74 Musicman HD130 From Hell (HELL I TELL YOU!)

[fdesalvo]

I know lol. That’s why I’m using a length of 18ga bus bar now. Jk!

Thanks to that schematic, I see I lifted the wrong wire and the ot ct remained in circuit. When I return home I’ll try the ot test again, along with the diodes on the rectifier board. I see the caps I replaced are oriented correctly.

[fdesalvo]

Progress of sorts. The kind that lead to new issues. I’ll take it Found two suspect diodes on rectifier board. Removed and replaced and no buzzing PT or blowing fuses.

Now the power tubes aren’t conducting. Heaters are glowing, but no blue electron party. More testing tomorrow; can’t wait to get this sucker off the bench. Have some fun work behind this one.

[fdesalvo]

Update: uneven readings from heaters at the loaded tube sockets. Unsoldered filament leads:

Lead 1: 84vac to ground
Lead 2: 88vac to ground
There is continuity between both leads

V2-5: el34
1 1.8mv
2 1.7 - 7mvac
3 480v
4 235.7v
5 -20.31v
6 -20.35v
7 4.41vac
8 288v

V1: PI (12ax7)
1 204v
2 10v
3 22.3v
4 31.1mvac
5 31.1mvac
6 167.2
7 0v
8 .8v
9 4.41vac

[Helmholtz]

V2-5: el34
1 1.8mv
2 1.7 - 7mvav
3 480v
4 235.7v
5 -20.31v
6 -20.35v
7 4.41vac
8 288v

288V at the cathodes! Should be 0.5V. Are the 10R cathode resistors open/disconnected?

(Voltages should be measured in Hi power mode. In low power mode all PT secondary voltages are reduced.)

[Enzo]

Measure the heater wires to each other, not to ground.

[fdesalvo]

I’ll post up my findings. Allred are the help. I’ve learned so much thanks to you guys and this amp.

[olddawg]

As Enzo wisely said.. it whatever the HV is connected too. Schematic? How is the B+ rectified and filtered? I would suspect that area before an OT.

[g1]

Schematic? How is the B+ rectified and filtered? I would suspect that area before an OT.

Do you not see the other posts? Do you have your view set to 'linear' so you see them in order? ('display' button above first post at top of this page)

[fdesalvo]

The plot thickens - added lo/hi measurements and addressed suggestions.

V2-5: el34 lo/hi
1 1.8mv/3.5mv
2 1.7 - 7mvac/cycling from 13mvac to 300mvac
3 480v/701v
4 235.7v/342.5v
5 -20.31v/-29.8
6 -20.35v/-29.8
7 4.41vac/6.5vac
8 288mv/.5v

V1: PI (12ax7)
1 204v/200v
2 10v/10v
3 22.3v/23v
4 31.1mvac/31mvac
5 “
6 167.2v/161.4v
7 0v/0v
8 .8v/.8v
9 4.41vac/4.4vac

- [x] Voltages should be measured in Hi power mode. In low power mode all PT secondary voltages are reduced.
- [x] Measure heater wires to each other: 7v
- [x] 288V at the cathodes! Should be 0.5V. Are the 10R cathode resistors open? Typo. Corrected to mv.

[Helmholtz]

Are you sure about the heater voltage at the PI tube in hi power mode?

[olddawg]

Do you not see the other posts? Do you have your view set to 'linear' so you see them in order? ('display' button above first post at top of this page)

Many times it’s just looking at something on my phone and not seeing the whole thread. My apologies.

[fdesalvo]

Are you sure about the heater voltage at the PI tube in hi power mode?

Yes, it makes no sense, right? I’m getting ~7vac from heater wire to heater wire. I was measuring the pins to ground.

[Helmholtz]

Yes, it makes no sense, right? I’m getting ~7vac from heater wire to heater wire. I was measuring the pins to ground.

What matters is the AC voltage measured between pins 4/5 and 9. It must change with hi and low power settings as with the power tubes.

[g1]

Many times it’s just looking at something on my phone and not seeing the whole thread. My apologies.

Ok, just checking. Sometimes people have the other settings and don't see all the posts.

[g1]

The plot thickens - added lo/hi measurements and addressed suggestions.

V2-5: el34 lo/hi

5 -20.31v/-29.8
6 -20.35v/-29.8
7 4.41vac/6.5vac
8 288mv/.5v

Can you recheck pin5? Or maybe something funny is up with your meter. Schematic shows around -46VDC bias, with -30V there, I'd expect much higher voltage at pin8, though that voltage looks good.