• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

LK-48 over voltage

After it went silent I replaced the 5AR4, the 3 wire wound resistors, the 4x475 can-cap and did the UF4007 tweak. Fired it up and it sounds great but...all the voltage measurements are high. Where it should read 430V I'm getting over 500, with other lower voltage areas similarly high.
I checked all the resistors, and besides most being ancient, all test good.
Before I sacrifice another can-cap or the new quad of EL84Ms I could sure use some advice.
Thanks in advance for any help
Karl
 
1. can you tell me, what is your mains voltage when you got that reading?

2. 500 volts is indeed scary...

3. found the schematics: is this exactly how the amp is wired now?
to lower voltage, you can put series resistors to pins 4 and 6 of the 5ar4 rectifier, you can start with 2 x 47 ohms 5 watt resistors...

An externally hosted image should be here but it was not working when we last tested it.
 
What's the power section signal tube AC filament voltage? I think AJT is on to something about your AC mains being above 125 VRMS.

If that's a more or less a continuous problem, You can add a small transformer to buck the primary side of the OEM power trafo down. Yes, your power company is supposed to keep the mains not higher than 125 VRMS, but ...
 
yes Eli, that is the only logical explanation i could think of....being a vintage amp, the power traffo could have been meant for 110volts....

is 125 volts the new USA line voltage standard? over here, lines can be from 220 to 240...so i design my amps to run on 240 volts...
 
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Hi Karl,
You can't run the amplifier without the output tubes. Your HT is far too high for the condensers now. The power supply must be loaded down.

Before you go too far, replace the grid coupling capacitors right away. Do not increase their size, but a higher voltage will be fine. Do this before applying power again.

-Chris
 
Tony,

Since before Noah's Flood, the North American standard for house current has been 105 to 125 V. Average line voltages have crept up, over time.

BTW, Tom Edison's DC house current was pretty good at holding at 110 V. Believe it or not, some areas of NYC's Greenwich Village had DC house current into the 1970s. They were near Washington Square Park, where Edison did his 1st lighting demonstration. While I have no proof, it stands to reason that the original equipment was on the job for nearly a century, not bad at all.
 
After it went silent I replaced the 5AR4, the 3 wire wound resistors, the 4x475 can-cap and did the UF4007 tweak. Fired it up and it sounds great but...all the voltage measurements are high. Where it should read 430V I'm getting over 500, with other lower voltage areas similarly high.
I checked all the resistors, and besides most being ancient, all test good.
Before I sacrifice another can-cap or the new quad of EL84Ms I could sure use some advice.
Thanks in advance for any help
Karl

Did you measure the voltage before the transplant? How do you know it was 430 to begin with?

Jan
 
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Hi Jan,
Yes, that is the question for sure. A section could have shorted due to age, or gone low resistance. That would have heated the other sections up and caused failure of all the sections. The resistors were just left as evidence to show where the current went. Under no circumstances should 430 VDC ever damage a 475 VDC capacitor. For these reasons, I'm pretty certain that his later voltage measurements were taken with no output tubes in place.

Failed outputs could have also caused the old capacitor to fail due to high ripple currents as one or more output tubes was conducting far too much current due to a bad coupling capacitor. For those and other reasons, replacing those coupling capacitors would be #1 on my hit parade. Replacement of the damaged parts is a given.

Those would probably make up the top two reasons for his problems. Just out of curiosity, where does that second heater string get its power from? That's for V1, V101,V2 and V102?

-Chris
 
Thanks everyone for weighing in. I purchased the amp 3+ yrs ago, recapped & modded to bypass the sound shaping circuits. It has worked fine (1 EL84 went red a while back and I replaced the pair in that channel) until the 80ohm WW and can-cap fail. I'm was not running it unloaded, using a quad of EL84s (2 that came with it and the 2 replacements, all EH)
The voltage at home runs 120 to 125 every time I've checked. I don't know what the line off the 5AR4 (marked with the flashing star) was pre fail. When I first checked it after the "transplant" it was 480V. The AC leads off the transformer feeding the rectifier read about 375V each.
I'm not a circuit guy, I've built a couple of tube amp kits in the last couple of years but that's it.
Thanks again
 
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The current standard for U.S. AC line voltage is 120V. There are a set of voltage tolerance boundaries, but most modern equipment is designed to operate over a range of 108 - 132V. In my neck of the woods it typically hovers between 118V - 126V with it generally in the range of 123V.. These amps if memory serves were designed for a nominal line voltage of 117V which was the standard value of the time.

See: http://www.pge.com/includes/docs/pd...ergystatus/powerquality/voltage_tolerance.pdf

I assume the 5AR4 is still in place, note that the diodes should be in series with the rectifier plates and not across the rectifier. Solid state rectification in these amps is a bad idea.

Note that a lot of people sub 5AR4s with 6087 in small integrated amps with over voltage issues.
 
The UF4007 tweak prevents arcing at power on in a number of current production 5AR4s. The "sand" protects the rectifier tube. You must scrupulously follow that yellow background graphic, insofar as wiring is concerned. The setup shown places a SS diode in series with each 5AR4 plate. Check your work.

If, as kstagger suggested, you happened to wire the SS diodes in parallel with the vacuum rectifier, your PSU is SS rectified and that definitely would account for the observed over voltage.