Question about SE parallel power amp and OT impedance

Discussion in 'Shock Brother's DIY Amps' started by mountainhick, Sep 20, 2021.

  1. mountainhick

    mountainhick Tele-Meister

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    I like the idea of single ended single or double parallel output tubes selected by the flick of a switch. I have my eye on the "littlerock: power amp section here: http://home.polstra.com/amps/littlerock/

    Littlerock-6ak6-output.gif


    The pertinent info bold in blue:

    "The output stage is a cathode-biased parallel single-ended stage using two 6AK6 power pentodes. The 6AK6 is a 7-pin miniature version of the 6G6 octal tube, with essentially identical specifications. Its plate dissipation rating is 2.75W, and in a typical class A stage (the 6AK6) it can deliver 1.1W into a 10K load with 10% distortion. By running two tubes in parallel, I was able to use a fine Heyboer 5K output transformer that I bought from the nice folks at Doberman Music Products. The power tubes run with 180V on the plates and screens, and 15mA plate current -- values I took directly from the data sheet.

    I used separate cathode resistors and bypass capacitors for the two power tubes, for two reasons. First, with separate cathode resistors the tubes don't have to be matched as well. Each tube biases itself without affecting the bias of the other tube. When a shared cathode resistor is used, any mismatch between the tubes is magnified. If one tube is "hotter" than the other, then its extra current raises the cathode voltage and causes the other tube to run even colder than it would run on its own. Using separate cathode resistors, I found I could use any two random 6AK6 tubes without any problems.

    Second, the separate cathode resistors allow me to remove V4 and run the amp on just a single power tube when I want to. This has some interesting effects. It reduces the loudness of the amp, but not as much as you might expect. It also changes the harmonic structure of the distortion, giving much more 2nd harmonic content and much less 3rd harmonic. There is a graph in the RCA data sheet for the 6AK6 showing this.

    I like the single-tube option enough that I plan to make it switchable. That's easy to add. Just insert a 47K resistor in series with R16, with a switch that shorts out the new resistor when it is turned on. C10 should bypass both resistors. When the switch is closed, the circuit is identical to the schematic as drawn. When the switch is open, V4 is biased into cutoff and is effectively removed from the circuit."

    [​IMG]


    So, it appears that he runs the single tube also on 5K OT windings. Without knowing why, I assume that this influences the second harmonic content he refers to. But it also seems an impedance mismatch from the 6AK6 data sheet.

    So what are the drawbacks running through less impedance? AM I correct that this draws more current from the tube?

    And does this create tangible risk for tube failure?


    I am considering using a line transformer as OT which has multiple inputs. I could fashion a switchable impedance at the OT, 8K for a single tube, 4K for two, but then I expect that to negate the second harmonic aspect. Is this correct?
     
    Last edited: Sep 20, 2021
  2. andrewRneumann

    andrewRneumann Tele-Holic

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    If taken to the extreme, there is a greater risk of tube failure. I hesitate to quantify the amount, as I've never tried what you are proposing. My understanding is that it's fine to exceed the maximum current limits of the tube momentarily, as long as the average over time remains under the rated maximum for the tube. That being said, the closer to red plating you run, the less life you get out of the tube. They're expendable.

    When the second tube is cut-off, that should decrease the load on the power supply and increase the voltage available for the first tube. The bias point will change--but it's cathode bias, so probably not a big deal. I would set bias under single tube operation and accept a slightly lower bias for parallel operation.

    Yeah, switching to a different tap on the OT with an equivalent load on the power tube will revert back to the original sound--in theory. I think you are thinking about this the right way. If you really like the harmonic content of the single tube sound (4k load), you could look at using the lower plate load for both situations. Do you have a nice single-ended OT that can deal with elevated levels of DC current without saturating?
     
  3. mountainhick

    mountainhick Tele-Meister

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    The intended OT is a 70v line transformer capable of 10 Watts. Idle plate voltage should be 180v with 15ma current. I got some info from a friend that these transformers will run fine on a pair of 6V6's or el84s, I think that means adequate current rating to run at double the current of the 6AK6. Please correct me if this is flawed logic.

    I know that @printer2 has used these types of line transformers, but I doubt for less than half the intended impedance for the tube.

    I do also have a very oversized OT for the task, but I think it is 8K.
     
  4. bebopbrain

    bebopbrain Tele-Meister

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    Needing a mismatch to get the sound? I am skeptical.

    > Am I correct that this draws more current from the tube?
    > And does this create tangible risk for tube failure?

    Yes and yes. Might work fine at such low power. But who knows.

    > switchable impedance at the OT, 8K for a single tube, 4K for two

    Switchable on the primary? That would certainly work; go for it. Alternatively, attenuating a few watts is cheap and easy and doesn't create much heat.
     
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  5. mountainhick

    mountainhick Tele-Meister

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    Yes, switchable on the primary.

    Thanks for confirming my suspicions with the same logic but heaps more experience! Helps me know I am on track.
     
  6. 2L man

    2L man Tele-Holic

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    Single tube loadline comes steeper if OT impedance is not doubled and same grid drive make output flatten, when one tube can not pull current thru same load the two tubes can.

    You can see it in this loadline calculator when you select any tube and change "recttive load" value.

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/
     
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  7. andrewRneumann

    andrewRneumann Tele-Holic

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    Hey if you have a good word that this OT will work with the current of two tubes in parallel, then it’s all good. Make sure the friend’s info was from a single ended amp. Push-pull requirements are different.
     
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  8. mountainhick

    mountainhick Tele-Meister

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    Thanks, that does show almost scary looking current increase. I'll pass on the single tube at 4K design!
     
  9. printer2

    printer2 Poster Extraordinaire

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    Guess we are on the same page with the Loadline Calculator. 180V @ 15mA almost looks like it was made for 8k. A little distortion but between friends, not bad. I used the line transformers in P-P, I gapped one for SE but did not like the sound, ruined the transformer for P-P which was a shame. But I used a 6AQ5 which pulled a lot more current. It 'might' be ok in SE with a 6AK6. One way to find out.

    Playing around I see that 4k and a screen voltage of 180V and a plate of 140V may work. Whether the screens like it for long is a question.
     
    Last edited: Sep 20, 2021
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  10. mountainhick

    mountainhick Tele-Meister

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    Thanks for the info about your result with the air gapped line transformer. I think I'll bail out to a $15 generic SE 5K and run both tubes. Gotta keep this particular project cheap!
     
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