ONE STEP AT THE TIME.
Hi,
It wouldn't surprise me that the hum would disappear once you get the operating points right...
In case it doesn't, the magic wand doesn't always work, you may consider this:
POST # 54
If that doesn't cure it you can still try the DC heaters, keeping the same trick....if not effective at all you'll than know it is not heater related at all.
Sorry for the sarcastic tone...nothing personal, as you should know.
Good luck,
Hi,
It wouldn't surprise me that the hum would disappear once you get the operating points right...
In case it doesn't, the magic wand doesn't always work, you may consider this:
POST # 54
If that doesn't cure it you can still try the DC heaters, keeping the same trick....if not effective at all you'll than know it is not heater related at all.
Sorry for the sarcastic tone...nothing personal, as you should know.
Good luck,
Joe,
It doesn't matter where you measure the tube voltages, as long as the end result is that the grid reads 8V less than the cathode measurement.
Now, I'm assuming this whole time we've been talking about BOTH sides of a 6SN7 biased with a single resistor, right? Because needing 1k+ to get 8V sure sounds to me like you're only running the plate current of a single triode through it. 1k ohms is a standard bias value for half of a 6SN7.
Please double check that the cathodes of both halves are wired together, and then you have a single resistor from that point.
It doesn't matter where you measure the tube voltages, as long as the end result is that the grid reads 8V less than the cathode measurement.
Now, I'm assuming this whole time we've been talking about BOTH sides of a 6SN7 biased with a single resistor, right? Because needing 1k+ to get 8V sure sounds to me like you're only running the plate current of a single triode through it. 1k ohms is a standard bias value for half of a 6SN7. Please double check that the cathodes of both halves are wired together, and then you have a single resistor from that point.
Well, sh*t. I'm at a loss then.
The numbers are just too weird! To get -8V bias with a 1k ohm resistor takes exactly 8mA of current... exactly what we'd expect from a single 6J5/6SN7 triode with 250V on its plate!
So, somehow, from what you're telling me - you've got two triodes running at 4mA each.
The numbers are just too weird! To get -8V bias with a 1k ohm resistor takes exactly 8mA of current... exactly what we'd expect from a single 6J5/6SN7 triode with 250V on its plate!
So, somehow, from what you're telling me - you've got two triodes running at 4mA each.
Well, I'm at a loss also
1. I have the correct parts per your circuit
2. I have the correct placement of the specified parts
3. I do have the two sections connected/paralled
4. I have checked and re checked parts values
5. placement
6.wiring
I must be doing something wrong with the 12AY7 also because I have bias of 1.2 volts.
I have a Dale 10K 1/4 watt resistor on the grid. I have a 1.6K on the cathode. I have a 100K on the plate.
Someone pour me a stiff drink please.
cheers,
Joe
1. I have the correct parts per your circuit
2. I have the correct placement of the specified parts
3. I do have the two sections connected/paralled
4. I have checked and re checked parts values
5. placement
6.wiring
I must be doing something wrong with the 12AY7 also because I have bias of 1.2 volts.
I have a Dale 10K 1/4 watt resistor on the grid. I have a 1.6K on the cathode. I have a 100K on the plate.
Someone pour me a stiff drink please.
cheers,
Joe
Joe,
No, that's not correct. It should be the way it is in the schematic. A 12AY7 draws 3mA or so of current - so we don't need anywhere near 7.7k ohms to get -4V on the grid.
As I said, you've done something wrong. If you are unsure of that, please find me a schematic where a 12AY7 is biased with a 7.7k resistor! (hint: there are none)
What is your heater voltage like? Perhaps you put 5V on them, instead of 6.3V?
No, that's not correct. It should be the way it is in the schematic. A 12AY7 draws 3mA or so of current - so we don't need anywhere near 7.7k ohms to get -4V on the grid.
As I said, you've done something wrong. If you are unsure of that, please find me a schematic where a 12AY7 is biased with a 7.7k resistor! (hint: there are none)
What is your heater voltage like? Perhaps you put 5V on them, instead of 6.3V?
Joel
How about some constructive ideas instead of telling me that I made a mistake?
1. I have 6.3 volts for the heater not 5volts
2. I have the correct values at the correct points in the schematic.
3. I have the two sections of the 6sn7 paralled.
4. I have everything where it is supposed to be.
With the values given the bias is incorrect. With the bias at the correct points the gain is very low. Does this circuit even work? Have you made this circuit piece for piece? I would gather that you haven't or you would soon see what I am running into.
How about some constructive ideas instead of telling me that I made a mistake?
1. I have 6.3 volts for the heater not 5volts
2. I have the correct values at the correct points in the schematic.
3. I have the two sections of the 6sn7 paralled.
4. I have everything where it is supposed to be.
With the values given the bias is incorrect. With the bias at the correct points the gain is very low. Does this circuit even work? Have you made this circuit piece for piece? I would gather that you haven't or you would soon see what I am running into.
Burnedfingers, are you sure that your 6sn7's are good? Maybe half of the tube is bad, making it read like a single 6j5. I have been following this and am curious to the problem also.
I remember my first tube amp I built, I wired all of the tube sockets wrong. Because I look at sockets solder tabs wrong. Talk about a stuip mistake!
I remember my first tube amp I built, I wired all of the tube sockets wrong. Because I look at sockets solder tabs wrong. Talk about a stuip mistake!
Guiness
Thanks for the reply. As a matter of fact I have had a friend with experience building tube gear check my work. He has found my work to be correct with respect to placement and parts values. The 6SN7 sections are paralled grid to grid,plate to plate, and cathode to cathode. I have half the bias I should have with the resistor values. When I do refigure the bias to -8 volts on the 6SN7 and 4volts on the 12AY7 I get zip for gain. I have substituted another set of new 6SN7's GE to be exact, and the voltages are the same. I have also substituted the 12Ay7 and the bias remains the same. I have incorrect bias on both the 12AY7 and the 6SN7. The pin numbers have been checked as were the parts values and placement.
Frank
I made some stupid costly mistakes along the learning curve thru the years. Forgetting to check/replace the bias transistor was one of my first ones that cost the entire bank of new output transistors. I'm bound to make mistakes here when I am attempting to learn the tubes I should/could have learned many years ago. Thank you for your patience and understanding here as I stumble along.
Joe
Thanks for the reply. As a matter of fact I have had a friend with experience building tube gear check my work. He has found my work to be correct with respect to placement and parts values. The 6SN7 sections are paralled grid to grid,plate to plate, and cathode to cathode. I have half the bias I should have with the resistor values. When I do refigure the bias to -8 volts on the 6SN7 and 4volts on the 12AY7 I get zip for gain. I have substituted another set of new 6SN7's GE to be exact, and the voltages are the same. I have also substituted the 12Ay7 and the bias remains the same. I have incorrect bias on both the 12AY7 and the 6SN7. The pin numbers have been checked as were the parts values and placement.
Frank
I made some stupid costly mistakes along the learning curve thru the years. Forgetting to check/replace the bias transistor was one of my first ones that cost the entire bank of new output transistors. I'm bound to make mistakes here when I am attempting to learn the tubes I should/could have learned many years ago. Thank you for your patience and understanding here as I stumble along.
Joe
Well, I am still lost on this post. But I now have to build one , I have also checked Joel's schematic and it is correct. I am going to build it excalt as shown on Joel's schematics. The only thing I am going to change is to substitue 2 2c22/7193 in for the 6sn7. They look cooler and hopefully will sound better too. (electrically the same as half a 6ns7). I will post my finding and pictures for you to see. But you will have to wait until last part of March
Good Luck
, I have also checked Joel's schematic and it is correct. I am going to build it excalt as shown on Joel's schematics. The only thing I am going to change is to substitue 2 2c22/7193 in for the 6sn7. They look cooler and hopefully will sound better too. (electrically the same as half a 6ns7). I will post my finding and pictures for you to see. But you will have to wait until last part of March Good Luck
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