2204 Bias 6550 factory setting???

Discussion in 'The Workbench' started by MSOUND, Apr 24, 2012.

  1. MSOUND

    MSOUND Member

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    I know this has been discussed a LOT. I have searched a LOT and asked a few guys I trust,yet I still do not really have a solid answer.
    Mainly 2 different ideals from what I have gathered. Each comes up with much different numbers.

    1983 2204 6550's 442 plate volts approx

    When biasing:

    #1 Treat like a 25 watt tube because that's what the circuit was meant for originally.

    #2 Go with 35 watt tube at about 65% max diss.

    I would REALLY like to know what these amps were set for from the factory. Does anybody know?? One of these two ways has to be wrong.

    The amp sounded killer when I got it, then for safety, I checked the bias one tube at .066 1 @ .102 (it was redplating). I turned it off!

    But it made me wonder if I liked the HOT setting and that's what made the amp stand out to me.
     
  2. MSOUND

    MSOUND Member

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    No techs at the workbench?
     
  3. big dooley

    big dooley Well-Known Member VIP Member

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    the 6550 itself running at a bias, that is calculated with a 35watt max disspation rating, in a 100 watt amp, is something that is possible, but there are a few things that have to be taken into account...
    in an ideal circuit, the 6550 would run with 600Vdc on the plates and 300Vdc on the screens... however, with 440 on the plates in a marshall design, the screens would run at around 425Vdc, which is at or beyond the limit for these tubes... also the max screen dissipation of the 6550 is rather low at 6 watt (the EL34 is rated at 8 watt)
    also, since we have a power supply that is meant for passing on 100 watt, running these tubes at 35 watts, puts some strain on the transformers, as there is quite some current flowing trough

    for these reasons, most people advise to bias the amp like a 100 watter, at 25 watts per tube...

    btw 66mA at 440Vdc is WAY too high... i won't even start on the 102mA reading
     
  4. mickeydg5

    mickeydg5 Well-Known Member

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    Hi MSOUND
    I would like to know if when your plates are at 442V and bias is .066A and .102A, what was your negative grid voltage?

    How are you measuring bias?
     
  5. MSOUND

    MSOUND Member

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    The 2204 is a 50 watt amp. Does that matter?
     
  6. MSOUND

    MSOUND Member

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    With a bias meter that tests plate voltage also.

    Dual BiasTester-MPD
    [DBTP-MPD] $79.95 [​IMG] [​IMG]
    Click to enlarge T
     
  7. MSOUND

    MSOUND Member

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    Thanks for the reply's. So does any one know the answer to my questions?
    50 watt amp 442 plate volts,6550 tubes in a marshall 2204 (originally designed for el34?)
     
  8. mickeydg5

    mickeydg5 Well-Known Member

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    I have seen different claims for negative grid voltages and for tube current.
    I have never seen any Marshall documentation for idle current of a 2204 6550 amplifier.

    I am not aware of any published Marshall factory setting for your amplifier.
    I believe that neither method is totally correct.

    It seems your amplifier was converted from EL34 to 6550. Correct?

    From what I gather a 6550 in that amplifier should have an idle current somewhat higher than its counterpart EL34.

    Did you note or can you tell me what your negative grid voltages may be?
     
  9. HAMPAMP TUBE AMP SERVICES

    HAMPAMP TUBE AMP SERVICES Member

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    You will never know the factory settings for that amp. I doubt very seriously that Marshall even measured plate current for some magic number back in 1989, Manufaturers didn't give that much thought to bias. The method used now wasn't widely used until the HI-FI nuts started biasing their amps hot so that they would be able to get super clean sound at "low" listening levels, this craze found it's way into the guitar amp community. I will guarantee you that you would be suprised and dumbfounded at how "COLD" that amp was when it left the factory. Reliability and getting product out the door was more important than holy grail tone. Also, if Marshall had set bias as hot as a lot of people do now-a-days, they would have had to replace a lot of tubes under warranty, Set your amp to about 50% of the tubes maximum rated dissapation and you'd probably be close to factory. Even today with the bias settings printed right on the schematics, most people ignore this factory setting claiming it to be "too cold" so don't even worry about it. A lot of manufacturers just set the amps to the negative voltage indicated on the schematic or used an oscilloscope to set bias, which by the way is taboo in todays world. I had a customer in the shop today and the bias was set too hot on his Bassman, I tweeked it down a little at a time stopping to listen, and between saturation and cut-off there was hardly a noticable difference in the tone of the amp, sometimes it's a big deal, sometimes not. There is no magic number, it's about as accurate to set it where it sounds best without overheating the tubes.
     
  10. MSOUND

    MSOUND Member

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    My knowledge of the internal circuit will be limited to what the aforementioned bias probe will show.

    I agree with the cold bias from the factory thing.... but..some amps sound better with a little higher setting. USA bound Marshalls(inc.JMP)from mid seventy's to mid eighty's had 6550. Many had them converted. Mine was sent from England with them built in Dec. 1983. It's 100% stock and I intend to keep it that way.:cool:
    I also realize people make a bigger deal out of bias adjust than real life.

    Because this is a Marshall forum I was hoping someone out there was as fact gathering as myself and may have tested their new amp when they got it to see how it was set for future reference.( I always do) I have had brand new new old marshalls but no bias probe then otherwise I'd have this already!
     
