Marshall Plexi Bright Cap Clean Tone Comparison

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Marshall Plexi Bright Cap Clean Tone Comparison

Postby David B » Tue Jul 26, 2016 4:48 pm

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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby dai h. » Fri Jul 29, 2016 5:59 pm

thanks Dave!
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby David B » Sat Jul 30, 2016 12:57 pm

Handy if one doesn't have all the values in house just to get an idea what the cap will sound like, FWIW prefer a 100pf in that spot in most Marshall type amps.


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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby Brian Wallace » Sun Jul 31, 2016 12:13 am

Interesting how the 5000pf cap made the amp crunch up.
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby David B » Sun Jul 31, 2016 7:09 pm

Sure does & its a bit more aggressive with that cap.
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby bill bokey » Mon Aug 01, 2016 12:47 pm

Very useful video, thanks !

I like to use 100-120pf for lower gain amps (Super Bass, JTM45 etc...) and 5nf for a Super Lead and full on crunch !
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby David B » Tue Aug 02, 2016 9:05 pm

bill bokey wrote:Very useful video, thanks !

I like to use 100-120pf for lower gain amps (Super Bass, JTM45 etc...) and 5nf for a Super Lead and full on crunch !



Yeah I had a seventies SLA with that cap, savage beast :twisted:
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby dai h. » Wed Aug 03, 2016 7:43 am

since a 4n7 or 5n cap there is typically a poor (wide) tolerance ceramic, I wonder if there were Marshalls that ended up with an extra high value there (helping to give the perception of a "hotter one", possibly along with a linear taper volume pot).

FWIW I've measured a bunch of 4n7 'B' temperature coefficient disc ceramics, and (IIRC) while some were close or near spot-on, I didn't get any way over. I also measured a 100 pcs. of 'E' temp. coefficient (wider tol. than 'B') 1n disc ceramics, and every one was over and none were on the mark or under. The closest one (IIRC) was about 10 or 20% over.

(From memory) a couple of guys on the metro bbs shared their measurements of old 5nFs, and I think they found usually a bit under 5n. Also (FWIW) on my JCM800 4010 (1x12 combo ver. of 2204 with very slightly different circuit), a 5n disc ceramic was about 3n8 (or so).
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby David B » Thu Aug 04, 2016 9:32 pm

I would think if it's way off you would notice, Dai did you try swapping out that low reading cap for one that's in spec?

I don't have any ceramics in that value but I might have some 500pf & maybe some 5nF's all in a box somewhere, oh do I want to go measuring caps.

FWIW the 100pf's I use are old RBS labeled silver mica & all measure slightly , couple out of spec but most are in..also lost in the woodpile are some 100pf domino caps, somewhere :oops:

http://jptronics.org/Collins/silvered_micas/

kinda cool
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby dai h. » Mon Aug 08, 2016 10:17 am

hey Dave, thanks (interesting link).

re: my 5nF ceramic disc, this was a cap removed from my Marshall 4010 (from the tone stack mid pot--wiper to ground if memory serves). In my thinking I thought it was safe to assume that this would be the same exact ones used for their concurrent JCM800 1987/1959 models. (This amp basically ended up as a "guinea pig" with lots and lots of modding, and I (probably?) never tried it for a volume pot bright cap.)

Something (seemingly) weird about the 4n7 ceramic discs I measured is that the reading on an LCR meter looks rather strange, with the value creeping up and up for a while, then settling at some lower value over time (which I'm guessing is normal due to measurement parameters of temperature, frequency, and measurement frequency amplitude (the size of the voltage being applied from the LCR meter) needing to be controlled when measuring non-temperature compensating ceramic capacitors. (I'm guessing my biggest problem was not being able to control the amplitude of the measuring voltage.)

Anyway, that weirdness seems to be normal, but something that seemed a bit of further oddness was that I had measured some of these before and written on the caps directly (in ink), and while some showed the erratic-looking measurement, some seemed to be much more stable and easier to read, plus the value appeared to agree with the value I had written down previously. (Still totally possible that I got something wrong with my method or mis-perceived something, but) that was kind of a head-scratcher, leading me to wonder if there was some sort of difference with some of them (due to what I don't know but the caps have a mixture of "dopers" and "shifters" (not West Side Story street gangs! :lol: :oops: )--the ratio of which apparently determining how stable or not and how volumetrically efficient or not (i.e. how much C in a given space) they are. So wondered (since there is some mixture) if some of the Cs ended up mixed slightly different than others in the same batch.

