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  1. Pete Farrington

    Jubilee: 50w hum... 100w no hum

    I think it’s common for Marshalls to implement what they describe as a half power mode by switching the output valves to triode operation, eg https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm900_dualrev_50w_4500.pdf Have you checked that for typical valve guitar amps, triode...
  2. Pete Farrington

    Marshall JMP 2204 v1 voltages

    It seems unusual for there to be a higher voltage at the output valve anodes than at the OT CT? I suppose the mains voltage may have decreased in between the readings being taken.
  3. Pete Farrington

    Jubilee: 50w hum... 100w no hum

    Given the symptom, the 50/100W switch may be faulty such that one of the outputs valves isn't getting its screen grid connected when in 50W triode mode (but is connected when in 100W pentode mode). without screen grid voltage, there's no anode current, so the valve would effectively not be...
  4. Pete Farrington

    Jubilee: 50w hum... 100w no hum

    It may just be that the amp’s been unpowered for too long, and the ecaps have degraded. HT hum in the power amp should get almost completely cancelled out in either mode, though the triode 50W mode can be expected to have more ghosting intermodulation at high power output. If there’s a...
  5. Pete Farrington

    Marshall JMP 2204 v1 voltages

    So that’s the first, non cascaded version? Has it had the bias supply mod? If you don’t know, would you mind checking the VDC on pin5 of either output valve, whilst in standby? Umodddd will be below -1V, modded will be roughly -35V.
  6. Pete Farrington

    No speaker load and the output transformer

    The value of a safety resistor on the secondary winding was brought home to me when I was tinkering on the test bench with my JTM45 build, which has an RS DeLuxe OT. Even with a safety resistor on the speaker jack, I noticed that with the amp operational but idling, if the output load impedance...
  7. Pete Farrington

    No speaker load and the output transformer

    Thanks, I’d not noticed that. It’s tricky to know how effective they are, but the rationale behind them seems good to me. As per the AC50 (470ohm 3W across the 16ohm winding), I use about a safety resistor across the OT secondary with a value about 30x the intended load impedance, rating about...
  8. Pete Farrington

    No speaker load and the output transformer

    None that I’m aware of, though 2W may be a bit underrated on a cranked JTM45 16ohm output, and definitely would be on most 50 watters. Bear in mind that almost all black and silver panel Fenders already have it, by virtue of their negative feedback network (eg 820+100ohms). Though as they’re on...
  9. Pete Farrington

    First amp build (Ceriatone Plexi All Access 51)

    Some suitable wood blocks and shims could have been used to brace between the insides of the front and back panels, so providing some support, so as to mitigate that issue.
  10. Pete Farrington

    Need to replace the valves from my JVM 410HJS?

    I agree that it’s nowhere near a decent IR. To be fair, bear in mind that the amp design predates the widespread roll out of that technology. But it seems inaccurate to describe the emulation as simple RC filters, as IC3a drives a resonant filter gyrator circuit. I’d take claims of the...
  11. Pete Farrington

    Vox AC15 Reverb Issues

    There should be a massive degree of gain when doing that. If not, signal is being lost somewhere. The above ‘thump’ test should indicate whether it’s going astray on the drive or recovery circuit areas. Model id, schematic link?
  12. Pete Farrington

    Seller misusing the term "original" regarding sale of mid 60's JTM45. Beware !

    Crikey, that seems a piss take. Would it even have passed QC at the Harrison operation?
  13. Pete Farrington

    Questions about tube biasing

    Just to note that per se, idling the anode/s at the type’s dissipation limiting value does not necessarily equate to class A. Of course, if single ended, it’s going to be class A no matter where it idles. If interested, Aiken delves into the topic in several of his pages...
  14. Pete Farrington

    Can you DIY tube burn in ?

    Yes, no matter how well the vendor makes sure they're good, shipping can screw them up. Output and rectifier valves are best tested in an overdriven amp, preferably into an inductive load, soon after receipt. I don't think it's a good idea to just fit them into an amp and trust things to work...
  15. Pete Farrington

    Deep (resonance) mod DSL

    I think that would add a lot more low mids, so that the effect of the deep switch would be much greater. Is that really the goal? To reduce the low bass boost effect of the deep switch, I suggest to reduce the value of R93, from 220k to 100k, or 47k...
  16. Pete Farrington

    50W head - 1976 I think?

    Dunno, I recall seeing the hara-kiri standby in late 70s JMP 50 watters, the schematics seem to support that https://el34world.com/charts/Schematics/files/Marshall/Marshall_50w_master_vol_2204.pdf https://www.drtube.com/schematics/marshall/78331-2.gif
  17. Pete Farrington

    50W head - 1976 I think?

    The stock standby arrangement on 70s 50 watters is a joke, comedically awful, as it applies HT to hot valves before the bias supply has charged up. Hopefully most have been modded to a more sensible arrangement by now, but valve killer unmodded examples still turn up here. The bias supply feed...
  18. Pete Farrington

    Questions about tube biasing

    The currents you've measured shows that the 2 EL34s are conducting similarly. But even if they weren't, their anode voltages at idle would still be very close to each other. Because both anodes are tied to the HT supply node via the low resistance of the OT primary winding, eg about 50ohms each...
  19. Pete Farrington

    Need to replace the valves from my JVM 410HJS?

    I’m not sure about that, many valve testers are BS, what actually matters is how well any individual valve works in its intended application. All he need do is to buy a known good ECC83 from a reputable vendor, burnt in and checked, using a high spec tester. Then swap that into each valve socket...
  20. Pete Farrington

    Studio Vintage SV20 mods (and SC20 too)

    I suppose that with the amp set clean, it should be possible to get a broadly similar frequency response that way. But overdriven, that will change, as the different RC values in the grid circuits of the 2nd and output stages of the circuits will tend to affect their overdrive characteristics.


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