Blues Jr. Distorting-Questions on speaker, Voltage Questions

[Perkinsman]

I'm repairing a friends amp (he said it's a 2003) which seems to be clipping more at normal volumes. As with other posts here, I found that there were some bad solder issues with the power tube sockets but I believe I fixed them. I've noted all the DC voltages but I can't find the expected VDC's anywhere on the schematic for V4 & V5. I also found that V2 pin 1 was 0, which can't be right since it's the plate & V1 pin 1 was 172. Both sides of V2 checked out as borderline "good" on my tube tester. V1 & V2 voltages were good.

Another issue is my friend replaced the original with a Vintage 30 speaker. I believe the original is 50 Watt.

1. What should V2 pin 1 be?
2. What should V4 & V5 pins 2, 7 & 9 be?
3. Is the Vintage 30 a bad choice for this amp? Too under powered?

Btw, I noticed that there are alot of AC measurements in one of the schematics. Is that to be done with an injected AC signal & is the reason to find a possible problem in the AC path?

[Jazz P Bass]

As far as I can tell, the first half of V2 is not used.

The test points for the preamp Vdc is measured at the Cathode of the 12AX7's.

The Vdc of the output tubes is well stated on the schematic.
Pin 7/ Plate = B+ (329Vdc)
Pin 9/ Screen = Z (309Vdc)
Pin 2/ Grid = C- (-10.6Vdc)

The AC test points rely on a 10mv input signal at 1K.

BluesJuniorIII_schematic_Rev-D.zip

[Dave H]

Another issue is my friend replaced the original with a Vintage 30 speaker. I believe the original is 50 Watt.

3. Is the Vintage 30 a bad choice for this amp? Too under powered?

Vintage 30 is 60W. Blues Junior 3 is only 15W.
I've used a 15W, 2 x EL84 amp and a Vintage 30 for over 10 years without any problems.

[Randall]

Blues Jr is only ~15 watts, I doubt it is overpowering that Vintage 30 speaker.

[Perkinsman]

As far as I can tell, the first half of V2 is not used.

The test points for the preamp Vdc is measured at the Cathode of the 12AX7's.

The Vdc of the output tubes is well stated on the schematic.
Pin 7/ Plate = B+ (329Vdc)
Pin 9/ Screen = Z (309Vdc)
Pin 2/ Grid = C- (-10.6Vdc)

The AC test points rely on a 10mv input signal at 1K.

[ATTACH]54719[/ATTACH]

Thanks for this, I didn't know how to read that part of the schematic and didn't realize that C-, Z & B+ were supplying those parts of the tube, but that's why I'm here. I'm ruling out the speaker. I also think I may have shorted a resistor when I scraped away too much of a trace off one of V5's traces.

With power off, I'm getting a reading of OL on R36. Is that a good indication I fried it?

[Dave H]

With power off, I'm getting a reading of OL on R36. Is that a good indication I fried it?

If it's measuring OL across the resistor then it's open circuit (unless there's a bad contact). It should measure 100 ohms. Check R35 measures 100 ohms. If either R35 or R36 is open circuit then the associated tube won't conduct making the power amp distort prematurely.

[Perkinsman]

If it's measuring OL across the resistor then it's open circuit (unless there's a bad contact). It should measure 100 ohms. Check R35 measures 100 ohms. If either R35 or R36 is open circuit then the associated tube won't conduct making the power amp distort prematurely.

Thanks, R35 is 97ohms so I’ll replace R36.

[Perkinsman]

Success, R36 was OL. I replaced it, cleaned up a couple of the power tube socket joints and voila! Amp sounds great.