Not understanding these schematics

[bob493]

Howdy; Im utterly confident you guys probably have these questions all the time, so I'll try to make it super easy and painless. I have a fairly decent background in engineering; but I've never done anything with tubes before and these schematics drive me bonkers lol. Im also not understanding why there appears to be like 2, 3, more, "loops" of the same signal going through the output transformer; that or I have a pretty big concept error going on. I've done a fair amount of looking into this stuff already, but explanations are either way too vague, or dont cover what Im not getting here.

So, altered image on left, if my "flowpath" is wrong, please lemme know lol. Untouched schematic on the right


Anyway thanks any help!

[Helmholtz]

ad 2: The long tailed phase inverter (LTPI) has some inherent asymmetry, so different plate resistors are used to improve balance of output signal.

Generally I don't understand what your blue/red lines and the arrows mean.
If they are meant to indicate current paths, you need to distinguish between DC and AC (signal) currents.
E.g. a cap doesn't pass DC current, large filter cap are AC shorts, supply nodes are signal grounds etc.
Power supply nodes (ideally) don't carry AC (signal) voltages, but signal currents do flow through them to ground..

It is easier to follow signal voltages than signal currents.

The output of the DC coupled cathode follower (V2) is taken from the right side cathode, the plate is connected to B+ (supply voltage), which is a signal ground.

ad 3: This is where the negative DC bias voltage is supplied for the power tube grids.

[Jazz P Bass]

I am seeing 'signal' paths in the incorrect spots (like on the power supply rail)
Question #4 is the feedback loop.

[eschertron]

This ^^^
the orange line (1) is power supply, not signal.

The RH triode of tube V2 under the 'orange arrow' is a cathode follower; the signal leaves the valve on the cathode connection, not the plate. Somewhat confusing, but you'll never make that mistake again

next to that tube, the network with the left-most (4) is the tone stack. signal gets filtered here. It leaves the preamp and goes to the power amp (Phase splitter) after the master volume pot labeled 'M'. be careful about crossing wires versus joined wires. Your light blue seems to have made that error.

(5) is the ground reference for the power tubes. Cathode grounded, and also here, supressor grid. some tubes are tied internally, this shows an external connection.

(3) I think you are asking about the bias circuit. google 'tube bias' for more than you'll ever want to know.

the RH (4) is next to the output transformer's speaker taps (different secondary impedances are provided here, other OTs may only provide a single tap). The negative feedback (NFB) is tied to one of the taps. HTH

[Helmholtz]

When speaking about "signal flow", it should be defined if this means signal current or signal voltage.
Mostly the term is used for signal voltage (which actually doesn't flow - just nitpicking here ).

To understand how signal voltage is processed, it's essential to identify inputs and outputs of amp stages.

I recommend to always start with the power supply structure when analyzing a schematic.
Once you recognize the power supply (including ground) wiring, you know where there can't be any signal voltage.

[bob493]

Thank you guys. I guess I have like a decade of "unlearning" to deal with here; the closest I've ever gotten to combined AC/DC circuitry are stepped sine inversion for UPS systems.... I'll figure it out; just gotta start fresh here I guess. One of my biggest complaints was the lack of standardization of electrical schematics; and holy moly these amp ones are just taking it to a whole new level lol.

[The Dude]

When speaking about "signal flow", it should be defined if this means signal current or signal voltage.
Mostly the term is used for signal voltage (which actually doesn't flow - just nitpicking here ).

To understand how signal voltage is processed, it's essential to identify inputs and outputs of amp stages.

I recommend to always start with the power supply structure when analyzing a schematic.
Once you recognize the power supply (including ground) wiring, you know where there can't be any signal voltage.

^^^^^^ That's good advice right there.

[bob493]

ad 2: The long tailed phase inverter (LTPI) has some inherent asymmetry, so different plate resistors are used to improve balance of output signal.

Generally I don't understand what your blue/red lines and the arrows mean.
If they are meant to indicate current paths, you need to distinguish between DC and AC (signal) currents.

Im CLEARLY messing something up here; but I'll explain my thought process I guess;

Starting from "LOW" as the input signal -> goes to grid of valve 1 -> output to the anode section, goes through filter cap; I guess that resistor is used for ripple reduction?

Feeds into grid of valve 2; then cylces back to get amplified again(???) (this is where it starts getting confusing to me; why is the output of the left side of valve 2 connected to the right side, but also loops?)

Then here it just totally breaks down for me... Im clearly going the wrong way... frustrating :-/


Then we have the power transformer; Im assuming the dual contact switch is the standby switch; then goes through the rectifier into an inductor? Is that for noise reduction or something? But it has that connection to the output transformer? I genuinely cant figure a reason for that; unless thats just a shared line for the AC output or something. From there Im ok until that resistor on the top left; it has a filter capacitor on both sides? Why? Valve 2 has a massive resistor in front of one plate vs the other; again im lost. Finally, I cant figure out what that weird "wye" looking section is before valve 3 on the cathode/grid side is.


A good nights sleep; a cup of coffee and some reading is in my future I think lol

thanks guys!

[g1]

You're still running the signal into the power supply. How about posting the clean schematic (or tell us the exact model and version).
One of us can draw in the signal path and you can ask about whatever you don't follow.

[Enzo]

Adding to what Helms said, Your phase splitter has two different plate resistors The tiny 47pf cap is not inline with one plate. it is wired between the two plates.

#4, note the lower line is connected to the C or COMMON terminal - the terminal usually grounded. The upper line is the NFB line.

#5 just shows the very common internal cathode to suppressor grid connection in most pentode power tubes. And cathodes are grounded.

#1 The signal leaves the cathode not the plate, to drive the tone stack, like a zillion MArshall amps.

The other #4, is the tone stack, and is fed by the cathode of the previous triode.

[Jazz P Bass]

My feelings are that the OP does not understand how a tube amplifier actually works.
No offense intended, just an observation.
Try this link for a start: Marshall (robrobinette.com)

[Dave H]

Then we have the power transformer; Im assuming the dual contact switch is the standby switch; then goes through the rectifier into an inductor? Is that for noise reduction or something? But it has that connection to the output transformer? I genuinely cant figure a reason for that;

Yes, it's the standby switch. The inductor (choke) is for ripple reduction, it forms an LC filter with the following capacitor.

You don't get “owt for nowt” (anything for nothing) in this life. If you want to take 50W of power from the output transformer secondary for the speaker then you have to put (at least) 50W of power into the output transformer primary. All that power comes from the amplifier's power supply (the clue is in the name). It enters the output transformer via “that connection to the output transformer” as you said. The signal voltage on the grids of the output valves controls the valve's plate current and therefore the OT primary current just as your hand on a tap leaver controls the flow of water.