2 questions- Tube values and pot value

[pbman1953]

I'm rebuilding another Super Twin into a bass amp.


1-I want to add a bias supply , like I have in the past, before the Output control.  I was told to use a 2.5 pot with a 1.5k resistor. I have the resistor but I see 2k and 3k pots but not many 2.5.  Can I go with the 2 or 3 k? If so do i need to change the resistor value?




2-I have a Sencor TC142 tester. I know it's on the low end but it will at least tell me if a tube is bad. I'm testing the amp's 12AU7 and the reading is questionable. All the other tubes, 12ax7, 12at7 & 6l6's tested fine. From my stash I tried another AU7 and it had the same questionable results. I thought my stash tube was fine. Is the 12au7 different in a way to fool or not read right on a tester?


Thanks
 

[pbman1953]

Sending the correct Schematic


In the past , my other models had a 820 ohm resistor to ground of V1B's pin 3. This one has a 1.5k. Members, in the past, have suggested to change that part to a 680 ohm. 


Should I go with that, or leave it?

[pbman1953]

Anyone?

[astronomicum]

Is the 12au7 different in a way to fool or not read right on a tester?

No. It is always possible that both tubes read low especially if they are both well used. I use a TC142 and find that used 12AU7s typically read a bit lower than used 12AX7s but not questionable (I assume by "questionable" that you mean between 50 and 70%). Testing a new tube might provide some insight. It is also possible that the issue is in your TC142. You may want to service it if you have not already done so.

[pbman1953]

Is the 12au7 different in a way to fool or not read right on a tester?

No. It is always possible that both tubes read low especially if they are both well used. I use a TC142 and find that used 12AU7s typically read a bit lower than used 12AX7s but not questionable (I assume by "questionable" that you mean between 50 and 70%). Testing a new tube might provide some insight. It is also possible that the issue is in your TC142. You may want to service it if you have not already done so.

Yes more like in the 50-70% area. I think the Sencor is ok, all the other tubes test fine, only the 12au7's test weaker


Can you answer the other questions above?


Thanks

[astronomicum]

In the past , my other models had a 820 ohm resistor to ground of V1B's pin 3. This one has a 1.5k. Members, in the past, have suggested to change that part to a 680 ohm. 

Should I go with that, or leave it?

I assume that you want to keep this stage clean since you intend to use it for bass? If I where to be faced with these choices and knowing what "I" wanted the stage to do, I would run load lines on all three and pick the one that is closest to center biased, or change the resistor value to do just that.

I would need a sketch as to what you intend to do in question one (sorry but I don't understand what you are asking) in order to provide comment.

Happy Thanksgiving 

[pbman1953]

Look at V1B, pin 3. There's a 1.5k resistor to ground. In the generation after this one,  they changed it to a 820 ohm. On my last unit upgrade the forum suggested to use a 680 ohm.


My question is should I leave the 1.5k or install the 680 ohm?

[astronomicum]

1-I want to add a bias supply , like I have in the past, before the Output control.  I was told to use a 2.5 pot with a 1.5k resistor. I have the resistor but I see 2k and 3k pots but not many 2.5.  Can I go with the 2 or 3 k? If so do i need to change the resistor value?

It was this "question 1" I was referring to. This is the one I do not understand.

[pbman1953]

1-I want to add a bias supply , like I have in the past, before the Output control.  I was told to use a 2.5 pot with a 1.5k resistor. I have the resistor but I see 2k and 3k pots but not many 2.5.  Can I go with the 2 or 3 k? If so do i need to change the resistor value?

It was this "question 1" I was referring to. This is the one I do not understand.

ok, I've done this in the past but I have a 20k pot so I'd need to know what value resistor to use

[astronomicum]

ok, I've done this in the past but I have a 20k pot so I'd need to know what value resistor to use

You do not need to change the value of the 1K5. The 20K will give you a more negative voltage if you sweep it up to 20K, but at 0(zero)K, it will give you a voltage similar to what the 2K5 pot would give you at 0K.

[pbman1953]

Doe this look right?