BASSMAN WITH VERY LITTLE VOLUME

[ViperDoc]

Hi all, new member here. I have just completed a Mojotone Bassman that has very little output volume. I've built a few amps previously with much success. The only changes I made were I converted the back to a single shorting output jack and wired my 410 cabinet with a spider junction cable so it is hardwired for 2 ohm output to match the Bassman OT. I also built this up using a turret board, which I prefer. Everything powers on fine, no tube red plating issues. When I turn either channel volume up, I get very low output to the speakers and only when the volume pot is cranked. It is very distorted sounding as well.

Notes:

The Mojotone Bassman wiring diagram shows no detail on how to wire the PT green/gold wire. Other diagrams show it wired to ground, so that's what I did.

The far right 250 uF cap that goes to ground is polar on the wiring diagram, but my kit supplied a bipolar "+/-/+" 250 uF cap. Would this matter?

The OT black wire goes to ground. I wired it to the grounded lugs on the output jack as I've seen in other tweed amp diagrams. Is this wrong?

I originally wired the build for individual V4/V5 adjustable bias and used Rob Robinette's diagram as a guide. I originally wired it all up and got the same result as I have now (I reverted it back to fixed bias with the stock 56K bias resistor, still no real output power). So no difference between fixed and adjustable bias. I did hook up a TAD Bias Master to read the current and the stock current was 18 mA on each 6L6 tube, which I thought was low. As I turned up my 56KL adjustable bias pots, the current went down. ???

Thanks for your help.

Brian

[drew]

Photos and voltage readings, please.

[ViperDoc]

PHOTOS!

[ViperDoc]

VOLTAGES AS READ THIS EVENING ON MY VOLT METER (INSERTED IN TEAL--ALL VDC)


I originally wired two pots on the back of my chassis for individual V4 and V5 adjustable bias, but decided to bypass them for now and rewired it to fixed bias per original spec. The pots you see on the back in the pictures are out of the circuit completely. The wires I had run to them I removed from the pot lugs, soldered together and heat-shrunk the connections (black). Each grid resistor wire runs to the bias divider turret now. The reading at the Bias Divider Turret was "-59 VDC". I forgot to put it on the picture.

What do you think of these readings? I have nothing to go off of since the kit I bought gave me no such information.

Thank you!

[statorvane]

Are your heater voltages measuring 6.0 - 6.5 VAC?

Hard for me to tell, but did you measure 0.03 volts at V1 - Pin 6?

Also, check that the cathode resistor in the LTPI is 470R instead of 470K - that will reduce the volume significantly increasing the dirt - I have made this mistake more than once.

I am not sure what the bias voltage should read, maybe somebody else can check that.

[statorvane]

Are your heater voltages measuring 6.0 - 6.5 VAC?

Hard for me to tell, but did you measure 0.03 volts at V1 - Pin 6?

Also, check that the cathode resistor in the LTPI is 470R instead of 470K - that will reduce the volume significantly increasing the dirt - I have made this mistake more than once.

I am not sure what the bias voltage should read, my annotated 5F6-A layout indicates -48V, plates and screens around 430V. Maybe somebody else can check that.

Sorry for the double post!

[ViperDoc]

Thanks for your reply, Statorvane.

The Heaters all measure 3.1-3.2 VAC to ground, so that adds up.

CORRECTION, V1 pin 6 leads to a turret that reads 173 VDC. This is just like the turret to its right, like 150-175. The Bassman was designed when outlet voltages were 110V, not 120V. So all of these readings should be higher than the vintage diagram.

So now what?

[martin manning]

What is the bias voltage on the 6L6 pin 5's (or the 220k's on the board) now? While you're measuring that, do you get a pop in the speaker? If so, work your way back to the input measuring voltage on the preamp tube grids (pins 2 and 7, but you can measure on the board to make it easier and safer) and listen for the pop.

