Marshall 1974x - Tremolo channel plate resistors

[ZehQuait]

Hi guys!
I'm looking at the Marshall 18 watt (1974x) schematic and trying to understand how the plate resistors work on the tremolo channel... Does V3A (on the attached schem) see a 320k resistance (220k+100k) at its plate? I don't understand this arrangement where R11 is in series with R12 and the plate of V3B is connected to the junction of the two. Can somebody help me understanding this circuit?

Also, it looks like some people actually prefer the sound of the tremolo channel, and say it has a little less gain, more note definition, and a slightly different mid frequency response. I'm wondering if this could be related to the different plate resistors or there's more to it? Maybe C15/R29/C12 are responsible for this difference in gain/mids?

Thank you!

[frus]

you can look at it this way: the left triode is a gain stage (with 220k plate resistor), which is "enclosed" within the right triode's stage. when the tremolo is off, the right triode is in idle and pin 1 sits at some 150V constant voltage, so that is the "B+" voltage for the left triode. C7 bleeds all audio signal from this node to ground. When you turn on the tremolo, the right ("outer") stage starts oscillating at low frequency, and the voltage at pin 1 goes from zero to 200-something volts, so (as it's B+ for the left gain stage) it modulates its (guitar) signal.

what's interesting in this circuit is that the cathode bias resistor is shared between these two stages, and that left triode actually biases very cold, its idle plate current is some 0.3 mA (as opposed to 1.2 mA on the usual "fender type" 100k/1k5 gain stages)

[ZehQuait]

Thank you very much, frus! Now I see how it works!
Then maybe the cold bias in this channel could be responsible for the differences in tone mentioned by various people... hmm

Also there are lots of reports about how finicky and unstable this tremolo channel is. I've built an amp using the Matchless Hurricane tremolo and never had a problem with it not starting or whatever, and I tried many 12AX7's. Maybe this arrangement with one triode contained inside the other is too dependent on the specific tube used and thus not very reliable?

[Enzo]

The two channels are very different to start with. The other channel has two triodes in parallel, the trem channel a single. They face different plate loads, the tone controls are different, there is some tone shaping between the trem triode and volume control that is not on the other channel, and even the coupling cap into the phase inverter is different. I am not surprised at all that they sound different.

[ZehQuait]

You're right, Enzo. Many differences between the two channels after all. Especially on a relatively simple signal path like this, where every little change seems to affect the final tone considerably. To me the simpler circuits are the best for learning the effects of tweaking a part value here and there and listening to the result besides learning the theory! Thank you for sharing your opinion!