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Rectifier/Power Tube questions for 12W Class AB amp

Discussion in 'Amp Central Station' started by tah1962, Jan 21, 2021.

  1. tah1962

    tah1962 Friend of Leo's

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    About a year ago, I picked up a used Morgan PR12 amp head off of Reverb.com. I have been very happy with the amp but it’s ready for new tubes. It came with a Ruby 5AR4-C rectifier tube, and a pair of JJ 6V6S power tubes.

    The amp runs a pretty high plate voltage. The last time I biased the amp, plate voltage was 460 and I biased the tubes at 21.5 mA. The JJ’s are a very bold and punchy tube and I am considering alternative tubes. In my research, my options seem limited due to the plate voltage. The amp is a bit stiff and I’m looking to tame it a little if possible.

    So my first question is, what experience/recommendations do you have with alternative 6v6 tubes to the JJ’s that could handle the plate voltage?

    Question 2 is, what alternative rectifier tube would work to bring down the plate voltage so I could try other 6V6 power tubes. What successful tube combination has worked well for you?

    Thanks in advance for your support.
     
    Last edited: Jan 22, 2021
  2. 2L man

    2L man Tele-Meister

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    When rectifier tube is used without a Choke there should be current limiter resistors on anodes. They form first RC filter stage with first capacitor although there are diodes in between. They "flatten" sine wave peaks and then current flows longer time and peak current does not come too high and rectigier tube can last the time it is specified. Look datasheet to verify this. There usually are examples for RC and LC filtering. If your amp does not have a Choke after rectifier possibly you could install it if thee is room on chassis? Choke lowers operative voltage.

    JJ is a 6V6 which is said to handle most Voltage but I think that high would need at least 10 000 ohm output transformer impedance.
     
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  3. 2L man

    2L man Tele-Meister

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    If your amplifier uses 6V6 typical 8000 ohm impedance and has 460V plates using fixed 21mA bias its loadline looks lime this which makes it very powerful and hot. It won't produce 40W because 6C6 can't deliver 200mA but when loadline runs that much above 12W power curve tubes overload although they work push pull.

    If amp power supply 6,3V filament could deliver 0.9A more current you could install 6L6 tubes.

    PHOTO-2021-01-22-10-50-15.jpg
     
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  4. kbold

    kbold Tele-Afflicted

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    Replacing the 5AR4 rectifier tube with a 5Y3 will drop voltage by around 40 volts.
     
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  5. tah1962

    tah1962 Friend of Leo's

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    Thanks for your response and information. I appreciate it.
     
  6. tah1962

    tah1962 Friend of Leo's

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    Thanks. After doing more research I believe this tube would create more “sag” then a 5V4, or 5R4, is that correct?
     
  7. Dacious

    Dacious Poster Extraordinaire

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    5V4 will also drop a dozen-20 volts.
     
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  8. 2L man

    2L man Tele-Meister

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    The tables where voltage drops are listed are when tubes rated max current is used and I believe they are also equipped with current limiter resistors and a capacitor what datasheets define. Still they are nowhere exact but rectifier tube which is rated for more current usually drops voltage less when same current is used than rectifier which is rated for lower current.

    However datasheet often show U / I graphs for few rectified AC voltages. Current limiter resistors have minimum value and they could or perhaps they should be bigger when modern transformers are used which are machine wound over plastic bobbin which make windings tight which make voltage drop less when current increase. I use tube rectifier only when I want sag effect and I also use bigger current limiters and sag comes stronger but also because I use toroidal power transformers which voltages seem to stay stable better than EI core transformers.
     
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  9. kbold

    kbold Tele-Afflicted

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    Have no knowledge of this. Maybe wiser minds ........ You may soon find out through experience.
    I thought the main component related to sag was the caps.
    More 'sag' may improve this.
     
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  10. tah1962

    tah1962 Friend of Leo's

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    So assuming I stick with the 5AR4 rectifier tube, and considering the JJ’s 14 watt rating, it appears that the EH 6V6GT, or TAD-6V6GT-STR would be my best options. Any other recommendations, or comments on the above tubes?

    I would really like to to try the Tung-Sol 6V6GT tubes but based on the spec sheet it doesn’t appear that they will handle high plate voltages.
     
  11. Commodore 64

    Commodore 64 Friend of Leo's

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    I use a pair of (new prod) Tung Sol 6V6 at ~350V plate voltage and about 10.5W plate dissipation in a Lite 2 (6V6) circuit with 5V4 rectifier. They've been good for 3 years so far, with one of those years seeing VERY regular gig play.

    I've redplated JJ6V6 when biased at 95% in cathode bias at 375V.

    EH 6V6GT seem to hold up to same conditions as the JJs, and I tend to use them interchangeably.

    NOS tubes are ridiculously expensive, and not worth it IMHO. You really can't guarantees the "New" part of OS, and really inventories have been picked through by now, it stands to reason, in most cases. That said, I have a buttload of used 6V6, old stock. They are easy to source in pulls/old tube tech boxes etc. I just roll them, and if they sound good I use them until they don't (I've yet to get to the "don't" part with any pairs I've selected by ear).

    IMHO, I wont use a 5AR4 in any amp. If I want that type of response, I use diodes. I use 5Y3, 5V4 and 5U4 primarily.
     
    Last edited: Jan 22, 2021
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  12. lathoto

    lathoto Tele-Meister

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    I would try a JJ 5Y3 and 6V6 tubes. What does the tube chart say?
     
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  13. Chicago Matt

    Chicago Matt Friend of Leo's

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    This ^^^
    For the 6V6s, I've gotten great sound and longevity with new production EH 6V6GT tubes, they're the ones I usually get. The modern Tung Sol tubes sound great too, but they don't last long in my 5E3. The JJs always sound stiffer and a little harsh in comparison to me, though they are very robust.
     
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  14. tah1962

    tah1962 Friend of Leo's

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    Thanks.
     
  15. tah1962

    tah1962 Friend of Leo's

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    I agree with your comments on the JJ’s being a little stiff. Thanks for your response.
     
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