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Hi All, What would cause the heater voltage on V1, V2, V3 to intermittently fade to zero, only to fade back up a minute or so later. V4 and power tubes ( 2x EL 34) not affected. The audio fades as the voltage drops and then rises back up with it. Thanks
Are the voltage amplifiers powered from the same transformer winding as the output valves?
Irrespective, it sounds as though there is a bad joint somewhere.
Follow the heater wiring back to the transformer and re-make all the solder joints, including the ones where the transformer winding connects to the solder tag on the side of the transformer.
If the heater wiring uses the chassis as the return line for the heater supply, ensure that the screws holding the solder tags to the chassis are tight.
I have seen problems (in a different situation) where screw terminal were not "loose", but at the same time they were not "tight" and making bad contact, just nipping up the screws fixed a BIG annoying problem, (a computer system kept on crashing because of slight variations in supply volts to the disc drives).
Also check the tube pins, heat can cause them to expand.
Talk about not tight. I had a 30 KW electron beam power supply that kept breaking. Finally it became my responsibility and I said, I'll fix this for good. Locktite 222 to the rescue. There were a few 625K resistors that were out of tolerance, a 1 meg 200 W resistor that was out of tolerance and a LOT of loose screws, particularly on the divider. This thing was shunt regulated by a vacuum tube. to like 15 kV at 1.5 Amps. Incidentally, we ended up getting three of these things and all suffered from the same issue. One became a "parts mule" because I was "not allowed" to fix it by management. One also fell off a truck. Someone forgot that it was on casters, so it rolled off the lift gate.
Some thing with a 1940's 100 kV, 0.1A power supply used for an X-ray goniometer. Lots of loose screws. The researcher said, that the signal was the quietest that it has ever been.
I did the same thing to a precision vapor drier, but this time I had to replace and upgrade the wiring. All annoying.
There was a 6.3 VAC at 3000 Amps AC supply too as well as a 1000 W tube RF amp. The latter I fixed some stuff on.
But then there is the 1000 W Arc light bulb that operated at 22V, 42 A and needed a 40 kV pulse to start.
Not sure which one was the worst. The RF Amp was the worst to work on. The RF amp had variable Jennings air caps with a matching network and the RF target was water cooled.
The 30 kW E-gun was the easiest with keys to open the doors. The "filament" transformer was about a cubic foot in volume and the main transformer had a 3 phase primary. It was about 2' x 1' x 2' in dimensions. Once it got wet and had to be replaced, because someone installed it under a drip. Actually where a Liquid Nitrogen replacement water to refrigerent (2 stages) refrigerator valve was. It just plugged into the wall with 70 A 3 phase 208 power.
We had purchased a 3 source e-gun surplus where all of the wires were removed from a terminal strip and the HV wires were cut about 6" away from the transformer. There were connectors inside of one of the cabinets that were overlooked by the packers and the HV wires could have been unbolted. No one believed that I would manage to get it to work.
In the end, the wrong power source was installed. They put a breaker in the neutral in the machine, but the neutral wasn't run and the nameplate wasn't right. 100 Amp, 3 phase, 208 with a neutral. Lots of loose connections and the shunt regulator tube fell out of the socket. But the bigger issue was the hearths were a poor design, cooling wise, per the manufacturer. This particular unit was a 3-source electron beam evaporator system. It worked when I got done. I had no help on the electrical side.
The RF supply had exposed terminal strips in the rear and the X-ray supply had no back on the rack like table at all. The xformer sat in the bottom and was about 2' in diameter and 1' high, oil filled. The oil had to be replaced once too.
Thanks for the info. The preamp heaters are fed from 2 center taps via a bridge rectifier at 6.3VDC. It's a straight forward design with heaters V1 through V4 powered by paralell +/- rails. When I pull the chassis out again I'll determine the output tubes heater source. The OP tubes appear to be unaffected by the voltage loss. A couple of the soldier joints looked a little iffy so I touched them up. Is it poosble for a bridge rectifier to go open under certain conditions, eventually returning to normal function? In the meantime I'll be playing through the amp to see if the problem shows itself again. Right now the amp is working as it should. DrewV
Thanks for the info. The preamp heaters are fed from 2 center taps via a bridge rectifier at 6.3VDC. It's a straight forward design with heaters V1 through V4 powered by paralell +/- rails. When I pull the chassis out again I'll determine the output tubes heater source. The OP tubes appear to be unaffected by the voltage loss. A couple of the soldier joints looked a little iffy so I touched them up. Is it poosble for a bridge rectifier to go open under certain conditions, eventually returning to normal function? In the meantime I'll be playing through the amp to see if the problem shows itself again. Right now the amp is working as it should. DrewV
Heyo Jon, The amp is a 1995 JTM 60, two channels w/ master. A 12" combo currently loaded with a G12M70. The original speaker is a Heritage 12". I picked it up on Ebay five or 6 years ago. Ended up going through the whole amp, replacing all electrolytics as well as a handful of out of spec components, resistors, mainly. Never had an issue with overheating, but the amp does run hot. It drips tone! Play on!DrewV
OK...the output valve heaters are fed direct off the 6.3VAC that feeds the rectifier, which explains why they're not affected. The DC side feeds the preamp valves with +/-6.3VDC. The two 100R resistors, R142 and R143, form a "false center tap" that creates the dual polarity split. Have you tested these two resistors? I would also recommend replacing the bridge rectifier as well. You'll wanna use one with a current rating of 1A or greater.
FYI, here's a link to the heater supply schematic -
Jon, For the time being, I'm just going to play through the amp cranking it and see if it will cut out again. It could very well have been a solder joint or maybe something to do with temperature. I haven't put the ohm meter on the resistors since I blueprinted the amp. Visually they look fine, as does the BR, with no signs of overheating/burn. In my experience bridge rectifiers can last a really long time, though I have replaced a few on various amps. The part # KBPC602 indicates that the rectifier is 6 amps. I'll post an update in a few days unless I can make it shut down before then! Thanks DrewV
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