talbany wrote:Actually it would need to be wired like a variable resistor.. If you ground the bottom like a Volume control you will be shunting voltage to ground. For a Dumble style stack I would add a 100k linear pot in series with a 50k fixed resistor so that you could adjust it from 50k to 150k..Keep in mind it's going to scratch like a mofo..also if it's carbon trace it will effect tone.
Tony
Sorry Tony, I didn't mean to say that it should be wired like a volume pot. What I meant is that there is no DC on the slope resistor the same way that there is no DC on the volume pot.
But please explain to me what possible path there is for direct current through the slope resistor...
Last edited by dave g on Sat Apr 17, 2010 5:35 am, edited 2 times in total.
The problem with mounting it to the chassis is insulation break down on the pot cause an arch or short to the chassis fireworks.. If you decide to mount it to the chassis I would isolate it.. Good Luck
Tony
" The psychics on my bench is the same as Dumble'"
talbany wrote:Actually it would need to be wired like a variable resistor.. If you ground the bottom like a Volume control you will be shunting voltage to ground. For a Dumble style stack I would add a 100k linear pot in series with a 50k fixed resistor so that you could adjust it from 50k to 150k..Keep in mind it's going to scratch like a mofo..also if it's carbon trace it will effect tone.
Tony
Sorry Tony, I didn't mean to say that it should be wired like a volume pot. What I meant is that there is no DC on the slope resistor the same way that there is no DC on the volume pot.
But please explain to me what possible path there is for direct current through the slope resistor... :roll:
Sure..The slope resistor is connected right off V1A plate resistor so sure there is voltage there.... Signal enters grid is amplified (Voltage) at the plate resistor/voltage divider is then sent through the slope resistor/ because the tone stack is effectively a collection of RC filters you're changing the "R" in RC which changes the center frequency of the filters and the filter's Q (the narrowing or widening of the mids scoop).. The signal you are referencing rides on the DC through the slope.
.If either side of the slope resistor finds ground bye bye fuse..
Sorry I didn't mean to imply that you would wire the pot like a Vol control..Just didn't want anyone to get confused.. I know I can be short and at times comes off in-sincere..
Tony
Last edited by talbany on Sat Apr 17, 2010 4:10 am, edited 1 time in total.
" The psychics on my bench is the same as Dumble'"
I think I see what you're misunderstanding. You are saying that because one end of the resistor is connected to a positive DC voltage, there is DC on the resistor.
I'm saying this is fundamentally incorrect - hence my analogy to the volume pot. What if you defined +200 volts to be "ground" and the chassis to be "-200 volts"? Does the volume pot now have DC on it?
There is no such thing as a voltage "on" something. There is only voltage ACROSS something. V = I*R, right? V is the voltage ACROSS the resistor. If you flipped on standby and applied no signal to the input, BOTH sides of the resistor will be at 200 volts. So, no DC current flows! Honestly, if you don't believe me please check it with your DMM...
.. I see what your getting at.. Sure without ground potential on either side 0 swing..However Not sure I follow the volume pot analogy.. With the bottom grounded your shunting voltage to ground if your talking about wiring it like a bias pot..or volume control.. If you wire it like a variable resistor no ground potential.. The signal still rides on the DC there as the resistance of the slope effects signal not voltage..
Tony
" The psychics on my bench is the same as Dumble'"
talbany wrote:.. I see what your getting at.. Sure without ground potential 0 volts.. Not sure I follow the volume pot analogy.. With the bottom grounded your shunting voltage to ground if your talking about wiring it like a bias pot..or volume control.. If you wire it like a variable resistor no ground potential.. The signal still rides on the DC there as the resistance of the slope effects signal not voltage..
Tony
I'm sorry, but I really do think you need to review basic circuit theory.
When I say DC, I am talking about a potential difference. This is absolutely the ONLY meaningful use of the word. If you connect one side of a resistor to a fixed voltage with respect to some reference, and leave the other side of the resistor flying, the flying side of the resistor will stay at the very same potential (with respect to the same reference) as the side connected to the DC voltage. Do you understand that? Current flows THROUGH a resistor when there is a DIFFERENCE in voltage ACROSS the resistor. So, there is no more or less current flowing through that same resistor if it was connected to "ground", because there is still no potential difference across it! That was what I was trying to get at with my volume pot analogy.
