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  1. Pete Farrington

    Still not sure about some JVM mods

    Ok, I wasn’t clear whether you meant the HT power supply, or the amp’s max audio power output, or something else. The lower ripple, potentially stiffer HT provided by a choke is only really going to affect the screen grid’s HT supply node, and is only really going to make much difference at...
  2. Pete Farrington

    Still not sure about some JVM mods

    That could have several meanings, could you rephrase it?
  3. Pete Farrington

    Help me identify mods made on JMP '81

    As the front panel and back panel are both from a 2203 then, it’s reasonable to say that’s what the amp was originally.
  4. Pete Farrington

    Position stand-by switch on Hiwatt / Sound City style amps

    It may be worth considering a protection diode for V3, the DCCF, see http://www.valvewizard.co.uk/dccf.html Or use a GZ34 rectifier, so that the HT has a nice slow ramp up, that doesn’t happen until after the other valves have warmed up some :)
  5. Pete Farrington

    Help me identify mods made on JMP '81

    The volume controls are labelled ‘master’ and ‘preamp’, the inputs (left hand side pair of the array of 4 front panel holes by the preamp volume) are labelled ‘high’ and ‘low’ ‘sensitivity’. Those things would be different on a model #1959 super lead.
  6. Pete Farrington

    Position stand-by switch on Hiwatt / Sound City style amps

    Thanks, it’s just that I inferred from post #6 that you had access to IEC standards, and I was interested in checking the one mentioned, but not so interested that was prepared to pay hundreds of ££ for the privilege :rolleyes:
  7. Pete Farrington

    Help me identify mods made on JMP '81

    Just to note that from the front panel, I suspect it may have been a 2203 originally.
  8. Pete Farrington

    Strange sine wave

    Using the low input? How about if it's turned up to the onset of power amp clipping? Turn gain up, treble, presence and bass to min, mid up full (for flattest freq response).
  9. Pete Farrington

    Help me identify mods made on JMP '81

    Someone’s gone to town implementing the ‘WTF’ mod :) It seems there’s an fx loop, and perhaps a second channel with its own gain and master vol? And relay switching for them, with an additional power supply. I’d think there was a footswitch jack somewhere? Otherwise the relay switching would be...
  10. Pete Farrington

    Strange sine wave

    What does the wave look like at the master volume? With presence at min, what does the max sine wave output look like, at the onset of power amp clipping? Consider injecting the sine input at the power amp input (eg as if there was an fx loop), to avoid the preamp confusing things.
  11. Pete Farrington

    NAD 1986 Bass 50w from '71 - a few questions

    A HT can cap is falling out and and circuit board looks to have a lot of carbon deposits around the bias supply, probably from a valve shorting and taking out its screen grid resistor. I suggest to take it to a competent tech for appraisal. Just to note it’s a 50W 1986 bass model, not a 100W...
  12. Pete Farrington

    Position stand-by switch on Hiwatt / Sound City style amps

    Thanks! Do you have a copy of / access to, IEC60335-1? I’d very much appreciate a scan of it :)
  13. Pete Farrington

    Jcm 2000 Tsl 100 Volume Drop Issue

    As per post #2
  14. Pete Farrington

    Position stand-by switch on Hiwatt / Sound City style amps

    DC is more demanding on a switch than AC, as arcs with the latter tend to self extinguish. Whichever, the switches used for standby are being subjected to voltage far beyond their rating. Dunno how that works with UL, CE etc approval?
  15. Pete Farrington

    Marshall JMP 2204 v1 voltages

    Yes, that’s possible. Maybe try a higher value anode load resistor if you want a couple of dB more gain, eg 470k. Due to the big 1nF bright cap, that’s to be expected. The tone should get a bit fatter as the gain is turned above 5 though.
  16. Pete Farrington

    Better AC Wiring Layout?

    Oh, and some sort of anti vibration fastener for the safety earthing screw, eg an additional locknut / nylock etc.
  17. Pete Farrington

    Bringing the 1974x closer to the 1974

    Happy days indeed when these could be bought. Such a shame that Ingo isn’t making stuff anymore. Perhaps he should have charged twice as much, that may have kept the workload manageable, and it would still have been good value.
  18. Pete Farrington

    Better AC Wiring Layout?

    It looks nice, good job :thumb: Is there a star / toothed washer between the green earth crimped lug and the chassis? Unless it was crimped with an industrial grade tool, I suggest to crimp AND solder that earthing lug to the wire.
  19. Pete Farrington

    Marshall JMP 2204 v1 voltages

    Without knowing which specific model it applies to, that chart is of limited utility. Note that the other 50W chart there has rather different V1 voltages. I don’t think that ‘ampchart’ is an official Marshall thing, rather it seems most likely to have been cobbled together from various...
  20. Pete Farrington

    Better AC Wiring Layout?

    It seems as if that would wire the fuse between live and neutral? Neutral should be wired directly to one primary leg, with a layer or 2 of heatshrink over the joint. Incoming live to the back of the fuseholder, out of the side of the fuseholder to the switch, out of the switch to the other...

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