Ad Widget

Collapse

Announcement

Collapse
No announcement yet.

JCM800 6550 tube substitution and fuses

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #16
    'grid feed resistors to the output tubes which are 22k for EL34, 150k for 6550'
    Oops, that should be 220k for EL34, 150k for 6550, sorry.
    But still, there's no benefit from messing with them, they don't affect the bias, they just feed it to the grids. As the grids don't draw any bias current, there's no V dc across the grid feed resistors.
    It seems that your bias supply voltage is too high, so change the resistor that feeds the bias pot, R27 currently 15k, to 22k. That should bring the bias voltage down a bit and so allow the tubes to pass more current.
    http://www.schematicheaven.com/marsh...l_50w_2204.pdf
    Adjust the pot to it's max value before you power up with tubes in. Peter.
    My band:- http://www.youtube.com/user/RedwingBand

    Comment


    • #17
      update

      I swapped R27 from 15K to 22K. I still get no readings on my bias tester.

      I have the bias trim pot set at max. at 22K. I slowly turned it all the way to the other end and have zero reading at any setting of the trim pot. Prior to the R27 change I read a nominal .006 mA.

      Maybe my bias tester is shot but I doubt that. It is fairly new and has been used very little.

      I am still getting noise through the speaker when I test an instrument cable. Should I just go for it, try running the amp with a guitar plugged in?

      Thanks again. MC

      Update... plugged in guitar with poor results...

      If I plug into the low input I get no sound. Plug into the high input I get very scratchy and distorted sound.

      Is this because I am unable to set the bias?

      Please help!!! MC
      Last edited by misterc57; 07-16-2009, 07:30 PM. Reason: plugged in guitar

      Comment


      • #18
        Still struggling

        Due to issues I figured I should make all the official changes per the schematics for an EL34 layout.

        Per http://www.schematicheaven.com/marsh...d_50w_100w.pdf
        there are 4 resistors that need to change...

        I have changed R24 and R25 from 150K to 220K

        R28 is already at 56K, according to color band, and when I remove from the circuit is reads 56K, in circuit it reads 46.6K

        R27 I placed back to 15K

        R8 is confusing me. It is a 10K 1 watt. Some schematics show 10K yet in the notes state it should be 120K for 6550 tube and 220K for EL34.

        My voltages for this amp should be in the following ranges.

        pin 3: 383 VDC
        pin 4: 382 VDC
        pin 5: -30 VDC

        I am reading

        pin 3: 450 VDC
        pin 4: 449 VDC
        pin 5: -73.5 VDC to -57 VDC (depending on setting of the bias pot)

        Thoughts on R8, voltages, etc?

        Thank you!

        Comment


        • #19
          Originally posted by misterc57 View Post
          My voltages for this amp should be in the following ranges.

          pin 3: 383 VDC
          pin 4: 382 VDC
          pin 5: -30 VDC

          I am reading

          pin 3: 450 VDC
          pin 4: 449 VDC
          pin 5: -73.5 VDC to -57 VDC (depending on setting of the bias pot)
          Your bias voltage is way too high, causing the output tubes to turn off. This is why your bias current meter reads zero.

          You need to get the bias voltage level down to around -40 volts in order to get the output tubes turned back on. This will in turn also reduce the plate and screen voltages.

          Comment


          • #20
            Bias Voltage Supply

            Would that be done by increasing R27 alone? I had the 22K in the there with pretty much the same bias voltage readings.

            Again I am still learning so please bear with me,,,, Is this the bias voltage circuit?...

            RV1 (22k) to R26 (56K) to R27 (15K) to R24/R25 (220K) to R31/R32 (5KB)

            Which do I increase and by what value?

            My total resistance from RV1 to pin 5 ranges (depending on RV1 pot) from a total of 300K to 279K resistance.

            I want to half my bias voltage so do I need to double my resistance?

            Thank you! MC

            Comment


            • #21
              Originally posted by misterc57 View Post
              I picked up a JCM800 2204 with a pair of 6550 tubes. One of the tubes is bad. I have several matched EL34 tubes in stock so I am wondering if I can substitute with EL34. If I do so, do I need to do anything besides a bias adjustment?

              Also I need to replace the fuses. Do I use fast or slo-blo?

              They are 500 ma and 3 amp fuses. Are the radio shack .5 AMP and 3 AMP 250 Volt fuses ok to use? They only have slo-blo for the .5 AMP fuses.

              Thank you! MC
              The Tube Amp Book 4th Edition by Aspen Pitman has a discussion and detailed instructions on this conversion on page 763 if you are interested.

              Comment


              • #22
                You can increase R27, but you should also reduce the value of R26.

                Comment


                • #23
                  now working with some hum

                  I have the Tube Amp Book, Volume 3. The book had suggested switching over to the 220Ks and one 56K for the bias circuit, or as an alternative placing (2) 47K resisitors in parallel in lieu of making any other bias circuit changes.

