5. Hiwatt DR103 EQ (Plate Driven)
Hiwatt designed an EQ that is somewhat similar to a basic FMV circuit but include a number of extra components, and difference circuit. This EQ has a different response then the FMV. The traditional DR103 design had the EQ driven off the 2nd gain stage's plate. The plate is one of the terminals of a vacuum tube. It is the higher impedance of the 2 output terminals (the other being the cathode). This is similar to a Fender FMV as seen in the schematic below.
The EQ is a bit more advanced then the regular FMV but it has a similar structure. Unlike the FMV, the controls go (from top to bottom), Treble, Mids, and Bass respectively. The pot value for the mids is 100k whereas Marshall's had a 25k and Fender had a 10k.
Here is the input and output signals of the EQ when all the of the knobs are at noon.
This EQ has a similar amount of insertion loss as a Fender FMV, as they are both plate driven. However the EQ loads down the high end a bit more than the fender FMV does. The mid cut is not as sharp in this design and is more broad looking.
Now lets take a look at the 3 control sweeps
Bass control sweep
Mids control sweep
Treble control sweep
The EQ has some similarities to the Fender response but differs in someways. The mid cut is not nearly as strong and you cant cut as much mids. The mids also only really controls the mid range unlike in a Fender which also affects bass and treble to a much greater degree. The location of the mid cut is in a different location as well. The Bass and Treble responses are more similar tho.
Here is the EQ with max on all controls
There is a pretty big mid cut centering around 250 hz or so and a lot of bass and treble there.
I wanted to include one last section about the Hiwatt EQ which is also to show more detail on how the impedance of the driver stage affects the response of the EQ. This is another way of highlighting the difference between the plate driven Fender FMV and the cathode driven Marshall FMV.
6. Hiwatt DR103 EQ (Cathode Driven)
According to the Hiwatt schematics on mhuss.com (I would say pretty much the authority on vintage Hiwatt information) a few rare models of the DR103 featured a cathode driven EQ. In this design the unused triode section of the V2 preamp tube is converted into a cathode follower and is DC coupled to the V2 gain stage. This is identical to what is done on the Marshall scheme.
Here is the scheme I created
Now what does the addition of the cathode follower do to the EQ response. Here is a comparision between the plate driven and cathode driven Hiwatt EQs with the controls all at noon
The Red and Dark Blue Lines correspond to the cathode driven and the Light Blue and the Green lines correspond to the plate driven. As you can see there is a significant decrease in the insertion loss and loading of the signal in the cathode driven EQ. In some cases there is between 5 to 10 dB increase in the output signal. That is a considerable gain in the signal. At noon on the controls tho there is not a huge difference in the shape of the EQ beyond the increase in the output signal.
Here are comparisions between the two EQs, just looking at the output of each while the controls are swept
So generally the cathode driven EQ has much more output then the plate driven EQ. There are some differences in the responses in terms of shape but not a great deal. The reason being is that I did not adjust the cathode driven EQ to be properly impedance matched with the cathode follower. One way to kind of show what this would do is to decrease the 100k resistor on the left of the tone stack to 33k.
Now there is serious differences between the cathode (blue) and plate (green) driven tone stacks. Decrease the 100k resistor to 33k resulted in a tone of more low mids where the center frequency of the mid cut moved up to 700hz or so. This is now looking a lot more like the Marshall EQ response. Ok so you might say well why dont I just make the 100k resistor in the plate driven stack 33k? Well I certainly can do that but at the cost of increased insertion loss and more mid cut.
That looks like this
So now the mids are being cut even more and there is even more loss in the tone stacks but the mid cut center frequency is not ~700 hz, nearly the same as the cathode driven stack. So whats happening here? Well the EQ is now loading down the tube gain stage even more and since its a fairly high impedance that will decrease the signal going into the EQ and subsequently the output signal as well.