Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on July 07, 2010, 03:49:11 pm
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I've got this new "rebuild" I am tweaking in and noticed the clean and OD channel don't match up very well compared to other builds I've done that are similar.
So, I measured the Mstr Vol pot and it's a 1M pot out of the circuit and closer to 180-220K max (IIRC) in the circuit across the outside lugs and from center to ground (no matter how I have it dialed).
I am trying to figure out why that's happening? Is the pot defective & shorting somehow when bolted on to the chassis or some other thing happening?
My guess is something is paralleling the pot to ground much as a resistor parallels a resistor and reduces value. I would think it's either the treble pot or the reverb pot doing that but can't see how that would be the case from the schematic? And again, it's not been an issue on other similar builds.
Any thoughts? As soon as I have time, I'll start jumpering across the tone stack and disconnecting things to see what happens.
With respect, Tubenit
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I measured the Mstr Vol pot and it's a 1M pot out of the circuit and closer to 180-220K max (IIRC) in the circuit across the outside lugs and from center to ground (no matter how I have it dialed).
The bottom leg is connected to ground. So you're measuring from "top" lug to ground. Middle lug to ground is identical to that, or less, depending on how you have the pot dialed. The 1M MV pot is in parallel with the tonestack AND maybe whatever follows, which is not shown. The tonesatck impedance is primarily set by the 100K slope resistor. 1M = 10X 100K so you should expect to read about 100K or a bit less. 180K must be resulting from circuit complexities.
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The tonesatck impedance is primarily set by the 100K slope resistor. 1M = 10X 100K so you should expect to read about 100K or a bit less.
Can you elaborate on how the tone stack is paralleling the 1M master volume? I can visually see where the tone stack and mstr vol both connect to each other and ground. Yet, I would think that there is enough resistance in the tone stack to NOT lower the 1M volume pot very much? I realize I may not be thinking thru this correctly which is why I'm trying to understand it.
On the "right" side of the tone stack drawing you have the resistance of two 10M resistors and a 500k bass pot etc....
On the "left" side of the tone stack you don't have any resistance straight to ground without a cap being in that path.
I am familiar with how to determine the value of paralleled resistors. One of the things that throws me off somewhat are the caps in the circuit.
thanks, Tubenit
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Hey tubenit, jjasilli speaks the truth. Flip that PAB switch up and your pot should read closer to 1M again. With it down, your tonestack is in parallel with the MV pot and you'll see a range of resistance from 10k to 400k depending on where you set the treble and bass pots. You are correct that due to the capacitors, you won't "see" anything on the left side of the tonestack.
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The caps cause trouble for meter reading.
DC Operation. An ohm meter reads resistance by putting a small DC voltage across the resistor(s) under test. To simplify I have drawn the Tonesatck as a fixed shunt resistor to ground; and the MV pot, turned to full resistance as a fixed 1M resistor to ground. So we have, for simplicity's sake, 2 shunt resistors in parallel. The Tonestack is really more complex -- it contains several resistors and caps in filter circuits. The end result is a frequency-selective shunt to ground. But DC has "no frequency", and it is blocked by the filter caps in the Tonestack. This screws with an ohm meter's reading.
And, the MV pot is probably also in a parallel with the grid leak resistor of the next tube stage. If you disconnect the wires from lugs 1 & 2 of the MV pot, & leave lug 3 connected to ground (or not), you should get a proper DC ohm's reading across the MV pot which matches its stated value, because it won't be in parallel with any other resistors anymore.
AC Operation. Signal is AC which will pass through the RC filters of the Tonestack with certain frequency attenuation. Any particular RC filter will act as as a certain fixed resistance to any particular frequency. But a simple ohm meter reading does not measure this, because the filter caps in the circuit block its reading.
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I've got this new "rebuild" I am tweaking in and noticed the clean and OD channel don't match up very well compared to other builds I've done that are similar.
With respect, Tubenit
Jeff, this may not help you directly w/ your amp but since they are somewhat similar, this info may be of some help in getting you someone else's experience and input?
As you know from my schem, in my amp I didn't feel the need to have a seperate vol pot after each tone stack and is also why my tone stacks were designed the way they are as they effectively compliment each other (when the OD is switched on). There is a redundancy in that design & this is an inherent issue I wanted to avoid, IMHO. Since the one vol control after the switch is plenty for both in either setting, I would otherwise have had 3 vol controls which seemed excessive and unnecessary and that's not counting if you wanted to add a VVR type or similar. The one vol pot controls either setting perfectly and the result is that the volumes when switched are very close and easily dialed in together. I can make them identically the same or have either one louder than the other when switching between them.