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  • December 29, 2016 at 9:07 am #84713
    Iron Mang
    Member

    I’m curious if anybody’s had problems with the 6CA7 EH shorting between the heater and the plate. The tubes have been in a Marshall style amplifier for about a year, when we all of a sudden had a problem with pin two and pin three shorting together, you could actually measure a DC resistance of 3.2 ohms between the pins. This amplifier doesn’t get moved at all it sits in somebody’s studio, so it’s not exposed to much physical movement. Plate voltage 475 V DC, screen grid 430 V DC.

    December 29, 2016 at 9:17 pm #122414
    EHX STAFF
    Keymaster

    It sounds like the tube arced between the pins in the base of the tube. This can happen if there is a shorted or open speaker cable or a speaker cable with an intermittent connection. This can also happen if the wrong impedance is connected to the output of the amplifier. What happens in these cases, a large signal will generate a flyback spike that is reflected back through the output transformer causing the tube to arc between the plate and heater pins that are closest together (#3 and #2) as the flyback spike (that could be several thousand volts in a short duration pulse) is looking for a path to ground (though the heater wiring). When replacing the shorted tube, it is best to inspect the tube sockets to make sure that there are no carbon traces between pins #3 and #2 where the tube arced.

    December 29, 2016 at 10:17 pm #122416
    Iron Mang
    Member

    Thanks for your response. I checked the tube sockets and there is no arc tracing on the top or bottom of the socket, “this has decent tube sockets in it (Belton).” and the correct load was connected at the time of the incident, all jacks and speaker cables are in good condition. I also opened up the back of the speaker cabinet to make sure there wasn’t anything wrong back there and again no issues. I took the output transformer apart to see if there is any obvious flyback burning or traces, and everything was okay there too. It’s just a really strange occurrence because the amplifier never moves and is always connected properly.

    December 29, 2016 at 10:26 pm #122417
    Iron Mang
    Member

    (Update) I replace the power tubes with some EL34’s re-biased the amplifier and fortunately all is well. I’m going to continue to run it under load and make sure everything is fine.

    December 29, 2016 at 11:40 pm #122419
    EHX STAFF
    Keymaster

    Well great ! its working and hope no more trouble.
    Just be careful with the high voltage.
    As mom would say…always keep one hand in your pocket when you working on the inside of a tube amp chassis :-)
    I personally have been thrown off my chair with 500 volts.

    December 30, 2016 at 12:08 am #122423
    Iron Mang
    Member
    Quote:
    Well great ! its working and hope no more trouble.
    Just be careful with the high voltage.
    As mom would say…always keep one hand in your pocket when you working on the inside of a tube amp chassis :-)
    I personally have been thrown off my chair with 500 volts.

    Well thank you very much for the input, I really appreciate it. Also thank you for the safety warning, but I will tell you that I’ve been building and repairing amplifiers for the last 25 years. I have a good zap story too, “when I was in college I was working on a Marshall major 200 watt, which had been unplugged for at least a week. Anyway there was loose B+ wire that bit me, the caps had so much charge in them that it cut a half inch long gash in my arm and cauterize the wound at the same time; I’m lucky that the same arm that got zapped was grounded to the chassis and it didn’t go through my chest.”

    This particular amplifier was a custom build that’s basically a 59′ Bassman front and with an EL34/6CA7 power section, and last year we installed the 6CA7 EH’s (which my customer loves) and I was wondering if there was any concerns or occurrences of heater to plate shorts in these tubes.

    Again thanks for everything’
    Iron Mang

    December 30, 2016 at 1:10 am #122424
    Iron Mang
    Member

    Pic 2

    December 30, 2016 at 1:36 am #122426
    EHX STAFF
    Keymaster

    OMG what a story! Glad you made it through.
    I am an old ham radio operator and we also get shocked (burned) by radio frequency (RF).
    I touched a set screw in an antenna tuner while transmitting a 2000 watt RF signal.
    RF rather than run along the top of the skin, it burns a hole through the the skin and runs along the bone up your arm!!!

    HAHA this is funny we both have old high voltage war stories.