                  I had already switched over to the 220Ks and one 56K for the bias circuit with no luck.

                  After reading the section of the book I tried two more things.

                  1. Moved the feedback wire to the 8 ohm leg of the output impedance selector.

                  2. Added another 56K resistor in parallel to the 56K in the bias circuit.
                  This step brought my bias down into the -30s range.

                  Was # 2 ok to do?

                  I set my bias to 36 mA to match the EL34 output tubes.

                  My voltages are now

                  pin 3: 433 VDC
                  pin 4: 427 VDC
                  pin 5: -39 VDC

                  This is about 15% over specs.

                  The amp plays with a lot of power and is loud. There is a constant hum in the background, the louder the pre-amp, the louder the hum. It is even worse when plugged into the high input.

                  Suggestions on the hum? The hum is from the amp as I have other amps I can run in the same location without noise.

                  Should I lower the bias to try and get pin 5 closer to -30 VDC? Would this eliminate the hum?

                  Comment


                  • #24
                    Hello,
                    your idle current seems fine. It sounds like you may have filter capacitors going bad.

                    If your power tubes were grossly mismatched, that could cause hum also, but not likely if the hum is really loud esp with only 2 power tubes as opposed to 4 or 6.

                    You will be dealing with very high voltages when you're testing for bad filter caps so be careful. Probably the easiest way to determine if your filter caps are bad is to actually jump a known good one across the old one. In the case of the Marshall, it will be jumping the different sections of the filter cap-cans with your test capacitor.

                    The first safest test is to jump the filter caps for the bias supply. They are smaller & only rated at something like 100uf @ 100V.

                    Unfortunately, I have to run & do deliveries...maybe one of the other guys can take it from here.....good luck, glen

                    Comment


                    • #25
                      If you turn the controls all to zero, does the hum go away? If it does, then it is not the power amp section, nor the filter caps.

                      Does it hum when NOTHING is plugged into the inputs? You mentioned it mattered which input you used.

                      If it only hums when a guitar is plugged in, does turning the volume control on the guitar to zero stop the hum?
                      Education is what you're left with after you have forgotten what you have learned.

                      Comment


                      • #26
                        2. Added another 56K resistor in parallel to the 56K in the bias circuit.
                        This step brought my bias down into the -30s range.

                        Was # 2 ok to do?

                        Yes, this is ok.

                        I set my bias to 36 mA to match the EL34 output tubes.
                        I don't understand what you mean here, what are you matching to?

                        Don't worry about the bias voltage number, just worry about the bias current draw of the power tubes. The voltage numbers listed on a schematic will only give you an idea of where the voltages should be, they are not meant to be absolute figure to match.

                        In the old days when a lot of the original schematics were created, everyone used an analog meter with sometimes wide tolerance and interpretation values.

                        Comment


                        • #27
                          Nothing connected to inputs...

                          It does not hum when no guitar plugged into the amp and adjusting the preamp and volume controls.

                          Guitar plugged into low input...

                          No hum until the pre-amp and master controls are set high, then some noticable hum.


                          Guitar plugged into high input...
                          Good amount of hum with pre-amp and master controls set very low, gets worse as you turn either control up. However there is no hum with both controls set to zero.

                          Comment


                          • #28
                            36 mA

                            The tubes were a matched pair with a "36" designation. With the tube inserted into my bias tester, I adjusted the bias pot until my meter read .036 VDC.

                            Comment


                            • #29
                              Originally posted by misterc57 View Post
                              The tubes were a matched pair with a "36" designation. With the tube inserted into my bias tester, I adjusted the bias pot until my meter read .036 VDC.
                              I don't know what tubes you have, and how they were matched, but I don't think that the 36 means to bias them to 36mA.

                              You should bias them to what is correct in your amp, not what the tube company matches them to. Their equipment does not know how many volts your amp will be putting on the tube plates and screens, so their numbers are meaningless to you.

                              What Enzo was getting to, is plug your guitar in to either input, set the controls to wherever you want them. Now turn down the volume control on the guitar down to zero. If the hum goes away, then the hum is in the pickups, wires, etc. not the amp.

                              Comment


                              • #30
                                the hum consists of two sounds combined

                                It is a weird hum that consists of two sounds combined....

                                1. A muffled "snow" sound we used to hear when we were younger on the tv sets when a channel was not broadcasting late at night.

                                2. A constant muffled locomotive steam engine rhythm sound.

                                The hum does not go away if I turn the vol to zero on the guitar.

                                High Input, Preamp at 10, volume pot low to high, the noise is extremely present and gets louder with rise in vol pot.

                                Low Input, Preamp at 10, volume pot 8 to 10, the noise is noticeable as a background noise. At lower volumes you barely notice it.

                                No instrument connected I get the same results as the low input connection above.

                                Under all conditions, turning the vol on the amp to zero the noise is gone.

                                Comment

                                Working...
                                X