In the tonestack we're talking about, the middle and bass caps charge up to the quiescent plate voltage, so there is no potential difference between the two ends of the slope resistor. Therefore, no direct current flows through the resistor!
I'm really sorry but I'm starting to tire of the widespread technical ignorance on this forum. I think I need to take a vacation for a while
While there is no DC voltage drop across it, the thing is still sitting at about 200VDC. I think what Tony was thinking about, which you mention below, is that one would have to be careful when thinking about doing something like putting a pot there and mounting it to the chassis. The case would be a DC ground while the rest of the thing would be a couple of hundred volts higher. It would be within range for even Alpha pots, but not something I would do myself.
Cheers,
Gil
dave g wrote:
ayan wrote:
kd wrote:Unless I'm screwed up on my thinking today(possible), there has to be DC voltage at the slope resistor. It would be seeing whatever voltage the plate is seeing. Am I wrong?
Ken
You're right, Ken. Exception would be if there were a coupling cap after the preamp tube and before the tone stack, which I believe Andy still does in the Fuchs amps.
Gil
Come on guys
No, there is no DC voltage across the slope resistor.
While one end is connected to a DC potential at the plate, the other end is only connected to the bass and mid caps - so how can any DC current possibly flow through the resistor?
There's just as much DC current flowing through the slope resistor as there is flowing through a volume pot connected to ground (read: NONE WHATSOEVER) , because after all "ground" is still "DC" (just at 0 volts).
The only potential hazard would be one of the potentiometer lugs shorting to the case, and hence the chassis. Personally, I'd be more afraid of dielectric punchthrough in a coupling cap, or the grid of V1A shorting to the plate...
talbany wrote:.. I see what your getting at.. Sure without ground potential 0 volts.. Not sure I follow the volume pot analogy.. With the bottom grounded your shunting voltage to ground if your talking about wiring it like a bias pot..or volume control.. If you wire it like a variable resistor no ground potential.. The signal still rides on the DC there as the resistance of the slope effects signal not voltage..
Tony
I'm sorry, but I really do think you need to review basic circuit theory.
When I say DC, I am talking about a potential difference. This is absolutely the ONLY meaningful use of the word. If you connect one side of a resistor to a fixed voltage with respect to some reference, and leave the other side of the resistor flying, the flying side of the resistor will stay at the very same potential (with respect to the same reference) as the side connected to the DC voltage. Do you understand that? Current flows THROUGH a resistor when there is a DIFFERENCE in voltage ACROSS the resistor. So, there is no more or less current flowing through that same resistor if it was connected to "ground", because there is still no potential difference across it! That was what I was trying to get at with my volume pot analogy.
In the tonestack we're talking about, the middle and bass caps charge up to the quiescent plate voltage, so there is no potential difference between the two ends of the slope resistor. Therefore, no direct current flows through the resistor!
I'm really sorry but I'm starting to tire of the widespread technical ignorance on this forum. I think I need to take a vacation for a while :roll:
Dave
If not being able to understand your volume pot analogy right off makes you upset.. Than so be it...I admit it I don't know everything..
I do know this...the amp sitting between Brad Paisley's Trainwreck and Bruno was built by yours truly.. The technically ignorant..so you say..
Gill
Good to see you back.. Been a while... Always enjoy your posts!!Stick around
Tony
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Last edited by talbany on Sat Apr 17, 2010 3:50 pm, edited 1 time in total.
" The psychics on my bench is the same as Dumble'"
talbany wrote:If not being able to understand your volume pot analogy right off makes you upset.. Than so be it...I admit it I don't know everything..
I do know this...the amp sitting between Brad Paisley's Trainwreck and Bruno was built by yours truly.. The technically ignorant..so you say..
The volume pot analogy is about the DC voltage potential across a volume pot that is sitting at 0 VDC. This is the same as the DC voltage potential across a slope pot sitting at 200 VDC. The same DC current will flow, according to accepted theory.
If you built an amp that made it onto a stage it means you can build an amp and get it onto a stage. This is all well.
This discussion, however, is about understanding Ohm's law and how a capacitor works, and then applying this 'basic circuit theory' on the components of a guitar amp.
