'68 Bassman amp volume

[The Dude]

And, while I'm asking seemingly dumb questions: The speaker is plugged into the "Speaker" jack (switched) and NOT the "External Speaker" jack?

[Doc Stitches]

tone controls not down

no DC on heaters

speaker in speaker jack not External

good points form all.....I wonder how much "egg" I am going to have on my face when I find the problem ????

[Chuck H]

tone controls not down

no DC on heaters

speaker in speaker jack not External

good points form all.....I wonder how much "egg" I am going to have on my face when I find the problem ????

A little. We've all done it.

Trace the signal. Keep a guess at it's likely level in mind as you follow the circuits. If anything looks off, check it. I did notice some iffy solder joints in the pics. A little fresh melted, flux core solder and a retouch on the worst looking joints can't hurt and won't take long once you're at it. It also gives you a reason to be looking at the circuits, so you may see something.

[Doc Stitches]

I don't know how to trace a circuit.

I'll resolder the joints.

Is a 10K resistor OK on the bass channel bass pot ? I don't have an 8.2 K ( Or is a 7.1K OK ? )

I have a 12 in. Scorpio speaker from an old Triumph 60. It reads 5.7 ohm ... Is it a 4ohm or 8ohm ?

[Enzo]

That would indicate an 8 ohm impedance speaker.

[Doc Stitches]

Thanks Enzo .....

[Justin Thomas]

Thanks also to Enzo for the explanation of how to properly measure & check the bias on the PI. I remember that often when I try to measure it the "normal" way, the voltage will start high and then go down, etc. So thanks for the refresher course!

Justin

[Doc Stitches]

Playing amp on both channels now, Bass channel is louder and very clean.

Normal channel is like volume #1 when set on #10 as well as a little distortion.

Found a preamp tag board wire was missing.

Here are the new tube voltage details : Pins 2 and 7 on V4 are strange, ( even for the west coast ) #2 is 84v and #7 is 75v

I am thinking there is a wiring issue under the tag board ... remember, someone had worked a messy looking mod on this amp

Photo # 5 is V4 with the strange voltage

Here are some photos :

[The Dude]

V4 is the phase inverter and last tube before the output tubes. If there were a problem in that stage, it would effect both the bass and normal channels since it's common to both. The problem is likely going to be in the 1st gain stage in from the normal channel input.

[Doc Stitches]

Thanks Dude ... so is 84V on Pin2 and 75v on Pin7 OK ?

I'll check the 1st gain stage in Normal channel

[The Dude]

Yep. Those voltages are fine. One thing to check: Occasionally, grounds come loose from that brass grounding strip under the controls. Check those - particularly any that might be related to that first normal channel gain stage.

[Doc Stitches]

will do

[Chuck H]

You can use a layout drawing to trace/test the under-board wiring. Readily available with an image search. I suggest you test ALL the under-board wiring for accuracy/existence.

I know you've been looking it over. I don't expect to "see" something you missed, but I'd like to see a clear photo of the pot wiring anyhow.?. Thanks

[Doc Stitches]

so Dude ... you da man !

check out this "evidence" in photo :



faulty volume ground on bass channel ....

I looked at it MANY times but thought it had an inner core wire which would be good ... so I did not worry about the "frayed"

look. After all .... it is that Vintage look isn't it ???? I'll let everyone know the final results.

Thanks again to everybody.

[Doc Stitches]

OK Chuck ... I'll get out the pot photos tomorrow