Marshall jtm-45/1987t schematic question

[Sowndman]

As I look over the various marshall schematics, I noticed some unusual.

The signal that feeds the tone stack com off the grid of the 12ax7 with a 100 k restistor tied to ground. The plate is wired direct to the b+.

This setup appears different as my bugera, fender HRD, etc all take the tone stack feed from the plate like the other gain stages.

Is there a tonal advantage to drawing signal from the grid ? I assume this is giving the tone syack an inverted signal. Just wondering if this is another secret to the "marshall sound"

[DummyLoad]

that arrangement is called a cathode follower. it has a gain of unity (1) e.g. no gain. however, as compared to the grounded cathode gain stage, it has a very high input impedance and a very low output impedance which is ideal for driving high loss circuits such as the tone stack. the cathode follower does not invert signal.


signal polarity is usually only relevant if applying negative feedback.


the cathode follower has more effect on the character of tone than any other type of preamp circuit, with a pentode gain stage being the exception.


--pete

[Sowndman]

Well, it appear this will be my next mod for my bugera v55. I already changed the tone stack input resistor from 82 k to 33k.

I will get a test tommorow to see how far I've come as I have a friend with a marshall 1987 50 watt.

The drive circuit still has less bass than the clean for now but I have to take one step at a time.

[PRR]

> secret to the "marshall sound"

The secret is that all this was stolen adopted from Fender 5F6a.

When the English did the 5F6a they used EL34 and british speakers in a non-Fender box (suggesting different NFB), so it was different from the start.

Later Marshalls had some of their own tricks.