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  1. mcentee2

    Lightbulb Limiter Wattage

    Any clues as to what wattage bulb to use here ? Am building a JTM50, 2xel34, 3x 12ax7, gz34 rectified (or SS), so idle bias current should be about 100mA, a bit less maybe. So, a 25W bulb would be effectively on all the time (24W=240V X 100mA). A 100W bulb would probably be dim/off all the...
  2. mcentee2

    Rectifier Toggle Switch Rating

    What sort of toggle switch do you all use for s SS/gz34 rectifier option like @robrob shows ? I've got loads of the usual DODT/SPDT miniature ones already in my drawer, they only show ratings for 240AC 2A or 125VAC 5A. As it will be in the B+ DC path, will these be ok for around 400V DC, at...
  3. mcentee2

    VTADIY Power numbers

    Can someone help me understand how the vtadiy Power figures are derived? I have set up a scenario, taken from Merlin's SE example for el34, and can't see how those power numbers are derived, especially the A/A2 one ?
  4. mcentee2

    Screen Dropper Vs Stopper ?

    Hopefully a simple one in my recent flurry of learning! I think I understand what role screen dropping resistors play in quiescent mode, dropping to a designed screen voltage as well as being part of the power supply RC filtering. ..and I understand what role screen grid stoppers play under...
  5. mcentee2

    Merlin's el34 SE example

    Just been re-reading Merlin's el34 SE example and this question struck me. He chooses a starting point vA of 300v, then works through it all, starting at the characteristics curves at that 300v and finishes up finding the cathode bias resistor that gives the required bias voltage of 17v...
  6. mcentee2

    SS rectifier with standby switch?

    Has anyone got any strong views about using a standby switch with solid state rectifiers ? I understand about tube rectifiers and standby switches :) Am interested on what good folks think about them with SS rectifiers from "cold" startup. Is it just the peak cold voltage/current to the first...
  7. mcentee2

    AA1164 - Presence Control

    I've recently built a Modulus Supa Deluxe, which has elements of both a 5e3 PI Power section and Super Reverb preamp, both with tweaks as designed and also now with my own tweaks added. I have played with an NFB loop and rather like what it does, and am now looking at a presence control for it...
  8. mcentee2

    OT Primaries

    Been doing a lot of reading on the JTM45 but can't get to the bottom of this one! Just why is it that 2x el34 like an OT primary of about 3.6k, and 2x kt66 like one of about 8k ? I think it's to do with things like transconductance values, anode resistance, max power transfer Vs distortion and...
  9. mcentee2

    What can I do with a 120ma PT

    Theoretical question here. Am trying to find out how to work out the "working", average or even max/peak current draw from a PP stage from the tube charts. If I have a PT rated at 120ma, what should I be looking for on the charts to tweak to get that useable ? Let's say 2x 6v6, 350v on the...
  10. mcentee2

    vtadiy load lines ?

    Been playing with my brain today trying to out into practice Output Transformer choices and Load Lines. I came across vtadiy and thought I would plug in some "standard" values. I chose el34, PP, 400v plate, 45mA bias and a 3600r OT. Doesn't look right to me.... that goes way over the...
  11. mcentee2

    B+ drop with zeners (not CT!)

    I've installed a SS/Tube rectifier SPST toggle in my amp (as per RobRob's idea), and am now getting a too high B+ for my 450v filter caps. I have a fair few 5w 10v zeners to hand, so 3 of those will be great.. I know about the zener on the PT CT approach, but I only want the zeners in when I...
  12. mcentee2

    Fixed Bias Grounding point

    I've added a simple fixed bias circuit to my essentially 5e3 6v6 power circuit. It's based on a single 22uF/100v cap, with parallel 22k resistor and 50k pot, coming off the PT via a 220k dropping resistor and diode. I haven't yet increased the cap, or built an expanded version like the jcm800...
  13. mcentee2

    JTM45 Screen Filter cap ?