[Stevem]

A quick and simple test is with the amp on and the master volume up ( if it has one) yank the preamp tubes out one at a time starting with the PI tube and you should here a pop that gets louder as you get back towards V1.

Where the pop stops then you found your dead gain stage.

[Ten Over]

A quick and simple test is with the amp on and the master volume up ( if it has one) yank the preamp tubes out one at a time starting with the PI tube and you should here a pop that gets louder as you get back towards V1.

Where the pop stops then you found your dead gain stage.

That's pretty funny and everything, but I just don't think that it's cool.

[Stevem]

Well you may think it's very funny, but when you do repairs for a living and you need to get things done fast to give a accurate estimate of the time / labor that will involved in the repair to the owner, this method works fine and no harm comes to the amp!

Yes " it's cool"
Just don't plug the tubes back in with the standby switch on so you don't set the tube back in wrong.

[pdf64]

Thanks for your reply, Statorvane.

The Heaters all measure 3.1-3.2 VAC to ground, so that adds up.

CORRECTION, V1 pin 6 leads to a turret that reads 173 VDC. This is just like the turret to its right, like 150-175. The Bassman was designed when outlet voltages were 110V, not 120V. So all of these readings should be higher than the vintage diagram...

If that was the case then the heater Vac would also be proportionately high; hence your PT may have been designed to accommodate modern mains voltage, whilst producing the period correct secondary voltages
Schematics generally have a note indicating that any voltages on there have a 20% tolerance; whilst that's a nonsense for PT secondary Vac, by the end of the HT dropper string that tolerance will be very relevant, as individual tubes will vary, perhaps significantly so, from their type bogie, and hence the exact current they draw will likewise vary.

Generally, when this 'newly built amp only passes tiny distorted signal' problem happens, the input or output jacks have been miswired; the shorting contacts are a favourite for new builders to get in a muddle with
Try disconnecting all of the jack shorting contacts and review the issue.

[ViperDoc]

What is the bias voltage on the 6L6 pin 5's (or the 220k's on the board) now? While you're measuring that, do you get a pop in the speaker? If so, work your way back to the input measuring voltage on the preamp tube grids (pins 2 and 7, but you can measure on the board to make it easier and safer) and listen for the pop.

Hi Martin,

The bias voltage at the 220K resistors is -58 VDC with no audible popping at all. There is popping when I check other areas of the circuit, say towards the preamp side of things.

[ViperDoc]

Thanks for your reply, Statorvane.

The Heaters all measure 3.1-3.2 VAC to ground, so that adds up.

CORRECTION, V1 pin 6 leads to a turret that reads 173 VDC. This is just like the turret to its right, like 150-175. The Bassman was designed when outlet voltages were 110V, not 120V. So all of these readings should be higher than the vintage diagram...

If that was the case then the heater Vac would also be proportionately high; hence your PT may have been designed to accommodate modern mains voltage, whilst producing the period correct secondary voltages
Schematics generally have a note indicating that any voltages on there have a 20% tolerance; whilst that's a nonsense for PT secondary Vac, by the end of the HT dropper string that tolerance will be very relevant, as individual tubes will vary, perhaps significantly so, from their type bogie, and hence the exact current they draw will likewise vary.

Generally, when this 'newly built amp only passes tiny distorted signal' problem happens, the input or output jacks have been miswired; the shorting contacts are a favourite for new builders to get in a muddle with
Try disconnecting all of the jack shorting contacts and review the issue.

Here are photos of my input jacks:


Here's my single output jack; green is OT hot, black is OT ground. The braided green wire goes to the NFB resistor.

[pompeiisneaks]

Quite a few of those turrets look like they've got little to no solder in them. They may need to be touched up to get better connection. In some cases if you don't have a good physical connection and there's not enough solder, the flux can create a resistive effect causing less current to flow through those joints. You may want to hit them again with a bit more solder. here:

somesolder.png

Also I think I see one of the input wires not soldered at all, here:

nosolder.png

~Phil