Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: musicman3858 on October 15, 2012, 03:05:08 pm
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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?
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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 ?
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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.
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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
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Remove all tubes and check .
I suggest that to see if a tube have a short heater
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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
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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.
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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
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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.
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-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.
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- pin 2 to ground: DC high voltage
Not to ground. :w2:
Pin 2 and 8 are the rect. tubes heaters, 5acv and are rectified dcv. Tap the dcv from pin 8 for B+ PSU.
Brad :icon_biggrin:
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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.
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Willabe: reading from pin 2 to gnd = dc high voltage, I did not mean to connect DC high voltage to gnd !
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Well now that's a horse of another color than.
I was wondering what you ment.
Brad :laugh: