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'71 Marshall JMP50 - hums when switched ON.

BIG Dave

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Aug 18, 2001
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Went to play the old '71 JMP50 today and after letting her warm up for a minute, I threw the Standby switch to ON and she produced a loud hum for about 5 seconds, then the hum faded away. After that, she played fine, no humming. The filter caps are the original DALY 50x50 from 1971, making them 44 years old. They look fine, no bulging or leaking, but looks can be deceiving.

I'm going to go ahead and replace them, what are the best choices currently available? I know F&T are well respected. What about ARS? I've heard Marshall uses them in some of their Handwired series.

Disclaimer: Yes, I know to discharge the filter caps before preceding.
 

duaneflowers

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Aug 13, 2013
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Went to play the old '71 JMP50 today and after letting her warm up for a minute, I threw the Standby switch to ON and she produced a loud hum for about 5 seconds, then the hum faded away. After that, she played fine, no humming. The filter caps are the original DALY 50x50 from 1971, making them 44 years old. They look fine, no bulging or leaking, but looks can be deceiving.

I'm going to go ahead and replace them, what are the best choices currently available? I know F&T are well respected. What about ARS? I've heard Marshall uses them in some of their Handwired series.

Disclaimer: Yes, I know to discharge the filter caps before preceding.

You sure its not failing glass? :hmm
 

BIG Dave

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You know what they say, always check the fuse and tubes first! I swapped the Mullard XF2's with an unused pair of Siemens. Fired her up, no hum! Thanks DuaneFlowers!

But back to the Filter Caps. They're approaching a half centry old and it's probably time they go. What are we using for replacements?

* F&T have been getting great reviews. They're quality caps built in Germany.

* ARS is a reseller based in California. These caps have also received good reviews and have been used as OEM in the Marshall Handwired series. Built in China by the Shanghai Tianhe Capacitor Co., Ltd. In addition to the ARS rebranding, they can be found labelled "Tianhe" and "TMC".

* JJ - Some people like them, some don't. From what I've read, they are not as reliable as the other listed above. I won't be considering them.

What do we recommend?
 

Pat Boyack

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Don't change the caps if they are fine. My old '56 Deluxe had half of it's original caps when I sold it last year. They were fine. :peace2
 

duaneflowers

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Don't change the caps if they are fine. My old '56 Deluxe had half of it's original caps when I sold it last year. They were fine. :peace2

I was thinking of replacing the caps in my '89 1962 Bluesbreaker and the John Wiley BB Book recommends F&T and JJ... after receiving this exact same advise (don't change them unless there is a reason) I backed off and left them as is. There is nothing wrong with the way it sounds now (except the fact that it is a reissue that doesn't sound like the original) so it will stay as is. :salude
 

BIG Dave

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Don't change the caps if they are fine. My old '56 Deluxe had half of it's original caps when I sold it last year. They were fine. :peace2

Wow. When I got my '67 Deluxe Reverb a few years ago, it sounded tired and lifeless. I replaced the five original 16uF Mallory filter caps (dated 67-01), with fresh F&T's and she immediately sprang back to life! She's now one of the gainiest and most responsive DR's I've ever played. I guess filter cap life comes down to how often the caps have been "exercised" over the years.
 

duaneflowers

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Wow. When I got my '67 Deluxe Reverb a few years ago, it sounded tired and lifeless. I replaced the five original 16uF Mallory filter caps (dated 67-01), with fresh F&T's and she immediately sprang back to life! She's now one of the gainiest and most responsive DR's I've ever played. I guess filter cap life comes down to how often the caps have been "exercised" over the years.

