Help restoring baseman AA165

[Lizard King]

I picked up a vintage bassman aa165.  Previous owner had 'modified' it to the point where it was a basket case.  Fortunately he had most of the original parts.  I've put everything back the way it should be, the amp powers up & plays but doesn't even get half as loud as it should.

I've changed tubes, checked all resistor values, everything looks good.  I'm not very big on tube theory.  Any suggestions on where I might try?

Thanks.

[LooseChange]

Probably a mistake on the rewire (Although, could be a bad part that you re-installed). Check every wire against the Schematic/layout. Use a highlighter to keep track.  Take voltages at every pin and post. Post a high quality picture or two of the insides.
One last thing. Be sure the speaker is plugged into the correct output jack.

[Lizard King]

Between the phase inverter & power tubes are a 100k & 82k resistor tied together to b+...my VOM isn't the best but it's reading 450v for what should be 414 v where the resistors tie together.  The voltages on the tube sides of the 100k resistor should be 220v -it reads 310v.  The voltage on the tube side of the 82k resistor should be 225v- it reads 160v.

That doesn't seem right.  I've checked the two resistors and they read close to correct values.

I'm not sure what else could be the problem...two .1 caps lead to the power tubes...those are new 718 orange drops...

[bigsbybender]

Insert Quote
Between the phase inverter & power tubes are a 100k & 82k resistor tied together to b+...my VOM isn't the best but it's reading 450v for what should be 414 v where the resistors tie together.  The voltages on the tube sides of the 100k resistor should be 220v -it reads 310v.  The voltage on the tube side of the 82k resistor should be 225v- it reads 160v.

That could be explained by a difference in voltage from your wall outlet. The amp would have been rated for 117v usage (assuming you're in N. America). Today's wall voltages often read over 125v in many places in the US and Canada.

If I take the actual voltage divided by design voltage it's 450v/414v= 1.086    Then I'll take 117v x 1.086, I get ~ 127v which is completely possible.

Measure the voltage at your wall outlet and see what you are getting (this voltage may vary at different times of the day).

This probably won't solve the issues you are having, however it may provide an explanation of the "odd" voltage readings... it's something to rule out.


j.

[Lizard King]

I wasnt so concerned with the values as much as I was with the one resistor measuring about 50v high and the other 50v low...

[HotBluePlates]

The voltages on the tube sides of the 100k resistor should be 220v -it reads 310v.  The voltage on the tube side of the 82k resistor should be 225v- it reads 160v.

That doesn't seem right.  I've checked the two resistors and they read close to correct values.

I think what you're pointing out is that you expect a bigger voltage drop on the 100k side, but you actually see it on the 82k side.

Have you tried a different tube? Are you sure the 500pF and 0.1uF caps going to the tube grids are not leaky?

What I'm thinking is whether there is some error in bias due to a leaky coupling cap, but on the grids of the phase inverter. The other thought is a tube with sections with very different Gm; in other words, one section close to normal and the other very worn out. If one side is very worn out, it might not draw the current it should.

So check input caps to the phase inverter, and the tube itself. The voltage should be higher than the schematic, but both halves should have very similar plate currents, so you'd expect a little more plate voltage on the 82k side.

[Lizard King]

Thanks...I replaced the tubes and the .1 cap....I'll try replacing the 500pf cap.

I dropped the 82k resistor down to 56k and it brought the voltage up to 200....sounded a little louder but nowhere near where it should be...


I REPLACED THE PI TUBE & changed the 500pf CAP TO .001uf.  (I'VE TRIED  TWO DIFFERENT .1uf CAPS)
THE VOLTAGE ON THE 100k RESISTOR WENT TO 400 AND THE 82k (now 56k) WENT TO 230v.

Something t'aint right

[HotBluePlates]

Don't focus on replacing parts, because you may find the part didn't need replacing.

Check for a leaking cap by unsoldering one leg, then seeing if the tube stage after the cap suddenly settles down.

Don't change to different-valued parts to get the amp to function properly; it should work as expected with stock values.

I strongly suspect wiring funkiness or other changes. If it were me, I'd probably unsolder enough wiring to remove the eyelet board, and be certain there are no underboard wiring changes/errors/solder blobs.

Due to wall voltage changes, you will never find any voltage in your amp that exactly matches the schematic, however, they should all be similar to the schematic voltages and be higher/different by similar proportions.

[Lizard King]

I'll do that next....check eyelet board...thanks!!!

[Lizard King]

You were spot on hotblues....I checked with the previous owner and while the tube chart reads aa165 he thought it had begun life as an ab165....I looked under the tag board and there was a connection to ground right in the middle of the PI circuit....I pulled that and it's quite a bit louder.  My voltages still seem wrong - 300+ on one PI output resistor and 100+ on the other resistor, but it sounds a whole lot better.

I found a circuit for an aa-ab165 hybrid....will probably go for that.  Only change is both preamplifier circuits go through the second tube stage instead of only the bass channel on the aa165.

THANKS ALL!  I hope other beginners learned a valuable lesson like I did.

[HotBluePlates]

You might have to check everything with a fine-toothed comb, then.

The wiring you described sounds odd, for all versions of the blackface Bassman. I wouldn't be surprised to find other "improvements".