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12AX7 in 122?

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  • 12AX7 in 122?

    Looking for more overdrive..... is a 12AX7 compatible, and is it advisable from a damage point of view?

    (Yes, I know the speed switching valve must be a 12AU7)
    Current:
    1971 T-202 with Carsten Meyer mods: Remove key click filters, single-trigger percussion, UM 16' drawbar volume correction. Lower Manual bass foldback.
    Korg CX3 (original 1980's analogue model).
    1967 Leslie 122 with custom inbuilt preamp on back panel for 1/4" line-level inputs, bass & treble controls. Horn diffusers intact.
    2009 Marshall 2061x HW Plexi head into Marshall 4x12 cabinet.

    Former:
    1964 C3
    196x M-102
    197x X5
    197x Leslie 825

  • #2
    Since the 12AX7 has higher plate resistance than a 12AU7, you can do this swap and see how it sounds. The 12AX7 will draw less current in the same circuit.
    I'm David. 'Dave' is someone else's name.

    Comment


    • #3
      Originally posted by Papus View Post
      Looking for more overdrive..... is a 12AX7 compatible, and is it advisable from a damage point of view?
      (Yes, I know the speed switching valve must be a 12AU7)
      Hi Papus.
      In an attempt to get more gain and overdrive through the Leslie amplifier, I did try out a 12AX7 valve both in a stock Leslie 122 and in a stock Leslie 147, and in both occasions the 12AX7 did not work out well.

      In the Leslie 122, a 12AX7 valve does produce more gain levels and therefore more overdrive levels, however the overdrive effect has an unpleasant sounding "coughing" effect or a blocking distortion effect whereby the initial attack of the organ note is louder and then the level abruptly goes down and it sounds weak before raising back up in level. This is an unpleasant non musical sounding distortion effect.

      After experimenting with my own heavily modified Leslie amplifier, I found out the reason for this unpleasant "coughing " effect or blocking distortion is because the high 1 mega ohms input impedance at the input of the Leslie 122 amplifier causes the 12AX7 to produce the blocking distortion effect.

      I found out that lowering the input impedance to around 10 K ohms or thereabouts removes the blocking distortion effect and thus allows a Marshall - like level of smooth overdrive to be produced.

      However a stock Leslie 122 amplifier would not work out well with a low input impedance such as a 10K ohms input impedance because of the particular circuit which uses the speed switching signal superimposed on to the balanced audio input before going to the speed switching 12AU7 valve.

      My own Leslie amplifier was originally a stock 147 amplifier, but I have modified it to have a balanced input but I have kept the 147 style direct speed switching set up which does not require the speed switching signal to be superimposed on to the balanced audio signal. Because of this, I have wired up the balanced audio input to in effect be like a dual or a "stereo" version of the 147 amplifier set up whereby I have wired up a 10K ohms log dual ganged potentiometer as a regular stereo type volume control for the Leslie amplifier, and the two balanced audio signals coming from the middle wiper pins of the 10K dual ganged potentiometer go directly to the grid pins 2 and 7 of the 12AU7 or the 12AX7 valve without needing the input coupling capacitors which the stock 122 circuit uses so therefore the resulting 10K ohms input impedance allows a 12AX7 to be used in my Leslie and this produces a dirty Marshall like level of Jon Lord style overdrive levels without any unpleasant "coughing" effect or blocking distortion when the Leslie volume control is turned up to full volume.

      In the stock Leslie 147 circuit, the cathode bias 150 ohms cathode resistor and the 200 uf cathode capacitor are wired at the center tap of the secondary winding of the output transformer and the signal then goes to the cathode pins 8 of the 6550 valves which brings the speakers into the audio feedback circuit, and when I tried out the 12AX7 valve, the additional gain of the 12AX7 caused a microphonic feedback howling noise and the speakers acted as a microphone whereby tapping the speaker cone created an audible thump noise.

      If the 147 cathode bias set up is rewired so that the 150 ohms cathode resistor and the 200 uf cathode capacitor are removed from the center tap of the output transformer and then rewired directly to the cathode pins 8 of the 6550 valves, the same as in the Leslie 122 amplifier set up, then the speakers would no longer be in the cathode bias circuit so therefore the extra gain from the 12AX7 would no longer cause a microphonic howling from the speakers. As well as that, the removal of the speaker signal from the cathode bias circuit results in more amplifier gain and therefore more overdrive levels.

      As for the possibility of causing any damage with a 12AX7 valve, constantly overdriving the Leslie amplifier with a 12AX7 valve would most likely more quickly wear out the valves and possibly strain the output transformer as well as risk warping or burning out the voice coil of a Jensen V21 treble driver, but because your Leslie has a higher wattage rated Etone treble driver, I assume that it will be able to handle the extra gain and distortion without the Etone driver voice coil warping or burning out.

      All the best.
      Kon.

