Marshall 60s to early 70s 50 watt - Which circuit variations get breakup at lower volume?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 9, 2020.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I'm still debugging my Plexi build, or at least trying to get the sound I want out of it (see https://www.tdpri.com/threads/how-t...-down-weird-overtones-in-plexi-build.1025094/ and https://www.tdpri.com/threads/not-sure-if-my-plexi-build-has-problems.984480/ for more detail).

    People on the forums have mentioned a lot previously that some variations on the plexi/metal panel circuit from around 67 to 73 get breakup at lower volume, and some stay clean like a Fender Twin all the way up the dial.

    My plexi build is basically a clean machine for some reason, which I don't really want. It has a very clear clean tone all the way up until 8 on the dial. Then at 8 the distortion kicks in, and the distortion sounds buzzy as hell, not nice (I can also get the buzzyness to kick in at 5 or 6 if I have a fuzz face on). But I built the guitar to try to get Marshally crunch out of it, not be clean.

    Yesterday when I was playing it at somewhere between 8 and 10 on the volume and was also using a fuzz face, I actually blew the HT fuse.

    I'm wondering if part of my problem is that whatever variation on the amp I built is too clean. Because it's so clean, I've had to run it super high-volume to get distortion out of it, which makes the distortion sound bad, and also can blow fuses. I've tried changing the main filtering and screen filtering caps from 32+32+32+16 to 50+50+50+50, and that hasn't really helped. I've tried changing from shared cathode to split cathode, and maybe that's helped a little, but not enough. Next I plan on trying to change the two 8k2 dropping resistors in the power supply to 10ks, and we'll see if that helps.

    So what do others think? What are the main circuit variations which cause a plexi/metal panel Marshall to have more or less distortion at lower volumes?

    Note: I'm talking only about variations that were actually used by Marshall in the original amps in the late 60s to early 70s, not mods like PPIMV, or cascaded channels, etc
     
  2. dunner84

    dunner84 Tele-Meister

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    I don't know the Marshall circuits very well, but I have built a few 5e3 using identical components except for power transformers. I noticed that increasing the B+ by even 40 volts had a drastic effect on clean headroom in that particular circuit, and more robust transformers can amplify the effect.
     
  3. Guitarteach

    Guitarteach Poster Extraordinaire

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    Turn it into 2204/JMP 50w MV :)
     
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  4. scelestus

    scelestus Tele-Holic

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    There was a transition period from the 45s to the 1987s where there were some small circuit changes and a shift from tube to SS rectifiers. I think if you can handle the sag, the tube-rectified ones will break up sooner than the later SS-rectified ones. You'll lose some of the "bark" and immediacy, though.

    Maybe start reading up on Black Flag JTMs. I'm no tech but I dig that era!
     
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  5. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Back in the day my '69 50 watter was one thing: LOUD
     
  6. itsGiusto

    itsGiusto Tele-Meister

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    Which way had less clean headroom and more crunch? Higher or lower B+? I assume lower, but please correct me if wrong

    I actually originally built this amp to be a hybrid with a switchable tube rectifier, each with its own separate bias pot. And I used the Classictone power transformer that had output taps for each type of rectifier. I honestly didn't hear much difference in sound just by switching from diode to tube rectifier. However, one difference it made was that if I really cranked the volume on the tube rectifier, it'd almost always blow the HT fuse.

    Well, I'm trying to deal with it by using an attenuator. I use attenuators with all my amps to bring them down to where they are getting break up at listening level, and this is the only one which had given me trouble, where the break up doesn't sound good.
     
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  7. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I dont remember what model it was, I dont remember a MV though....
     
  8. itsGiusto

    itsGiusto Tele-Meister

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    Here's a related question: is the amount of breakup at lower volume inversely proposal to the B+ voltage? As in, if I use a lower output power transformer, or use zeners or resistors to drop the B+, will that result in more distortion earlier, at lower volume?

    If so, is the the main reason certain Marshall amps of the time had more gain and less clean headroom than others because they used certain power transformers that were lower in their B+ voltage?
     
  9. printer2

    printer2 Poster Extraordinaire

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    You built a non-MV amp. Amp design has the last stage overloading first, then the PI, then the preciding stage, then the input stage. If you have an early stage clip before you get enough signal to drive the output stage to full output, well you will never drive the output stage to full output. So it will be loud. How are your output tubes biased? Too cold and you might be getting crossover distortion. Other than that, if you want more breakup out of the amp at full tilt but do not want the volume go to a lower voltage. The output tubes will be driven down to, say 50V. If you have a 450V supply that says you have 400V for the signal to live in before clipping. If you have 400V, then you only have 350V to play with, it will distort sooner.

    Also the lower the voltage the 'browner' the sound. Warmer if you will. Seems you need a lower wattage of amp. Running at lower voltage and idling at a lower level will also make the tubes last longer. Might even be the last set of tubes you will ever buy (your age dependent). Or for kicks, skip the fixed bias and go cathode biased.
     
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  10. dan40

    dan40 Tele-Afflicted

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    Higher B+ will cause the amp to break up a bit later. In the late 70's, Marshall dropped the B+ from mid 400's down to upper 300's because of the poor quality of EL34's being produced. These lower plate voltage amps did break up a bit earlier but I think Marshalls sound best at slightly higher plate voltages.

