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Fixed to Cathode Bias On The Fly

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  • #1

    Fixed to Cathode Bias On The Fly

    I am building hopefully my best amp yet. It's a 2 channel switcher with the lead being a SLO and the other channel is a crunchy almost hi gain amp that my buddy wanted based on a 5w single ended marshallesque/Jose (minus the clippers)/AX84 hi octane style. Here is me playing the 5w beast in my garage.
    http://vid587.photobucket.com/albums...1861688336.mp4

    I didn't realize how much crunch I was getting from the EL84 because I built the same preamp with a PP 6V6 PA and it sounded like dog shit. Now I don't want to build two separate PA's for this amp so I was wondering how switching to cathode bias when I switched to this channel would work. Will there be any problems for the brief moment it switches. I am using relays.
    AND!!!! Will cathode biased 5881's give me some of that mojo I was getting from the SE 84?
    Tags: None
  • #2
    Quick question for clarity: do you mean Vintage-Style 5881 (I include the TS "reissue 5881" here), or the Sovtek "5881" that are really 6L6GCs? They're quite different, and it might make a difference in answers. I really hate the way manufacturers (or maybe it's the marketers?) reuse old tube designations...

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

    Comment

    • #3
      That would be the Sovtek 5881. I heard that's the only one that can withstand the high plate voltage. And I hear it breaks up earlier than most 6L6's so I thought I might get more crunch from them in cathode bias.

      Comment

      • #4
        The reason the el84 sounds like it does is because it is a very unique tube compared to pretty much any other used in guitar amps. It only takes (about) 8V to 13V input to start clipping an el84 tube where you'll need more like 25V to as much as 40V to clip a 6V6. A 5881 would require a little more still. Cathode bias will do NOTHING to correct for this. In fact, it can make things worse in some regards. If you can increase preamp voltage output by adjusting an inter stage voltage divider or something then you might have the option of getting similar performance with other tube types. Otherwise you'll need to drastically modify the design, possibly even adding a gain stage, or stick with the el84. You can always put two el84's in parallel for the same power out as a 5881 too. So there's that. So the answer to "Will cathode biased 5881's give me some of that mojo I was getting from the SE 84?" is no. But it's possible the question was a little of the mark of the goal. The answer is above. That is, you'll need more signal voltage going into the grid of the 5881 (with ANY bias circuit) to get it to crunch similar to you're el84 prototype.

        HTH
        "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

        • #5
          Chuck knows his EL84s. Do what he says.
          I usually have the opposite problem - I like pentode preamps, and I don't use any attenuation between stages or Master Volumes, so my initial builds come out doing vintage AC/DC on "2" cuz the power tubes (whichever they are) get slammed right off the bat. Then I add tone controls or switch to an LTPI & that fixes it... ;

          Chuck - would cutting out some of that inter-stage attenuation help? Seems like it's a matter of lifting one end of a resistor to try...

          I think maybe some of the reason those Sovtek 5881s distort "earlier" is because everyone knows you can drive the ever-loving ?!$$ out of them, run them at 600V, bias them at 30+W, etc., without them self-destructing. Look up nearly any tube-torture-test-amp, and the tester will be using those Sovteks as guinea-pig burner tubes...

          Justin
          "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
          "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
          "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

          Comment

          • #6
            Fully agreed! The older Sovtek 5881's, while they don't respond just like NOS types, are VERY tough. I can't comment on the current offerings because I'm not familiar with them. But I can't see why you would fix something that isn't broken so perhaps they're still the same or similar.?.

            As far as mitigating preamp gain... If you're using el84's and the preamp is clean all the way up to some typical 100V PP at the PI, then yes, your el84 power amp will be squashed long before any preamp clipping mojo can interface with the tone to the point where it's moot by the time it happens and you're amp is clipping at 2 on the volume control. You can get more "big bottle" type performance out of el84's by reducing drive voltage at the tail end of the preamp or even after the PI but before the power tube grids. Something about attenuating PI output bothers me aesthetically I usually run relatively low PI plate voltage on my el84 designs to limit swing and allow the preamp and/or PI to get into clipping only shortly after the power tubes. This is more similar to the type of performance people have come to expect from guitar amps with other tube types. But I do use quite a bit of pre PI attenuation in my personal amp, which is a proto for most of my other el84 designs that ever came later. In fact I'm working on a refinement of that design right now and getting the right balance of PI drive/clipping relative to enough drive at the power tube grids to get a little bias shift due to blocking (cap charging) and just really dialing the whole thing in... Well, I'm a little compulsive and insane as a result. But I'm taking my time and, obviously, I sort of like it.

