parallel caps in Marshall curcuit

[Chuck H]

Funny, in the History of Marshall amps book they recommend a 500pf.

I removed the bright cap in my 69 1959 and 73 1987.

Typo, or wrong.

I don't have that book. I have the Teagle and Sprung book about Fender amps. There are a few errors in there (but a whole bunch of stuff I didn't know too). I chalked it up to the difference between enthusiasts and techs. History is full of situations where someone who was an accepted authority on a subject said something erroneous and the world took it for fact because "It was in a book".

Someone show me a stock 1959 with a 500pf bright cap or a schematic with that value indicated and I'll eat my words (and ask for seconds ).

[J M Fahey]

Sorry you don't have that book, and in that case don't know how you can refer to "all the known Marshall literature" while at the same time claiming to ignore that collection of schematics and notes from all eras.

In page 197, there is a JTM45 reissue schematic, obviously showing then modern version, but it is hand corrected by Steve Grindrod, quoting Ken Bran, who should know and stated:

Lead had a 500pF across the volume control and 500pF across the 470k9270k on early models) on the same channel.
The Bass model just had one 500pF across the 470k.
The organ model had a 100pF across the volume and a 500pF across the 470k.
The PA amp had neither capacitors.

Those are the amps I can call "original original" , although it may sound like an oxymoron, to emphasize that they were the original, earliest ones from which everything else derived.
And obviously, if later 5000pF started to be used on the Lead models , then

But at some point .005/4n7/4700pF started to be used .

becomes a Logic Truth.

As of writing units with the leading zero, it's the proper way to do in Math notation, including Physics, of which Electronics is but a branch.
That lazy Americans in common use miss it, it's a local quirk, not valid in the rest of the Scientific World.

In fact, even ibn USA it's illegal to omit it in cases where misreading maight cause big problems (including death), so *maybe* that means something

Some relevant quotes:

FDA Guidelines (U.S. Government FDA/ORA Compliance Policy Guides)
http://www.fda.gov/ora/compliance_re...pg140-500.html

"A zero before the decimal point should be used in numbers between
1 and -1 to prevent the possibility that a faint decimal point will
be overlooked.

"Example: The oral expression "point seven five" is written 0.75."

Is the omission of leading zeroes an american thing? I have only encountered it in US literature and software. It is completely unheard of in Swedish, and I have not read enough British technical literature to see any trend in it. – Ahlqvist Aug 19 '14 at 12:12

Good Prescribing Guidelines (Westmead Hospital Department of
Pharmacy - Khai Bui)
http://www.westmead.nsw.gov.au/allie...s/goodpres.htm

"Never leave a decimal point naked, such as .5 mL. When the decimal
point is not seen, a tenfold overdose may occur.

"When a decimal fraction must be prescribed, always write a zero
before the decimal point.

"Never put a decimal point and zero after a whole number such as
2.0 mg. This should be written as 2 mg. If the decimal point is not
seen, a tenfold overdose may result."

Medicine, Malpractice and the Law (a paper by Raymond Wacks)
http://www.medicine.org.hk/bma/programme.htm

"The expression of drug dose and units should be clear. For whole
numbers it is better to avoid following with a decimal point and a
zero which may be misinterpreted as ten or one hundred times the
appropriate dose. For numbers less than one it is essential to
place a zero before the decimal point."

In the modern and precise Metric System, even in the USA:

Metrics the Right Way (George Sudikatus, ICF KH Metric Coordinator,
Pacific Northwest National Laboratory)
http://www.pnl.gov/ag/usage/metrics.html

"In the United States, the standard decimal marker is a dot on the
line (i.e., a period or 'decimal point'). When writing numbers less
than one, add a zero before the decimal marker. For example, on a
drawing you might define a small length in English units as
.032 in., but write the metric length as 0.81 mm."

and so on.

[MarkusBass]

If you go to DrTube site: Marshall Schematics and browse the schematics, you will find some interesting information.
The first Marshall amp JTM-45 (initial version) was a copy of Fender Bassman. The schematic used ".0001" notation as a bright cap (100 pF) and ".00025" as a treble cap (250 pF).

Then, you will find schematic for JTM-45 tremolo version: http://www.drtube.com/schematics/marshall/jtm45tr.gif that use 100 pF as the bright cap:

You will also find a schematic of the version 1963, which did not use any bright cap:


This is another schematic of Marshall 1959 that uses 100 pF as a bright cap:
http://www.drtube.com/schematics/marshall/1959t-66.gif

And finally, you will find schematic of 1959 reissue from 1993, which uses paralel capacitors (4.7 nF + 1 nF) as bright capacitor and 470 pF + 120 pF paralel to 470k resistor following the Volume pot.
http://www.drtube.com/schematics/marshall/1959sprm.gif
Here are the details:

It appears that Bob is working on the reissue version from 1993 and the two capacitors in paralel are correct.

