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Old 15th June 2011, 06:21 PM   #1
tbie is offline tbie  United States
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Default Tubelab SE fire, help needed.

Though I said fire I think I know what happened to cause it. One of the multimeter clips from R7 (B-) touched R6 and seems to have shorted the circuit. I was dumb enough to wiggle and play with volume pot while the circuit is live. Anyway, result is I saw some fire around the C6 C7 area and when I checked I saw R32 burning bright and was black and charred when I checked it without power.
Question is how will I know which other parts (c6 C7 need replaced). How will I know if other parts are fried?
Another question though unrelated is, before frying it, I measured output grid voltages and set to -84V with only rectifier and with 5842 in the grid voltage went to 120's even while playing music? is this normal?
Qestion 2, Plate voltage at the postive terminal of coupling cap(C9, C11) was weird? Left plate voltage was 42-43 V nomatter however I changed the cathode pot (R9)? Reading was 0V on Right channel plate voltage. What is wrong? Why would this happen? Can anyone help me fix this.
Output for right chanel is very faint, to the level of no sound with left speaker dominating. With volume pot sound levels reduced by atleast 3 times. even with volume pot turned all the way to full. Why is this?

Last edited by tbie; 15th June 2011 at 06:23 PM.
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Old 15th June 2011, 10:48 PM   #2
tbie is offline tbie  United States
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Guidance, anyone!
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Old 16th June 2011, 04:16 PM   #3
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Quote:
Originally Posted by tbie View Post
result is I saw some fire around the C6 C7 area
Fire, do you mean spark?
Quote:
and when I checked I saw R32 burning bright and was black and charred when I checked it without power.
That resistor will have to be replaced.

Quote:
how will I know which other parts (c6 C7 need replaced). How will I know if other parts are fried?
Does the top of capacitor look like bulging out? If not, it may be ok for now.

Quote:
Another question though unrelated is, before frying it, I measured output grid voltages and set to -84V with only rectifier and with 5842 in the grid voltage went to 120's even while playing music? is this normal?
Qestion 2, Plate voltage at the postive terminal of coupling cap(C9, C11) was weird? Left plate voltage was 42-43 V nomatter however I changed the cathode pot (R9)? Reading was 0V on Right channel plate voltage. What is wrong? Why would this happen? Can anyone help me fix this.
Output for right chanel is very faint, to the level of no sound with left speaker dominating. With volume pot sound levels reduced by atleast 3 times. even with volume pot turned all the way to full. Why is this?
Follow the "check out" steps on Tubelab website step by step and if anything does not match what Tubelab shows, something is wrong. Try the steps again and write down the voltages per each step and let us know.
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Old 16th June 2011, 05:52 PM   #4
tbie is offline tbie  United States
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Yes, I meant sparks coming around the area of C6 C7. I checked the caps, they look ok no bulging or signs of burning.
I am in the process of procuring some more 4.7K ohm resistors, I don't have any spare ones.
I am building this for using 300b tubes.
I followed tubelab checkout steps, and here are the voltages before I shorted the board.

B+ with load 382V
Filament voltage at pin 2 on output tube socket is 5.02V both sockets. (B+ was negative 0.07 V at thsi point)
Output tube grid voltage was adjusted at R33 and R34 and set at -82V and -83V R and L (This was with rectifier tube in place).
5842 plate voltage measured at + end (lead closest to 5842) of C9 and C11. At C11 it measured 45V turning cathode pot (R20) all the way. Voltage was 0 at C9 no matter which ever way I turned the cathode pot(R9).
With 300b in place and playing music, I adjusted plate supply of output tubes at R 18 and R29 to 63mA and 64mA.
At this point L speaker has good output, R speaker it seems had some output ( I came to know of it when L speaker went dead after I shorted the board) but sound was very feeble.
Thats when I shorted the board and fried R32. I will reset the board and report once I chekout again.

Everharmonics thanks for the reply.
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Old 16th June 2011, 11:07 PM   #5
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Originally Posted by tbie View Post
Voltage was 0 at C9 no matter which ever way I turned the cathode pot(R9).
Something is wrong right there. Until you can fix that, this amp shouldn't be used. As to what is causing it, perhaps one of the gurus here can answer.

Quote:
I will reset the board and report once I chekout again.
See if you can fill in the step list below.

1) w/ no tubes in,
B+ =
B- =
Output tube filament supply left = , right =

2) w/ rectifier tube in,
B+ =
B- =
Output tube grid to ground left = , right =

3) w/ rectifier tube and input tubes in,
B+ =
B- =
Input tube plate to ground left = , right =

4) w/ all tubes in and set the output tube bias,
B+ =
B- =
Input tube plate to ground left = , right =
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Old 18th June 2011, 10:52 AM   #6
tbie is offline tbie  United States
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1) w/ no tubes in,
B+ = 0.07
B- = 292
Output tube filament supply
left = 4.99,
right = 4.99

2) w/ rectifier tube in,
B+ = 462
B- = 248
Output tube grid to ground (R33, R34)
left = 224,
right = -84.3

These measurements are after replacing R32, R25 which appeared burned. Can someone suggest a way of checking whats wrong with the left channel to get output grid voltage so high and positive. I will post board pictures later in morning when I get home from work.

Thank you.
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Old 18th June 2011, 03:50 PM   #7
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Quote:
Originally Posted by tbie View Post
1) w/ no tubes in,
B+ = 0.07
B- = 292
It may not be a big deal but B+ should be negative voltage at this step. How long did you have it on when measuring?

Quote:
2) w/ rectifier tube in,
B+ = 462
B- = 248
Output tube grid to ground (R33, R34)
left = 224,
right = -84.3
B+ and B- are about right. Output tube grid to ground should be both around -80v. I'll look at the schematic and see what could possibly cause this, unless one of the gurus respond before I do.
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Old 19th June 2011, 02:35 AM   #8
tbie is offline tbie  United States
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Everharmonics, Thank you for your support. I figured as much looking at those numbers. I was checking resistors on left channel today comparing with right channel. They appear to be in good standing except R33 which is measuring 1.55Mohms. I am not an electronics expert only a hobbyist and I am having trouble checking out what is going wrong. I appreciate all the help. Hopefully I can enjoy music from this experiment.
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Old 19th June 2011, 04:22 AM   #9
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Quote:
Originally Posted by tbie View Post
R33 which is measuring 1.55Mohms.
R33 and 34 both should be 100 Ohm 1/4 W resistor.
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Old 20th June 2011, 12:31 AM   #10
tbie is offline tbie  United States
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Changed R33 with a new 100 ohm 1/4w resistor. Still grid voltage on left is 272V. Only other thing that seemed to drive the voltage when I consulted schematic appears to be th Mosfet 2sk3563 (used Vishay IRF830). Could a blown Mosfet cause raise in voltage? From my understanding Mosfet in this circuit is used as cathode follower to reduce voltage presented to output tube to negative so that it won't go into Class B when driven to clipping.
Everhormonics, do you think changing Mosfet is useful. I tried to test it with multimeter in diode testing mode. The minute I put probes to source and grid multimeter is making noise. can you tell me whats going on?
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