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5E4A 6k6 vs 8k OT?

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  • #1

    5E4A 6k6 vs 8k OT?

    Hi there

    Goin' to build one, and wondering if anyone had opinions/views on the load resistance for the 6V6s.

    (I was thinking to use a spare 340-0-340 Pt I had, which would give a B+ about 415 - BFPR/DR territory. My thoughts were that the BFDR used 6k6 Pr Z, therefore it'd probably be appropriate to go there as the plate voltages were going to be highish anyhow)

    Cheers
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
    Tags: None
  • #2
    Hi TW
    I've had a look at the 5e4a and the 5f4 schematics. They're identical except of the output tubes (and bias resistor) and a different tube in second position.
    Depends on what you like. Do you already have an OT? Using an 8k OT will match the 6V6's. A 4.2K to 6.6K will match the 6L6's. How much headroom do you want?
    I've tried the 5F4 circuit with two 6L6's and it sounds georgeous.

    Comment

    • #3
      Tubes,

      If you feed the power tubes through a choke and use a 5u4 rectifier as in the schematic I think your Vp will be around 400 at idle. Not sure about the screens. Don't these amps have a rep for having the screens the same or even higher than the plates? Bruce explained once how it's Ok and gives the amps a specific quality to the tone but I can't remember the specifics now.

      I think most of the tweed and brown amps used 8k for 6V6's. Also IIRC as the plate volts go up it's better to lean toward the higher primary (not that 400Vp is considered high for 6V6's anymore). If you already have a 6.6k I'd use it. But if you have to buy one I'd go with 8k.

      JMHO

      Chuck
      "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

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

      Comment

      • #4
        The DR's 6.6k gets my vote. 8k is more appropriate for 6V6's when the screen voltage is lower than the plates, like in the tweed Deluxe.

        Mercury Mag sells a 5E4A OT, so they probably got it right.

        - Scott

        Comment

        • #5
          Originally posted by ThermionicScott View Post
          8k is more appropriate for 6V6's when the screen voltage is lower than the plates
          And by this I take that you mean 6.6k is better if the screen voltage is equal to or higher than the plate voltage?.. Why?

          Chuck
          "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

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

          Comment

          • #6
            Originally posted by Chuck H View Post
            And by this I take that you mean 6.6k is better if the screen voltage is equal to or higher than the plate voltage?.. Why?

            Chuck
            Given two different idle plate voltages for the same screen voltage, the higher plate voltage will need a shallower load line (higher load) to intercept the knee of the curves.

            And if you push up plate and screen voltages together, the ideal load resistance actually goes down, rather than up, because the knee is gradually moving to the right.

            But I'm mostly basing my recommendation on the fact that 6.6k is known to work well in the Deluxe Reverb with the plates and screens both in the low-400's for volts.

            - Scott

            Comment

            • #7
              Well I can't say that I see the same thing in the sheets I found on line. But I can see that at the typical test voltages (250Vp) that 6600 is best for almost everything. And I would consider a 6600 for myself for that reason. But I also know that when charted at higher Vp (315Vp) that the designated plate resistance goes up to 8000. This seems universal for all the data sheets. And when the screens are raised to the plate voltage (screens equal to plate as in our example) and high (315 plates, 315 screens) which I only found on one data sheet, they indicated the test plate load resistance at 10,000! So whether you or I can read from the sheets and see why more volts equals more load resistance, AND more screen volts equals even more load resistance, it seems to be the method of choice by the tube MFG's...

              If Steve Connor sees this we may get out answer (note my use of his name so this post will come up if he searches)

              Cheers

              Chuck
              "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

              "If you're not interested in opinions and the experience of others, why even start a thread?
              You can't just expect consent." Helmholtz

              Comment

              • #8
                Originally posted by Chuck H View Post
                Well I can't say that I see the same thing in the sheets I found on line. But I can see that at the typical test voltages (250Vp) that 6600 is best for almost everything. And I would consider a 6600 for myself for that reason. But I also know that when charted at higher Vp (315Vp) that the designated plate resistance goes up to 8000. This seems universal for all the data sheets. And when the screens are raised to the plate voltage (screens equal to plate as in our example) and high (315 plates, 315 screens) which I only found on one data sheet, they indicated the test plate load resistance at 10,000! So whether you or I can read from the sheets and see why more volts equals more load resistance, AND more screen volts equals even more load resistance, it seems to be the method of choice by the tube MFG's...

                If Steve Connor sees this we may get out answer (note my use of his name so this post will come up if he searches)

                Cheers

                Chuck
                The book examples are nice if you just want to replicate those conditions (juke boxes, radios, etc), but if you're striking out on your own, they're nigh worthless. How many of us are building tube amps that only put 250V on the plates?

                I'm still kinda new here, but do you guys play around with load lines very much? They're great for taking out the guesswork, and having a pretty good idea what to do if you're improvising or pushing things in a new direction.

                I'll post up a pic of why the ideal load is so dependent on the relationship between the plate and screen voltage (not just plate voltage) in a little bit.

                Take care.

                - Scott

                Comment

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