      Comment


      • #4
        Or one could try a 12AT7, which has higher mu than the AU7, but less than the AX7. There is also the 12AY7 with an even lower mu than the AT7.
        I'm David. 'Dave' is someone else's name.

        Comment


        • #5
          Thanks Kon and David.
          I finally resurrected my 122RV after two and half years (horn bearing plate was wobbling....)
          I'm going to reinstate a 12AU7 into the amp at the first opportunity!

          You may only view thumbnails in this gallery. This gallery has 1 photos.
          Current:
          1971 T-202 with Carsten Meyer mods: Remove key click filters, single-trigger percussion, UM 16' drawbar volume correction. Lower Manual bass foldback.
          Korg CX3 (original 1980's analogue model).
          1967 Leslie 122 with custom inbuilt preamp on back panel for 1/4" line-level inputs, bass & treble controls. Horn diffusers intact.
          2009 Marshall 2061x HW Plexi head into Marshall 4x12 cabinet.

          Former:
          1964 C3
          196x M-102
          197x X5
          197x Leslie 825

          Comment


          • #6
            Experimenting with different brands and types of 12AU7 will also get different responses out of your amp. The vintage 12AU7's especially. Or the mil-spec equivalent varieties like the 5814 and 6189.

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            Comment


            • Papus
              Papus commented
              Editing a comment
              Wasn't aware of those 12AU7 variants, thanks.
              The beauty of the old military gear is they often over-compensated for potential warfare scenarios by manufacturing ten times the required quantity of everything.... there are probably warehouses full of this stuff all over the place.

              "Stalin's Great Plan" is a good example. Didn't matter that there was a Soviet factory on the eastern side of the USSR which made one million pairs of military boots every year, and a different factory on the western side making 200 million pairs of army socks every year - as long as every citizen had a job to go to.
              That's how we've ended up with enormous quantities of Soviet-era mil-spec valves and other gear trickling through to the global market since the collapse of the USSR.

          • #7
            And from what I hear they still use tubes in their jets because tubes are not affected by jamming attempts.

            Yes a lot of the JAN tubes comes from the military and the stocks are still plentiful.

            Comment


            • #8
              Vacuum tubes keep working after a nuclear event if not in the direct blast field.IC's and such aren't in a vacuum so they are affected by such an event,EMF blast radius etc.
              Turns out those 'dumb' Russians were pretty smart.
              I recapped the BCV genny filters with PIO glass encased metal jacketed caps.....USSR 1989-1991 vintage.
              Turned out fantastic!
              A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

              Comment


              • KC9UDX
                KC9UDX commented
                Editing a comment
                It's not the vacuum that saves the tubes. They can handle enormous amounts of power for very short periods, just like solid state devices. But the amount of time, though miniscule, is much, much longer. It mostly has to do with the size of the gubbins. You can see all functional parts of a tube without a microscope. Not so for an integrated circuit.

              • David Anderson
                David Anderson commented
                Editing a comment
                It's also the case that vacuum tubes were always inherently high voltage devices and can tolerate a much higher momentary voltage spike that will destroy most solid-state components. Being able to survive 400-500V is a lot different from having a 60V applied voltage limit.

                I fixed a SS preamp for a guy who had lightning hit his house. The only electronic device completely undamaged was his Hammond Organ. The strike took out the chip that handled the source switching in the preamp.

            • #9
              Originally posted by Sweet Pete View Post
              Vacuum tubes keep working after a nuclear event if not in the direct blast field.IC's and such aren't in a vacuum so they are affected by such an event,EMF blast radius etc.
              Turns out those 'dumb' Russians were pretty smart.
              I recapped the BCV genny filters with PIO glass encased metal jacketed caps.....USSR 1989-1991 vintage.
              Turned out fantastic!
              Wow didn't know about those glass caps.

              Yes them Russians ain't dumb.

              Comment


              • #10
                Smart enough to defect that Mig in Japan on purpose......
                Not quite as smart as operation Fortitude was,they took cues from us,who can blame them!
                Planned by a Brit,carried out by the Allies.
                Biggest deception of 1944.
                On July 17 it will be the 75th anniversary of Canuck Spitfire pilot Charley Fox ending Rommels command.....
                last I heard they were all friendlies these days.....
                A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

                Comment


                • #11
                  I heard the whole WW2 thing with the Teutonics was based on economics. They, Churchill, wanted to block Germany from becoming a fast major business empire as they were real good at what they did and "threatened" the rest of the area.

                  Comment


                  • #12
                    I like the 6189 in a 147.Never tried the other ,maybe someday.

                    Goff,hearing that type of talk is quite common in the USA my friend.
                    Seldom hear about anything else from there,money is king.
                    A big vacuum for sure.....insulating them from the reality of the outside world.

