#124 Questions

[erwin_ve]

Modman: are we going on this all over again? I remember you had some diffuculties on a thread understanding the concepts of low pass and high pass filters. If a 350k of 500k trimmer doesn't make any difference: WHY is the a resistors in front of it? What does it do?
The best way is to solder the circuit in with fixed resistors, record it and then desolder and put the trimpot circuit in, record it. Post your findings. Does it match with the theory?

[modman]

Modman: are we going on this all over again? I remember you had some diffuculties on a thread understanding the concepts of low pass and high pass filters. If a 350k of 500k trimmer doesn't make any difference: WHY is the a resistors in front of it? What does it do?
The best way is to solder the circuit in with fixed resistors, record it and then desolder and put the trimpot circuit in, record it. Post your findings. Does it match with the theory?

Yes, I had difficulty understanding the snubber caps, which is still something else then a high or low pass filter. Nobody on the board could really explain why a cap in that position functions as the filter that it is.

The 120k inline with the signal reduces it.
The trimpot is a simple volume control, exactly like the one in a guitar.
the 68k is a grid stopper.

So, Mr Know It All,
Why is the grid stopper necessary, considering the fact that there is always at least 120k in line?
Where is the filter? I don't see any capacitance except the grid to plate capacitance like you mentioned, but we are talking about the resistors and the trimpot now.

[erwin_ve]

Modman: I'm not Mr Know It All. I'm learning just like you. But considering your remarks I'm afraid we are on the same path as on the thread with the snubbers.
On grid stoppers: http://www.aikenamps.com/index.php/grid ... -they-used
Does this anything mean to you: f= 1/(2xpixRxC) ? The capacitance is about 150pF for the tube.

[pompeiisneaks]

As he stated, the tube itself has some capacitance (Miller capacitance I think) and so you can look at that capacitance on the data sheet and see the frequency range via that ugly calculation (thus why I wanted an app to do it )

V(out) 1/CR
-------- = -------
V(in) s + 1/CR

or whatever

~Phil

[pompeiisneaks]

Oh and modman I hadn't noted the 'Mr Know it all' comment, that's quite uncalled for. Thank you Erwin for not taking the bait on that. Erwin is trying to help, I don't see how being rude is consistent with a useful attitude.

~Phil

[Bombacaototal]

Many thanks Erwin for sharing your knowledge and helping as always. I really appreciate your insights!

[M Fowler]

There are a few guys on this site that definitely know a lot and they share their knowledge with us thankfully.
I won't give names because they are very shy!

Mark

[modman]

By the way, is there a consensus that the high plate overdrive (ie #102) is nicer than the low pate overdrive (ie #124). Also on the #124 it must have been a 500K trimmer to have it set at 346K, correct?

This was the original question, but looking at the 124 schematic, it hints at a 500k trimpot

Schermafdruk van 2018-10-05 21-32-48.png

Why does it say 'cmpt marked 500k'? what is cmpt? Was the board marked 500k? Was the trimpot marked 500k but in reality 350k?

But there is also an asterisk, which refers to this:

Schermafdruk van 2018-10-05 21-33-02.png

So it is a 250k pot? with a 220k in series? That's 470k

Anyway, there is a lot of confusion about these schematics and this is only natural, but it does not help to consider them the Bible.
I don't understand why all this series resistance is there, and still convinced a 500k trimpot will work too. Or a 250k. I don't think a 350k trimpot ever existed, but that's beside the point.

[erwin_ve]

It's not hard to try for yourself:
1. #102: 220k+100k trimpot(set about 25k)
2. #124: 220k+345k trimpot (set about 100k) trimmer was marked 500k but measured 345k one of Mr. Dumble surprises.
3. Your option: ditch all the resistor and use a 500k trimpot.

Let your ears be the judge!

Edit: #124 had the right values now.

[Charlie Wilson]

I am Curious, where does the 120k resistor in reference to #124 come from?
Cw

[norburybrook]

I am Curious, where does the 120k resistor in reference to #124 come from?
Cw

me too. All the layouts and schematics I have show a 220k.



M

[Bombacaototal]

I was actually wondering the same as well

[erwin_ve]

Youre right it should be 220k.
Edit my post

[Bombacaototal]

And just a curious is there any difference between 1M pot and a 500k resistor across or a 500K pot and a 1M resistor across to get to 333k as far as taper, etc?

[martin manning]

On my 124, I used a 100k trim pot, 220k series in front, and 68k grid stopper, like the other dicumented ODS circuits. The 350k used on 124 is an outlier IMO. In the original, the pot was marked 500k, but appeared to have been modified. My recommendation is to go with a 100k trimmer.

That network (the two resistors and the trimmer) is shaping the frequency response in several ways. Firstly, it is part of the AC load on clean2 when OD is on. Secondly, combined with the clean2 coupling cap it is a high pass filter, and finally, when combined with the Miller capacitance of OD1, it forms a low pass filter.