1968 Fender Bandmaster Help Please

[jukelemon]

Hi all.

I have a 1968/69 Fender Bandmaster on the bench and i am seeing something I have not seen before and hoping for some help.


Schem: https://cdn.shopify.com/s/files/1/2325/2729/files/Bandmaster-AC568-Layout.pdf?5926397210715640943


The amp has been worked on before and looks to have been done by a good tech.  All blue molds were retained, preamp cathode caps were all replaced and all power supply filter caps were replaced.  V1 and V2 plate resistors were replaced at some point.


Tubes are healthy and I have sub'd in both power and preamp tubes prior to writing. I have checked all grounds and they measure out fine.


So..when I turn the amp on and then off standy by and with no input plugged in, all voltages are in spec.  I have measured all pins and they match within tolerance and given the extra wall voltage to the schem.  I also measure the heater voltage.

then...and regardless of channel, if the amp receives any kind of input for longer than 10 seconds (guitar or just me taping the end of the cord) two things happen that I have measured and/or have seen.

First, the plates on V1 and V2 shoot up to ~400 vdc and then measure as if there were no plate resistors at all.  That is...like it was directly off the power filter rail.


Second, all tubes (both power and preamp) lose their heaters DESPITE me measuring 6vac on each tube.  In other words, the heater circuit is not compromised yet all tubes go dark.  I can literally see all of them dim out.


The amp will stay this way i.e. tubes not heated and V1/2 plates ~400vdv until the amp is turned off.  Wait another minute, turn back on and the amp will be stable until an input is introduced.

I have never seen an issue like this and hope someone can point me in the right direction.

Thanks!

[mresistor]

 thats weird for sure   just reread your post
seems like the heater circuit is warming up and losing ability to carry any current load.



Have you checked the artificial center tap 100 ohms resistors?  measure them

[tdvt]

seems like the heater circuit is warming up and losing ability to carry any current load.

This sounds right to me,

But to clarify, there is still 6VAC at the heater pins even when the tubes are dark?

I might try running the heaters off a second source (battery or other power supply) to see if things are otherwise working.

[Dave]

To add to what Mresistor said, if there is an almost open short in the filament winding and it still reads 6 volts, but can no longer hold the load, when the tubes filaments go dim or out they are no longer drawing current to operate. Then, if they are not drawing current to operate, then there would be no load on the power rail which would account for the plate voltage rising.


The problem is, it is difficult for me to imagine a short like that that would continue to show 6 volts even the winding fails.


Can you hook up a DMM and read if there is a change in the 6.3 voltage when the amp starts to "shut down"?


Dave

[jukelemon]

Hey all.

Thanks.

There is no change in heater voltage when the issue occurs...which is why it is so odd to me.


I left my meter on the heater voltage while I observed the tubes all going dark.  I even tested each side of the heater voltage.


If I read my own post out loud I'd think I was nuts.

On the 100 ohm question...this amp has a heater center tap on the PT so not a concern.

Also..on the heater circuit warming up...the amp can sit without input and not be affected.  It is only when an input is applied that I am seeing this issue.

[Dave]

Do you have a tube tester to test all the tubes for shorts?

The tubes dimming out, I think, is the root of the problem. When the heaters are not hot, the tubes are not drawing current. That accounts for the voltage being high on the plates. So, I think, the symptom to diagnose/fix is the heater problem.


By the way, if it were me, I would disconnect the CT on the heater winding and replace with artificial CT. Because, if there is a short somewhere (in a tube or elsewhere), the winding will eventually burn up, but if you replace that CT, it will give you a bit of a fail-safe.

Dave

[jukelemon]

Do you have a tube tester to test all the tubes for shorts?

The tubes dimming out, I think, is the root of the problem. When the heaters are not hot, the tubes are not drawing current. That accounts for the voltage being high on the plates. So, I think, the symptom to diagnose/fix is the heater problem.


By the way, if it were me, I would disconnect the CT on the heater winding and replace with artificial CT. Because, if there is a short somewhere (in a tube or elsewhere), the winding will eventually burn up, but if you replace that CT, it will give you a bit of a fail-safe.

Dave

I don't have a tester but I have sub'd in all tubes.  I don't think it is a tube.

