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Resistors in tube heater circuit burning up as soon as powered up. Definately not tube heater.
I can provide schematic if needed.
I can provide schematic if needed.
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Are we talking the two resistors to ground from either side of the heaters? Like fender does?
Assuming no one has rewired the heaters or put in a new power transformer, only one way to get those resistors to burn up, and that is run excess current through them. Normally there is no current through them to ground. They form a 200 ohm series resistor across the 6v (using Fender example), which draws about 30ma. SO there would be 30ma or so flowing through the resistors from that, but not to ground.
What usually burns up those resistors is a bad power tube. Something in the tube shorts to the heater. Probably the most common reason for this is having the tube's center post snapped off. This allows the tube to be inserted in the socket the wrong way.
Remove the power tubes and see if those resistors no longer burn up. And make sure that no tube socket has the plate lead shorted to the heater lead. In a 6L6 amp for example, if pin 3 shorts to pin 2 ( or arcs there), those resistors will burn up.Education is what you're left with after you have forgotten what you have learned. -
Resistors burning up
There are 2 resistors in series going to the tube heater but coming off those resistors is a whole other PCB which I don't understand. It is my understanding that something in that circuit is loading the resistors.
How can I upload schematic?Comment
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Either click on Post Reply button under lower left or if you use the Quick Reply box, click Go Advanced. Under the main dialog box, will be Manage Attachments. CLick that, and a box opens. You can browse your own files for the one to upload. pdfs up to 2MB. Other formats as stated there. Or if your schematic is on some web site, you can enter a link there and it will retrieve it. or you can just post a link yourself.
yes, we'd like to see that. Doesn't sound like the basic hum resistors.Education is what you're left with after you have forgotten what you have learned.Comment
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R68 and R69. Going through them like crazy in search of problem. Tried tube and checked for shorts between resistors and heater.Comment
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There is a service bulletin on dc offset mod for power supply. Would that cause this problem?Comment
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Oh, solid state amp.
They do then appear to be the standard pair of 100 ohm resistors to make a virtual center tap for the heaters. Leave them out for now. Does the amp work without them?
Looks like there should be about 12VAC between pins 4 and 5 of the tube socket. Is there? More important, is there any voltage AC or DC from either pins 4 or 5 and ground? And pin 9 while you are at it.
I have no idea about the mod, post it or link to it. Certainly if that was not implemented properly or failed, it could do this. The schematic shows those two resistors just going to ground.
SOmehow they are a path for unwanted current.Education is what you're left with after you have forgotten what you have learned.Comment
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I'm going to guess that someone has been in here previously, disconnected the power supply board from the power amp board, and reconnected it the wrong way. So the rails are all mixed up, and the heaters are getting 50V instead of 6.3 or something equally awful. The voltage could be common mode, in which case they wouldn't even light."Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"Comment
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One lonesome 12AX7 in this amp.Education is what you're left with after you have forgotten what you have learned.Comment
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15 Volts AC across the tube. Randall said that the only time they saw this was when heater wires were backwards. Makes no sense if they are AC.
Amp does not work after burn. If it is turned up full it Burps out sound intermittently.Comment
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RG 200 Still burning resistors
I pulled this amp back out of the closet today. Did a little more playing. Heater voltage is correct.
If I disconnect CN2 no problem! I opened the two resistors and reconnected cn2 and once again burning fuses. While at it could you give me a lesson on what the network of caps is doing near CN2?
Thanks very muchComment
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Hi,Originally posted by reflection77ca2000@yahoo.ca View Post
that network is there for supply filtering purposes, the smaller caps are there to suppress faster transients that the larger one might "miss" due to their higher time constant.
One thing I find strange about the schematic you provided, is that QC6 and QC7 are labeled as HTR+ and HTR-, which makes me think about a DC heater supply, while the two 100 Ohm resistors ( R68-R69 ) are typical of an AC heater supply arrangement, providing an artificial center tap to GND as already noted, so they shouldn't be there if the heaters are supplied with DC IMHO.
Is it possible for you to post the power supply section schematic as well?
Cheers
BobHoc unum scio: me nihil scire.Comment
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Randall RG200
Tube heater is AC. Strange because randall said only time they saw a problem was when tube heater was wired reverse. Same tech told me it was AC. I will post power supply soonComment
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Randall RG200
Here is power supplyComment
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Heater is dc. Bridge, filter caps. Measure across the tube heater pins (4,5) with yer meter set to dc. You should have ~6vdc. Each pin to gnd will be -3 and +3vdc.
Maybe you have a different revision that doesn't match the supplied schemo?The farmer takes a wife, the barber takes a pole....Comment
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