So, as you can see, the amp had many different versions starting from no capacitor, then 100 and 500 pF, and finally 4.7n + 1 nF. There was also reissue version with 4.7 nF capacitor only (I was fixing this version).

Mark

[dai h.]

for the 1959 (model with split cathodes, 33k slope resistor, etc.) I would agree that 5000pF(5nF) is generally accepted as the standard value for the bright channel pot cap. (As mentioned previously) actual amps could be different. (While I've looked at many old Marshall chassis pics online) one of two I've seen in person (a split cathode, 33k slope R, etc. 1959 trem head) had no pot cap and 100nFs coupling caps out of the phase inverter (instead of the expected 22nFs).

[Chuck H]

Sorry you don't have that book, and in that case don't know how you can refer to "all the known Marshall literature" while at the same time claiming to ignore that collection of schematics and notes from all eras.

Easy now! Don't get all Juan on me. I didn't say "all the known Marshall literature". I said "absolutely ALL the Marshall schematics" and "neither do any official Marshall documents". Both statements inaccurate because I didn't follow them with "that I have seen." Which I should have.

I'm not disputing anything Ken Bran said about the JTM45.

Since I don't have the book, perhaps someone who does could share what it says about the 1959 model?

As far as I can tell from the schematics available on line () the 1959 model may have been the era that you say the amps bumped the volume pot cap to .005u (5000pf). Meaning that the only "correct" value for the 1959 (and not the earlier JTM45) is indeed .005u (5000pf). That has been my position. I'm not at all saying Marshall didn't use a different volume cap value on other models.

[Chuck H]

The model is question is the 1959.

AFAIK that is not the same as the Fender 5f6a, the JTM45, the 1959T or the 1963.

[drewl]

Arguing about such nonsense, classic!

Marshall used anything from 100pf to 5nf.

Page 39 Steve Grindrod is comparing JCM800 versions of the 1959 to originals and the 1959x reissue:

"Electrically the only difference aside from the choke being replaced by a resistor on the 1959x where the 0.005uf cap in channel one was replaced (on most) by a 470pf. This reduced the mid band gain at lower volumes making it sound fuller and more controllable. This incidentally is a worthwhile mod on any 1987 or 1959 to make it cleaner and more contollable, without affecting the sound with volume flat out"


Can't believe I typed all that.

[drewl]

Okay, he may be talking about your amp as he says it was based on a specific circuit only made in lat 68 to 69, based on one that sounded great.

"To accommodate this circuit and to be able to build up caps to the desired value means the SLP has it's own unique circuit board, non pot-mounted.

[Randall]

For what it's worth

I don't claim to know, but I currently have a dead stock, AFAIK, '68 plexi 1959 in my shop owned by a top level guy. It has a 500 pF bright cap. It was too bright for him, so I lifted one side. He now jumpers the two channels and is happy. Maybe the most rock amp I've ever had the pleasure of meeting. Has the original tubes, too. And they are tightly matched.

I think the more interesting argument is why the two channels were engineered to be SO different, beginning with the radical pre amp cathode bypass cap values.

[Enzo]

Well, if the two channels are more or less the same, what would be the point. make them sound like different channels.

[Chuck H]

I don't claim to know, but I currently have a dead stock, AFAIK, '68 plexi 1959 in my shop owned by a top level guy. It has a 500 pF bright cap. It was too bright for him, so I lifted one side. He now jumpers the two channels and is happy. Maybe the most rock amp I've ever had the pleasure of meeting. Has the original tubes, too. And they are tightly matched.

I think the more interesting argument is why the two channels were engineered to be SO different, beginning with the radical pre amp cathode bypass cap values.

That's the kind of thing I can sink my teeth into! I said I'd eat my words... Yum!

Too often it can turn out that common experience doesn't cover the whole reality. I never mind being wrong, or even a bit embarrassed, when I can learn something like this.

[dai h.]

would be cool to see chassis pics (if possible)!

[52 Bill]

I think the more interesting argument is why the two channels were engineered to be SO different, beginning with the radical pre amp cathode bypass cap values.

Isn't this because the circuit was originally copied from the 4-10 Bassman, which had a "Bass" channel and a normal channel.

[Chuck H]

would be cool to see chassis pics (if possible)!

Hear, hear!

[MarkusBass]

Well, if the two channels are more or less the same, what would be the point. make them sound like different channels.

The channels are still different because of the serial capacitor, which is either 2.2nF, or 4.7nF in the bright channel compared to 22nF in the other channel.

I don't claim to know, but I currently have a dead stock, AFAIK, '68 plexi 1959 in my shop owned by a top level guy. It has a 500 pF bright cap. It was too bright for him, so I lifted one side. He now jumpers the two channels and is happy.

I usually solve the problem by installing a switch that allows switching in different capacitors (including 4.7n).



As Juan explained, the amp can still be used with 4.7nF because the users usually link both channels and mix them (and they also play very loud). In my opinion lower value of the bright capacitor allows for more sound options. Even with e.g. 330pF the bright channel still sounds very bright.

Mark