I may be wrong (won't be the first time) but I thought the caps were there basically to smooth out the signal and filter out the low frequency hum. I didn't think it even affected tone all that much. Guitar cap values tend to drift with age, so perhaps that is what was happening with those caps... hence, they were filtering out progressively higher frequencies as they aged. A tech would have to chime in on whether or not that is the case as well as if the different caps have different jobs other than filtering. Also, when you replaced those Mallorys, did you replace the glass as well? Are you sure the results were strictly due to the fresh F&Ts? :hmm
 

Wally

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Filter caps filter the AC hum, but they also store and provide the voltage for the circuit. AS they age, their ability to store and thereby deliver that voltage on demand changes...decreases. This change in the caps' abilities does affect the sonics of the amp....and most of those who have experience in hearing the 'before and after' have no problem at all hearing the indicators of weak filter caps in an old amp.
Big Dave gives fair take on the results of weak filter caps.
Generally, the low end gets loose and exhibits out of tune subharmonics. The high end doesn't sparkle as it should, and the amp doesn't yield a three dimensional sound stage....the sonics are very flat and two dimensional.....as Big Dave said.... "tired and lifeless".
Electrolytics exhibit these weaknesses long before they show any physical sings of deterioration, ime....and long before they stop filtering AC hum, too. the last Marshall I recapped was a combo version of the 2203 from 1977, IIRC. Sad sounding...if one knew what they were hearing. When I was through, the amp was much better...stronger low end, sparkling high end, lots of 3-D in the sound stage. HE left happy. He came back the next week just to tell me that the amp had NEVER sounded as good. His Tele was now yielding good tones above the 12 fret....where before it was dead and lifeless.
I personally would not play a valuable old amp very much before I recapped it....both for sonic reasons and for reasons that pertain to the safety of the amp's major components. YMMV...
 
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duaneflowers

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:salude:salude:salude


Filter caps filter the AC hum, but they also store and provide the voltage for the circuit. AS they age, their ability to store and thereby deliver that voltage on demand. This change in the caps' abilities does affect the sonics of the amp....and most of those who have experience in hearing the 'before and after' have no problem at all hearing the indicators of weak filter caps in an old amp.
Big Dave gives fair take on the results of weak filter caps.
Generally, the low end gets loose and exhibits out of tune subharmonics. The high end doesn't sparkle as it should, and the amp doesn't yield a three dimensional sound stage....the sonics are very flat and two dimensional.....as Big Dave said.... "tired and lifeless".
Electrolytics exhibit these weaknesses long before they show any physical sings of deterioration, ime....and long before they stop filtering AC hum, too. the last Marshall I recapped was a combo version of the 2203 from 1977, IIRC. Sad sounding...if one knew what they were hearing. When I was through, the amp was much better...stronger low end, sparkling high end, lots of 3-D in the sound stage. HE left happy. He came back the next week just to tell me that the amp had NEVER sounded as good. His Tele was now yielding good tones above the 12 fret....where before it was dead and lifeless.
I personally would not play a valuable old amp very much before I recapped it....both for sonic reasons and for reasons that pertain to the safety of the amp's major components. YMMV...
 

BIG Dave

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Filter caps filter the AC hum, but they also store and provide the voltage for the circuit...

Thank you Wally for that explanation. You mentioned you recapped a '77 Marshall. What brand filter caps did you use?

Thanks again,
BIG Dave
 

BIG Dave

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...Also, when you replaced those Mallorys, did you replace the glass as well? Are you sure the results were strictly due to the fresh F&Ts? :hmm

Yes DuaneFlowers, I know where you're coming from. Make more than one change at a time, and you can't be sure which change created the results. I work in IT by day so I know we always make ONE change, then see what the result is (if any) before making a second change.

When I first got the DR, I checked the tubes and they appeared stock to the amp. I tried replacing the power tubes first with NOS RCA's and a NOS Mullard GZ34 and although it felt and sounded better, it still wasn't there. Tried replacing the pre's with (mostly) NOS RCA, and there was some improvement there too. But the most improvement came when I installed new F&T's. Night and day improvement!
 