    That amp looks killer, aluminum chassis?

    December 30, 2016 at 1:57 am #122427
    Iron Mang
    Member
    Quote:
    OMG what a story! Glad you made it through.
    I am an old ham radio operator and we also get shocked (burned) by radio frequency (RF).
    I touched a set screw in an antenna tuner while transmitting a 2000 watt RF signal.
    RF rather than run along the top of the skin, it burns a hole through the the skin and runs along the bone up your arm!!!

    HAHA this is funny we both have old high voltage war stories.

    That amp looks killer, aluminum chassis?

    Yeah I’ve never messed with any high-powered radio (RF) stuff. The highest powered AF tube driven equipment I worked on was a single channel 1500 W power amplifier. One of my uncles in the 60s was a sonar technician in the Navy and I got to go on board a ship that had three 50,000 W tube driven sonar amplifiers, “it was scary just being around those things!

    Yes I hand bent the aluminum chassis from scratch, my grandfather left me an old bench brake/bender. Sounds like we’re a couple of the older guys here! lol

    December 30, 2016 at 2:11 am #122428
    EHX STAFF
    Keymaster

    Were not old were cool!
    HAHA
    Happy New Year!!

    Bill

    December 30, 2016 at 2:21 am #122429
    Iron Mang
    Member
    Quote:
    Were not old were cool!
    HAHA
    Happy New Year!!

    Bill

    Ha ha! Well I think I’m a little old because I can still explain an ultra-linear output section to the young guys and they kind of get it.

    Happy new year and best wishes!

    John

    December 30, 2016 at 3:57 am #122430
    EHX STAFF
    Keymaster

    One more note on the original post I made.
    That answer was from was from our resident tube genius Alan Otto.
    This man KNOWS hie stuff and is a radio buff like me.
    He had an additional comment you will find interesting.

    “The vintage Ampeg V-4 and V-4B amplifiers actually had damper diodes installed from the plate connection to ground at the power tubes to suppress any flyback spike that occurred to reduce the chance of the tubes arcing. We also incorporated the same type of damper diode circuit in the Sovtek Mig-60, Mig-100, and Mig-100H for the same reason. As far as I know, Marshall never used the damper diode flyback spike suppression in any of their amplifiers.

    Otto”

    The best part about working at EHX is you get to hang with all these people are brilliant in their field.
    ALWAYS work with people smarter than you. They pull you up:-)

    December 30, 2016 at 4:29 am #122431
    Iron Mang
    Member
    Quote:
    One more note on the original post I made.
    That answer was from was from our resident tube genius Alan Otto.
    This man KNOWS hie stuff and is a radio buff like me.
    He had an additional comment you will find interesting.

    “The vintage Ampeg V-4 and V-4B amplifiers actually had damper diodes installed from the plate connection to ground at the power tubes to suppress any flyback spike that occurred to reduce the chance of the tubes arcing. We also incorporated the same type of damper diode circuit in the Sovtek Mig-60, Mig-100, and Mig-100H for the same reason. As far as I know, Marshall never used the damper diode flyback spike suppression in any of their amplifiers.

    Otto”

    The best part about working at EHX is you get to hang with all these people are brilliant in their field.
    ALWAYS work with people smarter than you. They pull you up:-)

    Those are good suggestion, what you are calling damper diodes I would call transient suppression and it does work well. Although in this case I actually took the tube and broke the envelope and disassembled it, “there was a loose piece of metalwork inside the the plate housing.” So in this particular circumstance I don’t think it would’ve saved anything.

    Another thing you can do and this only pertains to EL34 power tubes (true power Pentode) only, “you can connect the suppressor grid (G3) pin 1 to a negative supply i.e. the main output of the bias supply. This keeps an EL34 on a shorter leash and really extends the life of the Tube. With the quality of today’s current production tubes this is not a bad idea. But you have to remember that you cannot use this on a 6CA7 as it is actually a beam type Tube and you could have some major fireworks. If you do attach the suppressor grid to a negative supply you will need to re-bias the amplifier and a small heads up is that the amp will sound a little different, what I would consider “tighter”.

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