  11. MSOUND

    MSOUND Member

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    The amp is not a Mod. Mid seventy's to mid eighty's Marshalls destined to the US Had 6550 stock tubes. The trannys were same as the EL34 models (per model) 1983 2204 (50 watt) 6550 100% stock

    I can only give data that I can procure with the aforementioned probe.


    I figured someone along the way was like me and test new amps when they get them to "see where they are" for future reference. When I bought Marshalls new back then I didn't have a probe but I bet techs did. And techs see/buy new amps also. I just thought someone would know this.

    The .066 & .102 numbers were taken when I noticed a tube redplating so they probably mean nothing but reading bad tubes. of course the .102 was the crimson devil.

    Really the the whole thing boils down to: Do you treat the tubes as if they were 25 watt tubes or 35 watt(generic 6550 rating) or the actual tube rating (42 watt)
    and would biasing them up to 65% max of say 42 would harm the trannys marshall designed around EL34 but shipped with 6550 (of course resistors were changed to get bias current where it needs to be)
    This is the questions that I have only seen speculated answers to.

    I called a Famous amp (800 Marshall style)designer I know and he told me go with 35W and take them to 65%. He didn't seem worried about the transformers as some have on this forum that alerted me to this potential issue initially.
     
  12. MSOUND

    MSOUND Member

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    Marshalls from mid seventy's to mid eighty's destined to U.S. were shipped with 6550 tubes. My marshall is JCM800 1983 50 watt 2204 Stock /w/ 6550. Just like most JMP from '74 on.
    I am not worried about the tube life as much as I am the transformers. There are 10 years plus of these amps out there with the same configuration so I am probably just listening to too much internet banter making me worrying about transformers not really being meant for a 6550 amp thus you must bias cold. For all I know the transformers arn't even affected by tube bias.
    If your transformers stay cool is that an indicator you are not being mean to them?
     
  13. mickeydg5

    mickeydg5 Well-Known Member

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    MSOUND, there is nothing to worry about. Those transformers work just fine with either EL34 or 6550 (and other power tubes as well). Your 2204 is 1983 stock Mk.2 for US.

    Transformers can be cool or warm and some a little hot. A really hot transformer may indicate excessive current and malfunction.

    Back to the main subject. Have you bought or replaced the power tubes? Those bias measurements indicated earlier were way up. Where are you know with dealing with the situation?
     
  14. American Viking

    American Viking Well-Known Member

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    Don't bias at 35W dissipation. That would put a strain on your PT and make it run way too hot. I learned that the hard way. Just like the others said, treat all tubes in that amp like EL34's.
     
    yladrd61 likes this.
  15. MSOUND

    MSOUND Member

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    Well... I am pretty sure the one tube was shorting. I ordered a matched set and also went to the store and dug through their box of (used?) tubes. I found 2 Russian Groove tube 6550's with the same #s and thought I would be in luck to be able to see if it is amp or tubes ( I had already presumed tubes because it followed the tube not the socket with results.) I put the 2 groove tubes in and they were 30ma apart!!. I switched them to see if results followed tubes and it did. so I switched one with the good one of the other set and got them reading .12ma apart. I played it at practice that way and it sounded great. (I usually buy valve art tubes and all the sets I have had bias under 1ma in my SPLAWN!) So now basically I am awaiting a true trusted matched set and hoping to get closer together. ( the results always follow each tube so I do not think it is my amp causing imbalance,just the random tubes I'm grabbing!)
    What concerns me is the two groove tubes had the exact same numbers on them but were WAY off. But one could have been used/abused or any possibility.
    I am gonna shoot for 50 ma. (using 35watt & 445 plate volts)
     
  16. MSOUND

    MSOUND Member

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    This is what confuses me. One one side I hear use 35 watts and others say it harms things. Both ideals can't be right!!!

    If I'm running biased at 65% of 35 watts and the transformer is staying barely warm I should be OK right?
    I knew by my research prior to posting this was going to be tough to get people to agree on.
     
  17. mickeydg5

    mickeydg5 Well-Known Member

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    AV, how was your amplifier set bias/current wise for 6550 tubes and what happened?
     
  18. mickeydg5

    mickeydg5 Well-Known Member

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    What were the negative grid voltages while doing this?

    A reading of .12 is quite close. Did you mean 12mA?
     
  19. MSOUND

    MSOUND Member

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    Yes I would be interested also. So far most Marshall amp guys & tube company's seem to agree with the 35 W thing.
     
  20. MSOUND

    MSOUND Member

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    Yes I meant 12ma. It sounded fine like that with 2 bastard tubes!

    This is the first amp i have had with 6550s and I must say the tube glass gets freaking hot enough to brand cattle!!! Transformers staying cool :dude:
    When these amps came out in the 80s, I was using a JMP and thought it was OK but needed gain. I HATED the 800s, because everyone I knew that had one had to have it modded for the "high gain sound" at the time. (why not just buy something ready to go?) So I traded my JMP for a LANEY. (my biggest regret gear wise).
    It has been interesting going from wanting a huge high gain sound to appreciating a clearer,meaner,chimier sound.

    This 2204 just kills in that department. You can go from ripping to awesome clean sound with your volume knob keeping the perfect balance between the two.(like no amp I've ever used tube or other) I usually need 4 presets to do what I can do with this amp and a pedal!
    The search for something better and gillions of dollars in amps takes me right back to where I started!
     

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