Power tube arcing inside

[Locrian99]

Hello,

I have fender super 112 a friend of mine picked up and brought to me.   I believe for several reasons this amp was exposed to a moist environment, some corrosion on the circuit board, mold on the speaker and the mdf on the baffle is crumbling in one area.

Initially the amp would pass signal but it didnt seem to be amplified.   Pin pointed r112 was fried and we had signal passing.    Though it seemed like the output was a little low.   On the gain channel it seemed about what I would expect.    I tried two new 6l6gc’s just to try and volume was the same but then it cut to a higher volume (sounded what i would expect) and then cut back out.   A pop followed.   Turned it off, waited a minute went to pull v104 and it burned me.   Which point i wondered if it was redplating.   Decided to flip back on for a moment.  It did the pop thing right away and i noticed an arc when it did it in the tube. 

I powered it down, removed the tubes.   Noticed the arc actually left a burn. 

Appears to be at pin 4.

Checked voltages all looked good.   Without any tubes in i was reading 500v at pin 4, and 503 at pin 3.   Which is 20-25v higher than listed on schematic.   Going to go through and reflow all these solder joints that have some sign of corrosion.

Im assuming this is a bad output transformer at this point.   Im just not sure the best way to test to confirm this, or if there is something else to look at.   This board is a real PITA so far to get out of this chassis so hoping i might be able to confirm OT as the issue.   These spade connectors just about seem welded together lol.

Between the OT red wire and the OT green/yellow I’m showing 27k ohms.   Im assuming that doesnt bode well for the OT.   

   

[Latole]

1- What are the bias on each 6L6 with 1 ohms resistor on cathode ?

2- The voltages on the schematics are for reference only. They are never identical to what we will measure on an amp.

3- There's only one way I use to measure/test an output transformer ;

________________________________

Testing Output Transformer and finding Impedance

1- Signal generator set to 1000 hertz AC 0.5 volts

2- Send this signal to OT secondary , check with accurate meter to have 0.5 VAC on OT tap. Or close

3- Read AC voltage at OT primary

4- For exemple if you read 14.35 VAC you divide by 0.5  ( or what you read at 1 )

    14.35 / 0.5 = 28.7

5- Square the result  ; 28.7 X 28.7 = 829.67

6- Multiply by secondary output impedance where you are connected  , say 8 ohms
    829.69 X 8 = 6631

7 - This reading 6631 is the primary input impedance

If you use two 6L6 in push pull , look on Tube Book for Plate Load resistance . You’ll see you that you need 6000 to 3800 ohms  for two 6L6 in PP class AB

The reading we got 6631 is on spec
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If transformer is no good your reading will be very far from what we should get .

By experience these test do not work on Super Reverb Transformer or other with 2 ohms Output Transformer impedance

You can do this test on unknow OT to see how / where you can use it

[Latole]

I've often done this test on transformers I wasn't sure about, and the results have always given me a good indication of the transformer's condition.

[Locrian99]

I've often done this test on transformers I wasn't sure about, and the results have always given me a good indication of the transformer's condition.

Thanks for taking the time to explain, ill attempt to check that tonight.   I thought perhaps showing the reading of 27k the red and green/yellow was enough to show that it was bad, rather than needing to verify the impedance etc. 

[sluckey]

You don't need to verify the OT impedance ratio. Waste of time.

Between the OT red wire and the OT green/yellow I’m showing 27k ohms.

Disconnect the OT red, blue, and brown wires. Do you still measure 27K between red and green/yellow wires?

[Latole]

You don't need to verify the OT impedance ratio. Waste of time.

I think I understand what you mean. My test may be useless at this stage, but with my test, measuring the impedance ration, it tells us the complete state of the transformer.

[Locrian99]

Thank you both, I’ll try that tonight after work. 

[sluckey]

...with my test, measuring the impedance ration, it tells us the complete state of the transformer.

Not so. It doesn't tell you if any turns are shorted to each other, or another winding, or to the frame. All it tells you is the Z ratio. That test is great for determining the Z ratio of an unknown OT. But this is a known Fender amp and is a safe bet that Fender did not use an unknown OT.

[Beezerboy]

hes a link to a tranny tester:

https://www.premierguitar.com/the-super-secret-transformer-tester

basically you are cycling a low voltage source (battery) applied from the output side and getting a high voltage pulse on the input side via transformer action. it takes something like 80V minimum to light the neon bulb, so thats the test.   shorted windings are in effect, a load resistor.   the test pulse simply dissipates as heat and won't light the bulb.

I built this. it works

[Locrian99]

You don't need to verify the OT impedance ratio. Waste of time.

Between the OT red wire and the OT green/yellow I’m showing 27k ohms.

Disconnect the OT red, blue, and brown wires. Do you still measure 27K between red and green/yellow wires?

Nope no connection between them now.   

Going to go through and pull the board tonight so i can physically examine all the components.    Anything i should maybe look at first?   

[Locrian99]

Well went to go through and start pulling thr spade connectors.   Removed negative wire for the speaker behind it was the positive barely touching the male connector on the board.   Wonder if i accidententally pulled it loose before.


Would that cause thr arcing?    Im sure it would cause the volume and popping…

[Locrian99]

Well shoot brand new 6l6’s.   I decided to try swapping positions of the tubes for s&g’s.    The arcing followed the tube. 

But they worked fine in my fender 75…