Dating my JMP50

Discussion in 'Let's Talk Vintage' started by obx351, Mar 22, 2015.

  1. brets9

    brets9 New Member

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    I own 11086..! I am interested to know what your pot codes read..?
     
  2. Viking62

    Viking62 Well-Known Member

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    Nice score man..!
     
  3. stratburst

    stratburst Active Member

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    Really? I'll pm you tomorrow. I'd love to see some pics.
     
  4. SG~GUY

    SG~GUY Well-Known Member

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    -you have some nice gear-

    -I have a soft spot for those 8x10 cab's-
     
  5. chilipeppermaniac

    chilipeppermaniac Member

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    Neikeel, you must know the jerk that worked on my amp.
    I was so pissed when he said he pitched the original Marshall Drake Power transformer instead of saving it like I requested. I suspected he did exactly as you said. Told me it was bad then sold it off.
    Now I am seeing some less than ideal .22uF caps wired in series on my board and amp still blowing fuses after a recap, retube and new Power Transformer installation. I bet it was these bulging ,22 uF caps all along and he did a 1/2 ass job fixing other things, and a full ass job screwing me in the process.
     
  6. chilipeppermaniac

    chilipeppermaniac Member

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    The bulging caps are the brown ones on the right. which are wired in series instead of parallel, plus a few other issues I am hoping to rectify now that I am skilled in doing stuff on amps personally

    Caps Bulged.JPG

    capsbulged2.JPG

    Instead of having to seek a supposed pro.
     
    Last edited: Jan 31, 2017
  7. neikeel

    neikeel Well-Known Member

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    Two things I would say about that amp. I would be very tempted to replace both bias caps (for 10uF @ 200v rating as they will be the right size, and remove the white wire off the diode block and use a new longer white wire from the bias feed 220k turret to one of the stand by switch lugs that is fed direct off the PT (or, if like the earlier 1202-118 Drake fitted amps) to the lug on the PT. That way the power tubes see bias current before you hit them with B+ voltage to the plates.
    You are also missing 1k screen resistors which is fine if you are using NOS Mullard xf2s but otherwise they would be a good idea with current production EL34s.
    If your amp is a daily player then getting some UF5408s and ditching the snubbers altogether would be a good idea.
    Depends what you need from it.
     
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  8. chilipeppermaniac

    chilipeppermaniac Member

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    Neikeel, Thank you very much for addressing my questions and hopefully we can continue to converse.

    I have been discussing my amp with a couple other guys who know amps and electronics who live in UK and Australia.

    I believe you are seeing a number of things we also discussed and was suggested to me by the Aussie.

    1st remove the short white wire and run a longer one from the resistor and relocate the other end to the standby switch.

    2nd your idea about a UF5408, Again, the Aussie suggested a UF4007 setup which I presume is similar concept as UF5408 and as he said, ditch the snubbers.

    Here is what he said, "Hi Chilli, its 8pm where I am at the moment, no "daylight saving" adjusted time. Down south over the border (another state) where they do have daylight saving its 9pm. Ok, the snubbers we've been talking about are the two .22uf, swollen brown caps. These were used in the old days when diodes weren't so good to reduce or eliminate diode switching noise, or "hash" & also power transformer ringing caused by diode recovery (I think,, Don???). As diode technology got better the snubber caps weren't needed. The snubbers in the link you provided (from the little I read) are caps bypassing the plate load resistors of pre amp tubes where squeal is a problem. I have such a snubber on the added stage of my Tim Caswell #39 mod amp. Ok, the "UF4007" rectifier upgrade is very easy. You will need four UF4007 diodes (1 or 2 bucks). These are "ultra fast" switching versions of the common 1N4007 diodes & feature a "soft recovery", so suit our purpose well. To make the full wave rectifier you need to make "a pair" of "two diodes in series". The body of each diode will have a silver band at one end. This is the "cathode" end of the diode. The other end without a silver band is the "anode". To make a series pair you need to arrange the cathode end of one diode to the anode end of another diode. Do the same to your other two diodes. Ok, I will find a pic of how they are arranged on the board to make things a little easier for you ok"


    3rd I am not sure I understand the 1K screen resistors. I will need you to explain.
    I am currently running EL 34's

    I also have a known slightly messy resistor scenario of a 47 and 10 ohm resistors soldered together and going to the trim pot. I was advised to ditch that and simply put a 56 ohm one in.
    So, you will have to walk me through the 1k Screen resistors.