            I'm not sure I answered your question.?. I decided to run down my thinking on the matter WRT to circuit construction instead. I hope that's better actually.
            "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

            • #7
              "Something about attenuating PI output bothers me aesthetically..."

              Is that code for "Master Volumes are the Audio Devil!"?

              Justin
              "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
              "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
              "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

              Comment

              • #8
                Chuck thank you! You answered a lot of my doubts. I love the sound of the crunch channel running thru the SE 84 and to hear you say the 84 is unique in its response and sound is comforting but yet still leaves me in a quandary. The punch and string dynamics from the 5w amp I had him play will be very hard to reproduce with a bigger PA. I am shaking my head as to what I should do to remedy this. The same preamp with a PP PA is very sterile and does not sound well. I guess this is where the big dogs lie and I could sure use some advice.

                Let me edit here. The SLO sounds awesome as always but to emulate a low wattage amp in the same chassis has left me wondering what I can do. The punch and string dynamics are so organic and alive with the SE 84 but I will bow down and say I don't know how to reproduce that with a bigger PA. Ughh!!

                Comment

                • #9
                  If you can show us a schematic I might be able to make some suggestions. But please don't default to "Well it's basically a SLO preamp with 'this' circuit at the end and a ..." That sort of circuit description leaves too much out for making circuit adjustments.
                  "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

                  • #10
                    Originally posted by Justin Thomas View Post
                    "Something about attenuating PI output bothers me aesthetically..."

                    Is that code for "Master Volumes are the Audio Devil!"?

                    Justin
                    Shamelessly, yes!

                    Something else I'll mention that is unique about el84's is how much driven abuse they'll take without melting down. IMHE you can drive el84's more at the grid relative to their bias. You can clip them more than other types and the screens somewhat self regulate, to a point. If you try to drive other typical tube types as hard the screens melt down. I never realized how much I abuse el84's before I tried building similar amps with different output tubes and the screens started melting at drive levels in proportion to the bias similar to what I do to el84's. It's been a S ton of trouble and fun learning my way around it and some really good sounding amps that DON'T use el84's have come from the effort
                    "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

                    • #11
                      Well you could always run the amp into a dummy load and then use a big SS power amp

                      Comment

                      • #12
                        Originally posted by Zozobra View Post
                        Well you could always run the amp into a dummy load and then use a big SS power amp
                        That was one of the first thought's I had It really does work a treat. Just the same el84 amp as the prototype into whatever wattage power amp you want for A LOT less money than building with bigger power tubes. The tone and performance will be just what is expected too. I often add an OT driven line out to amps I build or mod that are 20W or less. This gives the owner the option of plugging into a PA for additional watts. Somewhat similar idea.
                        "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

                        • #13
                          I don't know if you can still buy them, but the bad cat unleash did all that in one convenient box. It's just a resistive dummy load and a B&O ICEpower class D module from what I've read. It wouldn't be all that hard to make one. The Fryette powerstation is an option if you want to keep it tube.

                          Comment

                          • #14
                            I'd rather Hertzog it into a Twin Reverb... Or a DR40_!

                            Justin
                            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                            Comment

                            • #15
                              Originally posted by Chuck H View Post
                              The reason the el84 sounds like it does is because it is a very unique tube compared to pretty much any other used in guitar amps. It only takes (about) 8V to 13V input to start clipping an el84 tube where you'll need more like 25V to as much as 40V to clip a 6V6. ... . Cathode bias will do NOTHING to correct for this. In fact, it can make things worse in some regards. If you can increase preamp voltage output by adjusting an inter stage voltage divider or something then you might have the option of getting similar performance with other tube types. ...
                              Agree. Another option, which might be worth a try, is to remove the cathode bypass cap on cathode-biased EL84s. This introduces local negative feedback around the tube and means you need 2 or 3 times more input swing on the EL84 grids to reach clipping. It makes them sound more like 'big bottle' tubes, if that's what you want.

                              EDIT: Re-reading, I think OP is trying to do the opposite of this!

                              Comment

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