EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: gooseEL34 on July 09, 2016, 01:44:42 pm
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hey folks
I have a buddy with a tube guitar amp that was blowing fuses. It is a Reverend Kingsnake and has a 2x6L6 push pull power section with 4 12ax7 preamp tubes.
The amp (I think) had a power tube failure and pulled a lot of current, burning up the fuses and the first major voltage dropping resistor in the B+ filter sections (1st one post-standby switch).
So after replacing all the components and testing (with no power applied), I decided to check the AC transformer. The AC transformer is 120v AC primary and per the schematic has two secondary windings: 370v AC at 110 mA (B+) and 6.6v AC at 2A for the tube heaters.
The 6.6v heater taps is the concern. The power tubes (6L6x2) are heated with AC, then the AC voltage is rectified with a full wave bridge rectifier and is sent to the preamp tubes as DC (presumeably, for hum suppression reasons).
Neither the high voltage nor the heater windings have an external center tap wire (nor is one shown on the schematic).
So here is my concern.... I lifted the 6.6v taps from the circuitboard and measured the resistance from end of wire to end of wire and from each wire to ground (Fluke 189 meter)
Resistance from end of each green wire is 0.04 ohm
Resistance from one wire to chassis ground 0.04 ohm
Resistance from other wire to chassis ground 0.04 ohm
So this leads me to believe the failure melted the 6.6v windings. I realize that the 6.6 volt windings would have relatively little resistance but the above readings lead me to believe that the windings are shorted to the tranny chassis.
There is no connectivity between either green heater tap wire to the red high voltage tap wires and using the meter, no connectivity between either high voltage winding red wire to chassis ground.
Here are the questions:
1. Can I assume the tranny is toast as relates to the 6.6v winding?
2. If #1 is YES, could I tie off those taps and get an external small tranny 120vAC to 6.0vAC heater tranny and fell safe?
My concern is this: when the secondary of an AC tranny has one winding that is shorted to ground, but the high voltage winding seems fine, does that secondary short mean that the tranny will pull massive current and pop the mains fuses or can I still use this tranny to supply the B+ and just use a secondary tranny to heat the tubes?
Schematic here:
http://www.themondellos.com/gear/KINGSNAKE-SCHEMATIC.pdf (http://www.themondellos.com/gear/KINGSNAKE-SCHEMATIC.pdf)
Thanks guys!
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I would expect the resistance of the 6V windings to be very low. There's probably only 10-20 turns of thick gauge wire in them. Can your meter reliably resolve such low resistances?
1. Can I assume the tranny is toast as relates to the 6.6v winding?
I would disconnect the other secondary windings and power-up the transformer. A shorted turn will over-heat very rapidly. You could also use this opportunity to measure the output voltage of the windings.
2. If #1 is YES, could I tie off those taps and get an external small tranny 120vAC to 6.0vAC heater tranny and fell safe?
No. You will need to remove the short on the main transformer - otherwise it will go up in smoke.
Be careful of those high voltages!
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The filament winding is clearly shown to have a grounded center tap on the linked schematic. I would expect a low resistance from either end to chassis ground unless you unground the CT. Then it should read infinite resistance to ground.
Quickest way to find a shorted turn would be to measure the voltage across the disconnected ends of the winding. If you have 6.6 Vac or slightly more (being unloaded), then the filament winding is fine. A shorted turn in the filament winding will result in low output voltage on that winding. Check voltage from either end of the winding to chassis ground. You should see 3.3 V on either side.
A short in a low voltage transformer winding is a VERY rare failure unless the winding is floated at a high voltage to ground (like a CRT filament transformer in a scope).
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As already pointed out, there is a center tap connection for the 6v heaters shown on the schematic. You would need to disconnect both secondaries fully, both HT and LT and then power the transformer to test it.
Some test powering options:
Use a smaller than normal mains fuse in line, like 1amp. If there is a short it will blow instantly.
Use a mains lamp in series with the primary mains. If the lamp glows at all, there is a short.
Use a Variac and slowly bring up the mains while measuring the secondaries.
With both secondaries disconnected, there should next to zero current flowing in the primary.