MoJo Brit 800 Build: At My Wits End!!!!

[pdf64]

The Mojo layout shows a yellow wire connected to the 100k series feedback resistor, other end of the yellow wire connects to the impedance selector, ie the speaker output circuit. That’s where you measure 9 to 12V, when the OT secondary should be pulling it down to 0V.
The low heater voltage is weird though, check the solder joints.

[sluckey]

We need to look at this from the start...not bounce around.
First of all, why does my rectifier voltage drop so much when I install the power tubes...since this is the first thing the power goes through, voltage should not be affected. Why would the bias voltage also drop?

I'm not bouncing around! I'm looking at this from the start. The ***first*** thing you should do is get the correct bias voltage on pin 5 of the output tube sockets ***BEFORE*** you ever plug in the tubes! This goes for any fixed bias amp.

With only -29V on pin 5 of the output tubes, they are running extremely hot. So much current is flowing through the tubes that they are creating a very big load on the power supply and causing the B+ to drop. The low bias voltage is causing the B+ to be low when the tubes are plugged in. Not the other way around. Did you notice that the bias voltage remained the same with or without tubes?

If you do what I suggested you will probably fix the low voltage situation. However, those KTs may well be toasted by now.

[nflrage]

I wouldn't be assuming as a new amp builder how to tell the experts what they should 'bounce around to' as they intimately know what's needing review and what isn't.

Voltage drop with tubes in is normal. Once they start conducting, they create a voltage drop due to adding load to the circuit. How much they drop is a different story, and dropping too much may mean you're burning up tubes because something's wired wrong and allowing way too much current to flow. This negative bias being off is one of the primary ways you can do this. This sets the operating point of the power tubes and if they're pulling too much they'll redplate and die really fast.

The bias voltage is dropping because of the tubes starting to conduct.

The question about the speaker voltages were to determine if there's something flowing to the speaker through the OT or not.

The HT voltage can be measured where it first exits the diodes providing rectification. (Edit: I see you did measure HT voltage in both cases, so not sure who missed those)

~Phil

First of all I am a certified aircraft mechanic and have deep knowledge of circuits and I am considered an "expert" in my field. This is my first amp build but assuming the schematic is correct and the components are placed where they should be (rechecked too many times to count), there should not be any issues. That's why I am seeking advice. This thing is making me crazy right now.

I did place a 100ohm and not 100k NFB resistor...that has been taken care of.

[nflrage]

The ***first*** thing you should do is get the correct bias voltage on pin 5 of the output tube sockets ***BEFORE*** you ever plug in the tubes!

So, how is this accomplished...as I stated at the start, this was a complete kit from MoJo and I would assume that everything would work as designed.

[pdf64]

Twiddle the 10k bias pot.

[sluckey]

The ***first*** thing you should do is get the correct bias voltage on pin 5 of the output tube sockets ***BEFORE*** you ever plug in the tubes!

So, how is this accomplished...as I stated at the start, this was a complete kit from MoJo and I would assume that everything would work as designed.

I already told you what to do. Read it again if you don't understand. The fix is simple.

If you had put EL34 tubes in this amp (like Mojo did!) rather than KT77s, we probably wouldn't even be having this discussion. I know you think changing from EL34s to KT77s is just a minor thing, but it ain't! You cannot make a major change like this without having to also modify the bias supply. This is probably the reason the PT failed and will cause you some more grief if you put those tubes back in without providing proper bias voltage for the tube type you plan to use.

FIRST THINGS FIRST... FIX THE BIAS!

[nflrage]

The ***first*** thing you should do is get the correct bias voltage on pin 5 of the output tube sockets ***BEFORE*** you ever plug in the tubes!

So, how is this accomplished...as I stated at the start, this was a complete kit from MoJo and I would assume that everything would work as designed.

I already told you what to do. Read it again if you don't understand. The fix is simple.

If you had put EL34 tubes in this amp (like Mojo did!) rather than KT77s, we probably wouldn't even be having this discussion. I know you think changing from EL34s to KT77s is just a minor thing, but it ain't! You cannot make a major change like this without having to also modify the bias supply. This is probably the reason the PT failed and will cause you some more grief if you put those tubes back in without providing proper bias voltage for the tube type you plan to use.

FIRST THINGS FIRST... FIX THE BIAS!

KTs and ELs are interchangeable they are the basically the same. I will tweak the 220k to a lower value.

[nflrage]

Twiddle the 10k bias pot.

Twiddle??? What does that mean?

[sluckey]

KTs and ELs are interchangeable they are the basically the same. I will tweak the 220k to a lower value.

Just because they both use a similar pin out and octal socket does not make them interchangeable. The KT77 needs a much higher bias voltage for proper operation than the EL34. This is exactly what you are experiencing.

[passfan]

The ***first*** thing you should do is get the correct bias voltage on pin 5 of the output tube sockets ***BEFORE*** you ever plug in the tubes!

So, how is this accomplished...as I stated at the start, this was a complete kit from MoJo and I would assume that everything would work as designed.

Except that you installed the wrong value part , and made an output tube substitution that you didn’t disclose from the get go. Relax though. These guys will help you get it going one problem at a time.

[passfan]

The ***first*** thing you should do is get the correct bias voltage on pin 5 of the output tube sockets ***BEFORE*** you ever plug in the tubes!

So, how is this accomplished...as I stated at the start, this was a complete kit from MoJo and I would assume that everything would work as designed.

Except that you installed the wrong value part , and made an output tube substitution that you didn’t disclose from the get go. Relax though. These guys will help you get it going one problem at a time. Always build it without modification first . When it’s working , then modify it.

[nflrage]

You guys are harping on me about using KT77s instead of EL34s. That point is moot as far as the impact on bias...they are close. The bias without tubes is way too hot. I changed the 220k bias resistors all the way to 82k and the voltage is still only -34.

[dgrainger]

What is it with a jumper instead of a resistor?

[sluckey]

Monitor the voltage on pin 5 of an output tube. Turn the bias pot from one end to the other. What is the highest and lowest voltage reading you have at each end of the pot?

Post a hi-rez pic that clearly shows the bias pot and the bias circuit on the left end of the board.

[martin manning]

You guys are harping on me about using KT77s instead of EL34s. That point is moot as far as the impact on bias...they are close. The bias without tubes is way too hot. I changed the 220k bias resistors all the way to 82k and the voltage is still only -34.

I believe you're adjusting the wrong resistor(s). Change the 220k next to the rectifiers to 150k and you should get negative voltage in the range you need.