    In 1972, Marshall made a few changes to the 50 watt circuit which caused these models to be a bit more aggressive than the earlier versions. One thing they did was to raise the nfb resistor from 47k up to 100k. The other change was to add a .68uf bypass cap on the v2a cathode resistor. These two changes made the amp more aggressive and they broke up a bit sooner. They were still running the higher plate voltage in these years but those couple of changes made a big difference in the amp. Running a 5000pf bright cap on the bright channel's volume will also help the amp to overdrive a bit easier. Earlier models ran a 500pf value which passed a lot of highs but the 5000pf cap allowed more of the midrange frequencies around the volume pot. It does make the volume pot react very quickly though. The volume goes from a whisper to a roar with just a small change.

    Even with these small circuit changes, these amps must be run very loud to achieve any overdrive. I built several versions of the circuit before I realized they were just too much for home use.
     
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  11. itsGiusto

    itsGiusto Tele-Meister

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    Thanks! I have them running at between 68 and 70% dissipation. Is that too cold? I usually err on the side of being lower than 70 as opposed to above, due to my fear of red-plating, don't wanna damage the tubes, but I could try hotter if it might help.

    I'm very fascinated, but I'm not sure I understand. Could you elaborate? Are you saying that there's usually a 50v gap underneath the plate voltage in which distortion starts, and it stays constant across different plate voltages?
     
  12. itsGiusto

    itsGiusto Tele-Meister

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    Do you know how they achieved the drop in B+ voltage? Did they use different power transformers? Or did they change some other component instead?

    I have a 27k resistor + 250k pot in place of the NFB resistor, and it's coming off the 16ohm tap. I like that it gets way ganier when I decrease the NFB, but it also allows the weird sounding buzzy distortion overtones I'm getting in more.

    One thing of note is that in the later years, Marshall not only raised the value of the NFB resistor, but he started connecting it to the 4ohm tap instead of the 8ohm tap, making it have even less NFB. By using a 100k resistor on the 4ohm tap, it's equivalent to a 140k resistor on the 8ohm tap, i think.

    That's interesting, I thought that the .68uf cap came earlier, in the late 60s. And I thought that by 72, they started not putting the bypass cap in as much anymore.

    I'm basing my assumptions off of the Ceriatone layouts, and this database of Marshall amps: http://marstran.com/Historic Data Base.htm


    I have mine switchable between 100pf and 5000pf. Having the 5000pf one is insanely aggressive, it's true.
     
    Last edited: Aug 9, 2020
  13. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Have a look at this. Especially the last part about power output:
    http://www.valvewizard.co.uk/pp.html
     
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  14. Dacious

    Dacious Poster Extraordinaire

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    A post-phase inverter dual ganged MV is probably the go. Simplest and best. Turned to ten it's out of the circuit.

    What many think Marshall 'headroom' is, is really preamp-clean. The eqs are gain controls as much as the preamp gain. The power section of most Marshalls doesn't really sag until way louder than most people play. That's Plexi tone.
     
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  15. radiocaster

    radiocaster Poster Extraordinaire

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    I don't know about Marshalls, but I borrowed my friend's Wangs 5W head, and it has a solid-state/tube rectifier switch.

    The solid-state rectifier produced way more distortion, it wasn't even a contest. The tube rectifier produced mostly clean sounds, nice bassy jazz sound.

    Now that is not conclusive, because it was the same amp and I suppose with the same power supply caps. In a more standard amp, the values are somewhat different based on the rectifier.
     
  16. rze99

    rze99 Poster Extraordinaire

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    I’m sorry I can’t help technically, but my quest for power tube breakup on all my amps - including a 70s Marshall - and at the right volume for a given occasion was solved with attenuation. Better than pedals.

    Of course it’s not the same physics with as the impact of sound waves on your body etc., but it’s still the tone and break but at least deafness and tinnitus is avoided.
     
  17. Wally

    Wally Telefied Ad Free Member

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    Blowing fuses when pushing the amp is problematic and indicative of a deeper problem, imho. Are you running this amp on an attenuator at all times when these fuses blow? Have you ever run this amp without the attenuator to see what it does without that attenuator involved? As for the quality of the distortion, the Md. 1987 is not an amp that creates harsh distortion, ime. The earlier tube-rectified should have an even smoother And earlier breakup.
     
  18. itsGiusto

    itsGiusto Tele-Meister

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    Thanks! That's good to know about the distortion quality. I have not tried running this without an attenuator, but I suspect that the attenuator is not the problem, because I can still hear the buzzy harsh distortion even when I'm just probing places within the amp, like post PI.

    I've long suspected that a faulty power transformer is the problem, but been too cheap to buy another one and test my hypothesis. But I finally just bought another one last night, so hopefully this weekend, I'll be able to hook it up and finally test this theory.
     
    Last edited: Aug 10, 2020
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  19. Wally

    Wally Telefied Ad Free Member

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    I would have suspected the OT before the PT if all voltages were correct.
     
  20. itsGiusto

    itsGiusto Tele-Meister

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    Do you have any particular insight into what sort of problems may cause HT fuses to blow?
     
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