Bell tube PA amp giving really low output. - diyAudio
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Old 25th February 2013, 10:46 PM   #1
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Default Bell tube PA amp giving really low output.

I just bought a Bell Carrillon PA amp to restore into a guitar amp. I knew it'd need a cap job so I went ahead and turned it on to see if there were any other problems and the hum came in. The problem was that, with an iPod plugged into 2 channels as bridged stereo it still didn't give much volume. The preamp tubes are good and the power tubes are good RCA black plate 6L6's. Should I worry about it until after I replace the caps or is it something I need to address seperately?
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Old 26th February 2013, 12:42 AM   #2
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Even bigger troubles. I was running it with tubes through an 8 ohm dummy load and it all of a sudden cut off. I didn't test the power trans yet but it smells funny and probably just blew for no apparent reason. All the voltages were good and I have no idea what just happened.
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Old 26th February 2013, 12:45 AM   #3
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Very, very sorry for triple posting, but I just checked it out and it blew the fuse. I'm looking into all of this. Can anyone help anything?
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Old 26th February 2013, 01:00 AM   #4
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What kind of rectifier does it have?

If they are the old selenium type they can be shorted as well.


jer

Last edited by geraldfryjr; 26th February 2013 at 01:11 AM.
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Old 26th February 2013, 02:13 AM   #5
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Quote:
Originally Posted by geraldfryjr View Post
What kind of rectifier does it have?

If they are the old selenium type they can be shorted as well.


jer
Normal diodes that are correctly continuous. I fixed the fuse and it was a faulty power tube I substituted for the RCA's. The old RCA's are still messing up and I can't test it with any other 6L6's because I don't have any on hand.
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Old 28th February 2013, 02:08 AM   #6
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I check my B+ voltage on old tube amps as soon as I turn it on. If it is not to spec, or not 85% of the rating of the output tubes, the wires come off the e-caps for testing with a dvm. Good ecaps charge up on ohms a certain voltage per scan and discharge at a certain rate on 2 v scale, normal voltage per scan. bad caps are slower to charge up and discharge faster. The amount depends on the cap value, so having a good (new) one around is good training. The payoff for not doing this, is burning a dropping B+ resistor, or a power transformer. If the cap is okay and the B+ is still bad, it can be a tube shorted, or a paper or film cap to the plate or grid shorted, but I start by checking the B+ e-cap. I've had to change the B+ cap 3 times in 40 years in my first tube amp- the ST70. Last time the cap had 2 hours operating time and 18 years shelf time, then I bought the new output tubes, slow started it at 70 vAC mains, then it ran fine for two hours, good B+ voltage. The next night it blew the fuse; cap slime was in a puddle in the bottom from the B+ cap. Oxygen attacks rubber cap seals, running or sleeping. Put a meter on B+ first, if you don't like buying new transformers. Use one hand on the probes, and a clip lead on negative, B+ can stop your heart if the current crosses from one hand to the other.
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Last edited by indianajo; 28th February 2013 at 02:13 AM.
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Old 11th March 2013, 11:03 PM   #7
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"I knew it'd need a cap job so I went ahead and turned it on"
WHY!?@!

This is why you don't just plug in old tube gear. If one of your caps shorted, you may have lost a trans.

I would replace all the electrolytic, paper and wax caps, check the trans for opens and shorts, and having your tubes checked for basic function before attempting to plug it in again. Then your attention should be to biasing.

Before you go full-bore and re-engineer this into a guitar amp, make sure your tubes and transformers are still good or you may have a frustrating (and expensive) time.
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