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  1. gibson17

    Marshall JCM800 2205 help needed

    Sticker says '89
  2. gibson17

    Marshall JCM800 2205 help needed

    Thanks for all the replies, this got put on the back burner as I had a lot of household issues that took precedent. Here's the schematics I was using: https://www.drtube.com/schematics/marshall/2205prem.gif https://www.drtube.com/schematics/marshall/2205pwrm.gif I did switch the power tubes...
  3. gibson17

    Marshall JCM800 2205 help needed

    Hello DanFrank, The tubes were bought from Viva tubes as matched and burned. Perhaps I've fallen victim to poor matching? I realize that the measurement of GM is really theory only. My understanding is that tubes are in inexact science and that the only real measurement was the theoretical GM...
  4. gibson17

    Marshall JCM800 2205 help needed

    Hello Radio, I do have a scope. I have no operable signal generator, the one I have is broken. I do have all the junk I need to safely work inside an amp. I do understand that the voltages are lethal. I have decent mechanical ability but very limited electronics theory. Thank you for the vote...
  5. gibson17

    Marshall JCM800 2205 help needed

    Hello Pete, The tubes were bought as a matched set. I measured the tubes on a calibrated Hickok TV-7 D/U. The GM was one digit apart at 46/47. I measured the currents with a dual bias probes from SRS into a Fluke 115. My plate voltage is 464, so I'm aiming for 37mA but the pot is bottomed...
  6. gibson17

    Marshall JCM800 2205 help needed

    Hello all, My friend brought me a JCM800 2205 that was having issues producing any sound. I took the amp in and disassembled it. There was a bunch of cold solder joints on the board that I reflowed. The resistor on R59 was burned and failed open. I replaced that with a new resistor and also...
  7. gibson17

    JCM900 4100 Correctly Biasing

    Just reporting back for some closure to the additional content I posted to this thread so that it might be helpful to someone later on down the road. I dislike it when I am researching an issue and find a thread that just goes cold. Like did you solve the issue? I wound up recapping the amp...
  8. gibson17

    JCM900 4100 Correctly Biasing

    A few questions: If I change the Screen Grid Resistors should I go with 1.5k 10% 4w? Is the value of the two grid stoppers that I measured cause for the imbalance you were referring to? Did I miss two grid stoppers on the PCB? The only resistors I saw that looked connected to pin 5 were the...
  9. gibson17

    JCM900 4100 Correctly Biasing

    R28 in this amp is a 56k. R29 is 15k. These values differ from the schematic that was linked by anitoli. R31/32/33/34 are 2.2k 10% 4w. I checked the resistors on the tube sockets pin 5. They are using one resistor on each of the outside tube sockets with a jumper to pin 5 on the inside sockets...
  10. gibson17

    JCM900 4100 Correctly Biasing

    It had EL34's in it, so that's what I put back in it. After some research I see that 5881's might have been a thing when this amp was produced.
  11. gibson17

    JCM900 4100 Correctly Biasing

    Thank you for your reply. I reconnected R30. That resistor is a 22.5k (red, red, orange, gold) in my amp. I pulled the tubes and turned the amp on. The measurements on pin 5 are -46.01vdc and -58.25vdc. I am using a dual bias probe from SRS into my DMM which is a Fluke 115.
  12. gibson17

    JCM900 4100 Correctly Biasing

    Hello, I'm resurrecting this thread because I am having the same issue with the same amp. I have a JCM900 DR 4100. The bias adjustment at full is about 23ma. I clipped r30 which in my amp was 22.5k and put a decade resistor in place to find a suitable replacement. I then moved the bias pot to...
  13. gibson17

    Marshall Jubilee 2558 Bias Question

    Hello All, Just a follow up to close this thread out. I swapped sides of the winged C's and the low bias reading followed the tube. I swapped them back again with the same result. I then installed a new set of matched Svetlana EL34's and the bias balanced out. Thank you all for you advice and...
  14. gibson17

    Marshall Jubilee 2558 Bias Question

    Thank you for the response. When I tested the tubes they were within 2% of each other. I guess I could try to swap sides and see if the bias reading follows. When I had one tube biased up to 37, the other tube was 27, which seemed cold to me. The tubes are winged c's, so when they tested good I...
  15. gibson17

    Marshall Jubilee 2558 Bias Question

    Hello all, I have a marshall 2558 Jubilee combo I recently picked up. I love it sound and have been using it to Gig. I recently checked it out to service it. I tested all the tubes and they test well on a tv7 tester. I calculated what the bias should be at with 65% dissipation. I get about 37...
  16. gibson17

    Marshall TSL 100 Footswitch

    Got it. They are indeed SPDT MOMENTARY switches. You can get them on ebay By searching SPDT Momentary 112p. I figured i would come back so the thread had a happy ending!
  17. gibson17

    Marshall TSL 100 Footswitch

    Yes, does anyone have the appropriate part numbers for the PC mount switch type. I can tell you that it is NOT SPDT. I bouht some off of ebay that fit and look the same, soldiered them in, and they work, kinda. You have to click them twice to get them to work. So the search is on, cause i have 3...
  18. gibson17

    The Tsl, the bias drift, the pcb board.

    Subscribing.
  19. gibson17

    jcm2000 volume fade when switching channel

    anyone ever find a cure to this problem? i have two jcm2000's and one of them has this issue... any help would be appraciated.
  20. gibson17

    Trouble with TSL

    Retube the whole thing, i would suspect your power tubes and or your preamp tube, i.e. V1 preamp which is the one with the metal cover. Good luck.

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