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Wally

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Duane, I have heard many a vintage amp with 'new glass' that still didn't yield good sonics. Everybody wants to replace tubes...easy to do, right? Sometimes the old glass is just fine after a recap...sometime new glass is needed in addition to the caps. I also cannot count the times that someone has brought in an old amp that they just got..saying "It sounds great. I just want it checked out.". We fire it up, and I ask them if they hear this or that aspect of the sonics...and they do. Note: IT is strange that many times we don't 'hear' until we 'listen'...two different things, right? We recap it and do the service..whatever tubes are needed it gets....and the owner understands when they hear the amp why it needed the service.
I find that amps that are properly serviced...new electrolytics and whatever else is needed...provide years of good service. I read often about people 'giving up' on their vintage amp because it gives them trouble after trouble. I don't have time for that. IF someone insists that the elelctrolytics are 'good enough' in an old amp, I inform them immediately that all bets are off as far as warranty is concerned. So many problems are 'cured' by the simple act of replacing old electrolytics....filters, bias and bypass caps. Reverb...vibrato....many times things get straightened out when the voltages are good and strong...and can hold up to the demand. Analogy...old spark plugs don't allow an engine to run well....it might 'run'...but it isn't doing what it should. Those old plugs might even burn an engine down if the gap gets too wide! Whoops...there goes my matching number engine in my '67 corvette!!! You can't replace those transformers...and that is where the vintage value is...not in those electrolytic capacitors. I know from experience. I can recall hawking 2 '59 Bandmasters and a '56 E7A Twin at a show back in the late '90's....no interest at all..even though I had them priced according to their originality. The top market value...and some buyers...want only all-original when the cab, the covering, the speaker/s, and the transformers are considered. Replaced electrolytics don't make or break the value of a vintage amp...but the cab, the covering, the trannies, the speakers???? Those do make or break the amp, ime. There is a big difference---value-wise--- between player amps and truly collectable amps. Fresh e-caps help one protect those transformers, ime....even if one doesn't hear what is coming out of the speakers.
 

duaneflowers

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Duane, I have heard many a vintage amp with 'new glass' that still didn't yield good sonics. Everybody wants to replace tubes...easy to do, right? Sometimes the old glass is just fine after a recap...sometime new glass is needed in addition to the caps. I also cannot count the times that someone has brought in an old amp that they just got..saying "It sounds great. I just want it checked out.". We fire it up, and I ask them if they hear this or that aspect of the sonics...and they do. Note: IT is strange that many times we don't 'hear' until we 'listen'...two different things, right? We recap it and do the service..whatever tubes are needed it gets....and the owner understands when they hear the amp why it needed the service.
I find that amps that are properly serviced...new electrolytics and whatever else is needed...provide years of good service. I read often about people 'giving up' on their vintage amp because it gives them trouble after trouble. I don't have time for that. IF someone insists that the elelctrolytics are 'good enough' in an old amp, I inform them immediately that all bets are off as far as warranty is concerned. So many problems are 'cured' by the simple act of replacing old electrolytics....filters, bias and bypass caps. Reverb...vibrato....many times things get straightened out when the voltages are good and strong...and can hold up to the demand. Analogy...old spark plugs don't allow an engine to run well....it might 'run'...but it isn't doing what it should. Those old plugs might even burn an engine down if the gap gets too wide! Whoops...there goes my matching number engine in my '67 corvette!!! You can't replace those transformers...and that is where the vintage value is...not in those electrolytic capacitors. I know from experience. I can recall hawking 2 '59 Bandmasters and a '56 E7A Twin at a show back in the late '90's....no interest at all..even though I had them priced according to their originality. The top market value...and some buyers...want only all-original when the cab, the covering, the speaker/s, and the transformers are considered. Replaced electrolytics don't make or break the value of a vintage amp...but the cab, the covering, the trannies, the speakers???? Those do make or break the amp, ime. There is a big difference---value-wise--- between player amps and truly collectable amps. Fresh e-caps help one protect those transformers, ime....even if one doesn't hear what is coming out of the speakers.


Thanks Wally! Great stuff to know there!! :salude:salude:salude
 

Dishimyuh

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Nov 4, 2005
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One thing is certain, if your power/filter caps fail, you are going to have a bad day.
 
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