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  1. Good point - I used an uncommited 5V winding,...

    Good point - I used an uncommited 5V winding, that is, ONLY my little supply was on it.

    Cheers,
    Ian
  2. Just for info: If the 5V winding is not used...

    Just for info:

    If the 5V winding is not used or has spare capacity:
    Bridge rectify the 5V winding using Schottky Diodes - 4 off something like 1N5821 (30V @ 3 Amps) into a 10,000uF electrolytic....
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    Just a bit of a caution - I have never fully got...

    Just a bit of a caution - I have never fully got my head around the 2nd cap bank , the one after the standby switch.

    Stacked caps will share voltage according to their capacitance. The same charge...
  4. Yes - minimise stray inductance is the goal AND...

    Yes - minimise stray inductance is the goal
    AND
    Yes - metal oxide resistors would be better.
    Cheers,
    Ian
  5. The Screen Grid (g2) exhibits gain when a signal...

    The Screen Grid (g2) exhibits gain when a signal is applied to it with respect to the cathode - just like the Control Grid (g1).
    The 1st function of the screen resistor is therefore as a grid stop...
  6. Not applicable according to your post just above ...

    Not applicable according to your post just above

    BUT for future reference:

    Hum or noise being max at mid setting on a volume pot or guitar volume control is a big clue. Actually with a log pot...
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    The Application Report has details of B+ of 200V...

    The Application Report has details of B+ of 200V operation - that will do.

    Cheers,
    Ian
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    Also remember that the screen supply is far more...

    Also remember that the screen supply is far more critical than the anode supply so make sure it has additional filtering.
    Cheers,
    Ian
  9. Replies
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    Slightly Off topic or an extension of it: The...

    Slightly Off topic or an extension of it:

    The GROUNDED Faraday Cage thing is why when using isolated input jacks with the cold (cable shield) side tied to 0V at the input tube it is a good idea to...
  10. ECC99 is a good tube but is really "just" a bit...

    ECC99 is a good tube but is really "just" a bit more linear 12AU7 with 3.5W vs 2.75W plate dissipation.

    So for circuits it is just an option (a good one) to use in place of a 12AU7 or 6CG7, it...
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    The extra 50 Ohms in the "strong" tube cathode...

    The extra 50 Ohms in the "strong" tube cathode will degenerate (reduce) its gm to be closer to the "weak" tube and hence give better signal balance in the push pull - PROVIDING that the extra 50 Ohms...
  12. Probably way more than you ever wanted to know...

    Probably way more than you ever wanted to know about driving MOSFETS. An application note explaining why you could/should use gate driver ICs and TI's Gate Drivers in particular....
  13. Grid stop Resistors do 2 things: 1) essential...

    Grid stop Resistors do 2 things:
    1) essential prevention of parasitic oscillations. The required minimum resistance is proportional to the inverse of the tube gm. EL84, EL34, KT88 etc higher gm...
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    jmaf, A problem - the curves you are using are...

    jmaf,
    A problem - the curves you are using are for a screen grid voltage of +250V.

    The data Dave H linked is more suitable in this case.

    The "rule of thumb" for the 3rd harmonic cancellation...
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    Get out your toothbrush and isopropyl alchohol...

    Get out your toothbrush and isopropyl alchohol and clean all those board solder joins.
    Why? - because then you will be able to tell if any off them are dry joints or fractured joints which may be...
  16. HV current is a bit light on for 4 x EL84/6BQ5,...

    HV current is a bit light on for 4 x EL84/6BQ5, also light on for 2 x EL34
    The 2 x 6L6WXT seems a better fit
    OR
    4 x 6V6GT into a 4K3 to 5K Raa output Tranny would also work.

    Weirder:
    2 x KT66...
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    897

    OR Be a "smarty trousers" and run 330 Ohms...

    OR
    Be a "smarty trousers" and run
    330 Ohms with 220uF/63V bypass on each tube but instead of returning these to 0V then
    For each push pull pair add a common 68 Ohms unbypassed under the 330...
  18. Replies
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    There is a Weber kit 5G15 (expensive) based...

    There is a Weber kit 5G15 (expensive) based entirely upon the Fender 6G15 Standalone Reverb
    https://www.tedweber.com/media/kits/5g15_schem.jpg

    There is a thread here on current mode drive of...
  19. gm for a 12AT7 is a bit over twice that of a...

    gm for a 12AT7 is a bit over twice that of a 12AX7 so you can get away with 4K7 for gridstops.
    Cheers,
    Ian
  20. A quick note on grid stop resistors. The design...

    A quick note on grid stop resistors.
    The design "rule of thumb" from the golden age of tubes was to use 8/gm for the gridstop value for immunity to parasitic oscillation.
    For a 12AX7 that means 5K...
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    Guitar Amp example - the HiWatt 400 Wiring...

    Guitar Amp example - the HiWatt 400

    Wiring colours:
    Green - grids
    Yellow - anodes
    Blue - Cathodes

    Cheers,
    Ian
  22. Replies
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    Hooray Justin, About time someone made this...

    Hooray Justin,
    About time someone made this point specifically.

    The AT needs to run at x2 the current of a AX and has significantly more grid current

    The AU has a bad rep with the hifi guys...
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    Randall, P23 obviously feeds the autobias...

    Randall,
    P23 obviously feeds the autobias circuitry but not shown on your posted schematic.


    From the Super_Sonic schematics (assuming they are the same) then the cathode voltages (from the...
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    Fan will help, I've done it often - you typically...

    Fan will help, I've done it often - you typically don't need to blow massive amounts of air - just put a air pressure bias in to assist convection.

    Rectify the 6.3V heater to get around 7VDC and...
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    Thanks nickb, I thought I was close. I used...

    Thanks nickb,

    I thought I was close.

    I used this as a guide.
    http://www.valvewizard.co.uk/acltp.html

    As Helmholtz says, the circuit has to be treated as a differential amplifier where V3a...
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    You are right of-course - not a concertina but a...

    You are right of-course - not a concertina but a LTP.
    https://schematicheaven.net/boogieamps/boogie_subwayblues.pdf
    The cathodes of V3 will sit at around, or just less than +2V giving a total of...
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    The Subway Blues has another thing happening tone...

    The Subway Blues has another thing happening tone wise, check out the deliberate imbalance in the concertina PI. 68K and 120K for the anode and cathode loads.
    I found the sound actually improved...
  28. Yes - I checked the linked datasheet above. The...

    Yes - I checked the linked datasheet above. The Class B data was certainly fixed bias but Class B infers very low idle current which in turn infer very low negative/reverse grid current and low bias...
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