69 Bassman with low volume output …fixed.

[TubeGeek]

Hello,


I have a 69 Bassman on the bench that I am converting from AC568 to AA864.


The problem I am experiencing is a lower than expected volume output level. 


The amp has a low level hum, not sure if this is part of the problem but something tells me it's possible.


The amp sounds normal, voltages are a bit higher than the schematic but I expected that.  I have attached a pic of my whiteboard notes on the voltages.  The black marker is actual voltages and the red is schematic values.


I have gone over everything a couple times, tried different tubes and all the normal troubleshooting steps and am now stuck.


What could cause low output, while the amp still semi-works?

[PRR]

V4 plates:

158 on one and 294 on the other is VERY un-balanced.

We expect them the same; or ~~20% different if Fender used offset plate resistors to balance the gains.

Re-re-re-check resistor values. Change the tube. Look for wiring errors which would bleed DC off the plates.

[eleventeen]

Agree with PRR. Those PI voltages aren't right.

[TubeGeek]

The difference did alarm me too.


I have literally rebuilt or replaced every component in and around the PI, but no matter what it measures ~150vdc!


I replaced the 82k resistor, verified it is 82k with dmm, replaced the 0.1uF cap, replaced the wire lead to the socket pin1, replaced the socket entirely, checked under the board for loose bits that may be connecting, verified the eyelet is not conductive to any other eyelet, measured the board for dcv, tried another 12at7, tried a 12ax7, measured all board resistors, checked the filter stages, verified the PT, OT and Choke are correct, went over and over and over the schematic and layout.…this is not the first bassman I have converted either.


    


I am really scratching my head on this right now.

[sluckey]

V4-2 voltage looks funky also. How did the amp sound before you did the conversion?

[TubeGeek]

It had some issues that I never worked through. The owner asked me to do all this work on it so I just got to work on it.  I may regret that decision.

[TubeGeek]

Ok found the problem.


It was a jumper under the board connecting the 220k channel mixing resistor's to the v4 grid pin 2. The 500 pF cap was there but under the board, where I could not see was a jumper in the same position.


Thanks guys for the help.


The amp's power is now what it should be.

[p2pAmps]

Awesome.  Sluckey taught me wiring under the board is a accident waiting to happen.  I now use his idea of running the wire up and through the top of the turret and bending it over.  Its a perfect way to know you connection is good and almost failsafe. 

[TubeGeek]

Awesome.  Sluckey taught me wiring under the board is a accident waiting to happen.  I now use his idea of running the wire up and through the top of the turret and bending it over.  Its a perfect way to know you connection is good and almost failsafe. 

That is a good practice for sure!


I am not a fan of under the board wiring for this exact reason too.


Since this was a conversion, I overlooked this jumper. Live and learn.

[Willabe]

Ok found the problem.

You always find it!   

[6v6Pin1]

Are under the board wires just for cleanliness or is there an electronic need to run under the board?

[John]

Usually for neatness, although sometimes it's easier (sometimes necessary) to run underneath instead of winding the jumper in/around turrets on top of the board.


The Sluckey Hook is the way to do those underneath ones.

[PRR]

> Are under the board wires just for cleanliness or is there an electronic need to run under the board?

The idea is that parts will fail in a few years, wires *never* fail. Keep the wires out of the way.

This assumes that wires won't be damaged or drop-out when parts are replaced. This may have been a bad guess.

I like the plan of drilling a small hole *next* to the turret, bring the wire through the hole for a visible turret-wrap.

[p2pAmps]

> Are under the board wires just for cleanliness or is there an electronic need to run under the board?

The idea is that parts will fail in a few years, wires *never* fail. Keep the wires out of the way.

This assumes that wires won't be damaged or drop-out when parts are replaced. This may have been a bad guess.

I like the plan of drilling a small hole *next* to the turret, bring the wire through the hole for a visible turret-wrap.

+1 on "drilling a small hole *next* to the turret"

[TubeGeek]

This time i didn't realize there was a jumper under the board in the ac568 schematic.  When I converted to aa864, the added 500pF cap in the same position as the jumper….well I learned a lesson I won't forget if I ever convert another one of these.


I really like the tone of this setup with a p-bass.  Very recognizable tone's.


I own this cabinet at the moment but not the head.  I am hoping the head owner falls in love with my 2x12 cab and takes both home.