14 Comments

  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.
      http://chasingtone.com/dvnator/meta-4/
      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.
    http://imgur.com/a/1bBy8

    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.

  4. 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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