    More History on the amp, I have not been able to play it much the last 10 years as I found the tech I went to less than reliable and his repairs still left the amp in the state it is in now with me trying to rectify the failures myself.
    It is a 1972 European market 1987X 50 watt Lead. It has a Bulgin mains socket, it can be run at 240 or adjusted to 110/120 as well. Those snubbers are marked .25 400 240 so I was told I could replace them run in parallel not series with .22uF 400 or 600 replacements.

    My "tech" initially diagnosed a bad Drake 1202-118 so I had him replace it with a Marstran I got from Bryan Wallace. Next it still blew fuses so I had him diagnose it again. This time he said I needed 2 new Power tubes and he replaced the Electrolytic caps. I got it home and was still not happy how noisy the amp sounded but no blown fuses for a little while at least.

    Kind of shelved the amp awhile and primarily played my more reliable ones. Just fired it up Friday 1/27 and then it sounded awful hum at first, then as I twisted various knobs and played it awhile the hums reduced. But shortly later I lost sound.
    Shut amp down, detached cord and checked fuses. HT was ok, Mains was cloudy but still showed some continuity on Meter.
    I will try to post some more gut shots too.
     
  9. chilipeppermaniac

    chilipeppermaniac Member

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  10. chilipeppermaniac

    chilipeppermaniac Member

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  11. chilipeppermaniac

    chilipeppermaniac Member

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  12. chilipeppermaniac

    chilipeppermaniac Member

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  13. chilipeppermaniac

    chilipeppermaniac Member

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    Also, thanks to the Original Poster for permitting me to add discussion of my Similar amp in your thread.
    I hope it is ok as I would imagine you can learn stuff about your amp which is similar.
     
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  14. chilipeppermaniac

    chilipeppermaniac Member

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  15. chilipeppermaniac

    chilipeppermaniac Member

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    One member wrote in response to the OP.

    Slight confusion setting in here I think. The screen grid resistors are 1Kohm 5W resistors used to limit current and are pretty essential with modern tubes if you want them to live. These are usually connected across pins 6 and 4 (pin 6 is not used on an EL34 and is used as a terminal on the socket). This is fed from the far side of the dropping resistors, where the choke and the screen filter side of one of the filter caps also join.

    The 5.6K resistors you talk of are input grid stoppers. These should be connected in series on pin 5 of each power tube socket and are there to help prevent oscillation. Not always essential, but a very good thing. Especially on 100W amps.

    I wouldn't ever consider building a Marshall style circuit using EL34's without either of these, if using current production tubes. Only the very best Mullards or early 70's and earlier RFT's (or NOS tube types other than EL34) would ever be up to the task, and even then I wouldn't want to risk running such valuable tubes without adequate protection.


    Then someone said,"I'd be interested to see what would happen if he actually did hook up the PI outputs to Pin 4 of the power tubes.:naughty:"

    then damienbeale said "
    Well caught Eric.

    PIN 5. Better go back and edit that"
     
  16. chilipeppermaniac

    chilipeppermaniac Member

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    72 Marshall Marstran pics.jpg

    On this other guy's amp, What are the 2 brown cap like parts up by the power tube sockets?

    Also, does his have the 1k screen resistors?
     
    Last edited: Jan 31, 2017
  17. chilipeppermaniac

    chilipeppermaniac Member

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    I found this info at the link here..
    What is your take on what was stated below?

