Vox AC30 Problem

[Picaflor]

I have a 1999 Vox AC30 6/TB that has been in perfect operating condition for the entire seven years that I've owned it. About 5 years ago I started running it through a 16 ohm THD Hotplate and have had no problems until now. I typically use the bright channel with the volume at about 2:00 with the Hotplate set at 16 Ohms with the volume around 10:00. I usually use the AC30 a few times a week between 20 and 60 minutes per pop.

A few days ago, I started losing volume to the point where even when I cranked the volume on the Hotplate, I could still hear my open strings on the guitar as loudly as the signal through the amp. I immediately disconnected the Hotplate and played through the amp conventionally. The volume definitely reappeared without the Hotplate, but with a nasty hum, similar to a strat through a cranked Marshall. I use humbuckers and have never had the hum problem before with this amp.

With the severe loss of signal through the Hotplate, and the loud hum when playing through the amp without the Hotplate, does anyone have any idea what the problem may be?

Thanks in advance.

[ohmslaw]

Hot Plates eat power tubes. You have most likely fried yours. Replace them and have the amp rebiased. If the tubes are not biased properly they will burn out much more quickly. It is very unlikely that anything is wrong with the amp itself.

[phsyconoodler]

An AC30 is cathode biased (no biasing necessary)and the power tubes share one cathode resistor.If a tube goes down,the amp will hum.You need new tubes.If you haven't changed them in 5 years,count yourself very lucky.Most AC30 owners expect a tube change every year.While you are at it,get a new rectifier tube.They go down a lot because of the heat generated inside an AC 30 chassis.
I would try and use the same brand of tubes that come out,because of your incredible track record.Lets hope you didn't damage the output transformer by using that hotplate.

[Picaflor]

Thanks a million for the responses. Although I have owned this AC30 for several years, I haven't really played it all that extensively until recently (which may explain why the tubes have lasted as long as they have). I assumed the problem was probably in the power tubes and was hoping it was nothing more severe.

I am not really a Vox guy and am not familiar at all with cathode biasing. Does this truly mean that all I need to do is pop in 4 more EL84's and play without having it biased? If so, would that also be the same for pre-amp tubes and rectifier?

As much as I've started to love this amp, I really need to start doing some research on how they actually work!

[MinuteMan]

^ Correct, just acquire a matched quad of EL84s and insert. No biasing.

[Ronsonic]

Modern EL84s are consumables, if not disposable. Replace once a year or so. Use a matched quad since all are balanced on the same cathode resistor. The preamp tubes are probably fine. You might want to do the rectifier while you're in there, 90s vintage GZ34 are not all that good and it's a pain to get into the amp.

[paolojm]

Hot Plates eat power tubes.

That's simply not true.
Users, running their amp flat because they have a HotPlate to reduce the volume, ruin valves.
Running the same amp flat out without the HotPlate will produce the exact same results except that you will probably notice more ringing in your ears. :)