Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubedue1 on February 25, 2017, 01:52:50 pm
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Customer brought in a Jet II 12R. When I got it....not working.
I use TV7 D/U tube tester...all 4 tubes good.
V3 (El 84) the tube runs so hot it has discolored the board around it and burned 3 resistors, I'd have to look
again but they are R98, R99 (10 ohm) replaced ( they were readable) with 1/2w . Also removed and tested D4 and 5...good. Still V3 is untouchable...V4 touchable.
Now I have found R116 burned in the middle and can't read the color band. Can't find a schematic for this RI.
I tested the voltage on Pin 7 for the 84's....the 84 running extremely hot has a lower voltage difference of 10 compared to the cool running 84..higher voltage.
Could this 10 volt difference be the transformer loosing it?
The problem now may be this one R116 and not knowing its value. Anyone know?
Any schematic available for this Jet II 12R that anyone knows of ?
Could sure use some help..
Thanks Folks..
tubedue1
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May this be of any help ?
(http://music-electronics-forum.com/attachments/5361d1241845676-ampeg_j12t_jet_ri0001.jpg)
(http://www.thefenderforum.com/forum/showthread.php?47594-Screen-resistors (http://www.thefenderforum.com/forum/showthread.php?47594-Screen-resistors))
Franco
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> the 84 running extremely hot has a lower voltage difference of 10 compared to the cool running 84..higher voltage. Could this 10 volt difference be the transformer loosing it?
No. The hot '84 is just sucking more.
I bet the two '84s are not remotely the same current. One may be *dead*. Or the rest of the circuit may be faulty.
I would start by inserting one at a time and *quickly* measuring the cathode resistor voltage. With just one '84 on a resistor scaled for two, the one tube will run extra hot, so get a reading and turn-off again. Try both tubes in both sockets.
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PRR,
Thanks for that...the schematic is very appreciated.