                    Canadian 3rd Division Inf/2nd Division Arm'd. closed the Falaise gap,allowing the larger(and less experienced) US Army to outflank 50,000 combatants.
                    Largest surrender by the enemy to that point. Canada made it happen! Speaking fluent German over captured radios was the norm for Canada.
                    "Park a few right here Fritz"......as the Canadians aimed captured 88's the other way. No way the Allies would have accomplished that without Canada.
                    Execution of captured Canadian POW's by Waffen SS Col. Meyer was a big mistake.It was all the incentive Canada needed to finish the job.With a lot of timely help!
                    Eventually in September Meyer was captured by US forces,tried by Canada,sentenced to hang, commuted to life.....released in 1954.That sucks big time.
                    I heard there are lots of hate groups still active.I am glad to live in a relatively peaceful integrated environment,and perform in an integrated band.
                    The power of love will always win out though the misinformed continue with their heads planted firmly in the sand.

                    British Lt General Fred Morgan planned most of D-Day. Brilliant tactician. Had the Germans concentrate on defending the Pas De Calais......
                    Built a deep sea port somewhere else in a single day in Normandy,logistics genius.Gotcha!
                    Canadian 3rd Div were 11 kilometers inland,farther from the beach at the end of June 6/44 than any other Allies..
                    Which was not easy. Canada shrugged off it's losses,resolved it's duty and commitment,leading by example the whole way,to the astonishment of all but ourselves.
                    Outnumbered 150,000 to 80,000 or so......Allied ground forces got the jump on bad German intel. It was all about fake news then too!

                    USAAF/RAF/RCAF etc made sure the atrocities of 1940-41(The Blitz) were payed back in full....by the time 6/6/44 happened the Germans were in full panic mode.
                    The Allied airforce dropped ten times the ordinance they had received,demoralizing the Nazi's from the west,while the Soviets exacted even crueler revenge from the east.
                    Germany was finally schooled.Learned the lesson they failed to grasp in 1918.We are human and helped them recover though.They have been better behaved since.

                    Too bad a lot of those Nazi's were 'set free'.The west needed scientists.....and were willing to look the other way to maintain an advantage over the USSR.
                    The top seats of power knew all about it.The general public? Are you kidding me?. A nice cozy arrangement.
                    Played a gig in Uranium City Saskatchewan in 1971.That's where the fuel for the first A-Bombs was mined.
                    A plaque commemorated this at the mine there......yep,Canada was in on that too!
                    USA had uranium,not as rich a grade in fuel however.Would the new weapon have worked as well without that rich Sask. fuel? Not likely.
                    Flew in and out on the Eldorado Mines DC3(WW2 vintage) from Edmonton Municipal,a 'tail wheel' gave it that WW2 feel!
                    For all I know it was the same DC3 that delivered the fuel to the Manhattan Project,there are still DC3's flying in the arctic over 75 years later!

                    Hymns were played on my BCV at that time no doubt(1939-1945).....many parents no doubt sought consolation from their grief and loss in it's presence.
                    A truly magnificent little electric organ,Hammond should be very proud of their contribution to human sanity and emotional well being.

                    Love the 122 in a studio,will still use a 147 at a rock gig though! Prefer the extra headroom.
                    Some 80+ year old things still work just fine,even some of these tube Leslies are 50+ now.

                    I hope there is more music,and less war.We need another important discovery like insulin to be at our best!
                    Lets invent cures,instead of weapons.
                    Would be nice to say we avoided a major conflict,and helped all of humanity in doing so.


                    A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

                    Comment


                    • #13
                      I can honestly say that I've never heard of anyone on the USA claiming WWII was fought for money.

                      Comment


                      • Sweet Pete
                        Sweet Pete commented
                        Editing a comment
                        Me neither.Still,North America was better off than Europe after that squabble.
                        As certain as Wisconson has great ice cream!

                    • #14
                      Winston Churchill told Lord Robert Boothby:
                      Germany's most unforgivable crime before the Second World War was her attempt to extricate her economic power from the world's trading system and to create her own exchange mechanism which would deny world finance its opportunity to profit.


                      On topic:
                      GE 5 Stars 5814A's with triple mica discs to make sure the tube elements do not vibrate.
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                      • #15
                        Canada never had a five star general(or vacuum tube).However they had General Currie......who didn't need them and lead his troops to victory.
                        As a baby boomer born in 1953 I most certainly benefited economically being born in North America.
                        My friend Hans Staymer had to endure post war Hamburg.....not a very nice place at that time.He did get to hear The Beatles a bit though......
                        his school buddy did the Revolver LP cover.......he knew 'Astrid' too.

                        Shielded from vibration,those mil spec AU7's are the holy grail of their kind Goff? Prone to microphonics, 12AU7A 'OEM' harder to find than ever!
                        Always looking....will need a spare or two! Tucked away some 12BH7's,6GW8 too......this gear is like me,OLD.

                        The first things to touch the moon on Apollo 11 were the landing gear/legs? Built and designed in Quebec......meaning, 'who was there first' LOL.
                        India tried again today,aborted the launch.They'll get there too eventually.
                        A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

                        Comment

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