Mojotone BF Bassman AA165 - No Sound

[DRH1958]

Also, I notice that pin 5(bias voltage) of power tubes is at -61v. Isn't that too high? At the very least, this will be going towards cutting off the power tubes or a very cold bias. Shouldn't this be closer to -50v give or take? Probably not the source of the problem but should be addressed.

[Jazz P Bass]

Preamp tube V2/ Pin 1 does not have any voltage reading.

Aside: If you are going to troubleshoot this build, then you need to comprehend how a most preamp tubes operate.

They must have a load resistor on the plate, fed by the preamp high voltage supply, which results in a voltage on the plate.

There must be a voltage on the cathode.

[Briac]

Hi Justin
after reading your reply. I re-tested pin 8 = 0, then moved wire pin 8 = 2.3, resoldered pin 8 = 2.3. I edited my prior post, issue description and "gain" to bass.

Thank you

[Briac]

BF Bassman doesn't use both sides of V2.

Thanks for the reply

[Chuck H]

I'm going to resubmit my suggestion of drawing your circuit as it is in the amp. Either draw it to match the layout and check it against the layout as you go OR draw it to match the schematic and check it against the schematic as you go. In the end it will likely speed things along. Because you aren't entirely familiar with the circuits you are basically stuck working rote. Copying either the layout or the schematic correctly should result in a working amplifier. You may be hoping to find the one or two things that may be wrong to get it working, but even that can be tricky because a miswire isn't a failed circuit. It's an icorrect circuit and who knows what it's doing. Symptoms are often confusing. Even to experienced people that know how to fix proper, but broken circuits. So, because rote is what you can do, and you have rote information you can work with, you should do that. It's not as bad as it sounds. You draw the tube sockets and controls in the same locations as whatever diagram you're mimicking and then follow your own leads and connect the dots placing the components along the way. Check your work against the documentation every few components you draw. You WILL discover some errors this way.

[Briac]

will do, Thanks Chuck

[überfuzz]

will do, Thanks Chuck

Check for wonky connections along the way, i.e. is there a connection where there should be and is there a connection where there shouldn't.

[Chuck H]

And examine connections for iffy contact. Like cold solder joints. Though I have to say that for a first build that's some very good solder work. I saw one or two joints that didn't look just right, but I could find that on any of my own amps

[Briac]

Hi Chuck, with the drawing I found 2 mis-wires and a handful of iffy solder joints. fixed them, moved mainboard grounding away from filter grounding. Got rid of ground loop. Bass channel still has a slight volume hump. I found a Fender Bassman schematic Bassman-AA165-schematic.pdf and diagram Bassman-AA165-layout.pdf. I'll use the solid state rectifier until I have a working amp. Re-checked everything. Inserted tubes one at a time with dim-bulb limiter on/ off, all good until the power tubes. Blows fuse with dim-bulb off. The kit came with JJ's 6L6's. One of tubes has a warm glowing center coil the entire length. The other's coil is very dim except for the end which is bright orange. I think its shorted. I have Ruby Quad coming.

Thank you

[überfuzz]

It's often a good thing to test with known good tubes. :-)

[Chuck H]

It's often a good thing to test with known good tubes. :-)

When they send the tubes with the kit I think it's reasonable to assume the tubes are functional. Unfortunately WRT current production tubes this isn't always the case and new doesn't mean good IMHO It then falls to whomever is selling the kit to test the tubes for suitability so that the product they sell is indeed a whole kit as advertised. Unfortunately this isn't always the case either

Briac, if you swap the tubes, does the overly red glow follow the tube or stay with the socket? This may tell you if the problem is with the tube or the socket. I've seen the creeping bias thing you describe when only one tube is inserted into an AB (two tubes) circuit. So be certain that your OT is wired correctly and has continuity with the sockets and no shorts or faults.

[Briac]

Chuck, swapped tubes, it stays with the socket V6 . I'm confused with the output terminal wiring. with mojo diagram BF Bassman AA165 WD.pdf. with speaker plugged in, I get normal hiss sound. With amp off, no speaker plugged in, connection continuity to ground, ok. When I place a piece of paper between output contacts, still continuity to ground, shouldn't this be isolated? When I wire out plug as in fender diagram Bassman-AA165-layout.pdf. grounded and no sound.

added : Drew a pic, looks right. with power tubes in, dim bulb on: powering on the dim-bulb goes bright then dims to nothing. as the power tubes warm up dim-bulb goes bright again.

added: Drew a pic of fender output terminal. I had an extra connection. found another iffy solder joint on tube, fixed. with dim-bulb; very bright, dim, then less bright. removed dim-bulb. with normal power, No blown fuse. Now, I'm tracking down an intermittent "putt putt" sound.

[Briac]

Many Thanks and appreciation for everyone's reply and help. I've got amp.

Normal channel: Quiet shhh, no hum, good volume and attack punch. Yeap, there's dynamics.
Bass channel: Quiet shhh, no hum, good volume. (But, what is that farty distortion when pushed?)

The last of my problems were: a bad 12AX7 tube at V1 (BC volume hump) and PT plate bolts were very loose (ground loop noise).

Learned a lot.
Take away: 1) "All" solder joints are critical. 2) Proper grounding critical. 3) Trouble (draw a picture) 4) "New" doesn't mean good. 5) Lots more to learn...

Again, Thank you!

[Chuck H]

I think I speak for everyone when I say... What was the problem with the socket that was red plating the tube?

Oh, and congratulations too

The farty bass can be tweaked out. You have to determine if it's the amp or the speakers first (probably the amp, but not always). Of course, then it's not a Bassman any more.?. Most vintage designs will get farty with hot humbuckers or sometimes just fat sounding guitars with standard pickups. The amp is just trying to process more LF than it can handle. Set up the cranked gain where you like it. Then turn the bass knob all the way down. Now turn it up until it just starts to fart and then back it down a hair. That's all the bass the amp and speakers can make. Sorry. Dialing it out with circuit mods will affect the other channel and basically just do the same thing, except you will be able to turn the bass knob up higher creating the illusion in your head that there is more bass. If you actually need more clean bass from an amp like this a speaker/cabinet upgrade is probably the best way to go. JM2C.

[überfuzz]

Many Thanks and appreciation for everyone's reply and help. I've got amp.

Normal channel: Quiet shhh, no hum, good volume and attack punch. Yeap, there's dynamics.
Bass channel: Quiet shhh, no hum, good volume. (But, what is that farty distortion when pushed?)

The last of my problems were: a bad 12AX7 tube at V1 (BC volume hump) and PT plate bolts were very loose (ground loop noise).

Learned a lot.
Take away: 1) "All" solder joints are critical. 2) Proper grounding critical. 3) Trouble (draw a picture) 4) "New" doesn't mean good. 5) Lots more to learn...

Again, Thank you!

Very nice!!! Like I said, known good tubes. Funky hardware can be a pain in the BEEP if you're debugging a build.