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Taming a REALLY bright JCM800

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  • #46
    Re: Taming a REALLY bright JCM800

    Well it's the head and not the speakers. Does this head have 6550's? A lot of 6550 amps were never biased correctly, and they did have a bright cap from hell. Dolye once commented that the 6550 equiped JCM800's were among the brightest amps ever.

    50watt amps have a hard time pushing high wattage speaker cabs, and they almost always sound thin, when they try. The head isn't right and it's further disadvantaged by the high wattage cab. The speaker cab is rated at 270 watts RMS, so the ratio of amp output (RMS) to the speakers is like 1:5.5 This is why the T75's (300 watts in a 4x12) usually sound better with a 100 watt amp too.


    6550's can give a strong solid bass when biased properly. Perhaps you can get some help from a brother in such a large population area, and check the tube type and the bias.

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    • #47
      Re: Taming a REALLY bright JCM800

      Have you put tape over the speaker grill cloth yet? That is your simple fix to cut out high end. Simple and non-invasive.
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      • #48
        Re: Taming a REALLY bright JCM800

        Speaking of JCM800s, i'm getting back mine from the repairs this weekend.

        I agree that its bit bright in a way or so .. the pre amp tubes just changed to some svetlana and I think I would wanna replace them soon though
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        • #49
          Re: Taming a REALLY bright JCM800

          Lake Placid: He already stated that it has EL34's.

          Since they won't let you return it, do the tape on the grill cloth and try the low input. That should help. If you can borrow an EQ pedal from a buddy or if you have an overdrive pedal with a tone knob on it, try one of those before the input. On the overdrive just turn the gain down if you don't need much and adjust the tone to taste.

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          • #50
            Re: Taming a REALLY bright JCM800

            Originally posted by ErikH
            Lake Placid: He already stated that it has EL34's....

            .
            Oops! missed that. Sorry.

            If he can find some one that knows their way around Marshall innards, he could possibly get the bright cap temporarily fixed, and the bias checked though.

            I agree with putting some tape over the speaker centers too.

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            • #51
              Re: Taming a REALLY bright JCM800

              Originally posted by Boogie Bill
              Instead of keeping the midrange high, turn the mids DOWN then add just a little treble and presence, say T-4, P-2.

              JCM800s usually have a "bright cap" connected to the input gain--to add brightness and definition on your clean settings. You will want to run the gain up to at least 6 or seven to negate this feature. (I clipped my out of the circuit, but of course--you wouldn't want to do that with a rental.)

              I always liked the RECORDED sound of my Marshall when I plugged into the LOW GAIN input and cranked it. I would often add a little clean boost. This created a low volume Marshall with the most unbelievable FAT tone. MIne was a 50-watt JCM 800, so there wasn't quite enough power to gig with--but, but those old recordings still sound great.

              Otherwise a graphic or parametric EQ (as others have suggested) is about your only other option.

              And with a Marshall, louder IS better...mine always sounded best after it got hotter than Hades!

              Good luck.

              Bill
              Tried the eq trick you mentioned already. No dice. Now that I think about it... the only controls that really seem to work at all on this amp is the volume controls. The other controls ARE operational but their range seems like its almost nil.

              The low gain input seems to be broken... I can't get any sort of competitive volume out of it and theres a lot of noise.

              Cranking the amp without the attenuator doesnt really help round it out... it just gets that same annoying ear-shatteringly bright tone... just LOUDER.

              I checked the pre tubes again... turn out they're GT 7025s. I found a listing for them here: http://www.groovetubes.com/tubes_lis...ctGroup_ID=107 Are these real 12AX7s or not? I have some Sovtek 12AX7s lying around, I'll pop those in and see if it makes a positive difference. I'm not going to go clipping bright caps unless I've exhausted my beam-blocker/bias by ear options first.

              Stay tuned...

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              • #52
                Re: Taming a REALLY bright JCM800

                That thing just plain needs service. The JCM800's are bright by nature but still, they shouldn't be as bright as you're saying. And they fact that the low input isn't working is an issue in itself. I'd be raising some hell.

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                • #53
                  Re: Taming a REALLY bright JCM800

                  If the low input isn't working, the high gain input can't work correctly either. That amp is broken in such a case.

                  On the JCM 800 2204's the high gain input goes directly to V1A, then it goes to the low gain input through an isolation cap (cap3) before going through the gain sensitivity pot and on to V1B. In other words, the low gain channel shares the same signal path as the high gain channel, but just plugs in later, by passing the amplification of the first preamp tube triode.

                  If the low isn't hardly working , then there's a possibility that there's a short, or a bad cap, or something shunting the lows and mids to ground. Perhaps V1 (first 7025) is bad. The possibility exists that the RC network between the cathode of V1B and ground may be bad. This can cause crackling, static, and awefull metalic tone. I've seen this happen in older Fender amps, with worn out caps and resistors.

                  7025's are a military grade American 12AX7. There were ofton used in Fender amps in noise sensitive positions. It wouldn't hurt to try some other 12AX7 tube in V1 and V2 agian.

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                  • #54
                    Re: Taming a REALLY bright JCM800

                    Originally posted by B2D
                    Tried the eq trick you mentioned already. No dice. Now that I think about it... the only controls that really seem to work at all on this amp is the volume controls. The other controls ARE operational but their range seems like its almost nil.

                    The low gain input seems to be broken... I can't get any sort of competitive volume out of it and theres a lot of noise.

                    Cranking the amp without the attenuator doesnt really help round it out... it just gets that same annoying ear-shatteringly bright tone... just LOUDER.

                    I checked the pre tubes again... turn out they're GT 7025s. I found a listing for them here: http://www.groovetubes.com/tubes_lis...ctGroup_ID=107 Are these real 12AX7s or not? I have some Sovtek 12AX7s lying around, I'll pop those in and see if it makes a positive difference. I'm not going to go clipping bright caps unless I've exhausted my beam-blocker/bias by ear options first.

                    Stay tuned...
                    The Problem could be in pots!!
                    Try to scoop the mids if it doesn't work, the pots aren't grounded very well. The biggest difference is in the middle(am I right?) 'cos it's wired nearest to ground

                    Erm if the eq pots don't work you should mention it to the rental-place and they have to repair it!
                    My amp picks up a radio station - what can I do about this?

                    Tech Head response: Nuke the radio station?

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                    • #55
                      Re: Taming a REALLY bright JCM800

                      If its still too bright then just lower the treble and it will be ok I guess. heh
                      Gibson Songwriter Deluxe
                      Gibson 1959 reissue
                      Gibson 1960 reissue
                      Gibson EDS-1275

                      Freedom through the blood of Christ

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                      • #56
                        Re: Taming a REALLY bright JCM800

                        Am I the only one that sees Kevlar's posts?

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                        • #57
                          Re: Taming a REALLY bright JCM800

                          Originally posted by JOLLY
                          Am I the only one that sees Kevlar's posts?
                          Whose post ?
                          I've been driving all night, my hands wet on the wheel.

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