Conversely, temp. compensating types seem to be far more consistent and don't exhibit the measurement weirdness (much easier and straightforward).

(Some notes I took FWIW: )

one strip of 13 Murata 680pF 50V temp. coefficent CH (similar to C0G/NP0) MLC (leaded thru-hole types) taped on piece of cardboard (as packaged from factory) ("bandolier" type packaging? (not individual))

24.5℃

freq: 10kHz

caps placed directly into meter slots(i.e. attachment with alligator clips not used)

caps removed and measured consecutively as found on cardboard strip from one end to the other
(2nd meas done later(23.4℃))
1) 677.8-9pF
2) 678.8pF
3) 684.4pF
4) 681.3pF (681.3pF)
5) 676.4pF
6) 678.0pF
7) 680.1pF
8 ) 681.1pF (681.1pF)
9) 672.6-7pF
10) 681.8pF
11) 683.8pF
12) 677.5pF
13) 681.2pF (681.3pF)

1) 677.9pF +12) 677.5pF (0.4pF difference)
2) 678.8pF +6) 678.0pF (0.8pF difference)
3) 684.4pF +11) 683.8pF (0.6pF difference)
4) 681.3pF +10) 681.8pF (0.5pF difference)
8 ) 681.1pF +13) 681.2pF (0.1pF difference)

5) 676.4pF + 9) 672.7pF (3.7pF difference)
7) 680.1pF odd one out

(I grouped them into pairs for possible use in electronically balanced mic input end RF filters (usually made up of a small value R such as 10 ohms plus a small C (470p-2n2) for each side). The tighter the match between the two halves, the better the balance (expressed as CMRR, or common mode rejection ratio) at some frequency 1kHz, 10kHz, or whatever.
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby David B » Wed Aug 10, 2016 7:32 pm

That is odd on the shifting, what meter? BTW did find some 100pf dominoes & they all measure around 115pf with a Wavetek LCR55 , I also have a vintage Rohde & Schwarz BN 5100 cap tester which I should dig outta storage to compare the readings.
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby dai h. » Thu Aug 11, 2016 2:46 am

hey Dave,

these are the ones I have (not mine in the pic--this is just someone's blog with a convenient pic) :

http://blogs.yahoo.co.jp/saikihiroshi/31130878.html

I also have an old Philips ECG cap meter I bought from Antique Electronic Supply ages ago. (I forgot to add previously that I tried to re-measure some of the ceramics using this older meter hoping that the freq. and measuring voltage amplitude would be more favorable (couldn't find an instruction manual for it online listing the freq. and V and I'm sure if I have the original paper one), and (IIRC) the readings were more stable on some 560pF 500V 'B', but seemed stable and un-stable on some 2n2 and 4n7 (both 500V) disc ceramics depending on the cap (as mentioned above).

Also, here are a couple of threads over at aron's stompbox forum where the phenomenon is described:

http://www.diystompboxes.com/smfforum/i ... c=114456.0

http://www.diystompboxes.com/smfforum/i ... c=111236.0

Also (from what I gather), on the more expensive lab-use type (non-hand held-type) bench meters, there can be a lot more adjustments parameters such as for the measuring voltage amplitude.
------------------------------------------------------------------------------------
Murata article on measurments of ceramic caps:

http://www.murata.com/en-us/products/em ... 0120628-p1
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby David B » Mon Aug 15, 2016 8:12 pm

Yeah the RS has some adjustments, cool old tester. I still need to do some work on it.

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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby Brian Wallace » Tue Aug 16, 2016 8:07 pm

Oh I like that. :D
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Re: Marshall Plexi Bright Cap Clean Tone Comparison

Postby David B » Tue Aug 16, 2016 8:55 pm

eBay find cheap to boot, my recollection it's pretty accurate.
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