Vox AC30CC help

[rylanmartin]

:help

Calling all amp(h) techs...I've got an AC30CC that I use as my secondary amp.

As of recent I've been having a some issues with it that will require repair. After the AC30 heats up and has some sound going through it, every once and a while it'll fizzle out to either 5% volume or completely out, until a loud noise hits it again and it pops back to the original volume. The longer it's on, the worse the problem gets.

This is a poor description, but is this is a sign that it needs new preamp tubes? or what are other possibilities, or things I should be looking for??

Thanks!

 

[tunghaichuan]

You can find the schematic here:

http://www.mzm.us/AC30CC_Schematics.pdf

You might try changing out all the tubes, the power amp is cathode biased so you don't necessarily need to reset the bias for the purpose of testing tubes. Careful though, that amp is biased very hot. Let the power tubes cool down before yanking them and replacing.

If your problem goes away, then it was the tubes. If the problem persists, then it could be a number of things. I would suspect a loose connector. It looks like there are lots of modular connectors in that amp. It could be a flaky or cold soldered joint. It could be an intermittently working part about to fail.

It is best to take amps like this, i.e., modern construction, to a competent tech who has lots of experience servicing both PCBs and tube amps.

Good luck and let us know what you find out.

:help

Calling all amp(h) techs...I've got an AC30CC that I use as my secondary amp.

As of recent I've been having a some issues with it that will require repair. After the AC30 heats up and has some sound going through it, every once and a while it'll fizzle out to either 5% volume or completely out, until a loud noise hits it again and it pops back to the original volume. The longer it's on, the worse the problem gets.

This is a poor description, but is this is a sign that it needs new preamp tubes? or what are other possibilities, or things I should be looking for??

Thanks!

 

[rylanmartin]

I'm almost certain it's tubes. When I slave the amp to my Marshall (just run through the poweramp) it works 100% ok. So I know it's a preamp issue. I'll try swapping out the 12AX7's in the preamp and see how it goes.

Is anyone on TheFret.net a tube snob? I really know nothing about how particular brands or types of tubes sound.

 

[Radioboy950]

I'm almost certain it's tubes. When I slave the amp to my Marshall (just run through the poweramp) it works 100% ok. So I know it's a preamp issue. I'll try swapping out the 12AX7's in the preamp and see how it goes.

Is anyone on TheFret.net a tube snob? I really know nothing about how particular brands or types of tubes sound.

Good luck with the amp.

If you are so inclined to experiment with preamp tubes, I’ve tried the following with very good results.
I’ll refrain from describing how they “sound”, because it is very subjective. But they all seem to be solid performers.

New stuff:
Tung Sol
Penta Lab (Chinese)
Electro Harmonix
Sovtek (LP)
JJ

For older stuff :
I’ve purchased a few used tubes, and I like these:
Telefunken, Mullard, Brimar

Read up and consult with reputable vendors who can help you choose based on your amp, playing styles, and tones you're looking for.

 

[tunghaichuan]

I'm almost certain it's tubes. When I slave the amp to my Marshall (just run through the poweramp) it works 100% ok. So I know it's a preamp issue. I'll try swapping out the 12AX7's in the preamp and see how it goes.

Is anyone on TheFret.net a tube snob? I really know nothing about how particular brands or types of tubes sound.

I have lots of used 12AX7s of various varieties, Amperex, Brimar, Mullard, Telefunken, but I mainly use el-cheapo Russian and Chinese tubes in my amps. So I guess I'm not a tube snob.

You might want to email Bob at Eurotubes and ask him what 12AX7s he would recommend for what position in in your amp. [email protected] He only has JJ tubes, but there are a few different varieties of 12AX7s. At least you will get a tested, working set and if nothing else you can use them to confirm that it is actually indeed the preamp tubes failing in your amp.

 

[Duff]

Vox

I would stop plugging in the amp right away and get it checked out.

I had a Crate V50 do a catastrophic blow up when the power tubes went out and it was at first intermittent, not to last long in the intermittent state until it blew up, taking out numerous important parts including the power supply wihich was an unavailable switching power supply.

My new approach with suspected tube problems is to take the amp to a qualified tech without delay, in order to avoid very costly repairs to other components. At least until I become more knowledgeable of tubes. Maybe pre amp tubes are more forgiving and won't cause a catastrophic blow out of other parts. Is intermittent cutting in and out a sign of a bad pre amp tube? Or is this a symptom more common of something on the board or elsewhere.

I had my brand new Hot Rod Deluxe serviced under warranty after I bought it and the tech resoldered all the cold solder joints, because I knew that they could cause serious damage to the board and other components from overheating. That's about a three or four year old amp now. Never a problem, made overseas with the new lead free solder I believe. I don't think the factory floor workers had developed a lot of expertise with working with the new solder at that time. I can't work with it myself. I use leaded solder and am going to get a bunch more before you can't get it anymore.

If it's kicking in and out there is a definite problem that could become a major problem, considering the quality and expense of your amp. I would not play tiddly winks with it, no offense, if I was you. That's because I lost a great amp because I didn't heed the first warning.

I like to work on my own stuff but when it comes to suspected tube issues I'm burned by my Crate V50 wonderful amp. But I'm happy after all because Crate headquarters sent me a NOS V32 212 with Celestion seventy eighties in it free, an 800 list price amp. I paid 164 for the V50 during the blow out plus a coupon. It was a check for the 164 or a NOS V32 - that didn't take a micro second to jump on and it was here in two days.

 

[rylanmartin]

The biggest problems are always the simplest solutions.

So after taking the amp apart and testing each of the tubes and monkeying around for hours, it dawned on me what the problem was.

in the back of the amp, by the effects loop there is a little switch you can use to turn the effects loop on or off (Effects loop bypass). The switch is just really crappy. With the effect loops bypassed, the switch won't sit comfortably in the "on" position. So I just turn it off and plugged in a little patch chord and not a problem.

If I'd have taken this into a tech, who knows how long it could've taken to find the problem, considering I'd be paying by the hour. So to replace a potentially 5 dollar part would cost me probably 80 bucks.

Thanks for your input guys!

***If anyone ever has this problem in the future, (preamp stage cutting in and out) on a Vox AC30CC, check the effects loop bypass switch***

I bid you adieu.

 

[Duff]

Vox

True about the simple things. Especially if you can check things like this yourself you can save a lot of money.

My amp blew up because of a serious problem with the caps and bad tubes.

Stuff I couldn't figure out.

Glad you figured it out with no problem, that's a great amp.

I'm still, personally, shy when it comes to suspected tube problems and my tech is reasonable and probably wouldn't have chrarged me a lot to fix your problem because I'm a steady customer but it could have been frustrating and ran into some hours and winding up costing me a lot like you suggested.

Glad all turned out well with the great amp.