1. It seems that you’ve gone back and forth on the PI that you like to use with these 6f4p tubes. Is there a “best” PI in your opinion? Also – do you have a favorite 6f4p circuit from among your own designs? I attempted to Frankenstein together a few different pieces from a selection of your amps (damocles tone stack, TMC- power amp maybe?… not sure which pre-amp…) and I got sound out of it but the circuit obviously wasn’t right.

    • I probably do go back and forth but I’m leaning toward the cathodyne PI. I’m not sure what my favorite is but I’m pretty happy with the Meta 4. I just finished it and its a fairly simple amp but has some nice tone and works well with the guitar volume knob and picking dynamics.
      The two gain stages are plenty to drive these tubes really hard. The Meta 4 has an unused triode that could be used as a cathode follower to drive a tone stack between the second gain stage and the PI.

      • Thank you sir. How difficult to work with are these 6f4p tubes anyways? I copied your Meta4 PI and Power amp almost exactly, and fed it with just a single conventional Fender style pre-amp stage and a volume control. The results were not good. I think I probably have a layout problem but I figured its best to ask – are these tubes exceptionally hard to make them work? My very first amp – an 18 watt – worked just fine even though I substituted 6L6 and 6v6 tubes.

        • They are a bit tricky to work with. It’s easy to drive them with too much gain or too much bass.
          If you want to do something based on the Meta4 I would recommend building it according to the schematic and then tweak from there.
          My guess is that if you are using a Fender style 100k plate resistor with 1.5k/22uF on the cathode you are getting too much bass, especially if you are using a bigger coupling cap.
          I’d be happy to look at a schematic if you want.

          • I appreciate the offer. I have layout problems confirmed. I feel that I’ll be happiest if I work through that on my own. (It hums crazy loud) but I could hear the amp better after moving transformers and wires around, and I think I heard the distortion that you mentioned too. I do have the large coupling caps. Thank you for that advice it will help me.

          • SUCCESS!!!

            I built the meta4 to your schematic. I still didn’t get the feedback loop hooked up properly. It was reversed at first, so I had to cut the wire. I couldn’t wait to try it. The circuit sounds really pretty to my ears at about 9 o’clock on the volume. I’m playing through a 1×12 with a V30. There was some (faint) odd distortion at higher volumes which I hope is just due to the disconnected feedback loop. I’m looking forward to hearing it again after I get that sorted.

            I am grateful to you for experimenting and documenting your work with these cool Russian tubes.

          • Great!

            When you get the feedback loop sorted it should help the distortion and also help the low end. The overall volume will go down too.

  2. I have had the Meta4 working 100% and it was sounding great. None of my other amps make my guitar pickups sound so different from one another. I remembered your comment about the unused triode and I remember that you made a Cathode Follower suggestion, so I tried that. I must have done something wrong. I can hear it and it sounds good but there’s hardly any volume at all, even with the volume cranked. I marked over a copy of your schematic with my changes. I also marked a few other component changes for myself – reflecting the parts I had on hand and the ones that I couldn’t find. Please pay them no mind.

    Afterwards I re-read your suggestion and saw that you said it should go after the second gain stage. That’s not where I put it. I don’t know if I missed a component or if I misunderstand the cathode follower itself, but I’ll try to figure out what I did wrong.

    • “DC Coupled Cathode Follower” doesn’t mean I should put DC on the volume pot. Don’t know what I was thinking. Working again, but seems somewhat quieter than the original circuit. I think I will check voltages later this week.

  3. The amp is working great. I am happy with the change that I already made (adding the cathode follower) but I can tell that the circuit has lost just a tiny bit of – uh…. something… Well, I don’t know what it lost exactly but I wanted more distortion and I’m willing to accept some trade-offs to get it.

    I am wondering if I can get some interesting tone-shaping results out of a Bridge-T circuit by putting a potentiometer in series with the .002uF capacitor. I will have another triode available to me soon and I’d like to use it to juice the pre-amp just a little bit more. The plan would be: New V1a -> Bridge-T -> Old V1a.

    Would there be any problem with using a 1 Meg pot with the wiper connected to the ground side? At the max “mids” setting I imagine that it would act more like a treble-peaker while at minimum setting it should basically be the standard Bridge-T mid-cut circuit. I don’t know if it’s been done before but I’m looking forward to trying it. If it works, that could be a cool mid control.

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