Mojotone Bassman very low output

[Privatpig]

The -45 means you have bias voltage, that reading was with the rectifier in and power tubes out, so you don't fry the power tubes if the bias is wrong, but leaving the tubes in was fine.

I think you have the 4, 8 ohm speakers wired correctly, in parallel, this should give you 2 ohms total. I would double check the resistance of each individual speaker to make sure the male rca plugs are wired correctly and not shorted. Do this one at a time, unplugged from the amp. You should get somewhere around 6 ohms for each speaker.

Hard to see much detail from the picture, but some of the solder joints around the tube sockets look a bit rough, might want to touch them up, making sure to heat the wire and the socket pin at the same time.

Poking each solder connection with a non-conductive chopstick while audio is playing through the amp can help you find bad solder joints. Better to use something other than your guitar so you can safely use one hand for poking and keep the other in your pocket.

Thanks. Checked again no pops.

[Privatpig]

Negative bias voltage confirms the diode and the bias circuit are correct, filter caps not exploding confirms they are installed in the right orientation.

Thanks!

[Privatpig]

A fender Bassman schematic calls for -48 volts.

At -45 volts your output tubes will get getting very hot to the touch very fast, in fact in a darkened room you might see there plate structure glowing red and you do not want that taking place!

its been on for at least 30 minutes and I can still touch them. No bad smells or anything.

[Privatpig]

Mains power OFF Ohm checks:

1) Verify that the input jacks are wired correctly. Plug a guitar cord into one of the inputs. Measure Ohms between the sleeve and tip of the other end of the cable. You should measure either 1 Meg or 136K. Verify that there is a low Ohm reading from the tip to one of the 68K resistors on the eyelet board. Try all the input jacks.

2) Measure Ohms across the speakers at the jacks inside the chassis. Should be a very low reading, less than one Ohm. Now, unsolder one of the wires from the output transformer that goes to the speakers. It doesn't matter which one, choose the one that will be easiest to re-solder after the test. The resistance across the speakers should jump up to about 1.5 Ohms. As you unplug speakers one at a time, the reading should go up until you have no speakers connected.

3) Turn all the pots half way up. Measure the Ohms from the center terminal to ground. The Volume controls, Treble and Bass should have readings 100K or higher. The Mid and Presence controls should have lower readings about half the pot's rated resistance.

Dang I did all but the unsolder and I waited too long and before I could post it kicked my data.
Bright 1 tip to shaft=1.412, 2=133.9, norm 1 =1.343, 2=131.1

Bright to 68 1=1.411, 2=133.7, norm 1 is 1.345 2=134.5

Inside speakers .3 .2 .3 .2

Mid vol=12.15
Bass 117.9
Treb 139
Bright 70.6
Norm 66.2

[maxkracht]

Didn't notice the voltages on your schematic before. Double check how V2 is wired and how the 100k resistor is oriented, mojo has expected DC voltages written on the schematic.

Resistance readings need a multiplier. 100 ohms 100k ohms 100M ohms etc.

[LOUDthud]

Check the resistance from V2 pin 3 to ground. Should be 820 Ohms.

[angelodp]

For general reference. Do you have a bulb /current/limiter? Maybe look at all the voltages around the power tubes, using the RobR guide.

Here is a quote from Sluckey -

"On first power up of the few new builds I've done, I usually plug in the tubes except the recto, turn on and verify filament operation. Then I pull all tubes and insert the recto, power on and check for B+ in all the right places, ie, node caps, plates, screens, etc. Not looking for exact voltages, just that B+ is where it should be and ain't where it shouldn't be. This just gives another layer of confidence before powering up with all tubes in place.

It could be helpful sometimes when troubleshooting an amp that has a high B+ current related problem. Getting the tubes out of the equation, especially power tubes, is quick and easy to do and may help you to quickly get on the right trail rather than chasing a goose."

[Privatpig]

Are those Voltage readings you took in the layout picture of post #1 ? Check connections around the middle preamp tube. Is pin 1 connected to pin 7 ?

YOU SIR NAILED IT! AND SOMEINE ELSE SAUD THEY BET ITS IN MY PREAMP TUBES! THANK YOU GUYS! YOUR ALL AWSOME!! So I got to looking at the drawing and on the middle tube there is that 100 resistor you guys have been talking about. If you look at the pic it has the resistor between 6 and 1 and then goes to 7. I had the wire stretched from 6 to 1 AND THE RESISTOR ON THE OUTSIDE of 1 and then going to 7. That fixed it! Although I did this same thing on my Super and it works? Maybe I'm not getting it fully? But I am getting full volume on it? So when I got done it was loud, but it was breaking up. I pushed around on the wires, nothing, then I pushed in on the same tube and man it sounds great! Scared my wife! Lol! I can't thank you guys enough! I was giving up and just about to send it somewhere. You guys are great!
Thanks again all!!!

[Stevem]

If your output tube bias voltage on pins 5 is still only -45 then that’s why the amp is very loud and also more distorted then it should be.

Your also going to be eating up the output tubes very fast.

[Privatpig]

If your output tube bias voltage on pins 5 is still only -45 then that’s why the amp is very loud and also more distorted then it should be.

Your also going to be eating up the output tubes very fast.

I will check it when I get home! Thanks again!

[Privatpig]

If your output tube bias voltage on pins 5 is still only -45 then that’s why the amp is very loud and also more distorted then it should be.

Your also going to be eating up the output tubes very fast.

I will check it when I get home! Thanks again!

-53.8 and - 53.6. What shouzld I do now?

[angelodp]

You could implement the adjustable bias mod that Rob R has on his site. In that way you can dial in the bias current you need.

[Privatpig]

You could implement the adjustable bias mod that Rob R has on his site. In that way you can dial in the bias current you need.

I may try that. Thank you!

[Privatpig]

I could change the 56k to a larger resistor, correct? Would that drop the bias?

[martin manning]

I could change the 56k to a larger resistor, correct? Would that drop the bias?

Increasing the 56k going to ground will lower the bias voltage (make it more negative), which will reduce the idle plate current. Most people prefer to add a trimmer pot there to make the adjustment easier, something like a 27k in series with a 50k trimmer wired as a variable resistor.