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5150 Head Having a Hissy Fit

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  • #1

    5150 Head Having a Hissy Fit

    Wanted to get your opinion on this 515o head I am going to re-tube for a customer

    First it is an original 5150 head, all original except he reports the tubes "might" have been changed several years ago

    Besides some new glass needed, I hear a tremendous amount of hiss (white noise) out of this thing: really loud on the crunch channel and damn-near unbearable on the lead channel.

    I know this is a super-high gain beast and hiss would be expected but this amount does seam excessive really.

    Filter caps are original (~20 years old) but not likely causing this issue as its not loudly humming.

    Thoughts? Something else to look at in there while I have it apart? Hoping a new tube compliment will help. Also, should I go ahead and change filter caps as well as they are aging? Thanks

    Schematic for reference:
    http://www.blueguitar.org/new/schem/peavey/evh_5150.gif
    Tags: None
  • #2
    Why even take it apart till you try the tube swaps? Then if all the noise is just caused by tubes, you can ask the owner if he wants to go further with it.
    And try to change them one at a time to see if any of the old ones were actually the problem. Then you can return the old ones that are still usable as spares and mark the noisy ones as "bad".
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

    Comment

    • #3
      Originally posted by g-one View Post
      Why even take it apart till you try the tube swaps? Then if all the noise is just caused by tubes, you can ask the owner if he wants to go further with it.
      And try to change them one at a time to see if any of the old ones were actually the problem. Then you can return the old ones that are still usable as spares and mark the noisy ones as "bad".
      Sage advice, thank you.
      The customer is adamant to have it fully retubed with JJ pre and power tubes so just doing what he wants there. He also wants the adjustable bias mod which is ok so I have to have it apart for that anyway. So that being said, any other advice on the hiss while I am in there?

      Comment

      • #4
        If it turns out the tubes are not the source of the hiss, I would look at preamp plate resistors, also chopstick around to see if there are any bad connections that affect the hiss.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

        Comment

        • #5
          Just wanted to mention that hiss may be the number one complaint about the 5150. It's a fairly early uber gain type amp and some better design ideals for hiss have come along since then. Maybe look into a diode type noise gate. A lot of modern high gainers use them so examples shouldn't be hard to find. The trick is dialing the noise gate component values for the gain at the node where it's implemented. I've never added one of these, but I would. It could be done on a small perf board and just zip tied in place somewhere. If the experiment doesn't work just snip it out.
          "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

          • #6
            The noise suppressor is just a pair of diodes. Look at the Peavey XXX. It is very similar to the 5150 but it adds the noise fighter. Then look at their JSX, they made the thing variable by making the parallel resistor into a pot.

            The 5150 is not remotely a bedroom amp, it is meant to be played loud on stage. SO hiss will be at higher levels than most amps. Hiss is the other side of gain monster amps like this.
            Education is what you're left with after you have forgotten what you have learned.

            Comment

            • #7
              Found this thread. Good info for implementation. I had trouble finding the Triple X or JSX schems but that would be the easiest way to go since those circuits likely translate almost directly to the 5150.

              http://music-electronics-forum.com/t31796/
              "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
                If a person wasn't inclined to modify the amp, you could also insert a noise gate in the effects loop.
                "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

                Comment

                • #9
                  If the hiss is there with the gains down, you have a problem.

                  With gain & volume up, all bets are off.
                  They seem to be o/k about half way.
                  With gain & volume all the way up on the Lead channel, I have measured 2 volts of 'hiss' at the output on these beasts.

                  Comment

                  • #10
                    Originally posted by The Dude View Post
                    If a person wasn't inclined to modify the amp, you could also insert a noise gate in the effects loop.
                    Yes I did mention that to him and he said he runs a ISP decimator and it doesn't help the extreme hiss (I cannot confirm this). just taking his word.

                    I found another possible option searching the interwebz as well. Will bookmark it incase for later but not sure the rationale:

                    "V5B(middle preamp tube): add 10pf silver mica cap to pins 7 and 8. This will remove hiss, make the gain less fizzy, make the high mids clearer and improve clarity in chords. between pins 6 and 7 it’s even more efficient, but will also darken the amp a lot."

                    Thanks for all the info everyone. Always greatly appreciated.

                    Comment

                    • #11
                      Originally posted by Pryde View Post
                      Yes I did mention that to him and he said he runs a ISP decimator and it doesn't help the extreme hiss (I cannot confirm this). just taking his word.

                      I found another possible option searching the interwebz as well. Will bookmark it incase for later but not sure the rationale:

                      "V5B(middle preamp tube): add 10pf silver mica cap to pins 7 and 8. This will remove hiss, make the gain less fizzy, make the high mids clearer and improve clarity in chords. between pins 6 and 7 it’s even more efficient, but will also darken the amp a lot."

                      Thanks for all the info everyone. Always greatly appreciated.
                      If that pedal doesn't help, it COULD indicate that there is a problem with the power amp. As Jazz P hinted, is the hiss still there with the gains and volumes down? If it is, there is likely a problem. These things are normally biased pretty cold, in my experience, and should be reasonably quiet with all of the gains and volumes down.
                      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                      • #12
                        Yes its ok with vol/gains down. I checked that right away.

                        It appears some have had luck running a decimater G-string in the effects loop and into the front end x-style. might be an option for him.

                        Comment

                        • #13
                          Just another quick thought:

                          Ask your customer how he was running the decimater. I've had some customers with limited electronic knowledge who put it from guitar to amp instead of in the loop- not making the leap of thought to consider that this would do nothing for preamp noise. It seems obvious, but apparently not to everyone.
                          The reason I persist is because I know of several players who use this exact pedal as a solution and it seems to work quite well.
                          "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

                          Comment

                          • #14
                            Indeed. WRT guitar players there is sometimes, maybe seldom even, the tiniest possibility of user error
                            "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
                              I will ask him to bring the noise gate when I retube and bias it for experimentation. Thanks for the suggestion.

                              One last thing: Filter caps? Just leave them be for now? They don't appear to be causing issues at this time.

                              Thanks

                              Comment

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