I have a cj pv2a pre amp that has a problem with the phono tube section. After about an hour of play, I hear a very loud rushing water sound from the speakers. The second tube (5751 ) is much brighter looking, as though the heater is getting too much voltage. The first tube (12AT7A ) then runs cold, no illumination at all, and cold to the touch, as though no voltage at all is going to it. The line stage works just fine. Put in two new tubes, and worked fine for about an hour, now same thing I think i fried the tubes.
What should i check/ do?
Paul
What should i check/ do?
Paul
Possibly a cold solder joint problem in the heater wiring? If one heater goes open circuit, the other will get a little extra voltage (assuming non-regulated supply).
If you let the pre amp cool off, does it again work normally for a while?
You could check for correct heater voltage at each tube socket with a DVM. The trick will be to check it when the symptoms appear (ie before things cool off). Pull out the tubes and fire the pre up and check the pins in the tube sockets for proper heater voltage. Keep in mind that pins 1 and 6 will have high voltage on them since they are the anodes (probably 200-300v)
Both tubes heaters can be wired as 6V or 12V (series or parallel) heaters so you will need to check pins 4, 5 and 9 for voltage to determine how they are wired if you don't have a schematic. The heater voltage is most likely DC, although it could be AC.
The tube data for the 5751 is here:
http://tdsl.duncanamps.com/show.php?des=5751
and the data for the 12aT7 is here: (same heater wiring)
http://tdsl.duncanamps.com/show.php?des=12AT7
If you let the pre amp cool off, does it again work normally for a while?
You could check for correct heater voltage at each tube socket with a DVM. The trick will be to check it when the symptoms appear (ie before things cool off). Pull out the tubes and fire the pre up and check the pins in the tube sockets for proper heater voltage. Keep in mind that pins 1 and 6 will have high voltage on them since they are the anodes (probably 200-300v)
Both tubes heaters can be wired as 6V or 12V (series or parallel) heaters so you will need to check pins 4, 5 and 9 for voltage to determine how they are wired if you don't have a schematic. The heater voltage is most likely DC, although it could be AC.
The tube data for the 5751 is here:
http://tdsl.duncanamps.com/show.php?des=5751
and the data for the 12aT7 is here: (same heater wiring)
http://tdsl.duncanamps.com/show.php?des=12AT7
I don't know how this happened? Definitely not A/c to the pins 4,5 or 9. O volts register.
But in the range of 270 volts on all three phono tubes measured between pins 4 & 5, and 5 and 9 ( o volts between pins 4 & 9 ).After I had "lost"my phono section, a couple of weeks ago I took the unit apart and found the brown wires coming out of the transformer were dried and cracked the last 1/2 inch or so, and the wire (although still soldered well) was green and "frayed", so I cut it off, and resoldered it. thats all I did!
Guess I'll have to take the dang thing apart again.
No real stress on it turning it on with half or all of the tubes out of it is there?
Thanks again for your help
Paul
But in the range of 270 volts on all three phono tubes measured between pins 4 & 5, and 5 and 9 ( o volts between pins 4 & 9 ).After I had "lost"my phono section, a couple of weeks ago I took the unit apart and found the brown wires coming out of the transformer were dried and cracked the last 1/2 inch or so, and the wire (although still soldered well) was green and "frayed", so I cut it off, and resoldered it. thats all I did!
Guess I'll have to take the dang thing apart again.
No real stress on it turning it on with half or all of the tubes out of it is there?
Thanks again for your help
Paul
Sounds good to me....maybe one of the experts will chime in here. You've determined that your preamp uses 12.6V DC heaters (not surprising) and you possibly have a little AC ripple on the DC voltage (may be difficult to quantify with a multi-meter).
Were these measurements taken shortly after the problem occurs or when the pre is "cold"?
Were these measurements taken shortly after the problem occurs or when the pre is "cold"?
Cold solder joints and rotten, crinkled insulation???
I'm not familiar with that design so this is conjecture. In an old unit it is sometimes necessary to cover that rotten crinkled insulated wire with heat shrink tubing or nylon tube braided swording, or replace the rotten wire with new wire. It may be necessary to remove the end bells of the power transformer and investigate where the wires come out of the transformer if the insulation is indeed rotten. 12.6 volts means that the filaments of two sides of a 12ax7 are in series. I prefer parallel at 6.3v but that's just me. This avoids one side of the tube acting as a current limiter and glowing uncontrollably when first turned on. If you would post the schematic then we might be able to help you more. Ray
I'm not familiar with that design so this is conjecture. In an old unit it is sometimes necessary to cover that rotten crinkled insulated wire with heat shrink tubing or nylon tube braided swording, or replace the rotten wire with new wire. It may be necessary to remove the end bells of the power transformer and investigate where the wires come out of the transformer if the insulation is indeed rotten. 12.6 volts means that the filaments of two sides of a 12ax7 are in series. I prefer parallel at 6.3v but that's just me. This avoids one side of the tube acting as a current limiter and glowing uncontrollably when first turned on. If you would post the schematic then we might be able to help you more. Ray
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