Hi.

[musicman3858]

Just a question on the OT of a Fender Bandmaster, I am getting an a/c voltage of 4.5 on the filament pin. Is this normal?

[stratele52]

No

You must have 6.3 AC volts.  6 volts is good too , but I never see a Power Transformer ( not OT) give less than 6.3 volts.

Remove all tubes and check . Broken heater wire ?
 
Do you have a good meter ?

[jjasilli]

1.  Expected voltages are valid under load only.  That means with tubes IN. (If there is no load, then voltages will be higher - see Ohm's Law.)  Are the tubes IN?

2.  Filament voltage comes from the PT not OT, as stratele52 points out.

3.  Note that with a center-tapped filament supply you should be getting a pretty even voltage on Both filament pins.  Together they should add up to about 6.3VAC, under load.  One volt higher than that is acceptable.

[tubenit]

Make sure you have a voltmeter with a fresh new good battery in it.  I can't tell you how many times on this forum that it turned out to be a battery problem giving false readings.

I am presuming you are referring to the PT not OT,  correct?

With respect, Tubenit

[stratele52]

Remove all tubes and check . 

I suggest that to see if a tube have a short heater

[Colas LeGrippa]

I suggest that to see if a tube have a short heater

Good morning gentleman !

Could you re-formulate your suggestion in good english so that we can understand what you mean, please ?

Best regards

Colas

[alerich]

I suggest that to see if a tube have a short heater

Good morning gentleman !
Could you re-formulate your suggestion in good english so that we can understand what you mean, please ?

What he's suggesting is to remove all of the tubes and recheck the heater string voltage. It will probably be a little high in this no-load state but it's a good place to start. If the voltage looks ok then you can replace one tube at a time (powering off between each tube) to see if the trouble can be isolated to a particular tube or tube socket. A short circuit or partial short circuit somewhere in the heater string could cause the low voltage condition.

[stratele52]

I suggest that to see if a tube have a short heater

Good morning gentleman !

Could you re-formulate your suggestion in good english so that we can understand what you mean, please ?

Best regards

Colas

Good morning Colas
+1 alerich

[musicman3858]

Thanks and yes I was referring to the PT...sorry. The voltage I'm referring to is the number 2 and 8 on the rec. tube. I have the high dsc voltage but I'm also getting the a/c voltage. I tried with the tube in and out. Should I retry with all tubes out and replace on at a time? I can't see any shorts that are obvious. I have an digital Beckman meter with a new batt. in.

[Colas LeGrippa]

-2 and 8 are the heaters pin: 5V A/C
- pin 2 to ground: DC high voltage
-4 and 6 : A/C high voltage

If you have reversed heaters and ac high voltage, you will keep on blowing fuses.

[Willabe]

- pin 2 to ground: DC high voltage

Not to ground.      

Pin 2 and 8 are the rect. tubes heaters, 5acv and are rectified dcv. Tap the dcv from pin 8 for B+ PSU.


              
                  Brad    

[tubeswell]

Thanks and yes I was referring to the PT...sorry. The voltage I'm referring to is the number 2 and 8 on the rec. tube. I have the high dsc voltage but I'm also getting the a/c voltage. I tried with the tube in and out. Should I retry with all tubes out and replace on at a time? I can't see any shorts that are obvious. I have an digital Beckman meter with a new batt. in.

A tweed bandmaster with a 5U4G rectumfrier?  Pins 4-6 are the rectifier plates and that's where the High Tension VAC is connected. Pins 2-8 are for the 5VAC rectifier filament. Most 5V rectifier tubes will work with 5VAC +/- .5VAC. (As Willabe says, Pin 8 is where the reservoir capacitor for the B+ is connected to). If the amp is working as expected, then there should be no problem with 4.5VAC, although it is at the 'low-end' of normal. Try popping a fresh rectifier tube in.

[Colas LeGrippa]

Willabe: reading from pin 2 to gnd = dc high voltage, I did not mean to connect DC high voltage to gnd !

[Willabe]

Well now that's a horse of another color than.

I was wondering what you ment.

              Brad