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  • #46
    Originally posted by Valvehead View Post
    So...this is a popular mod for tube amps......what does everyone use then for a switch ? Ive never done this mod . With 420vdc or whatever on the power tubes...???

    Yes, but not one that I've never liked much. Triode mode when pushed just doesn't sound so great IMHO. Then consider the voltages, you'll routinely have 1KV across the plates side of the switch, and on occasions, could have considerably more. So arc-over is a risk unless care is taken. It's a tough call to find a switch that can operate with high DC voltages applied as any arc doesn't self extinguish as it does with commutated AC. That's why the switch should only be operated with the amp off or in standby. Forget to do it right and the switch may get toasted. Also half power is hardly any quieter than full power.

    I suggest that a better way to get "half power" and to get a useful new dimension to the sound, would be to use cathode bias. Your switch has to remove the bias supply and add two resistors (guess 680 ohm 5W) in series with the cathodes. For KT66's separate cathode resistors are required. This means using a 3PDT switch but only low voltages are switched. Still best to be in standby to avoid loud pops but if you forget, no harm is done.
    Last edited by nickb; 06-06-2020, 07:53 AM.
    Experience is something you get, just after you really needed it.

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    • #47
      Well.......I removed the switch and powered the amp on and off many times last night with no problem . It sounds really nice too . I went back to it today and the 3rd power on it popped the fuse . I cant find anything wrong ...set bias to 32ma per tube...swapped all tubes ... im going to have to put this on the back burner for now

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      • #48
        At the moment of switching, the screen grids will be open circuit and the plate current will cut off. That may cause a back emf spike on the plates. The slightest discrepancy between the switching action of 2 poles will cause those spikes to be differential, rather than common mode, possibly resulting in arcing between the plate switch terminals.
        My band:- http://www.youtube.com/user/RedwingBand

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        • #49
          As a general rule such half-power switches should only be operated when de-energized, meaning amp powered-off or in standby.

          Edit: I missed nickb's post above.
          Last edited by Helmholtz; 06-05-2020, 11:34 PM.
          - Own Opinions Only -

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          • #50
            Originally posted by Valvehead View Post
            Well.......I removed the switch and powered the amp on and off many times last night with no problem . It sounds really nice too . I went back to it today and the 3rd power on it popped the fuse . I cant find anything wrong ...set bias to 32ma per tube...swapped all tubes ... im going to have to put this on the back burner for now
            Does this amp have an HT fuse? If so, which fuse is blowing? If it doesn't and this is the mains fuse then the fuse popping would indicate excessive current draw. That could be an intermittent bias supply.
            "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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            • #51
              Originally posted by Chuck H View Post
              Does this amp have an HT fuse? If so, which fuse is blowing? If it doesn't and this is the mains fuse then the fuse popping would indicate excessive current draw. That could be an intermittent bias supply.
              just a main fuse , no standby switch , pops right at power up randomly , bias and B+ doesnt rise up until 10 seconds or so ....so what would draw current??

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              • #52
                Originally posted by Valvehead View Post
                just a main fuse , no standby switch , pops right at power up randomly , bias and B+ doesnt rise up until 10 seconds or so ....so what would draw current??
                Is the fuse a 'Fast' or 'Slow' blow type?

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                • #53
                  Any signs of arcing at the GZ34 socket?
                  - Own Opinions Only -

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                  • #54
                    Originally posted by Dave H View Post
                    Is the fuse a 'Fast' or 'Slow' blow type?
                    it kills 3a , 4a , 5a slo-blows

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                    • #55
                      Originally posted by nickb View Post
                      ... For KT66's separate cathode resistors are required.
                      Off Topic a Bit, but is there a reason for this?

                      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 -

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                      • #56
                        Surely, but I'm not sure if it's for distortion, mismatch or something else but see pg 2 of the datasheet :-

                        KT66_GEC.pdf
                        Experience is something you get, just after you really needed it.

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                        • #57
                          My guess is that the guidance wasn't meant to be specific to KT66, rather GEC were just trying to point designers in the direction of general good practice. It perhaps acknowledges that although sharing cathode bias had become widespread, it was liable to degrade circuit performance and tube life.
                          But then they were always an expensive tube type, so perhaps manufacturers would be more tempted to sell, rather than scrap, outliers, than they would with cheaper tube types?
                          It makes sense to try and head early failure warranty claims off at the pass. Can't then use the argument that shared cathodes were standard practice and you didn't say not to do it.
                          My band:- http://www.youtube.com/user/RedwingBand

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                          • #58
                            You said it popped the fuse on the 3rd power on. Was it warm at this time? Does it pop instantly?
                            I'm just trying to pin down whether the 10 second bias and B+ ramp up is a factor or not. Has a slow-blow fuse ever blown from a complete cold start, and if so, how long after turn on did it blow?
                            "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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                            • #59
                              I don't recall, did we ever monitor mains current to see if the fuse is idling just under blow current or if it is at a healthy level, and just spikes now and then to blow?
                              Education is what you're left with after you have forgotten what you have learned.

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                              • #60
                                Originally posted by Enzo View Post
                                I don't recall, did we ever monitor mains current to see if the fuse is idling just under blow current or if it is at a healthy level, and just spikes now and then to blow?
                                ammeter should be here soon

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