for pin 5.
V5 and V7 are reading -52.4v for pin 4 and 431v for pin 5.

This is after I replaced the 100 ohm resistor in the V4 position. Now what are the chances of the tube socket arcing or not making contact on the tube pin, will this cause the tube to run away?

Thanks

[llwhtt]

Are you sure you read those correctly? Pin 5 (control grid) should be a negative voltage and pin 4 (screen grid) would be positive. If there was no contact on pin 5 you would lose bias on that tube, which would be the worse case scenario. Another situation would be a bad connection on pin 3 (plate) , the screen grid would pass all of the current from the O.T., and the screen resistor would go. Also the pin configuration on EL34s, KT88s, 6550s causes arcing between pins 2&3 if given the slightest chance. Check the bottom of the bad tubes and see if they arced, check the socket too. Another scenario is that V4 is being over worked because V6 is not doing it's job. If the screen resistor is open on a tube that tube is basically shut down, but your voltages (+430) seem to verify that both screen resistors are OK. How long does a new tube last, dies instantly, couple of minutes, or longer. Do the plates glow red after a few minutes?

Craig

[kevinkr]

Originally posted by llwhtt
Are you sure you read those correctly? Pin 5 (control grid) should be a negative voltage and pin 4 (screen grid) would be positive. If there was no contact on pin 5 you would lose bias on that tube, which would be the worse case scenario. Another situation would be a bad connection on pin 3 (plate) , the screen grid would pass all of the current from the O.T., and the screen resistor would go. Also the pin configuration on EL34s, KT88s, 6550s causes arcing between pins 2&3 if given the slightest chance. Check the bottom of the bad tubes and see if they arced, check the socket too. Another scenario is that V4 is being over worked because V6 is not doing it's job. If the screen resistor is open on a tube that tube is basically shut down, but your voltages (+430) seem to verify that both screen resistors are OK. How long does a new tube last, dies instantly, couple of minutes, or longer. Do the plates glow red after a few minutes?

Craig

Suspect he just transposed the two readings or the pins. This amplifier is on pcb and there are no common components between pins 4 and 5.

[kevinkr]

I'd be a bit concerned about U3 which is a servo in the output circuit which acts to keep the output tubes in balance.. You need to make sure that this is still working properly. If everything is happy the voltage drop across R38 and R39 will be virtually identical, if not something is amiss. Watch out probing this as one slip may be instantly fatal to both you and the servo.

Clues that the servo is bad include things like the output slammed to one or the other of the rails, voltages across pins 4 & 7 less than the zener voltage, etc..

Seems likely that either V4 or V6 is bad, the substantially lower grid bias voltage indicates a marked difference in transconductance between the two pairs or the balance servo is fried. First swap the pairs and see if the imbalance remains. (The measured values will be different.)

Be very careful! You can make things much worse without much effort, don't ask me how I know!

[Serratoga]

Thanks , everyone, I basically replaced the 100 Ohm resister again , cleaned my board
and cleaned the V4 tube socket, I noticed some black carbon on one of the pin sockets , it
must of arched on that spot.
I scrubbed it all out with 99% Isopropyl Alcohol and 6550 pin brush, replaced the 2 driver tubes just in case.... Put some tested EH 6550 tubes in, and it's working flawless.

Thanks again...

[kevinkr]

That's good news.. I'd say you got off pretty lucky.