splatty-fizzy-chirp when playing through vox ac-30 hand-wired

[bmast160]

my amp stopped sending the guitar signal through the speakers recently. it still turned on the way it should and i could hear a little faint noise coming through the speakers. i figured it was a tube issue so i changed all of the tubes in the amp(preamp, matched power, rectifier) twice and it sounds good and works fine when playing lightly but when i add some attack or play harder (mostly on the bass strings or if full chords are played louder) the sound starts to compress to a kind of somewhat splatty-slightly-fizzy sound like if you adjusted an internal trim pot on an overdrive/distortion pedal so that it was tuned improperly...the sound also has somewhat of a high frequency chirp in it too. any ideas what's going on?

[WRGKMC]

What tube manufacturer did you use? Some tubes cant take the high cathode bias in an AC 30.

The AC30 has fixed bias and some tubes cant take the high current.

 

According with Vox should bias to measure 2.2 volts dc across each 47 ohm cathode resistors. If you use different tubes you may need to change the cathode resistors to cool the bias voltage down and prevent the tubes from being damaged.

 

The highest you'd want to run those tubes is 47ma. That's almost 24w out of a possible 25w per tube which is pretty dangerous. Some older AC30's can run as high as 50ma which is max current.

 

I believe the newer AC30s come with Electro Harmonix tubes. They do make some good tubes but I'm not as experienced with the EL34's first hand.

 

JJ tubes are very durable and can run those higher voltages/currents and they also sound good. If you're using some other tubes you could increase the cathode resistors to like 60 ohms and reduce the voltage. Going much higher will likely affect the sound quality removing the magic tone of that amp. This would need to be done by a good amp tech however.

 

 

If you are running budget tubes I'd definitely start there. I'd likely give it a look inside and make sure you didn't cook anything of the cathode resistors that connect to the tube sockets when those other tubes went bad. If so replace them and the tubes and don't run it again till you do or you may cause even more damage.

 

BE SURE you're running the right speaker impedance!!!! The quickest way to blow out power tubes is with the wrong speaker impedance (usually higher) The output transformer needs to see a balanced load. If the impedance is too high the current doesn't get consumed by the speakers and it doesn't just disappear on its own. It winds up staying in the power tube circuit an cooks them. Then things go south quickly like you experienced.

 

If you do have good tubes, then get it checked. Something's been damaged by the tube failure and continuing to run it may continue to do damage. Hopefully its a couple of dollars for grid resistors. At worst you may have a damaged output transformer. AC30 amps are high maintenance amps. You want to change the tubes before they fail. If you figure how long the tubes went since you bought the amp (say a year), cut that time by at least 1/3 and change the tubes at 9 months.

 

You can Google up what others use but you'll likely find a high consensus for a few types. luckly the JJ's are well priced so you don't bust the bank on them. Other NOS stuff, forget it. ASllot of those tubes are used and you Definitely don't want to stick used tubes in that amp.

 

Mullard may be OK as well as a few others too. just don't buy into the magic tone voodoo garbage. there might be slight gain differences but even allot of that is a placebo effect.

[bmast160]

tried lots of different kinds of combinations of tubes with the same result. all tubes purchased from a reliable dealer and all are tube brands i've used in the amp previously without needing to rebias the amp...sovtek and mullard el84's. jj, nos ge, mullard 12ax7s.

 

speakers are a 8ohm 40watt ceramic, 8ohm 15watt alnico

 

no other recent changes

[onelife]

Did you try the amp through a different speaker?

 

[WRGKMC]

The 15W speaker might be the culprit. Measure the resistance with and ohm meter and try moving the cone by hand. If you get a rubbing noise the coil may read close to 8 ohms but the voice coil may have seen too much heat and deformed the coil and its rubbing.

 

Some times you can work the cone by hand and get rid of the rubbing too. Be sure your mounting screws are even too. The stamped steel frames are very flexible. If you have uneven screw pressure because the screws loosened up or were never even when installed, or the wood is warped, it can bend the frame causing the cone to rub. It sounds like static but its actually the cone rubbing. removing the screws and turning the speaker 1/4 turn then tightening the screws firmly can often do the trick. Never wang one screw all the way down then the others. That side will be too low when you're done and torque the frame. Think of a drum head that gets each side tightened down a little at a time.

 

That amps rated for around 30 clean watts. The amp may produce up to 50 or so saturated watts peak. That would be closer to 25W per speaker so a 15W might have been damaged. A 15w speaker is usually used on a 10~12 watt amp max, even if you have a second one because the saturated wattage can be much higher.

 

They may sound terrific up until they go because you're getting actual cone flap but its a fine edge between sounding great and blowing. They just fade out and go crispy. (I know, I've blown a few in my time) Personally I'd go with at least a pair of 25's so there's a little extra headroom. You can stick with alnicos too. Those amps sound terrific with alnicos because they get a smoother saturation and better compression.

 

This is of course if you confirm the problem is the speakers, not the head. You may be able to run just the 40W for short periods. or just put your ear up to the cab and see if the problems coming from one speaker or both. Just be sure the total speaker impedance matches the head. Those speakers should be run in parallel too. If you have them connected in series, you have 16 ohms total and it can be the cause of the power tube failure running it that way. The transformer is like 4 or 8 ohms. 16 is likely too high unless you have an ohm switch on the head.