Results 1 to 10 of 10

Thread: OC3 screen regulator in Plush 3000B

  1. #1
    Member
    Join Date
    Feb 2018
    Posts
    46
    Thumbs Up/Down
    Received: 2/0
    Given: 1/0
    Rep Power
    0

    OC3 screen regulator in Plush 3000B

    I've got one these on the bench that i've revived. It's got got tubes in it and I biased it up, and one side of this (6) 6L6GC amp, was pulling waaaay more current than the other.
    I pulled the board to find burnt trcaes, and a burnt resistor that went across the OC3 regulator tube, for the screens.
    My problem is that I can't read the resistor, so I'm not sure how I should go about with a replacement.
    I've attached a schematic I found on the net, but all of these amps are inconsistent.
    The schematic shows R44 as 10K, across the OC3, but the burnt resistor I'm looking at looks like black, white, red, then some space, then some tolerance band I can't read....but I've never seen a resistor start with a black band
    Any ideas on what this resistor should be?
    Plush3000B.pdf

    0 Not allowed! Not allowed!

  2. #2
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,513
    Thumbs Up/Down
    Received: 526/1
    Given: 410/0
    Rep Power
    14
    Often when resistors get hot the colors fade/bake/change. If the schematic says 10K, I see no reason not to trust it.

    3 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  3. #3
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    681
    Thumbs Up/Down
    Received: 433/1
    Given: 322/2
    Rep Power
    1
    ^^^^That.

    With a working regulator tube, the 10k resistor will dissipate 1W. If the regulator fails (open) dissipation will increase considerably. You may find out by quickly measuring the voltage across the resistor without tube.
    What is the size of the burnt resistor?

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

  4. #4
    Supporting Member mozz's Avatar
    Join Date
    Oct 2015
    Location
    NEPA
    Posts
    636
    Thumbs Up/Down
    Received: 75/1
    Given: 64/0
    Rep Power
    4
    Burnt black, white, red,
    Unburnt brown white orange

    19k?

    I'd start with a high value and see what it does to that voltage, adjust accordingly. Screen supply i think?

    0 Not allowed! Not allowed!

  5. #5
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    681
    Thumbs Up/Down
    Received: 433/1
    Given: 322/2
    Rep Power
    1
    Quote Originally Posted by mozz View Post
    Burnt black, white, red,
    Unburnt brown white orange

    19k?

    I'd start with a high value and see what it does to that voltage, adjust accordingly. Screen supply i think?
    The regulator tube acts like a series wired zener and its dynamic impedance is much lower than the resistor. The influence of the parallel resistor on voltage is negligible, it does not adjust voltage. The current through the resistor stays constant at 10.5mA. The current through the tube adjusts according to current demand. Once ignited, voltage across the tube will always be close to 105V with or without resistor.
    The resistor may be required to protect the regulator in case of peak current demands >>40mA, e.g. caps charging.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 12-04-2018 at 11:33 PM.
    - Own Opinions Only -

  6. #6
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    31,076
    Thumbs Up/Down
    Received: 1,030/5
    Given: 0/0
    Rep Power
    52
    19k? Oh come on. How about not trying to decipher what burnt color stripes become and just stick a 10k resistor in there and move on?

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

  7. #7
    Senior Member
    Join Date
    Jul 2011
    Location
    Adelaide, South Oz
    Posts
    702
    Thumbs Up/Down
    Received: 27/0
    Given: 1/0
    Rep Power
    8
    10K looks right.
    The OC3 is a 105V gass regulator tube.
    As soon as the total screen current from 6 off 6L6GCs goes above 10.5mA then the voltage drop across 10K reaches 105V and the OC3 tube will conduct.

    Lets see - each 6L6GC screen current is 5.6mA at idle and 22mA at max output.
    Well that is ALWAYS.

    That resistor will dissipate 1.1Watt (from V^2/R) when the OC3 tube is good and 11.2Watts at idle (from 6x 0.0056 ^2 x 10K) if the OC3 goes bad.

    Replace the resistor with a 5W (minimum) type and I would be looking for a replacement OC3.

    Cheers,
    Ian

    2 Not allowed! Not allowed!

  8. #8
    Supporting Member mozz's Avatar
    Join Date
    Oct 2015
    Location
    NEPA
    Posts
    636
    Thumbs Up/Down
    Received: 75/1
    Given: 64/0
    Rep Power
    4
    Quote Originally Posted by Enzo View Post
    19k? Oh come on. How about not trying to decipher what burnt color stripes become and just stick a 10k resistor in there and move on?
    Just trying to be helpful, your response is? If you see a burnt resistor, and the schematics are known to have many changes or be outright wrong, what method do you use?

    0 Not allowed! Not allowed!

  9. #9
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    31,076
    Thumbs Up/Down
    Received: 1,030/5
    Given: 0/0
    Rep Power
    52
    The schematic shows R44 as 10K
    From your post, it seems you have a schematic. I see no reason to doubt the schematic. Other amps may have other values of this resistor, but we can presume the schematic you have works.

    I have seen tons of burnt resistors in my years, and indeed, the colors often change appearance after going crispy. And yes, a resistor can heat right in a confined area and toast just one color band among the rest, it does happen.


    I am not trying to be snarky, I just hate to see people bog themselves down in the tinyest details.

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

  10. #10
    Member
    Join Date
    Feb 2018
    Posts
    46
    Thumbs Up/Down
    Received: 2/0
    Given: 1/0
    Rep Power
    0
    Allrighty thanks everyone for the responses. I didn't entirely trust the schematic, given some errors, so was looking for a bit of a lesson there. 10K seems to be working fine. IIRC the screens were right around 100V lower than the plates, which were 560V with all the tubes in. In addition to the burnt rsistor(looked like a 2-3W), the B+ trace was burnt up, so had to deal with that as well, then rebias. I settled on about 34ma per tube. Amp sounds really great too, with a massive bottom end, imagine that

    1 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Plush P1000S
    By Rhodesplyr in forum Maintenance, Troubleshooting & Repair
    Replies: 8
    Last Post: 06-14-2018, 04:40 PM
  2. Space Equipment SPE-3000A, SPE-3000B
    By ARI in forum Schematic Requests
    Replies: 0
    Last Post: 02-06-2012, 07:24 AM
  3. protecting output tubes- screen voltage vs screen resistors
    By imaradiostar in forum Theory & Design
    Replies: 28
    Last Post: 03-07-2011, 06:01 PM
  4. Plush tube amp - please help
    By GlennW in forum Guitar Amps
    Replies: 5
    Last Post: 01-03-2010, 07:36 PM
  5. PLUSH OPT help
    By DirtyErnie in forum Repair and Restoration
    Replies: 2
    Last Post: 07-20-2009, 07:58 PM

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
  •