Results 1 to 16 of 16

Thread: Vox AC15C1 red plate problem

  1. #1
    Member
    Join Date
    Jul 2013
    Posts
    22
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Rep Power
    0

    Vox AC15C1 red plate problem

    Suddenly my AC15 stops to sound. I found that one of the EL84 burned. I put a new pair but I observed the one of them go red plate after two minutes or so. meantime the amp seems are working again, but still the problem with redplating. always is about the left position socket, even I inversed the tubes or I put a new other set.
    also the voltage on the cathode resistor is not 9V or so as I remember corect value. is 12V and rise in time. when reach 13v the tube goes red plating and very very red plate when reach 14v.
    the screen resistors are ok, cathode cap ok, cathode resistor. anyway I changed all with new components.
    on DC, I read around 100 ohms for each primary transformer coil, and around 1 ohm on secondary.
    I feel the OT have some problems...
    Any ideas? thanks for help!

    0 Not allowed! Not allowed!

  2. #2
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    14,864
    Thumbs Up/Down
    Received: 1,911/4
    Given: 2,148/0
    Rep Power
    29
    It's possible that the coupling capacitor feeding the left position is leaking DC voltage onto that power tubes grid. Another possibility is a bad connection for the grid to ground resistor on the left position. Maybe even a bad contact for the grid pin due to a dirty socket or bad solder joint.

    1 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  3. #3
    Member
    Join Date
    Jul 2013
    Posts
    22
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Rep Power
    0
    thanks Chuck. I suspected also the coupling caps and changed too. still the same.
    on DC, the grid voltage on the left rise from 0.2 V on starf to around 6 volts when tube go red. on the right side, the DC grid voltage gos to 2 volts

    0 Not allowed! Not allowed!

  4. #4
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,607
    Thumbs Up/Down
    Received: 1,195/1
    Given: 718/2
    Rep Power
    4
    Quote Originally Posted by sorin65 View Post
    Suddenly my AC15 stops to sound. I found that one of the EL84 burned. I put a new pair but I observed the one of them go red plate after two minutes or so. meantime the amp seems are working again, but still the problem with redplating. always is about the left position socket, even I inversed the tubes or I put a new other set.
    also the voltage on the cathode resistor is not 9V or so as I remember corect value. is 12V and rise in time. when reach 13v the tube goes red plating and very very red plate when reach 14v.
    the screen resistors are ok, cathode cap ok, cathode resistor. anyway I changed all with new components.
    on DC, I read around 100 ohms for each primary transformer coil, and around 1 ohm on secondary.
    I feel the OT have some problems...
    Any ideas? thanks for help!
    Did you get sound with the new tubes?
    If so there is no reason to suspect the OT. DCR readings look O.K.

    I would think of some leakage (to EL84 grid) issue: coupling cap, carbonized EL84 socket or circuit board... To verify you may ground the EL84 grids (pin 2) - one after the other - and monitor cathode voltage. If cathode voltage drops when grounding the grid, there was leakage in the corresponding grid circuit.

    Please post schem.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 07-07-2019 at 10:11 PM.
    - Own Opinions Only -

  5. #5
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,787
    Thumbs Up/Down
    Received: 428/2
    Given: 224/0
    Rep Power
    25
    "Please post schem. "

    AC15C1_.pdf

    1 Not allowed! Not allowed!

  6. #6
    Member
    Join Date
    Jul 2013
    Posts
    22
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Rep Power
    0
    @Helmholz, thanks for sugestions. I put the grids to ground, same problem. cathode voltage rise suddenly to 12-14 Volts. and left EL84 goes red. till then, yes I have sound, (when no grounding G1), but started to crack sounds in a minute or so. tommorow I will receive a new output transformer.

    0 Not allowed! Not allowed!

  7. #7
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,607
    Thumbs Up/Down
    Received: 1,195/1
    Given: 718/2
    Rep Power
    4
    Quote Originally Posted by sorin65 View Post
    @Helmholz, thanks for sugestions. I put the grids to ground, same problem. cathode voltage rise suddenly to 12-14 Volts. and left EL84 goes red. till then, yes I have sound, (when no grounding G1), but started to crack sounds in a minute or so. tommorow I will receive a new output transformer.
    I can't think of a way a defective OT could noticeably increase idle cathode current/voltage. Possibly bad tube socket contacts.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 07-08-2019 at 08:00 PM.
    - Own Opinions Only -

  8. #8
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,787
    Thumbs Up/Down
    Received: 428/2
    Given: 224/0
    Rep Power
    25
    Agree on a probable socket issue.

