Completely Out of My Depth - Modulus 50w 69 Build

[kingfredward]

Fuse back in and it’s good. I got HT voltage

This is without tubes

And clipped on the 1ohm resistor

Again that’s nothing and that’s with the standby OFF so now really scratching my head. The tubes glow up and warm up fine too

Anyone got any ideas?

 

[Marcomel79]

The tubes are not conducting because if they were, your ht would be lower. I cant see from the pics but is there a lead going to ground from pin1 of the power tubes?

 

[kingfredward]

The tubes are not conducting because if they were, your ht would be lower. I cant see from the pics but is there a lead going to ground from pin1 of the power tubes?

Sorry I should have clarified. that was without tubes. Shall I try with tubes? I can garantuee ill have the same result

Yup definately grounded on pin 1

 

[Marcomel79]

Man!! Without tubes you have no reading!!! Before you install the tubes, clip your black lead to ground, and your red lead to pin5. Adjust the trimpot to max negative voltage, so the tubes will be safe. After you do that, turn the amp off, drain the caps, install power tubes and measure voltage from pin 8!

 

[kingfredward]

sure thing thank you!

Okay pin 5 and maximum negative voltage done

Put in tubes



Not clear but they glowed up great

Then I put a clip on the corner of pin 8

And … still nothing

 

[Marcomel79]

Negative 162 volts?? Sounds like a lot!!

 

[Marcomel79]

Turn amp off, drain filter caps and with dmm on continuity test, check between this junction and pin3 of power tubes. Report back readings

 

[kingfredward]

Turn amp off, drain filter caps and with dmm on continuity test, check between this junction and pin3 of power tubes. Report back readings
View attachment 158974

Good advice. I have continuity between both pin 3s and I have a solid connection. Any other suggestions?

 

[Marcomel79]

Good advice. I have continuity between both pin 3s and I have a solid connection. Any other suggestions?

Yes, 162 negative volts is not right. The max is usually in the -70V range. That will stop the tubes from conducting. What are the readings from pin3 to center tap for both tubes?

 

[kingfredward]

thanks for your continued help and patience

Also noticed I never need to discharge this cap

Yes, 162 negative volts is not right. The max is usually in the -70V range. That will stop the tubes from conducting. What are the readings from pin3 to center tap for both tubes?

Great I’ll try that. Could you point me how to measure that reading? Not entirely sure what the centre tap is. Sorry still a complete noob

 

[Marcomel79]

thanks for your continued help and patience

Also noticed I never need to discharge this cap

View attachment 158987

Great I’ll try that. Could you point me how to measure that reading? Not entirely sure what the centre tap is. Sorry still a complete noob

Its on post #167. You want the readings in ohm. Write them down for both power tubes and report back.

Also, can you take detailed pictures of the area of the bias trimpot?

 

[kingfredward]

I think I found the issue!!!


Cold solder and loose connection could that be plagueing the whole system?

 

[PelliX]

It could explain a complete lack of sound, though I can't make the jack out entirely. It doesn't explain the >160V bias...

 

[Marcomel79]

I think I found the issue!!!


Cold solder and loose connection could that be plagueing the whole system?
View attachment 158988

Take the readings i asked you. And clean between those turrets, there are a lot of small solder balls in there

 

[NickKUK]

I think I found the issue!!!


Cold solder and loose connection could that be plagueing the whole system?
View attachment 158988

Bit of flux - a flux pen works great for sorting these out.

 

[NickKUK]

thanks for your continued help and patience

Also noticed I never need to discharge this cap

View attachment 158987

Great I’ll try that. Could you point me how to measure that reading? Not entirely sure what the centre tap is. Sorry still a complete noob

That cap is simply a cathode bypass, the bypass resistor will act as a bleed. It will only have about 2-3V on the cathode.

 

[kingfredward]

okay got 43 - 44 OHMs across between pin 3 to the choke on each socket.

Ill clean with a cotton bud around the turrents too

 

[PelliX]

I find (old) toothbrushes work well for this. There's nothing extremely ESD sensitive there anyway.

 

[kingfredward]

Here’s the trim pot I have cleaned all the Debry off it too (not seen in the photo)

Anyone got any suggestions what to look at next?

 

[Marcomel79]

Here’s the View attachment 158989View attachment 158990trim pot I have cleaned all the Debry off it too (not seen in the photo)

Anyone got any suggestions what to look at next?

Ok, its not clear from the picture, but is the center lug of the trimpot connected any of the other two lugs of the trimpot? If not, connect it to the lug thats connected to ground. Do that and measure again the max negative voltage at either pin5 of the power tubes