High plate - low plate

[Aurora]

It may be just my ignorance, but I can't find any explanation for the difference....? I'm assuming is deals with anode voltage on the preamp tubes, but right or wrong? Could anybody tap e few words about pre and cons with the different versions?

[pompeiisneaks]

The basics are that the anode(plate) resistors are higher, 100k for low plate and I believe 220k for the High plate. There are other changes as well, but that's the reason for the naming.

~Phil

[Aurora]

Yeah - just as I thought...and thanks.... but I did find some references where somebody talked about tonal differences...?

[pompeiisneaks]

Tonally biasing the 12AX7's at 220k instead of 100k puts them lower down on the load lines area, which means you get a bit more overall room, but have a bit less linearity. I think effectively low plate has a bit more distortion when pushed, but the low plate gives a bit more clean headroom.

I think that's how I understand it anyway.

~Phil

[Aurora]

Thanks

[martin manning]

Two things happen going to the high-plate configuration. One is you get more gain due to the higher Ra (about 1 dB for 220k vs. 100k), and you also get more distortion products, about double the 2nd, 3rd, and 4th, but that is more about the chosen Rk. The plate load is a secondary contributor to the bias point, which depends more strongly on Rk. Note Rk is different for each Ra value (1k5 for 100k, 3k3 for a 220k, e.g.). The result is that the operating point is positioned a little closer to cutoff in the high-plate circuit.

[pompeiisneaks]

Two things happen going to the high-plate configuration. One is you get more gain due to the higher Ra (about 1 dB for 220k vs. 100k), and you also get more distortion products, about double the 2nd, 3rd, and 4th, but that is more about the chosen Rk. The plate load is a secondary contributor to the bias point, which depends more strongly on Rk. Note Rk is different for each Ra value (1k5 for 100k, 3k3 for a 220k, e.g.). The result is that the operating point is positioned a little closer to cutoff in the high-plate circuit.

This sounds like I may have gotten it backwards? I thought when you had the slope down lower near the non linear area, it gave more amplification room, but less linearity, or am I conflating two things?

~Phil

[dbeasley]

With Dumble circuits isn't there a ratio he follows? I recall 1:1.16 or something like that?

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[martin manning]

There are a lot of things going on here. Yes a higher plate load will mean more available voltage swing from Vg1 = 0 to cut-off Vg1, and gain will increase, driving the following stages a little harder. But, the exact gain and the distortion produced will depend on the bias point, which is determined by Rk. Colder bias will produce more distortion and reduce gain (mainly because plate resistance ra increases). For example, if the 220k Ra stage's Rk is reduced from 3k3 to 2k2, it will clean up considerably, and pick up some gain. I think HAD chose his Rk values by ear, and the fact that they are all about 1.5% of the Ra value is just a coincidence.

[dariez75]

I’m not a technician as you but only using my ears, the difference between high plate and low plate are: low plate big low in clean and more gritty distortion (Marshall like), high plate more gain but a fuzzy thin distortion. This is what I found experimenting with my ceriatone FM 50.


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[erwin_ve]

There are several combinations seen on Ods:
V1a, V1b, v2a and v2b all Ra=100k, Rk=1k5 (as in early Fender, most lineair amplification) this is low plate.

V1a, V2a Ra=220k, RK=3k3 and V1b, V2b Ra=150, Rk=2k2 this high plate.

Hrm models have sometimes on V1 or V2 the Ra=180k, Rk=2k7 and Ra=120k, Rk=1k8 combination.
All Ra/Rk combinations are 66,6:1, I guess he had something with this ratio

[martin manning]

Could be, I suppose, but unless someone has some first-hand knowledge we'll probably never know. Higher Ra requires a higher Rk to recenter the operating point, but all these combinations go a little more toward cut-off from the more-or-less standard 100k/1k5. Since resistors are made in standard values the possible ratios will do the same.

[M Fowler]

Load lines chart

Plate load and cathode values chart.jpg

[Mr. dB]

Load lines chart

Plate load and cathode values chart.jpg

Those gain numbers are without a cathode bypass cap, right?

[M Fowler]

Correct no cathode bypass cap.