#124 Questions

[Bombacaototal]

I know I am overthinking but a quick question, for those using the GNFB on a switch (on the back plate), is it worth going coaxial given the proximity to the pre amp tube heater wire? I have mine with a unshielded hook up wire now but haven't wired it up already

[norburybrook]

I know I am overthinking but a quick question, for those using the GNFB on a switch (on the back plate), is it worth going coaxial given the proximity to the pre amp tube heater wire? I have mine with a unshielded hook up wire now but haven't wired it up already

don't you mean LNFB on a switch by V1? if so no it doesn't need coax, it's so short.



Marcus

[Bombacaototal]

Hi Marcus, thanks for the reply. I actually mean the global negative feedback (ie 4.7K) on the PI. I have it on a switch to be able to disconnect it as well! Mine covers some distance as I wanted to keep the switch on the low impedance part of the circuit (left)

EDIT: I do not have the trimmer on the PI plates yet. Does the advent of the trimmer make a good difference?

[norburybrook]

I don't think you need coax at that point in the signal chain. I put a GNFB on my wonderland with regular cable.


The PI trim, it's a subtle balancing tool. It's definitely worth putting on for the sake of a trim pot.




MC

[Bombacaototal]

I don't think you need coax at that point in the signal chain. I put a GNFB on my wonderland with regular cable.


The PI trim, it's a subtle balancing tool. It's definitely worth putting on for the sake of a trim pot.




MC

Thanks Marcus, I will keep the normal hook up cable then....I know I was overthinking this

Thanks for the feedback on the trim pot as well. I've seen HAD amps with 5KB, 10KB and 25KB. Any preference as far as the pot value? I m thinking about using the 10KB..I would like to be able to adjust the "dynamic balance" from outside like the ODS #121, and therefore using a bias pot instead of a trimmer. Would it be a really bad idea to have the pot between the power switch and the fuse right by one of the power tubes (pic below)? I am thinking yes...but I ran out of space lol...(by the way the pot on the right side of the pic is the bias)

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?

[Bombacaototal]

Overthinking inquisitive mind at work:
Would there be an audible difference between RN65D (1/2W) and RN60D (1/4W) the tonestack cathodes? Most of HAD seems to be 1/2W but I've seen some Two Rock with 1/4W..which got me to ponder that maybe the 1/4W would add to the compression (just speculating)

Also is there an audible difference between Orange Drop 6PS 400V and 600V?

[norburybrook]

Raphael,

The trimmer pot isn't set to 346K it measured 346K. It s probably a 350k trimmer but they're hard to find. If you do a search you'll find a way of adding a resistor across a 500k to get it in the ball park figure.


I wouldn't worry about the sound difference in 400 v 600 caps or 1/2 v 1/4 resistors. Just build the amp and enjoy it!


Marcus

[Bombacaototal]

Raphael,

The trimmer pot isn't set to 346K it measured 346K. It s probably a 350k trimmer but they're hard to find. If you do a search you'll find a way of adding a resistor across a 500k to get it in the ball park figure.


I wouldn't worry about the sound difference in 400 v 600 caps or 1/2 v 1/4 resistors. Just build the amp and enjoy it!


Marcus

Thanks a lot for clarifying regarding the trimmer.

Thanks for the advice as well, I do overthink more often than not!

[modman]

Also on the #124 it must have been a 500K trimmer to have it set at 346K, correct?

The trimmer pot isn't set to 346K it measured 346K. It s probably a 350k trimmer but they're hard to find. If you do a search you'll find a way of adding a resistor across a 500k to get it in the ball park figure.

I wonder what magic the 350k potentiometer does that a 500k one cannot reproduce? Especially when it's a trimpot and not a control on the front.
Putting 1M across a 500k pot will give you 333k, but it will mess with the taper.

[pompeiisneaks]

I believe on mine I couldn't find a 350k so I just went with 300k. From what I'd read the pot was set around 270ish so I figured it would work for what I needed. It does I too wonder if the exact value of that pot matters, as it's being used as a variable resistor anyway, and therefore only matters if you get the value you 'need' Unless that value needs to be 330k and then not having the range from 300 to 350 on mine 'could' be a problem. In my case I KNOW it's not because it is at shutoff at the higher end of the pot, probably 300-280 ish k range, so I have to dial it down a bit to get to a 'sweet' spot for me anyway. I didn't measure the exact spot I liked, but I like it

~Phil

[erwin_ve]

The trimmer (346k) is a voltage divider, it was set at 100k to ground which leaves 246k+220k(in front of the trimmer) resistor=466k.
So basically a 466k/100k divider.

It matters for this reason; the 466k is a low pass filter, the 100k defines how much signal is going to the grid of the od stage. So lowering the trimmer results in a lower signal going to the od stage but also raises the 246k part in front of it resulting in the alteration of the high pass filter.
Combining this with the capitance that a tube has you actually can calculate the frequencies affected.

[dorrisant]

Also on the #124 it must have been a 500K trimmer to have it set at 346K, correct?

The trimmer pot isn't set to 346K it measured 346K. It s probably a 350k trimmer but they're hard to find. If you do a search you'll find a way of adding a resistor across a 500k to get it in the ball park figure.

I wonder what magic the 350k potentiometer does that a 500k one cannot reproduce? Especially when it's a trimpot and not a control on the front.
Putting 1M across a 500k pot will give you 333k, but it will mess with the taper.

You are not going to be fiddling with the pot too much... 1M strapped across a 500k trimmer works well.

[pompeiisneaks]

Oh yeah you're right! Sorry that's completely correct and I'm up in the night. That being said, though, I'd love to have a tool that easily lets you see the frequencies at different levels. I know the duncan tone stack calculator does this 'somewhat' but only for pre-defined tone stacks. I can't setup just this one to see. Anyone have any ideas on how you could? Is this a spice thing?

Ultimately, though, that 100k to ground with that particular amp was what he devised for that player, for what they needed/wanted, correct? We don't know the exact reason 'why' but he may have had a default pot there, tested for the player and found they asked for more or less of the specific sound he was going for, so he knew to do the math and get the right frequency for them? I've read somewhere on here that people suspect he may have opened pots and scratched at them gently to adapt the resistance of the pot, but is that really what he did? Or is this just a standard 350k pot that has standard variances and thus why it measured 347k? That's within 1% of tolerance.

I'm trying to filter out the 'guesswork' and find out the facts I guess

~Phil

[dorrisant]

I think that the trimpot is there to get the overdrive to work correctly but also to adjust it to the players needs. Why not put a trim on the back of the amp, or put one on the chassis like a Fender bias pot... out of the way but accessible.

[modman]

The trimmer (346k) is a voltage divider, it was set at 100k to ground which leaves 246k+220k(in front of the trimmer) resistor=466k.
So basically a 466k/100k divider.
It matters for this reason; the 466k is a low pass filter, the 100k defines how much signal is going to the grid of the od stage. So lowering the trimmer results in a lower signal going to the od stage but also raises the 246k part in front of it resulting in the alteration of the high pass filter.

Schermafdruk van 2018-10-05 10-11-26.png

I really don't see a low pass filter, except in the sense that a volume control can act like one. Seeing such a network of series resistance doesn't seem logical or smart design. I suppose it did sound good in the amp in question, but if all these values were a result of tinkering, they shouldn't be adhere to as if it were the Bible.
The 500k trimpot will give you more gain at max, because of the extra resistance to ground.

I would first try and use a 500k trimpot and lose the 68k and the 120k...

Combining this with the capitance that a tube has you actually can calculate the frequencies affected.

that is the case for any 12AX7 stage...