Thanks for the suggestion on the center tap.

[AlNewman]

What about the lamp?  I once saw a factory defect which had a stray wire which shorted the socket.  All that did was make a hell of a hum though.
You may want to install a fuse inline for testing, and while you're in there check current with your mutimeter to see if it's drawing current at fault...  Unless you have a 5+ watt 1 ohm resistor you can temp in.

Edit-Sorry, wasn't thinking.  1 ohm resistor is a bad idea...

[mresistor]

So if no artificial center tap and 100 phm resistors then does this amp have a replacement PT?

[jukelemon]

So if no artificial center tap and 100 phm resistors then does this amp have a replacement PT?

Hi.

The PT is a 022814. I show that as accurate and original for the Bandmaster.  It has a green and yellow CT for the heater supply unless I am really missing something.

Essentially identical to: Fender Bassman Bandmaster Original 022814 Power Transformer 1967 Schumacher | Reverb

[jukelemon]

What about the lamp?  I once saw a factory defect which had a stray wire which shorted the socket.  All that did was make a hell of a hum though.
You may want to install a fuse inline for testing, and while you're in there check current with your mutimeter to see if it's drawing current at fault...  Unless you have a 5+ watt 1 ohm resistor you can temp in.

Edit-Sorry, wasn't thinking.  1 ohm resistor is a bad idea...

Yeah..no hum at all.  The amp is dead quiet i.e. I don't suspect a lamp issue.

[jukelemon]

Ok...
I am thinking one of the 6l6s is shorting out and my replacement set also had a bad one 


So I am going to get another set and make certain it is not the power supply.


Without the 6l6s in, the amp seems to be stable.  But I still have never seen a power tube short take down the heater circuit.  Would be interested in knowing how that can be without the fuse blowing.



I will report back after I find another set.

Thanks all.

[shooter]

But I still have never seen a power tube short take down the heater circuit.  Would be interested in knowing how that can be without the fuse blowing.

I wouldn't put any PA tubes in til you've verified bias volts over time.
not totally sure but cathode to heater seems a reasonable fail point.

[sluckey]

Resolder the filament wires at the pilot lamp and also on the first 6L6.

[jukelemon]

But I still have never seen a power tube short take down the heater circuit.  Would be interested in knowing how that can be without the fuse blowing.

I wouldn't put any PA tubes in til you've verified bias volts over time.
not totally sure but cathode to heater seems a reasonable fail point.

Hi.  Do you mean monitoring the bias during this event?

When I measured, the bias supply was -47.6v with the tubes idling at 29.7 and 30.2 milliamps.

[jukelemon]

Resolder the filament wires at the pilot lamp and also on the first 6L6.

Ok thanks and will do.

What are you suspecting?  Seen this before I assume?

[sluckey]

bad connection or broken wire.

[AlNewman]

I suppose if it's signal that creates the fault, you could pull all tubes except V1 and strum a few chords, see if the fault occurs, then move down the line.  It's definitely strange that the voltage remains constant fault or not... 

[jukelemon]

I suppose if it's signal that creates the fault, you could pull all tubes except V1 and strum a few chords, see if the fault occurs, then move down the line.  It's definitely strange that the voltage remains constant fault or not...

Hi.

Yes, good thought and I tried that.

V1 = Normal Channel


V2 = Vibrato

With either V1 or V2 pulled it just delays the loss of all heaters i.e. just takes a little longer to get to the failure point. I even pulled V3 and left V1/V2 in place to see what would happen. Same issue.

[jukelemon]

Resolder the filament wires at the pilot lamp and also on the first 6L6.

Spot on sir!!

I had chopsticked the amp several times but went back with my fingers and really gave things a move between the lamp and the 6l6.  Off popped one side of the filament circuit!  The wire had completely broken in half at the pin.

Cleaned the pin and resoldered. Amp is 100%

Much thanks Sluckey 

I guess the wire was attached enough to show voltage but could not handle any amperage?  Then the small physical connection failed altogether taking down all the downstream heaters/tubes.

[sluckey]

Resolder the filament wires at the pilot lamp and also on the first 6L6.

Ok thanks and will do.

What are you suspecting?  Seen this before I assume?

Just trying to get you to focus on the elephant in the room.