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Bogen CH8 to 3-knob champ with 6L6 conversion filter capacitor questions

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  • #16
    Originally posted by bmccowan View Post
    Yes - there are single ended Princetons; but there were never single ended 6L6 Princetons as all single ended Princetons were 5 watt 6V6 amps.
    Tweed Princetons - single ended - same as champ circuit, but with an added tone control. Same wattage (5 watts) as Champ, 1-6V6, but depending on vintage, a larger speaker.
    After the tweed era - all Princetons were push pull 2-6V6.
    Brownface Princetons - push pull, 2-6V6.
    Blackface & Silverface Princetons - push pull, 2-6V6.
    All Princeton Reverbs - push pull, 2-6V6.
    Scratch built, kits, or converted PA amps that people sometimes call Princetons - Any of the above and more, including single ended 6L6 amps. Some people even change the power tube cathode resistor and use a 6L6 in SE Champs and Princetons, but the power transformer is working harder than it was designed. Dave Hunter's two-stroke started out as a Champ type circuit with parallel 6V6 and then moved to a single 6L6. Weber's Maggie is the same. Yes people like them, I have built them and like them.
    The single ended Champ type circuit was used by just about everybody back in the day: Fender, Gibson, Valco, Webster, etc. It was not an original Fender circuit, they are all derived from a Western Electric circuit.
    Princetons are Fenders, and they are all good amps as far as I am concerned. I have rebuilt several and scratch built a few.
    Chemtooler referred to what DLabs had done with their Bogen CH8- gutted it and rewired it with a Champ type circuit (same as a Princeton type circuit.) Terry hooked up the preamp sockets for the use of either a 9 pin 12A_7, or an octal 6SL7.
    What I did with my CH8 was to use a Matchless Clubman type preamp 6AT6 (7 pin triad) into a 6SJ7 (Octal Pentode) with the 6L6 power tube. Different - not better, not worse, just different.
    I thought the issue was a "weird" power supply problem? My point simply was to compare some similar schematics and modify the power supply more conventionally if the old PA setup is causing the issue.

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    • #17
      Originally posted by olddawg View Post
      I thought the issue was a "weird" power supply problem? My point simply was to compare some similar schematics and modify the power supply more conventionally if the old PA setup is causing the issue.
      And my point was simply to correct some of the Princeton info you provided, so that Chemtooler would not spend effort looking for what does not exist; that's all. I get stuff wrong sometimes and others correct it - no offense taken. I have that Bogen CH8 - the voltage is higher than desired for the circuit and Bogen scrubbed off voltage with a couple of big wattage resistors to ground. In fact according to their schematics, they did that for several of their smaller amps. Why? I can only guess that with the huge number of models Bogen produced, they tried to standardize as much as possible on power transformers. That left some models with transformers designed for larger amps. Following a fender power supply schematic, would not fix that, unless you also replaced the PT itself. In many cases, you are right; follow a well know power supply/filtering architecture and you will be ok - but not for this one.

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      • #18
        Originally posted by bmccowan View Post
        And my point was simply to correct some of the Princeton info you provided, so that Chemtooler would not spend effort looking for what does not exist; that's all. I get stuff wrong sometimes and others correct it - no offense taken. I have that Bogen CH8 - the voltage is higher than desired for the circuit and Bogen scrubbed off voltage with a couple of big wattage resistors to ground. In fact according to their schematics, they did that for several of their smaller amps. Why? I can only guess that with the huge number of models Bogen produced, they tried to standardize as much as possible on power transformers. That left some models with transformers designed for larger amps. Following a fender power supply schematic, would not fix that, unless you also replaced the PT itself. In many cases, you are right; follow a well know power supply/filtering architecture and you will be ok - but not for this one.
        No offense taken. Iím obviously not an expert on all of the Champs and Princetons. For some reason I thought there were single ended production Princeton Reverbs. Guess I was wrong, probably confused it with some custom builds as you said. That said, thereís always more than one way to Egypt. With a high B+ I would be tempted to use a 6L6 and a couple of big zeners. But as you say... The OT might be an issue.

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        • #19
          Originally posted by olddawg View Post
          No offense taken. I’m obviously not an expert on all of the Champs and Princetons. For some reason I thought there were single ended production Princeton Reverbs. Guess I was wrong, probably confused it with some custom builds as you said. That said, there’s always more than one way to Egypt. With a high B+ I would be tempted to use a 6L6 and a couple of big zeners. But as you say... The OT might be an issue.
          "More than one way to Egypt." I've never heard that before - that's funny. I've never tried the zener route, but know of it. The CH8 uses a 6L6. I have dropped the voltage more than I need to to protect the tube, but I like that "brown" sound you get from lower voltage. And hey, if you are interested in checking out all the various Fender amps, check out the Fender Amp Field Guide; The Fender Amp Field Guide There are a few mistakes but its pretty interesting and comprehensive.

