driver biasing question

[catalin gramada]

Hello. does it any beneffit using individual cathode resistors against a shared one.I keept Gingertube ideea regards unmatched pair and hogging current from one to another. But supposed driver tubes are perfectly matched static condition, class A working all the way,of course. what are the benefits of common mode against individual biasing, please ? The second question regards bypass capacitor over shared cathode resistor. Why is necessary as it did almost nothing in common mode just raising the harmonic distorsion ? Can You enlight me please? I want to refere strictly at this situation: perfect matched pair, perfect balanced inputs and no constant current generator. Felt someone any sonic diferences and can share his impression,please ?Thanks.

[potatofarmer]

What do you mean by "driver"? I've seen that refer to gain stages between the phase inverter and the power tubes; is that what you're referring to?

[catalin gramada]

Yes. that.s right.Power stage driver

[J M Fahey]

What do you mean by "driver"?

Yes. that.s right.Power stage driver

Sorry but answering a question about a word by using the same word explains *nothing*
As in:
- "can you explain what you mean by kerfludonic sound?"
-"oh, simple, something that sounds kerfludonic"

so please describe at some length what does a power driver do in an amplifier circuit.
FWIW at this moment I am visualizing 3 different circuits , any of which could be called "power driver" , so a little schematic would be even better.

[catalin gramada]

Ok I presented graphic the situations I asked above. Suppose the tube have very good driving capabilities in all 3 instances and have not care to save components. The inputs are perfectly balanced and tubes perfectly matched. I asked from a design choice perspective







Sorry for confused question. Power stage driver means a driver for power stage in my understanding . What is the correct term to use in english , please ? Thanks

[mhuss]

I think you're using the correct English term, it's just that power drivers are unusual in guitar-sized tube amps -- I think that is what caused the confusion here. I've only seen them in 6+ output tube designs like the Fender PS400 and Hiwatt DR405.

The other thing is that the power driver designs I've seen either use a transformer in the anode circuit or a cathode follower design driving the following grids. Topologies like you have drawn (anodes driving the following grids) seem unusual.

[catalin gramada]

Hi. Thanks.I know what you mean as found more common to have enough voltage amplification from previous stages and buffering the inputs of power stages in case of "heavy loads" by own- in case of kt120 /150 or by summing.
But I.m particulary interested to drive by plates and curious to find the differences between situations I asked for in "real life". Thanks

[Mike Sulzer]

Ok I presented graphic the situations I asked above. Suppose the tube have very good driving capabilities in all 3 instances and have not care to save components. The inputs are perfectly balanced and tubes perfectly matched. I asked from a design choice perspective









Sorry for confused question. Power stage driver means a driver for power stage in my understanding . What is the correct term to use in english , please ? Thanks

I would choose number one. I would expect better balance and lower distortion, but only half the gain. For a guitar amp, you might sometimes get a surprise as to what actually sounds best with a given style. It would be easy enough to compare 1 and 2.

[catalin gramada]

Thanks Mike. Actually found the gain the same in common mode with shared resistor bypassed or not. but You-re right about distorsion. In theory at least the bypass cap should harmonics distorsions topped up

[eschertron]

Thanks Mike. Actually found the gain the same in common mode with shared resistor bypassed or not. but You-re right about distorsion. In theory at least the bypass cap should harmonics distorsions topped up

In my limited understanding, the current from the out-of-phase signals through the cathode resistor cancel each other out - in class A, and only on the chalkboard - so that the cathode voltage doesn't vary, and the need for the cap to prevent negative feedback is moot. That seems to be the result you came up with experimentally.

But only for pure class A, and then only as long as the output tubes are in a linear portion of their transfer. I would think that outside of that restricted range, the scenario without the cap would distort more due to uneven current flow causing a time-varying NFB. But isn't that the opposite of what you've stated? Help me understand.

[catalin gramada]

Hi. I think is commin from better balance, maybe... as I found the bypass cap (large enough for experiment proposal - 1000uF) reduced the gain a little bit. It was a tiny fraction but still recordable.
Does it have any sense, please ?
I don.t know nothing about time-varying NFB (pretty ignorant in this sense) Thought negative feedback is a voltage matter and from this point better balance helps a lot as time assure current consistency over shared resistor wich becomes common mode. These are my thoughts about

[Mike Sulzer]

Thanks Mike. Actually found the gain the same in common mode with shared resistor bypassed or not. but You-re right about distorsion. In theory at least the bypass cap should harmonics distorsions topped up

Yes, I forgot that you are driving both grids!

[catalin gramada]

with or without bypass cap output shows no big differences here at least at 20khz square:





have to play little bit maybe find some mojo in between. Thanks guys

PS: I still did not understood for what reason to choose one against the other as the amp works the same in both circumstances and is stable. Maybe will do a difference in overdriving condition but is not the case cause it is a clean kt120 power amp project

[catalin gramada]

...

[catalin gramada]

Driver balance. cathode or plates, please ?



Thanks