GRID LOAD RESISTORS

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BJF
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Re: GRID LOAD RESISTORS

Post by BJF »

Hi,

Screengridresistors and gridresistors both combat oscillation and it works since there are inherent capacitances in the tube. Usually parasitic oscillations occour at high frequency, often so high that it won't be heard through the speaker but it may cause the amplifier to run at full output but with no sound, there may also be metallic or ringmodulated sounds or high pitch squeeling all dependent on the frequency of oscillation. Since this is a high frequency problem proper HF lead dress may certainly help and so these resistors are best mounted directly at the tube socket.
The values for gridstopper resistors are determined by what gives appropriate safety marginal values generally range from 1K Ohm to 56K Ohm depending on amplifier
Values for screengridstopper resistors are defined by the screengridsupply structure and typical values range from 100Ohms to 2,2K Ohms- if the value gets too high it will lock the tube.

For gridstopper resistors in preamps interstage these can be used to isolate the workingpoint of the previous stage and also to limit current when recieving grid goes positive: when that happens the grid will no longer isolate but work as a diode positioned like anode to grid and cathode to cathode, and so at the cathode where there may be a capacitor a rectifier is formed and the ripple that gives becomes part of the output at the anode of the tube.
Perhaps this is called blockingdistortion these days?

Anyway, gain may be higher between two stages if the gridstopper has a higher value because the actual load at the first anode will be higher, but if value becomes too high there will be frequencylosses due to internal capacitances, while this can be to some part equalized by a speeding capacitor.

Generally low voltage to the preamp would give more saturated distortion, while higher voltages would give higher headroom, and there could be various sweetspots depending on what is to be achieved

Just a few thoughts
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novosibir
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Re: GRID LOAD RESISTORS

Post by novosibir »

Good delivery of insight, Bjorn!

Btw. welcome on this place!

Larry
Larry's Website now with included Pix's Gallery
heisty
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Re: GRID LOAD RESISTORS

Post by heisty »

Hi thanks stavarone , your right the voltages are all over the place to the preamp section , why so? and how would higher voltages improve , i know messa boogie uses real high voltages to their preamp section running the tubes right on the limit
CHEERS
heisty
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Re: GRID LOAD RESISTORS

Post by heisty »

Statorvane sorry for spelling ya name wrong , its early in the morning here on sunday and i have a bad cold , not thinking straight ,
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statorvane
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Re: GRID LOAD RESISTORS

Post by statorvane »

Generally low voltage to the preamp would give more saturated distortion, while higher voltages would give higher headroom, and there could be various sweetspots depending on what is to be achieved
Well, the JCM800 has substantially higher voltages at V1 compared to the 1987. Since this has an additional gain stage compared to the 1987, it is possible the preamp voltages are higher to improve headroom in the preamp as BJF notes above.

I don't know anything about the Mesas.
BJF
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Re: GRID LOAD RESISTORS

Post by BJF »

Hi,

Thanks for the kind words.

Some thoughts:
In a good sounding plexi you'd often find the voltage for the first two twintriodes around 190V's, giving about 100V's at the anodes. This is on the lowside but there aren't that many stages cascaded so saturation will occour at the second stage( second twin triode and due to the on the low side voltage and depending on the workingpoints,i.e the voltages at the electrodes of the first trodepair there may even be a bit of distortion here. A design like this will have a point of balance set by the total level through the stages, hence a bit of damping between the first stages and the summation amp( Stage 2) will aid definition: likely around midrotation with a logarithmic pot as Volume the most dynamic response can be set, meaning that distortion can be manipulated by the signal strength

To get more dynamic range voltage can be increased, and this would be the rout if there were many more stages involved.
With more stages, involved no stage need to run at maximum gain and even then as the gain of each stage add up logarithmically, like if first stage gives 30dB of gain and second stage gives 30dB of gain and third stage gives 30 dB of gain and losses between stages are -20dB the total netgain is 30dB+30dB+30dB-20dB=70dB as compared to say a standard two stage preamp that would give 27dB in first stage and 33dB at second stage and interconnection losses of say- 20dB which gives
27dB+33dB-20dB=50dB and if you'd force the latter to give a gain of 70dB it would choke on peaks
You'd find with the design, and there can be many a window defined by the workingpoints in which distortion can be controlled dynamically.

Ah Mesas, well those have many stages.......

I think you might be talking about blasting this amplifier and yes if you'd listen to the preamp of a Marshall Major it distorts slightly but to get it to overload and sustain you'd need to overdive also the poweramp and when the whole amp distorts you can address the various distortions that occour in each stage including the preamp.
Try to define what you don't like in the sound and try to isolate where that occours.
For instance if you play the amp and find that bass gets a bit muddy, try to balance this at e.g the cathodes of the triodes in the preamp or adjust the inputcap of the poweramp, then proceed to other levels and try to define what is wrong and how to get past that.

On voltages and loads, you'd find that lower loads give harser distortion and so does real high loads.
How a tube will distort, and yes sooner or later it will,especially if it sits in a guitar amp, is governed by the workingpoints of the tube and you will adjust that via the cathode bias and the plateload, the latter keeping in mind that the real dynamic load is the parallell sum of the plate load and the external load.
This is perfectly evident if you look at your amplifier from an AC point of view
Now if you don't have the loadgraphs you can use your ear, in fact even if you have the loadgraphs and know how to read and extract information from those you'd still first use your ear and then mathmatecally explain what you like as in drawing a picture in numbers.

Have fun
BJ
chris_d
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Re: GRID LOAD RESISTORS

Post by chris_d »

Thanks for both of those posts, BJF!

-chris
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