What tube is red plating, a output tube I will guess?
If you put a different output tube in that socket does that one red plate also?
If so than make sure that pin 5 on all the output tubes reads at least the -40 volts that the schematic calls for, if they all read such and only that sockets is acting up than the connection from pin 5 to that tube is bad and needs to be retensioned and or that socket needs to be replaced
If other output tubes are ok in that socket then that output tube is shorted!
The 470 volts you are reading is due to the power supply not being loaded down with powering all of the tubes with dc voltage!
Actually the schematic shows that 500 volts is normal on pin 3 and 370 on the screen being pin 4, but 470 is close enough!
Also note that these amps are notorious for being right on the edge of having enough bias voltage to keep the output tubes from red plating and at times if you have a really strong new set of output tubes in the amp the bias voltage will not be enough to tame them and you will need to mod the amp in order to get more - voltage to all those pin 5s!
In light of what I just posted I would get 4, 1 ohm one percent tolerance, 2 watt resistors and install them in place of the ground wire that now goes from pin 8 to ground and this will allow you to read the current draw of the tube directly with just a meter set for dc volts across that 1 ohm resistor.
1485 blowing fuses
Moderators: pompeiisneaks, Colossal
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Stevem
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- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: 1485 blowing fuses
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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hossymandias
- Posts: 59
- Joined: Thu May 22, 2014 2:54 pm
- Location: United States
Re: 1485 blowing fuses
Okay, I looked for the -40v on pins 5, and found none.
In my amateur brain, I thought, let's take it off standby and see if that changes anything. When I did that, the 1n3754 diode connected to C33 started smoking.
I turned the amp off.
Is C33 the culprit?
Needless to say, I'm in over my head, but I'm learning a lot thanks to this forum. I appreciate all the help!
In my amateur brain, I thought, let's take it off standby and see if that changes anything. When I did that, the 1n3754 diode connected to C33 started smoking.
I turned the amp off.
Is C33 the culprit?
Needless to say, I'm in over my head, but I'm learning a lot thanks to this forum. I appreciate all the help!
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: 1485 blowing fuses
Yes my first guess would be that C33 is very leaky or shorted,do you read a short across it when it's out of the circuit?
Replace that diode with a 1n4007 or even a 1n4004 which you can grab from radio shack if need be.
Do not put any of the output tubes in until you get that -40 volts back up and running on pin 5!
If that bias cap is bad replace it with one of more than a 50 volt rating, even using two 200 uf @ 50 volt caps in series will work great!
Note that if thus amp still has it's original electrolytic filters ( most of the main ones are red paper cased ) than it way passed the time to change them!
We are all over our head at first, we just need to Take the time to think things thru more sometimes and or walk away for a bit to clear our heads then come back and review what we have done that could be effecting what is going on!
The worst thing a newbie or even a more experienced tech can do is to trouble shoot and or make too many changes at once!, than if you get distracted chances are pretty good your gonna screw up!
Replace that diode with a 1n4007 or even a 1n4004 which you can grab from radio shack if need be.
Do not put any of the output tubes in until you get that -40 volts back up and running on pin 5!
If that bias cap is bad replace it with one of more than a 50 volt rating, even using two 200 uf @ 50 volt caps in series will work great!
Note that if thus amp still has it's original electrolytic filters ( most of the main ones are red paper cased ) than it way passed the time to change them!
We are all over our head at first, we just need to Take the time to think things thru more sometimes and or walk away for a bit to clear our heads then come back and review what we have done that could be effecting what is going on!
The worst thing a newbie or even a more experienced tech can do is to trouble shoot and or make too many changes at once!, than if you get distracted chances are pretty good your gonna screw up!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: 1485 blowing fuses
I don't see how flipping that standby switch is related to the smoking diode. Gotta just be coincidence.I thought, let's take it off standby and see if that changes anything. When I did that, the 1n3754 diode connected to C33 started smoking.
The STBY switch in that amp operates as a 'mute' switch, shorting the two PI drive signals together to cancel drive signal to the output tubes, thus producing silence. But no operating voltages are ever interrupted when in standby mode.
But your C33 (looks like C30 to me) is a likely suspect for smoking diode and no bias voltage on pins 5 of the output tubes.
If this amp has been turned on for very long without bias voltage I suspect all the output tubes are cooked.
Re: 1485 blowing fuses
If your voltage measurement was done with the tubes not-installed, the higher voltage is fairly normal. That is because no current is being drawn. But if you measured the higher voltages with the power tubes installed (which I don't think you did), hen it might not be a good sign.
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hossymandias
- Posts: 59
- Joined: Thu May 22, 2014 2:54 pm
- Location: United States
Re: 1485 blowing fuses
Okay, I needed a break from this guy, but I'm back.
I replaced the fried diode, C33, and installed a new on/off indicator bulb for the jewel. For some reason the previous owner had taken out the bulb.
This is important because here's what she's doing now:
With no 6L6's installed, she powers up fine (jewel turns on), the fuse holds, and +/- voltages on tube pins are correct.
WITH 6L6's installed, the jewel does not light, I hear a sinister ticking (like a radiator heating up), and I have to cut it off because one of the 6L6's begins to turn red.
I just installed brand new Mojotone output transformers and I really want them to survive this troubleshooting, so maybe I'm being overly cautious, but because the jewel isn't lighting there must be a short somewhere.
Can anyone give me tips on how to continue troubleshooting? Again, I am a novice and I really appreciate the baby-step walkthroughs.
Hossy
I replaced the fried diode, C33, and installed a new on/off indicator bulb for the jewel. For some reason the previous owner had taken out the bulb.
This is important because here's what she's doing now:
With no 6L6's installed, she powers up fine (jewel turns on), the fuse holds, and +/- voltages on tube pins are correct.
WITH 6L6's installed, the jewel does not light, I hear a sinister ticking (like a radiator heating up), and I have to cut it off because one of the 6L6's begins to turn red.
I just installed brand new Mojotone output transformers and I really want them to survive this troubleshooting, so maybe I'm being overly cautious, but because the jewel isn't lighting there must be a short somewhere.
Can anyone give me tips on how to continue troubleshooting? Again, I am a novice and I really appreciate the baby-step walkthroughs.
Hossy
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: 1485 blowing fuses
No, that voltage is on pin 4 is ok .
Your likely issue is not having enough or no negative voltage on pin 5 of each output tube, and then again just because you may have voltage there it must get to the tube pin and if the female reciver in 5 is too far sprung to make contact than any tube you place in the socket will red plate!
Also you need to confirm some how that your output tubes are good and not shorted out, even placing them in a buddy`s little Fender champ for instance will let you prove them out one at a time!
Your likely issue is not having enough or no negative voltage on pin 5 of each output tube, and then again just because you may have voltage there it must get to the tube pin and if the female reciver in 5 is too far sprung to make contact than any tube you place in the socket will red plate!
Also you need to confirm some how that your output tubes are good and not shorted out, even placing them in a buddy`s little Fender champ for instance will let you prove them out one at a time!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: 1485 blowing fuses
Some how this replay ended up here when it should be at the end of this string!???
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Bulb limiter
Why not clear all the tubes and run the amp through a bulb limiter to see if you have any shorts in the circuit.