Another Bassman Build With Issues (AA864)

[mazzas]

After several successful amp builds, I've graduated from screwing up 5F1's to screwing up AA864's. Anybody care to take a crack at why I have *very low* output levels here? At 10 (it doesn't go to 11) I can hear output on the speaker cabinet that sounds like I've got a 1/2W amp.

In trying to determine if the problem is in the pre-amp or power section, I touched the wiper on the treble pots, a technique suggested by erndawg, and heard almost nothing on the output. So... seems like a problem in the power section???

The amp is built based on these:
http://www.fenderholic.com/schem/bas...a864_schem.gif
http://www.fenderholic.com/schem/bas...864_layout.gif

I can supply a list of parts and pictures.

One obvious deviation from the schematic is the substitution of 100uF/350 caps for the 70-350 caps in the power section. Problem?

Also, the PT that I sourced does not have a center tap for the 6.3V heaters so I wired up a "virtual" center tap. Wiring 1/2 of the winding to ground (the way it's shown) produces no change. I've tried it both ways to no effect and so have left my vct in place for now.

I *think* everything else is to spec.

Here are some readings to get started:

Line voltage: 120.2V
PT Heaters: 6.49V
PT Bias tap: 51.2V
PT Power: 361V

Bias pot: -61.3vdc (biased cold)

B+: 480vdc

Filter caps (left to right on the schematic):
430vdc (high by about 50v)
464vdc (high by about 54v)
476vdc (high by about 56v)

6L6
pin 3/4: 476 (high by about 56v)
pin 5: -59.5

12ax7
pin 6: 285vdc (high by about 45v)
*I hear "scratching" when I touch these pins
pin 8: 2.1vdc (high by about 0.3v)

12at7
pin 1: 200.4vdc (low by about 20v)
pin 2: 109.2vdc *I hear "scratching" when I touch this pin
pin 3: 116.2vdc (high by about 20v)
pin 6: 285vdc (high by about 80v!!!) <==== PROBLEM???
pin7: 74.3vdc

[loudthud]

Check for oscillation. Set your meter to AC volts and connect it across the speaker. With no signal you should see a very low signal, something like .02 V. If you read several volts, the power amp is oscillating at an ultrasonic frequency. The fix is usually to reverse the OT leads to pin 3 of the 6L6s.

[mazzas]

Thanks much. It appears that I've got the OT properly hooked up.

[loudthud]

That isn't the only thing that can cause oscillation. Do the AC volts test.

[mazzas]

Sorry for my ambiguous answer. After performing the test it appears that my OT is properly hooked up. Actually, I'm reading 0.00 VAC across the speaker. There's a very little voltage that "drains" quickly... less than a volt.

I assume that I'm to have done this with the speaker (load) pugged in, lest I run the risk of damage to the amp (which is a stretch in my case).

[mazzas]

I couldn't help temporarily adjusting the bias to -44vdc and here are the readings:

12at7:
pin #1: 190v (spec: 220v)
pin #2: 94v
pin #3: 102v (spec: 95v)
pin #6: 262v (spec: 205v)
pin #7: 65v
pin #8: tied to #3

Note: I'm confused as to the readings on pins 1 and 6. If they were opposite I'd be more comfortable.

12ax7:
pin #6 of all of them reads 268v
pin #1 is within a volt or 2

Note: tapping on pin #6 of the "last" 12ax7 makes more noise in the speaker than tapping on pin #6 of either of the other 2.

All other voltages at filter caps, coupling caps, and PT look well within spec.

I'm beginning to think that my signal path is suspect.
I think it's also worth considering that I have a faulty PI.

[loudthud]

There is some kind of problem at the phase inverter. Normally you can't get a reliable voltage reading on pins 2 and 7 because your meter pulls the grid towards ground and that changes the bias on the tube. But you should get close to the same reading if you have a good tube and the grid leak resistors, 1 meg in this case, are the same on both sides. You should be able to measure the resistance of the 1 meg resistors pretty accurately when the power is off. If that's ok, try a different tube if you have a 12AT7 or you can try a 12AX7 to see if the voltage on pins 1 and 6 gets a little closer to what it should be. Another thing to do is measure the voltage across the 1 meg resistors when the power is on. You should get a very low reading, less than 0.1V. A larger voltage would indicate a problem with the caps hooked to pins 2 and 7 or a bad tube.

The scratching sound... Is it fairly loud? Can we be sure that there isn't a short across the speaker jack and that it's wired correctly? This is a hard thing to troubleshoot sometimes.

[mazzas]

Using my tube tester, the 12at7 looks good on the Short Tests and Gas Test. The needle deflects only slightly to the "GOOD" side on the Emission Test. There is some initial flickering on the Short Test for some pins but this is acceptable and the Short light goes out after a second or so.

Measuring the resistors, one is 1M on the nose. The other is 0.8M. How far off is acceptable?

In suggesting that I measure the voltage "across" the resistors while powered up I assume you mean in the same "style" as measuring resistance (except on the vdc scale). Or do you want me to take the difference of each side to ground?

I think the scratchy sound being louder on the "last" 12ax7 is due solely to the 2 gain stages of the Bass Instrument input. It's still not loud enough. Very low output means VERY LOW output... unfortunately.

I certainly appreciate all of your assistance.

[loudthud]

The .8 meg might indicate that there is a leaky capacitor on that side. Note that on the feedback side, the side with the .1uF your meter might take a while to get to a stable reading.

Measure the voltage directly across the resistor one lead to each end of the resistor.

[mazzas]

Okay...

Across the resistor on pin #2 I got about 0.5v
Across the resistor on pin #7 I got about 0.05v

[loudthud]

Going by the pin numbers on the schematic, the 500pF is bad. But that's not enough to cause an almost complete loss of signal (I don't think). Replace that 500pF. If you don't have one you can substitute just about any higher value while the part is on order. Keep looking up and down the signal path. There is another problem somewhere.

[mazzas]

How high can I go with this value? I'm not certain that I've got a suitable replacement available. I've got a .047 in an acceptable voltage... but that's pretty high.

How much trouble can I get myself in by swapping the OT leads... just to make sure I've got them right?

[Jazz P Bass]

Swapping the OT leads will either make the amp oscillate or not. Depends on the negative feedback loop.
What ac voltage swing are you getting on pin 5 of the 6L6,s?

[mazzas]

What ac voltage swing are you getting on pin 5 of the 6L6,s?

Can I measure this to ground? I'm not sure I know what you're asking me.

[Jazz P Bass]

Assuming you are inputting with a signal generator, 100mv input at max volume, should drive pin 5 to about 100 volts ac. Measure pin 5 to ground & sweep the volume control up & down. Look at the same point with your scope. The signal should be symetric ( top & bottom should look the same) If all is well, look at pin 3, the plate. Same thing. Meter on ac, use the scope. If the plate is behaving properly & you have a weak output, then your output transformer is bad.