    If the grid pins (pin #2) are knackered you would loose the bias & red plate the output tube.
    Or a slight conductive short between the cathode (pin #3) & the grid would send the grid positive with the same results.

    2 Not allowed! Not allowed!

  9. #9
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    14,864
    Thumbs Up/Down
    Received: 1,911/4
    Given: 2,148/0
    Rep Power
    29
    Quote Originally Posted by Chuck H View Post
    ...a bad contact for the grid pin due to a dirty socket or bad solder joint.
    And I'll add, a conductive socket. This sometimes happens with a catastrophic tube failure. Do we know if this may be part of the amps history? It hardly matters. Unfortunately testing the socket for insulation doesn't tell the story because conductive paths at HV can present at voltages much higher than any meter uses for measurements.

    0 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  10. #10
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,104
    Thumbs Up/Down
    Received: 417/4
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by sorin65 View Post
    tommorow I will receive a new output transformer.
    Which sadly won't fix the problem

    0 Not allowed! Not allowed!

  11. #11
    Member
    Join Date
    Jul 2013
    Posts
    22
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Rep Power
    0
    @Chuck, yes it was a tube failure. I changed the pair with a very very nos EL84 Siemens, all seems fine, but in two minutes os playibg the guitar, the sound stops and I saw one EL84 goes in a very luminous blue neon light. on cathometer the tube shows no plate current.
    the transformer arrived, but first will change the sockets - I dont have around PCB sockets, but I m waiting a friend to borrow sone in a couple of hours.

    0 Not allowed! Not allowed!

  12. #12
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,104
    Thumbs Up/Down
    Received: 417/4
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by sorin65 View Post
    @Chuck, yes it was a tube failure. I changed the pair with a very very nos EL84 Siemens, all seems fine, but in two minutes os playibg the guitar, the sound stops and I saw one EL84 goes in a very luminous blue neon light. on cathometer the tube shows no plate current.
    If one tube has no plate current the other tube will most likely red plate because they share a common cathode resistor. This could also happen if one side of the OT primary failed open but both sides measure 100 ohm.

    2 Not allowed! Not allowed!

  13. #13
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    31,626
    Thumbs Up/Down
    Received: 1,512/7
    Given: 0/0
    Rep Power
    54
    If one tube has no plate current the other tube will most likely red plate because they share a common cathode resistor.
    Um, I just woke up...and I am old... but that sounds wrong to me. If they share a common cathode resistor, one red plating tube carries way more current, that increases the voltage drop across the cathode resistor. That in turn serves to bias the OTHER tube colder, not hotter.

    1 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  14. #14
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,104
    Thumbs Up/Down
    Received: 417/4
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by Enzo View Post
    ...but that sounds wrong to me. If they share a common cathode resistor, one red plating tube carries way more current, that increases the voltage drop across the cathode resistor. That in turn serves to bias the OTHER tube colder, not hotter.
    I think we are both right

    I said if one tube is not conducting then the OTHER red plates.

    1 Not allowed! Not allowed!

  15. #15
    Member
    Join Date
    Jul 2013
    Posts
    22
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Rep Power
    0
    thanks guys, the problem was the socket. I changed and red plating disappeared. I let the amp working on for two hours. is back to life!
    thanks again!

    1 Not allowed! Not allowed!

  16. #16
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,607
    Thumbs Up/Down
    Received: 1,195/1
    Given: 718/2
    Rep Power
    4
    Quote Originally Posted by sorin65 View Post
    thanks guys, the problem was the socket. I changed and red plating disappeared. I let the amp working on for two hours. is back to life!
    thanks again!
    I vote for a bad/open grid (pin2) contact.

    0 Not allowed! Not allowed!
    - Own Opinions Only -

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Vox AC15C1 distorted
    By mikeskory in forum Music Electronics
    Replies: 12
    Last Post: 10-30-2018, 02:42 AM
  2. Intersting fix to Vox AC15c1
    By gwell1 in forum Music Electronics
    Replies: 2
    Last Post: 02-04-2017, 12:15 AM
  3. Vox ac15c1
    By pnut5150 in forum Guitar Amps
    Replies: 16
    Last Post: 11-15-2014, 04:00 AM
  4. VOX AC15C1
    By Jay D in forum Schematic Requests
    Replies: 2
    Last Post: 11-12-2014, 07:57 AM
  5. plate resistor problem....?
    By acorkos in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 10-08-2008, 12:59 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •