5E3 too clean, no overdrive

[printer2]

As said, you should get about a volt below that cathode. Do not have a amp with a cathodyne PI here at this time, basically it is a case of Ohm's law and the current through the cathode resistor, no different from reading it on an input tube, phase inverter, or output tube. You are measuring the voltage drop across the biasing resistor.

[pajur]

Guys,
Great thanks and I can't wait to check these things when I have the amp back together (very soon!).

However, I think I misled you with this grid voltage on pin 7 of V2.
I believe now that I made a measurement error.
I measured pin 7 to ground. After speaking to a friend here and analyzing your advise, there is no direct way of measuring grid voltage to ground. Please tell me if otherwise.

So I believe that I should measure the voltage across the cathode resistor and that would be the correct value, right?

[printer2]

You can measure it to ground. All voltages have to add up in a circuit. But why bother measuring it to ground as you are more concerned where the grid is biased at relative to the cathode.

[pajur]

Originally Posted by printer2

You can measure it to ground. All voltages have to add up in a circuit. But why bother measuring it to ground as you are more concerned where the grid is biased at relative to the cathode.

printer2,
I will investigate this further but I have never seen anyone give the grid voltage to ground.

Anyone has their pin7 voltage to compare?

[jjmantele]

You can measure the other end of the 1meg resistor that is on the grid (which is not grounded.) Both of those ends would have the same voltage at idle but measuring at the grid pulls a hair of current thru the 1meg causing some voltage drop. At idle, with no meter connected, there would be no current flowing thru the 1meg, so no voltage drop. If you have an analog meter with a big load inside it, measuring at the grid could be way wrong.

The voltage on the 1 meg should be within a few volts of the cathode's reading, as mentioned.

[pajur]

Originally Posted by jjmantele

You can measure the other end of the 1meg resistor that is on the grid (which is not grounded.) Both of those ends would have the same voltage at idle but measuring at the grid pulls a hair of current thru the 1meg causing some voltage drop. At idle, with no meter connected, there would be no current flowing thru the 1meg, so no voltage drop. If you have an analog meter with a big load inside it, measuring at the grid could be way wrong.

The voltage on the 1 meg should be within a few volts of the cathode's reading, as mentioned.

jjman,
Do you mean that I should measure these voltages:

- voltage across the 1M resistor legs (and not to ground),
- voltage across the 1.5k cathode resistor legs,

Compare the two and they should be within a few volts?

What values should I expect?

[printer2]

Originally Posted by pajur

jjman,
Do you mean that I should measure these voltages:

- voltage across the 1M resistor legs (and not to ground),
- voltage across the 1.5k cathode resistor legs,

Compare the two and they should be within a few volts?

What values should I expect?

I think maybe you should read this document. It gives a good overview of what some tube stages do and what you should be reading. Once you understand these the couple of differences in a P-P amp are easy to grasp. Without this knowledge it is all black magic and you are groping at straws.

http://ax84.com/p1/P1_Theory_Document.zip

[pajur]

Originally Posted by printer2

I think maybe you should read this document. It gives a good overview of what some tube stages do and what you should be reading. Once you understand these the couple of differences in a P-P amp are easy to grasp. Without this knowledge it is all black magic and you are groping at straws.

http://ax84.com/p1/P1_Theory_Document.zip

Thanks for the document. Will read for sure!

[jjmantele]

Originally Posted by pajur

jjman,
Do you mean that I should measure these voltages:

- voltage across the 1M resistor legs (and not to ground),
- voltage across the 1.5k cathode resistor legs,

Compare the two and they should be within a few volts?

What values should I expect?

Put the negative side of the meter to ground and the positive side to the 1 meg resistor that goes to pin 7. But not the side that goes to pin 7. Put that positive lead on the other side of the 1 meg. That point is the junction of the 1 meg and the 1.5k and the 56k. Write down the voltage.

Then move the positive to the cathode pin and write down that voltage.

Then subtract the 2 numbers on paper. This is the bias voltage and it should be approximately 1.5V.

Or put one end of the meter on one end of the 1.5k, and the other end of the meter on the other end of the 1.5k. Read meter. Result should be the same.

[pajur]

Originally Posted by jjmantele

Put the negative side of the meter to ground and the positive side to the 1 meg resistor that goes to pin 7. But not the side that goes to pin 7. Put that positive lead on the other side of the 1 meg. That point is the junction of the 1 meg and the 1.5k and the 56k. Write down the voltage.

Then move the positive to the cathode pin and write down that voltage.

Then subtract the 2 numbers on paper. This is the bias voltage and it should be approximately 1.5V.

Or put one end of the meter on one end of the 1.5k, and the other end of the meter on the other end of the 1.5k. Read meter. Result should be the same.

jjman,

Great thanks,
The amp is up and running now. Everything seems as before except that I lowered the B+ voltage. This is what I read:

- 41.8V at the junction of the three resistors to ground and 44V at pin 8, for the differential voltage of 2.2V,

- However measured across the 1.5k resistor, I get only 1.2V (???)

What do you think?

[pajur]

Originally Posted by pajur

jjman,

Great thanks,
The amp is up and running now. Everything seems as before except that I lowered the B+ voltage. This is what I read:

- 41.8V at the junction of the three resistors to ground and 44V at pin 8, for the differential voltage of 2.2V,

- However measured across the 1.5k resistor, I get only 1.2V (???)

What do you think?

I originally measured those voltage to a chassis ground. So now I tried to measure to the closest ground connection for those resistors and the voltage at pin 8 now becomes 43.7V with all else the same.

So the differential now becomes smaller: 1.9V

[pajur]

I think the above voltage inconsistency is a measurement error. I have tried a different digital multimeter and the result is similar. If you can measure your voltages at those points, it would be an interesting study as to why the meter behaves this way. I don't think it is a problem though.

[pajur]

Guys and Gals,

I wanted to thank everyone here who helped or inspired me to work on this project. As of today, I call it a success and it is yours as well!

I have put the amp back together and it is working fine. My complaint was the lack of overdrive. As it turned out, the V1 that I originally got is unusually weak (probably a factory defect). By modifying the preamp, I was able to achieve some overdrive. Then, trying a higher gain tube made an even more dramatic difference. The "harder to overdrive" V2 may be a factor but I think to a lesser extent.

I have also relocated parts and the board is not upside down anymore, which makes trouble-shooting easier. It is still a very different animal from the original 5E3 design-wise but it doesn't have hum and the sound is great.

For those who will be building. Some suspected the wrong speaker or the over-sized OT. I am sure now that they had nothing to do with the issue so no worries there.

Great thanks!

[Ricky D.]

I'm glad that worked out for you!

[pajur]

Originally Posted by Ricky D.

I'm glad that worked out for you!

Thank You!

[adkima00]

I've been following this thread as I also just completed my first build - a 5e3. It's a great amp, and cruisin home was also a great help to me! Glad you were able to get it going!

Sent from my iPhone using TDPRI

[hackworth1]

adkima00,

I would have personally offered my help, but Cruisin' was doing such an excellent job helping you, there was no need for me to jump in.

"Too many cooks spoil the broth", might be an apt metaphor, I think.

In any case, Congrats on a successful build. and Thank You.

[adkima00]

Originally Posted by hackworth1

adkima00,

I would have personally offered my help, but Cruisin' was doing such an excellent job helping you, there was no need for me to jump in.

"Too many cooks spoil the broth", might be an apt metaphor, I think.

In any case, Congrats on a successful build. and Thank You.

No worries! If I felt like I was really stuck and not getting anywhere I would have called you on the phone... You did include your phone number in the kit after all! I'll be keeping an eye on your web site for when you start to sell a Marshall 18 watt kit!

Sent from my iPhone using TDPRI