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          • #20
            This amp ended up coming to me for help. It was running waaayyy hot and burning up 6L6s with a 470 ohm 5 watt cathode resistor. It would go over 100mA. I had to double the resistor to get it down to 17 watts. Someone check my methods, please? Plate voltage = 440. cathode voltage = 39.8v. Cathode resistance = 931R. So, 39.8v/931R = 42.7 mA. (440v - 39.8v) x .0427A = 17.1 watts, correct?

            If this is correct, does 931 ohms seem like a high value for a cathode resistor?
            Last edited by Randall; 02-05-2019, 06:37 PM.
            It's weird, because it WAS working fine.....

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            • #21
              High? Low? Who knows? Does it work, sound good? That is the issue.


              Does a different set of 6L6 act the same?
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                It sounds about right. It's a single ended 6L6 Champ build, so no sets.
                It's weird, because it WAS working fine.....

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                • #23
                  Oh, it is single ended? You know that will be running class A, right?
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #24
                    I've had to go from 470R to 1K on the cathode of a single ended fender before. No big deal when considering how much the line voltage has gone up from 117V, and also that I wouldn't subject a modern tube to the hotter conditions they used to get away with.

                    edit: but that was 6V6, and that was to get it down to max. dissipation.
                    17W would be considered cold for 6L6 single-ended I think.
                    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                    • #25
                      Here's a bias calculator. You should be able to go 30watts diss with a6l6gc.
                      https://robrobinette.com/Tube_Bias_Calculator.htm

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                      • #26
                        "17W would be considered cold for 6L6 single-ended I think."

                        I always get confused with the class A and dissipation. What would be a good target to shoot for with a 6L6GC?
                        It's weird, because it WAS working fine.....

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                        • #27
                          Less than 100% say.

                          If it sounds good now, why fight with it? results matter more than anything I can calculate on a napkin.
                          Education is what you're left with after you have forgotten what you have learned.

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                          • #28
                            What they said^^^^^^^^^^^^^^^^^^^^^

                            Also, the max dissipation @ voltage and for a given load is what you'd be aiming for WRT idle dissipation for class A I think. That is, just because the tube is rated at 30 watts max doesn't mean that a standing idle of 30W is ideal at 440Vp into the load provided. The idea here is center bias when conducting with a nod to a safe idle dissipation. I'm sorry I don't know the formulas, but it can be done pretty easily on a scope with a big rheostat as the cathode resistor. Chances are that at 17W dissipation idle the tube will be cutting off ahead of saturating. That means you're not getting maximum efficiency. But as Enzo pointed out, if it sounds good you're done. It's often the case in single ended guitar amps that a center bias during conduction can't be achieved without an idle current over max dissipation. I think this might be because of high plate voltages. High WRT class A operation anyway. And it's also not uncommon for single ended amps to be dissipating more at idle than when conducting signal. Which is just saying the same thing in a different way I guess.
                            "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

                            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                            • #29
                              Originally posted by Randall View Post
                              This amp ended up coming to me for help. It was running waaayyy hot and burning up 6L6s with a 470 ohm 5 watt cathode resistor. It would go over 100mA. I had to double the resistor to get it down to 17 watts. Someone check my methods, please? Plate voltage = 440. cathode voltage = 39.8v. Cathode resistance = 931R. So, 39.8v/931R = 42.7 mA. (400v - 39.8v) x .0427A = 17.1 watts, correct?
                              17.1W is correct but there's a typo. It should read (440v - 39.8v)

                              What does the power supply look like now compared with the original schematic in post #1?
                              The original had a 500R pi filter, 5u filter caps and a 47k screen node dropper. If the pi filter is no more, filter caps 20u and the screen dropper 1k (as the AA764) then it will be running hot.
                              Last edited by Dave H; 02-05-2019, 02:08 PM.

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                              • #30
                                Originally posted by Dave H View Post
                                What does the power supply look like now compared with the original schematic in post #1?
                                The original had a 500R pi filter, 5u filter caps and a 47k screen node dropper. If the pi filter is no more, filter caps 20u and the screen dropper 1k (as the AA764) then it will be running hot.
                                Hot for a 6L6 single ended?
                                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                                Comment

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