Output Transformer Tap Switch?

Discussion in 'Shock Brother's DIY Amps' started by SerpentRuss, Oct 17, 2021.

  1. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    I'm bad about successfully searching this forum so I'm just going to ask this.

    I'm building / finalizing the plans for a high gain, singled ended EL84 amp. Cathode Biased.

    2 x 6N2P
    1 X EL84

    (Gain Stage - Gain Stage) - (Gain stage - follower) - James Tone Stack - Power

    ANTEK 240 volt Tube Power Transformer

    And this Output Transformer purchased from eBay (cross your fingers).

    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=154621949187

    B+ Will probably land around 300 volts.

    OK, my OT has two taps, 5K and 7K. I was thinking of biasing the EL84 at a modest 20 mA to bring the tube into a zone that gives more distortion earlier and to manage volume.

    Regardless of the bias point I pick, however, it looks like both the 5K and 7K tap work well and the 7K tap has less distortion.

    So, does anyone see any issues to place a switch in to use one or the other tap? It might give me another way to tweak the sound on this amp. I would place the switch in the back and only switch it with the amp powered down.
     
  2. 2L man

    2L man Tele-Holic

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    20mA is very low bias current for EL84 on A-class operation and operation comes far from balanced but for sure sound distort a lot :)

    When transformer output current is much lower than its rating low the Voltage comes higher than 300VDC.

    Many Antek datasheet list output voltages for few currents and then you can draw a voltage/current curve and there can get more accurate voltage estimates.

    Search Universal Loadline Calculator tool and you can test different impedance, voltage and current very fast. For cathode bias amp input Anode voltage which is g1 voltage less than PS HV is. Notice what effect Screen voltage has to g1 voltage. When you input "headroom" there comes also distortion values and power what that headroom produce.

    For A-class Single Ended I set the headroom so that 2nd and 3rd distortions come about the same and then I can compare different combinations at that "point"
     
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  3. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    I have the PT and bridge rectifier hooked up right now and I'm working on the filter caps. I have 339 volts "DC" un-filtered. You're right that there is going to be very little load on the PT, but I'm using resistors and a filter after the reservoir to bring the voltage down. I have put that array into LT Spice with my filter choices and simulated loads and it comes out to 304 volts. Of course, that is simulating a perfect AC source that can't get dragged down at all. I'm pretty confident that the B+ will be 300 and the B+1 Screen will fall in the neighborhood of 272 and the b+2 (6N2Ps) will be very close to 265.

    Regardless of what I bias the amp at, is there any harm in switching between the OT taps with a switch if the operating point works for both loads?

    5k_EL84.PNG 7k_EL84.PNG
     
    Last edited: Oct 17, 2021
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes you can switch them. I would position the switch so it would not be easy to switch while the amp was *on*. I think you will find you like it one way more than the other and it will be switched only on a rare occasion.

    EDIT: Remember there will be several hundred volts on the switch. Be careful.
     
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  5. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    I really didn't think about the voltage. I'll get a good switch and place it on the back. Perhaps since my bias point works for both impedances, I should just test them both and hard wire the one I like best. I can also test multiple bias points, I just don't want the complexity of adjustable cathode bias.
     
  6. andrewRneumann

    andrewRneumann Tele-Afflicted

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    There will be a great temptation to do an AB test with the switch while playing music. You don’t want to momentarily have an open circuit. I would advise some sort of make-before-break switch.
     
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  7. andrewRneumann

    andrewRneumann Tele-Afflicted

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    You might find that doubling the speaker impedance and going with the 5K tap for an actual load of 10K gives you more power (check it on VTADIY) and maybe even a better distortion tone. If you do this, consider lowering the screen voltage to hit the 0V grid curve just below the knee. Just another option for you.
     
  8. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    I can't seem to make that combination work without putting the screen voltage in the dirt and lowering the plate voltage as well. I may be doing something wrong.
     
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  9. andrewRneumann

    andrewRneumann Tele-Afflicted

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    No, you are probably doing it right. I wasn't looking at the curves when I suggested that as a possibility. Have you got a plan for protecting the screen from overdissipation?
     
  10. Telenut62

    Telenut62 Poster Extraordinaire

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    I always had 8K tap on my single EL84 amps.
     
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  11. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    Most data sheets for the EL84 list something in the range of 4 to 5k for Class A. Just about anything will "work" and it's interesting how changing the slope of that load line changes the sound. I'm still wrestling with how to set this up. It's a head, so I will probably leave it out of the box and play around with different set ups and speakers and then I will probably pick a tap and leave it. The switch just seems like a disaster waiting to happen.
     
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  12. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    I will probably put a 2.2K resistor on the socket, I'll have to see what I have handy.
     
  13. NTC

    NTC Tele-Holic

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    IF you put a switch in, it has to be rated for greater than the peak voltage. You should probably have at least a 500V rating, 1000V to be safe. Most of the standard switches we buy are rated for abour 250 VAC and less for DC (125 I think?).
     
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  14. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    I really didn't think this through completely. You're right, finding a switch that will break this amount of DC under load, which we've already determined should never be done, would be impossible without it being the most expensive component in the build. And just because I know it should never be switched while the amp was on, doesn't mean some future owner will. I'm not going to live forever, a fact my doctor reminds me of regularly.

    Just about any switch rated for 600 VAC would have contacts that would handle the voltage if it was never moved under power. High Voltage DC arcs quite a bit on make and break and usually takes metal with it.

    Maybe I should just put a big exposed knife switch on top of the head and put caution tape around it ;) A real Frankenstein mod.

    KnifeSwitch.jpg



    I don't think I'll be putting a switch in. I will test it extensively both ways and then make a choice.
     
    Last edited: Oct 18, 2021
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  15. Telenut62

    Telenut62 Poster Extraordinaire

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    I just like the sound with it in that set up......basically a 5F1 or GA5 circuit with an EL84
     
  16. rdjones

    rdjones Tele-Meister

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    Put the knife switch inside the chassis.
    This discourages accidental operation but allows quick solder free changes, at least until the "research" is done.
     
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  17. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    I said that tongue in cheek, but I really appreciate that you thought I was serious.
     
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