Troubleshooting amp without sound

[bmast160]

No sound from vox handwired ac30. Replaced all the tubes with two completely new sets of 12ax7s and el84s and i'm still getting the same result. Checked all 4 fuses and they're good. The only odd thing i noticed was that the rectifier and el84 closest to it are the only tubes that are lit...i know sometimes the 12ax7s don't light up but shouldn't all of the el84s light up? the only tube i didn't replace was the rectifier because i didn't have an extra but that one is lit. any suggestions? i was thinking it was a wiring issue

[WRGKMC]

Tubes wont work without heater voltage. Having some tubes lit at least tells you have some of the heater voltages, but that's about it. Tubes also require B+ and the only way to test that is with a high voltage meter. Its not stuff an amateur should be messing with because one slip and you can take the amp or yourself out (or both) The rectifier converts the AC to DC so its is an important item to try. It probably wont give you filament voltage however. Filament voltages usually come off a transformer tap and if you loose one you loose them all.

 

Some older amps used to wire tubes like a Christmas tree light. Pull one out and they all go out. Guitar amps are usually parallel so any one tube can go out and the others remain on. If the filament is low or hidden by the metal plates it should still heat up fairly quickly. You should be able to see the glow in the dark as well.

 

There can be many reasons for no sound. You can have an amp go DC and send AC110V through the speakers and blow them. Make sure the speakers move when you touch a low voltage battery to the speaker leads.

 

Not much else I can advise you on at this point. It can be anything from an open transformer, to open solder joint. AC 30's go through tubes quickly. You want to use new matched pairs and set the bias on any tube replacements. Never swap singles, even for testing purposes on Voxes. The amps run with a hot bias and you can blow something out like a screen or grid resistor if the tubes are unbalanced. We used to have stacks of the old AC 30's at the repair shop I used to work at. People would but the wrong tubes, unbalanced and melt the amps down. taking out power and output transformers. many would get fed up and just buy another amp instead of having their amp maintained every 6 months. I think some of the flaws were fixed on newer ones. Fuses were added etc but they still blow out.

 

if the rectifier doesn't breathe life back ito it, I suggest a trip to the amp shop and let a pro look at it. Hopefully its not a transformer or you may be looking at a hefty bill. Be sure you buy some durable tubes for it too. JJ's can usually take allot of abuse, especially in combos that see allot of vibration.

[bmast160]

i tried two different sets of tubes with 4 matched el84s and 3 12ax7s. i didnt replace the rectifier because it's lit and none of the fuses went. the only other lit tube is the el84 closest to the rectifier. the amp powers up fine and the speakers seem to be fine but i dont get any sound. if i touch the end of the input cable there isnt any sound (and i did try other cables). do still think the rectifier could be the issue or should i just take it in for service?

[WRGKMC]

Like I said when you turn it on, all the heaters on all the tubes should come on. The tubes do use AC 6.3 off the main transformer so all should be lit. They should all get hot to the touch quickly too. If they aren't heating up you have a filament circuit problem. This can be a cold solder joint.

 

If you want to get risky and know how to use a meter you can test the voltage at each tube socket. You should get 6~7V on each of the EL34 tubes on pins 3&4. Normally tubes have only green wires going to each of the pins and nothing else.

 

If you have voltage but no filament lights up and the tube doesn't get hot, the tube filament blew.

 

This is unusual but a big voltage surge might do it. Its even more unusual for multiple tubes.

 

Your ECC82's use pins 4, 9 & 5. There should only be two wires but two of the pins are likely soldered together.

 

The rectifier lights so it doesn't need to be tested. You don't want to get near that one anyway because it supplies the B+ voltage of about 360Vdc.

 

If the Rectifier is bad you wont amplify at all. The tube stages need high voltage to operate. It's filament may light but it may not pass the high voltage. The problem could also be the power transformer. There probably isn't a high voltage output fuse and if output tubes go it can back up the high voltage circuit. A shorted power cap can do it too.

 

Hopefully no one stuck a higher value fuse in the amp when it went out. That's sure death of a transformer if its outputs shorted. When bad speaker cords, guitar cords used as speaker cords, or additional speaker cabs of the wrong impedance can blow a head.

 

If the problem was the tube, and/or the transformer, the problem may also trace back to your power tube circuits. Like I said, these amps are biased hot and the tubes don't last very long. 6 months tops with normal use maybe for your best tubes before they should be replaced. Junk tube can blow in a very short period of time. They either get cooked or rattle apart.

 

I'd say check the tubes to be sure all are getting hot. The red glow isn't always a good indicator with some tubes. They all shoud become untouchable after 30 seconds or so but don't burn yourself.

 

If they all get hot, try the rectifier. If its bad you get zero sound.

 

After that, its off to the doctors office.

 

You may save allot with a complete batch of tubes too. Don't put them in, just provide them.

Techs have to cover their own shipping and handling and cant give them away for free. They normally mark up 100% so you'd pay double if he has to supply them.