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  1. M

    JCM900 4500 Extra 2.2k resistor soldered to pin 5 of V5 (EL34)

    Check it for yourself but you might be surprised to see that they are biased super cold. I don't know why Marshall used a 56k bias pot resistor but if you have to increase the current and max out the pot before you hit your target, try swapping a 47k in R30. Just be careful when de-soldering...
  2. M

    JCM900 4500 Extra 2.2k resistor soldered to pin 5 of V5 (EL34)

    Next time my chassis is out I will take a look at the build date. Transformers are Drakes and have 3/90 and 5/90 on them, so probably assembled around the same time. My serial number indicates it is a 1990. For bias I just use a single probe and DMM and adjust the hotter tube to ~70%...
  3. M

    JCM900 4500 Extra 2.2k resistor soldered to pin 5 of V5 (EL34)

    I thought the same thing. Super clean inside! I see yours is the same model from the same year; does yours also have a QC sticker inside signed by a "Denise" and/or "Karen"?
  4. M

    JCM900 4500 Extra 2.2k resistor soldered to pin 5 of V5 (EL34)

    It's still a mystery, but the voltages on pin 5 for both tubes are the same, so I have no idea. The fact yours has it too suggests Marshall did this from the factory, so there's that.
  5. M

    Gold Lion KT77, JCM900 4500 DR: Maxed Out Bias Pot

    Well, 220k plus 47k came out to 38.5k. Soldered in the new resistor pair and biased tubes at -36ma and -39ma. I can live with that. I think the pot has enough range in it that I can swap in my EL34s with just a bias adjustment. Amp sounds quite good with these tubes; definitely more bottom end...
  6. M

    Gold Lion KT77, JCM900 4500 DR: Maxed Out Bias Pot

    Thank you. I think a 220k in parallel with the 47k will get it to just a hair under 39k.
  7. M

    Gold Lion KT77, JCM900 4500 DR: Maxed Out Bias Pot

    https://imgur.com/gallery/M3cUUSu Here is the schematic for the bias circuit. The circled 56k resistor was changed to a 47k. PR1 is the 22k ohm trim pot. What might you suggest changing to allow for a slightly hotter bias?
  8. M

    Gold Lion KT77, JCM900 4500 DR: Maxed Out Bias Pot

    Thanks for this. The spec sheet shows 25w as the design max for the KT77 plate dissipation, and if I use the 22w figure my idle current is around 65-70%. I would still like a little room above the current settings for further adjustment, so that the pot isn't maxed out at least.
  9. M

    Gold Lion KT77, JCM900 4500 DR: Maxed Out Bias Pot

    Previously had some Ruby EL34BSTR tubes in my 50W JCM900 Dual Reverb (which worked well and sounded good) but wanted to see what these KT77s sounded like. Rubies were biased at -38.5ma or so and were surprisingly well balanced with about a 0.4ma difference between the two tubes. Put in the GL...
  10. M

    JCM900 4500 Extra 2.2k resistor soldered to pin 5 of V5 (EL34)

    Hi, this is my first post on the forum, and probably not my last! I have a 1990 JCM900 4500 DR that we've owned since ~1994, purchased used. This model has to JMP50B power stage board and came stock with EL34s (no reason to think this was modded from 5881 given the confirmed year of the amp...

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