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First trip to tube amplification.. That's broken.

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  • #1

    First trip to tube amplification.. That's broken.

    Hello there!
    Ive been a solid state amp player for many years, mainly USA 80s peavey combos
    I guess the reliability and pleasant tone has kept me to them. I recently received a crate BV120 head as a gift from my brother. It's broken, maybe needs power tubes, I changed the fuses as they were all broken. I'm a little skeptical on the value. It looks to be 1/4 amp fuses on the board. Kinda hard to read the schematic, also pardon my juvenile questions but this is all new to me. I have basic understanding of electronics but the tube world seems much more complicated. I would love any insight or suggestions to get this guy running again safely. Thank you all
    Tags: None
  • #2
    There are four 1/4 amp fuses on the board, one for each power tube, F2,3,4,&5.
    schematic here: http://music-electronics-forum.com/a...e_bv120_sm.pdf
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      Thank you very much for the prompt reply as well as the schematic. Now I'm trying to follow the path and and when folks refer to V1-V5 and so forth I'm assuming that they mean tube sockets? Again sorry I'm a new guy. I'm really here to learn, not to just fix this thing and flip it. This is an interesting field and I'm excited to gain some knowledge about this. Thank you again.

      Comment

      • #4
        The V1, V2 etc. moniker is typically applied as read on the schematic. If no V#s are offered on the schematic some guys will reference V1 as the "first" preamp tube. But in this case it's usually better to just say "first gain stage", "second gain stage", etc.

        First, I'm not sure what you mean by "I changed the fuses as they were all broken." I'll assume you mean blown? But more important, how did you replace them without knowing the value? I think it's important to note that there is probably a reason those fuses blew! Simply replacing them probably won't fix the amp. And you're probably right that it needs power tubes.

        On a side note, I think you'll be happy with that amps tone, especially if you like the high gain thing. but not so much the reliability. Not that it's a bad amp, but it's not exactly top of the line and certainly more prone to reliability issues than 80's SS Peavey amps. I use to call my Bandit 65 "the wrecking ball" because of all the door casings and furniture legs I whacked it into toting it around. And I joked that it was so tough you could hang it from a chain and do demolition work, then take it to a gig. The Crate is NOT going to be as reliable as you're use to.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

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        • #5
          Yes that's my go to amp, the bandit 65. Great amp. As for the high gain comment, I'm not big on hi gain tone maybe just a mild grit. Like a thin lizzy type sound. I can't get into this massive distortion they have going on today. Now to get back to the fuses, I saw 1/4 amp printed on the fuse itself, also when I say broken, means that the wires inside were all broken in half but none were burned. Odd. Also I will add that there was an oddball tube among the stock GT tubes. And possibly that could have caused some problems. Then there's the biasing which I think I have a basic understanding of that. But do you bias per the tubes or is it a preference setting? Thanks!

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          • #6
            I was also curious 120 watts is a lot of power and is it possible and if done properly, safe to run the bv120 (in a healthy state) at 60 watts without catastrophic results? What is the procedure for this if possible? Thank you again

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            • #7
              Pull two power tubes and run the impedance selector at half your load. If you're really diligent you can swap the tubes you pulled for the two in the amp every month or so. Then you'll have somewhat equal wear on the tubes if you find yourself in need of the extra power for awhile (remember to put the impedance switch back to normal). Otherwise just wait until you wear out one pair and then put in the other.
              "Take two placebos, works twice as well." Enzo

              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

              "If you're not interested in opinions and the experience of others, why even start a thread?
              You can't just expect consent." Helmholtz

              Comment

              • #8
                Chuck, thanks for the reply. I don't think I have any other options. Meaning that my imp selector switch is already at 8 ohms and there isn't a 4 ohm selection. And my speaker is 8 ohms. Is there a rewire involved? Thanks for taking the time.

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                • #9
                  8 and 16 are all you get. Just run the thing on the 8 ohm tap, it won;t hurt it.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    How would I bias the amp for 60 watt operation?
                    Thanks again for the responses

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                    • #11
                      Probably should not have to adjust the bias. How are you adjusting it with 4 power tubes installed?
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


                      ​

                      Comment

                      • #12
                        I would prefer to bias the amp to run a little on the cold side, I play mainly clean and use overdrive pedals, not much on big gain, I guess now it's set to factory specs but the adjustment pot is on the board, it's all new to me so I appreciate any and all feedback.

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                        • #13
                          If you are not aware of the lethal voltages and safety precautions for working on tube amps, do not proceed.
                          Pulling two of the output tubes will probably raise the B+ a bit so the 2 remaining tubes may be biased a little hotter.
                          You can check the idle current of each tube by measuring the current through the output tube fuses. This means removing the 1/4 amp fuse and clipping a current meter across the fuse holder. You also need to measure the plate voltage at pin 3 of the power tubes.
                          Calculate the idle wattage by multiplying the current by the plate voltage.
                          Do this with all 4 tubes installed. Then remove 2 of the power tubes and recheck. If the idle wattage has increased, you can decrease it with the bias adjust pot.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


                          ​

                          Comment

                          • #14
                            Concise and well explained. But I'll mention that that particular amp isn't the sort that overdrives the power tubes, typically. To get the big watts from a quad of 6L6's they are probably biased a little on the cool side now. It should be fine to just pull two tubes.

                            Since it wasn't mentioned yet the tubes you want to pull would be the two inside tubes OR the two outside tubes.
                            "Take two placebos, works twice as well." Enzo

                            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                            "If you're not interested in opinions and the experience of others, why even start a thread?
                            You can't just expect consent." Helmholtz

                            Comment

                            • #15
                              Hmm, replaced all fuses, got tubes, and flicked on the power thinking I was in good shape and now I have no led lights which means I guess no tube heaters ??(the goofy led lights behind the tubes are what I am referring to. They worked before all this and now nothing. It certainly is voodoo.

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