124 v2 180k grid resistor

[pottedplant]

Has anyone noticed that the 180k resistor going to the grid of v2 is possibly hand trimmed?

https://hackaday.com/2017/04/10/hackada ... resistors/

v2 grid resistor_cr.jpg

If you read the article he notes that it takes very little filing to alter the resistance once the coating has been removed. This resistor definitely has the coating removed. Is it possible that Mr. Dumble trimmed this resistor? What would finely adjusting the resistance (can only go up) do for this particular location? I suspect it might be a method of tweaking the input capacitance? Sorry if this has been covered before, I searched the 124 collection of information and found no mention of this so I figured I would ask here.

more info about trimming resistors from a secondary link in that hackaday article here: http://www.all-electric.com/b&c.html

edit: click on "THE FIELD PROGRAMMABLE RESISTOR" on the right

[dcribbs1412]

Interesting idea.
Thanks for the info!

[jabguit]

"The technique was taught to me by my late farther. "

[award70]

How does it work? Are you creating discontinuity in the layers of the carbon film ?

[jabguit]

How does it work? Are you creating discontinuity in the layers of the carbon film ?

read the link at the end of the OP

[martin manning]

How does it work? Are you creating discontinuity in the layers of the carbon film ?

The thickness of the film is locally reduced, which causes the resistance to increase. Interesting observation and certainly plausible... Maybe HAD was trimming that 180k up to reduce the high frequency response a bit.

[award70]

I see. Thanks Martin I didn’t notice the link in the OP.

[pottedplant]

How does it work? Are you creating discontinuity in the layers of the carbon film ?

The thickness of the film is locally reduced, which causes the resistance to increase. Interesting observation and certainly plausible... Maybe HAD was trimming that 180k up to reduce the high frequency response a bit.

Is it possible to sub in a pot to test how the value affects things? I have little practical experience or formal technical knowledge so that's how I'd go about trying to figure it out but I assume there are many caveats to doing such a thing?

[martin manning]

Is it possible to sub in a pot to test how the value affects things?

Sure, but I would use a trimmer pot and solder it close to the tube socket, much like where that resistor is located.

It could be that HAD just didn't have the value he wanted on hand, so he improvised. Might want to try a 220k or 270k there.

[pottedplant]

Is it possible to sub in a pot to test how the value affects things?

Sure, but I would use a trimmer pot and solder it close to the tube socket, much like where that resistor is located.

It could be that HAD just didn't have the value he wanted on hand, so he improvised. Might want to try a 220k or 270k there.

Ahh thanks, and for the mathy side of things where should I go reading if I wanted to learn about what is actually going on/calculate the filtering without experimenting manually? I know this is your forte so I figured I would ask. I can google, just don't know WHAT to google

[martin manning]

Look into Miller Capacitance and Grid Stopper. Here's a good article: https://www.aikenamps.com/index.php/wha ... apacitance

[pottedplant]

thank you, will do some reading!

[martin manning]

thank you, will do some reading!

Another one: https://www.valvewizard.co.uk/gridstopper.html

[pottedplant]

thank you, will do some reading!

Another one: https://www.valvewizard.co.uk/gridstopper.html

This is fascinating.

"However, because the anode voltage signal is an amplified version of the signal on the grid, the voltage across Cga will be much larger than if the valve didn't amplify. The resulting input current is therefore also larger. For example, if the gain of the valve is -60 and we input a +1V signal, the anode voltage will fall by 60V. The total signal voltage appearing across Cga will therefore be 61V. As far as the input is concerned it looks like we have an additional capacitive load that is 60 times larger than the Cga we already had. "

Aiken also says:

"The term "Miller capacitance" is often seen when reading about guitar amplifier circuit design. It refers to the effective multiplication of the plate-to-grid capacitance in a triode tube (or transistor) by the gain of the amplifying stage."

are these two passages saying the same thing? If i'm to understand this correctly? It is sort of a dynamic capacitance?

In the valve wizard quote he mentions that the anode voltage will fall by 60v if a 1v input is given, is this effectively seen as sag on the b+? Sorry to bombard with so many questions. This is quite a fascinating aspect of tube amps, further reading has led me to understand that you have to carefully balance how many stages you have with this capacitance in mind because, to my understanding, it will effectively compound the high frequency losses the more stages you have. I always wondered why tube amps didn't gradually bring the signal up to distortion over many series of stages and I suspect this is a major component of that? If i'm way off and you feel like letting me know please do. haha Thanks for the shares!

edit: i also had another thought, the signal that's coming back to the grid from the plate via the capacitance of Cga is inverted from the grid's signal right? There is so much going on...

[martin manning]

In the valve wizard quote he mentions that the anode voltage will fall by 60v if a 1v input is given, is this effectively seen as sag on the b+?

No, that's the voltage drop across the plate load resistor.

further reading has led me to understand that you have to carefully balance how many stages you have with this capacitance in mind because, to my understanding, it will effectively compound the high frequency losses the more stages you have. I always wondered why tube amps didn't gradually bring the signal up to distortion over many series of stages and I suspect this is a major component of that?

This mechanism is basically rounding off the corners of the clipped waveform at the source, removing high frequency content and "smoothing" the distortion.

i also had another thought, the signal that's coming back to the grid from the plate via the capacitance of Cga is inverted from the grid's signal right? There is so much going on...

Aiken and Blencowe are saying the same thing there, and you are right the Miller capacitance forms a negative feedback path from anode to grid, which limits the high frequency response. It's equivalent to a capacitor going to ground after the grid stopper, making a low pass RC filter. The C is (Cg-a + Cpar) * (1-A) + Cg-k, where Cpar is parasitic capacitance, for example the pin-to-pin capacitance of the socket, and Cg-k is grid-to-cathode capacitance, which is not subject to the Miller effect. In RF circuits this HF roll-off is often undesirable, but in audio it's an effective way of controlling high frequency content.