1st gen Marshall JTM60's and their clipping section: any comment?

[freefrog]

Posting about a JTM30 on another forum has reminded me something that I had noticed a long time ago then forgotten but that I find interesting right now, since I'm vaguely meditating on clipping diodes topology.

In the last JTM60 and JCM600 schematics, if memory serves me, clipping diodes were implemented after the output of V2A, as a diode bridge to ground following a capacitor (itself replaced with a simple jumper in some of the most recent amps).

But in the first gen JTM60's, things were different: the signal from V2A going through the 4,7µ cap then through the diode bridge was sent back to the cathode of V1B.

The same thing appears to have been put in the JTM30, but between the output of V1 and its own cathode.

I was wondering why Marshall had designed this circuit like this then modified it in the late JTM60/ JCM600...

..so I'd be glad to know how members here understand and would explain this feature.

Comments, anyone?

Thx in advance!

PS as a reminder: English is not my mother tongue. Thx for your understanding if ever my words or sentences are not correct.

FF

BTW, here is a schematic showing the relatively convoluted path that I mention:

http://www.prowessamplifiers.com/sch...-Poweramp.html


Oh, and... Happy new year to the forum! Even if my topic has for only function to express that, it will have been useful to me. :-)

[Helmholtz]

BTW, here is a schematic showing the "negative feedback clipping section" that I mention:

http://www.prowessamplifiers.com/sch...-Poweramp.html

V101B cathode bypass cap C117 essentially AC grounds the cathode, so no NFB action.

Why not directly ground the diode bridge? Don't know, maybe layout convenience.

[freefrog]

V101B cathode bypass cap C117 essentially AC grounds the cathode, so no NFB action.

Why not directly ground the diode bridge? Don't know, maybe layout convenience.

Thx for your answer...

...and Happy New Year!

Talking about "negative feedback clipping section" came to my mind as a spontaneous analogy rather than an analysis. Hence the quotation marks in my original post.

To put the discussion in perspective, here is why my curiosity had been excited:

-IME, the clipping diodes of first gen JTM60's were very close to a ground bus. Instead of a link between these parts of the circuit, the PCB featured a long convoluted track creating the path that I mentioned. i'm not sure if we can see it as convenient.

-In the late 90's, I've compared a first gen JTM60 to a JCM600. They were essentially the same amp minus a few differences like the implementation of clipping diodes. The JCM had clearly more gain and a fizzier buzzier voicing than the JTM. I understand that it could be attributed to components tolerance but my question was born from this comparison. Asking it more than 20 years later must be my way to celebrate this year 2020.

More later maybe. Beyond the precise subject of this thread, let's treat it as a way to talk about an old cheapo line of Marshall amps still used and appreciated on this planet (a few years ago, Andy Brauer described the JTM30 & 60 as sleepers & keepers).