    I found this info at the link here..
    http://tone-lizard.com/marshall-myths/
    • Marshall has a reputation for making very temperamental amplifiers, even though they are ‘borrowing’ a pretty tame Bassman circuit. This came about because although the original circuit design worked quite well with 5881s, the switch to EL34s did not come with many circuit design changes. In general, Beam Power tubes (such as the 6V6 and 6L6) will exhibit less of a potentially damaging Screen-Grid current rise at maximum signal excursions than Power Pentodes (such as the EL34 and 6BQ5/EL84). Marshall took the standard procedure of using 1K Screen-Grid resistors as opposed to the 470-ohm resistor seen with Beam-Power tubes. However, as the voltages crept up in the amplifier over the years, the output stage can still become unstable, especially with the available poor quality EL34’s. Intuitively, you may consider that fact that Marshall actually switched back to 5881’s in their amplifiers for a brief period as a silent admission of the previous statement. Why Marshall neglected the option of ‘fixing’ the circuit remains a mystery. Adding in the fact that many early Marshall amplifiers came out of the factory without any Screen-Grid resistor certainly doesn’t help today’s players replacing their tubes every other day! A final symptom of unstable Marshall amplifiers is the older examples that intermittently oscillate at very high levels. Check to see if your Marshall has grid stopper resistors on each output tube. Some amplifiers had none at all, while other 100-watt models had only one grid-stopper resistor per ‘side’. Either way, ‘modify’ the amplifier to have one grid-stopper resistor per tube. An engineering switch to 6550 output tubes (to try and increase reliability) didn’t go over too well with guitar players. You can make your Marshall very reliable without drastically changing the tone.
    • Speaking of changing the tone, adding any Screen-Grid resistor does just that. If you have really good tubes, in a well-designed amplifier, try it for yourself. When the Screen Grid supply is solid (no voltage drop across any resistance) the sound is a lot ‘tighter’. One modification idea is to place a 20uF/500WVDC filter capacitor right on each Screen-Grid, along with raising the resistor value to5K. This runs the tube as a true pentode, with a solid Screen-Grid supply. You loose a few watts in the deal, but some people like the tone and the extended life expectancy of their EL34s. Garnet, Traynor and Hiwatt (only in some 100-watt models) did it much smarter and their methods are my favorite way of running today’s poor quality EL34s in any Marshall. Does it change the tone? Not at all. These ideas are presented below.
     
  18. stickyfinger

    stickyfinger Active Member

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    Brown caps are the screen resistors. Should be 1K and you should add them.
    The 5k6 grid stoppers are not needed unless your amp oscillates and is unstable. I don't have them on my originals. I doubt you will need them as your amp appears to have a bit less gain as other examples. No v2 .68uf and 47k feedback resistor.
     
  19. chilipeppermaniac

    chilipeppermaniac Member

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    Thanks, sticky

    I am surmising what we see on that other guy's amp I posted is exactly what is stated here. Correct me if I am wrong.
     
  20. neikeel

    neikeel Well-Known Member

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    No you are not those are brown Ohmite resistors.

    What you need to do is desolder the yellow wires from pin 6 on your output tubes, clean the sockets of solder.

    Move the yellow wires to cleaned up (ie deoxidised) pin 4 of each output socket (suggest use the lower holes for the wires) then take a 1k 5watt (or 3w will do) ceramic resistor (I tend to use the green barrel shaped Welwyns, as this is what Marshall started using in 71, but the white block type or Ohmite 5w will be ok, just as long as they are flame proof).

    Ideally wrap the wire around the top hole of pin 4 and pin 6. Don't leave the wires too long. just long enough to span the gap with 1/4 above the socket is fine.

    Pin 4 is empty in the valve itself you are just using that pin as a solid mounting point for the resistor.

    You should not need swamp resistors on pin 5 on this amp, you only need those if the lead dress of your amp is poor or you have a modded preamp that is unstable.

    I do not suggest you mod your amp along the tonelizard line, it is unnecessary in your amp and ruins its resale.
     

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