• 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.

Low output from two 6L6's for no apparent reason?

I recently bought a new Bell Carillon PA to restore. It has two good RCA black plate 6L6's, three 12AX7's, and one 12AV6. I knew it would need a filter cap job, so I went ahead and turned it on to see whether or not it had any other problems, and sure enough, cranked, it's giving me a pathetic amount of volume. No other problems; the master volume still affects the gain, the mic knobs do the same, and it sounds sorta like distortion from unsaturated power tubes. Should I be worried about it or should a cap job fix this? What could potentially be the problem otherwise?
 
When recaping, try to do in a reduced group of say, 2 or 3 units and power the set between them, so if anything goes wrong, you can go back. Also use a incandescent lamp in series with any of the power line cord, so if a short circuit or a reversed cap, the lamp will bright powerful meaning an error has happened. The lamp, in this case acts as a current source protecting the equipment. Keep attention to extraneous sound on the speaker and or the light in the tubes.

The lamp or group of them will be about 5 times the power drained by the DUT.
 
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chd_diy

Member
2009-11-30 2:25 am
I had an amp recently that lost most of it's output, and sounded really distorted.
I changed the coupling caps, but it still sounded the same.
After more testing, I found that the plate resistor for the input tube had gone bad.
Changing that resistor totally cured the problem.

I would recap the entire amp, and if you still have the problem, look for a bad resistor somewhere.

Good luck.
Chris
 
Yes, good advice (post 5).

Thanks, DF!. Actually I'm 43, but from the secondary school (Here known as ENET's, Escuela Nacional de Educación Técnica, National School for Technical Education, some years ago disappeared), when in 4th year (To complete was 6 years), a professor say to use always in a doubt, a series lamp.

This actually includes a three phase test board here in my job, in where there is a 100W lamp in each phase, shunted by a switch which permits to normal voltage and current flow into the DUT, but in case of doubt, always first is triyed in series.

This avoids or almost reduces risk of personal and equipment injuries.
 
Thanks you guys! I have the new filter caps in place and will solder them in after I get back from New York. The coupling caps are all good, I tested all of them for voltage and resistance and they're all good. A few resistors are out of spec so I pulled the tubes and I'm replacing a few of them as well. I'll tell you all how it goes!