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Swap out different Preamp Tube Type ?

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  • #16
    I see that the amp has 2 independent global feedback loops, the second one leading to the vol pot at the input also providing RIAA equalization.
    You may try to ground the junction between C3 and the 2200 Ohm resistor to deactivate this loop.
    Also lifting C2 would increase treble and probably gain.

    Edit: If the amp is going to be used with guitar only I would remove the filter components at the input, change the vol pot to 1M log and add a 47pF to 100pF bright cap.
    Last edited by Helmholtz; 10-07-2019, 12:16 PM.
    - Own Opinions Only -

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    • #17
      Originally posted by Helmholtz View Post
      I see that the amp has 2 independent global feedback loops, the second one leading to the vol pot at the input also providing RIAA equalization.
      You may try to ground the junction between C3 and the 2200 Ohm resistor to deactivate this loop.
      Also lifting C2 would increase treble and probably gain.

      Edit: If the amp is going to be used with guitar only I would remove the filter components at the input, change the vol pot to 1M log and add a 47pF to 100pF bright cap.
      Thanks Helmholtz !

      I missed the second Global feedback loop. I will try the changes you recommended and let you know how it goes. I think I am going to leave the input filtration portion intact for now (Still trying your changes to C2 and C3), but I might change everything later for sure, once I understand the amp better. I will probably have a switch between the original input filters and the guitar style one you just suggested. Just need to buy some parts along with a switch first. Still have lot's of things to look at.
      Last edited by HaroldBrooks; 10-08-2019, 02:16 AM.
      " Things change, not always for the better. " - Leo_Gnardo

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      • #18
        Originally posted by Chuck H View Post
        My only caveat for the adapter would be to test the pinout to see if it isolates one triode or if it puts them in parallel for the dual triode substitute. Because the description doesn't really say. My guess is that it isolates one triode.
        Chuck, got the adapter today (took a long time), and promptly put adapter in place of the 6J5 V1 preamp, along with one of my Long Black plate RCA 12AX7. I've used that type of vintage tube in V1 in almost every amp I own, with the exception of one or two that use JJs.

        Ready ? The 12AX7 in this Zenith Hi-Fi amp doesn't sound nearly as good (to me) as the 6J5 does ! I realize it has little to do with the tube, and probably just the way the whole circuit accepts the 12AX7 at present, but there it is !

        Let me explain my qualitative assessment, just so you can see what I personally was looking for :

        The Original 6J5 has more lower midrange and bass, and is clearer given the same level of gain and that sounds better IMHO. I think it may have something to do with the fact that it may makes better use of the amp as it is, as in the 150vdc plate voltage, the 33k plate resistor, the global negative feedback, etc...

        The 12AX7 has considerably more signal gain than the 6J5. This in turn must hit the second 12AX7 gain stage in the PI tube harder than the 6J5 did and that's another element. The 12AX7 in the V1 position in this amp sound noticeably thinner, and a lot less lively and flatter then the 6J5, so much so that I would bet I could spot the difference blindfolded, easily.

        I trust my ear generally when making mods for tone and response, as I know what serves me best, and in this case the 6J5 will definitely stay put in this converted Hi-Fi to guitar amp ! I would have never guessed that in a million years, but the old octal tube sounds better in this amp !

        Thanks for your help in showing me where to buy the adapter. I would have spent a fair amount of time rewiring the socket for anther tube, but now I know the original 6J5 is the good one ! I have the rig "muscle" up front to drive the amp into insane levels of gain even with the 6J5, and while super high gain sounds good in bedroom playing (when you are pretending !) I usually back off a fair amount during actual gigs.

        Once again, thanks for your expert help, and feel free to comment on this latest test !

        Click image for larger version

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        Last edited by HaroldBrooks; 10-20-2019, 04:48 AM.
        " Things change, not always for the better. " - Leo_Gnardo

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        • #19
          That result doesn't surprise me. Consider that ANY tube may not sound good when operated in a circuit that's not idealized for it. But you could still plug a 12au7 or even a 12ay7 into that adapter and very likely get good performance. Which is something I would try because if it works out then you have an easy option whenever the 6j5 gives up.
          "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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          • #20
            I have a 12au7 and will do just that !

            Thanks for the idea !
            " Things change, not always for the better. " - Leo_Gnardo

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            • #21
              Find a 12ay7 if you can too. One of my favorite tubes! They are much happier at lower plate loads than 12ax7's and sound very (what I call) "tubey" The antithesis of "thin". And still twice the gain of the 6j5 or 12au7. It could be a very good thing.
              "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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              • #22
                Put in the 12au7 and it's ok, but so far the 6J5 is my favorite for the tone and response in this amp.

                I will definitely try a 12ay7 and see how it goes, I like the fact that there are so many tubes as subs for a 12ax7 !

                I've already tried different 12ax7 tubes in the second gain stage/PI tube, and a modern Mesa branded 12ax7 works best, again, despite what I have hoped would be an RCA or some other vintage variant. But I will keep rolling as long as it makes sense.

                Bumping the screen voltage higher worked out great (reduced DC node resistor size), but last night playing up a storm, I saw the screen on one tube glowing just a hair, so I upped the screen resistor on that one tube only from 470ohm to 1.5kohm. At first I tried increasing both, but I noticed a slight difference in compression (too much) by using bigger resistors on both, and when I switched back to the 470 ohm for the one "good" tube, it got better again. That was a subtle change, but I believe a real one. In any event the one glowing screen is gone, but the sound is still awesome with the high screen voltage.

                Question : Should one attempt to keep screen voltages as stable as possible ? It's my understanding that maximum power output is achieved with no sag, but I also realize it's not just about power output ! but tone and the interaction between all the components before, and down stream.

                Thanks for the help !
                Last edited by HaroldBrooks; 10-23-2019, 12:05 AM.
                " Things change, not always for the better. " - Leo_Gnardo

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