Observations on Plate Resistors and Bright Caps

[GlideOn]

Yes, the JMPs and early JCMs have three droppers!

Before the JCMs were more uniformly standardized in the later 80s; more mass produced going forwards. More demand for clarity and headroom under gain.

That's not my amp though - the Traynors follow early Marshalls the most.

There was a video from Headfirst where he compared the differences between the early Vertical input and the later Horizontal input.

He modded the 86' spec to early 80' spec and it sounded nearly identical to the early model amp - far more saturated and clearer to the average listener having added an individual 10K Dropper for V2, but he also removed a Hot Shield from V1, did a bit of lead dressing which can explain the difference in clarity too.

Proof that playing around with voltages and loads can have a positive effect!

 

[Pete Farrington]

Here’s a detailed but not too intimidating run down of the factors and equations that determine the operation of a common cathode gain stage https://aikenamps.com/index.php/designing-common-cathode-triode-amplifiers

 

[Taxman1966]

That's not at all what I mean lol. You're acting like this is some big discovery when, as you agree, every amp builder has been doing it for 117 years. Like I said earlier, might be a good idea to grab a couple books on the subject. TUT series is pretty cool and breaks down tons of mods and circuit design tricks you can use to tweak an amp to get it sounding how you want. Figuring out that plate resistor and bright cap values... like you know... do stuff... isn't a big revelation. I'm being snarky, but it's in good fun

Goosing V1a plate load is usually the very first step in "hot rodding" an amp, along with a bunch of other mods that work together. But if you think changing that one resistor yields a huuuuge difference, try messing with NFB. Replace the NFB resistor with a pot and you can hear the effects in real time. Same with slope.

Hey FourT6and2,
I really, really hope that you don't think of me as impolite or rude to get in contact with you like this, but the word on the street is:
"If you're still unsure from all the above help, shoot a PM to a member here named "FourT6and2"....he can close one eye, look over the opposite shoulder with a mirror and tell you what you need to know about your gut shots."
That got to me haha. I would apreciate it so much, if you could help me out with my '82 JCM800 2203 problem, which is so hard to dial in, with its unusual scratchy top end and overall slightly broken sound, but surprisingly sweet, clean Low Channel.
The topic: "Is my '82 Marshall JCM800 modded?"
(I took pictures of the inside of the amp)
If you don't have time for this, that's of course ok too!

Best regards!

 

[rocker68]

Fantastic thread. Very good resource. Thanks to all for posting.

 

[arthur.lowery]

We can think of a valve/tube as a voltage-controlled current source, so we might expect a higher-value load to give more gain (gain = transconductance x load resistance); however, the valve also has internal impedance (rp), which acts in parallel to the load. For an ECC83/12AX7 this is about 62 k ohms at 1 mA. Now resistors in parallel have a combined resistance of 1/(1/R1 + 1/R2)), which basically means that if the anode load is >> rp, it isn't going to make much difference to the gain. The datasheets give some useful examples.

Furthermore, the load is actually affected by the next stage's impedance (in parallel with it, so the effective loads is 1/(1/rp + 1/Rload + 1/next stage)).

Anyhow, another way to thing of a valve is that the anode and the grid both affect the current out of the cathode (Child's law?). This gives rise to mu - the amplification factor - which is about 100x for an ECC83, and essentially fixed by the internal mechanical arrangement of the valve (interestingly the ratio anode-grid capacitance over grid-cathode capacitance). You could think of this as internal feedback, which limits the maximum gain to rp * transconductance. I like this because valves are much more predictable than transistors, which are very temperature dependent!) The internal feedback also has advantages.

So, Pete is correct, but also adds the fair point that if the anode load is too high a value, the anode voltage is no going to be very large, leading to other issues. That said, some early car radios used anodes off the 12 V battery! (Early Mullard data books showed hybrid valve-transistor circuits like this!).

Dinner is ready
Arthur