    Having completed my Modulus Supa Deluxe, I have now got the bug - maybe funds won't allow it, but have always had a soft spot for a JTM45. Having looked at schematics, from posts etc over the years I understand a lot of that circuit, but never really looked at Chokes or Cap Cans in any detail...
  14. mcentee2

    Rob's site

    Is it just me, or is @robrob's web site down yesterday and today so far? Goes to show how much I rely on it to assess and make tweaks :) Have a fixed bias all set up to install and wanted to check some stuff :( Hope all is well with you Rob.
  15. mcentee2

    Bias Test Point thing

    I want to put a female type test point in my new build so I can put my DMM lead in and read the voltage across the power tubes' cathode 1ohm resistor. For the life of me I can't work out what those things are called so I can search for one on eBay or wherever? Test Point Jack or something?
  16. mcentee2

    Modulus Supa Deluxe Build

    Hi everyone, long time lurker and sometimes poster -usually of questions rather than answers, as I have yet to repeat a learning exercise enough to feel confident in proffering lessons learnt! Anyway - I've finally taken the plunge and will build a Modulus Amps Supa Deluxe - basically a 5e3...
  17. mcentee2

    Vibrato on a Vibro Champ Vs 6g2/3

    I love the 6v6 bias vibrato/Trem on the 6g2/3, but am really looking at a "quieter" home amp. I know the AA768 Vibro Champ is single ended, cathode bias 6v6, so the vibrato/Trem links into the second preamp triode cathode instead. I can't find many good videos to show if this has the same...
  18. mcentee2

    Marshall Input patching...

    I've just got a Marshall SV20 which is essentially a 20w 1959SLP with the 4 inputs, which is essentially a Bassman.....and 5e3 3etc !! I have been playing with the various High/Normal (hi/Lo) channel inputs individually and with patching them - I've learnt a lot from @robrob 's 5e3 input page...
  19. mcentee2

    5e3 Tweedle Dee - hot bias?

    Just looking at the Tweedle Dee mods and can see it uses a gz34 rectifier. I can’t see any changes to the 6v6 cathode resistor, or any other B+ reducing tweaks. Does this mean the Tweedle is usually running at a very large bias current / dissipation ? I have a gz34 to hand and wouldn’t mind...
  20. mcentee2

    5e3 to 5f2a ish

    Ok, been a while since I posred, and have now exhausted all my 5e3 experimentation. I borrowed a friend's 5f2a and was pretty much blown away, so am now thinking of tweaking my 5e3 to get closer to that single ended 6v6 sound. So, can I ask what gotchas there are in doing this? I know it's not...
  21. mcentee2

    5e3 Pots- manufacturer

    Hi Can anyone identify which manufacturer my 5e3 pots are from? I hope the picture shows, but the logo on the back looks like an "S" in a sort of infinity or hourglass shape.
  22. mcentee2

    5e3 - Solid State Rectifier

    Having modded and tweaked my 5e3 build to death, I've now thought about making it into a "very clean" pedal platform to see how far I can go. One of those tweaks is to swap out the 5y3 for a solid state rectifier,and of course this is as per @robrob site :) So, my build already has the...
  23. mcentee2

    5e3 Normal channel - bright cap ?

    Just linking to this thread in Amp Central Does anyone know if this would work ?
  24. mcentee2

    5e3: sweet pot mod pre PI MV

    I am about to remove my mod for pre PI master volume as a really I don't need the volume control and always have it at max. One or the other benefits of that mod is to place a variable grid stop resistance so as to reduce the blocking distortion from an overloaded PI grid. Now: I cannot detect...
  25. mcentee2

    OT Primary and Plate Load maths...

    For a standard 6v6 OT Primary (reflected of course) impedance: 1) What does a single 6v6 "like" to see ? 8k? 2) What does a simple push-pull 2 x 6v6 like to see *per tube*, is it the same as the above, and if so does that mean the total OT primary should be half that ? or twice that ? I see a...

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