Hello and a question

[burning tires]

I am new so hello

I have a vox ac15cc, it has a good deal of hiss. Do you think changing some of the carbon film resistors to metal film would reduce this? if so which ones would make the most difference? Oh, and yes I am ok inside the amp I know how to be safe and not get myself killed.

thank you

[darrellcheng]

hello and welcome. I think you should start by seeing where the hiss originates in the circuit. do this by seeing if the level of hiss changes with the volume knob, or any of the dials, reverb etc. does the hiss only occur when the instrument is plugged in? check to see if there are solid ground connections in the amp. try changing valves with a known good tester valve. Do this before trying to change hardwired components because you can severely alter the tone of your amp, and it is also easier to check these first.

[burning tires]

ok, no instrument, when I bring it out of standby there is a very light hum, if I bring up the gain I get hiss vol@12:00 gain@12:00 = bad hiss, if I raise the treble it gets worse, if I turn the reverb knob I get 60 cycle hum. It is not old and I don't remember it being this loud before. I tried changing the stock preamp tube with a few I have on hand, it didn't really help.

[darrellcheng]

Does it hum when the reverb is switched off? or when the tank is disconnected? what about moving the amp to a different place, maybe there is hum from electronics

[burning tires]

Does it hum when the reverb is switched off? or when the tank is disconnected? what about moving the amp to a different place, maybe there is hum from electronics

sorry for the long delay. it hums more with the reverb on. the higher the reverb level the more hum, even with the tank disconnected. but the hiss is always there, and its maddening.

[darrellcheng]

Okay have you tried pulling out the reverb driver tube as well as pulling the recovery tube? if one of the pulled tubes stops the hiss then just replace that tube. You should also use a known good preamp tube to 'swap and check'

[burning tires]

Okay have you tried pulling out the reverb driver tube as well as pulling the recovery tube? if one of the pulled tubes stops the hiss then just replace that tube. You should also use a known good preamp tube to 'swap and check'

there isn't a reverb driver tube, its a reissue, its tube pre and power amps with solid state rectifier and reverb driver. I thought about replacing the op amps in the reverb and trem with low noise op amps, that might be a little better but I'm not sure thats the main problem.

[darrellcheng]

hmm i dont know much about solid state, maybe try moving the thread to repairs and some of the pros might know what to do

[Chuck H]

The hiss is a typical by product of gain. I don't think changing from carbon film to metal film will help. It's not carbon film resistors that are notorious for hiss, It's carbon composition resistors. Carbon comp resistors have much lower density in the resistive element and tiny arcs in minute spaces translate to hiss. Thats the way I understand it anyway. No film resistor I know of has this problem to any noticable degree.

Some brands of preamp tube hiss less than others. Most of the time this is because they have less top end. You may as well turn down the treble knob in that case. Actually, some preamp tubes are noisy. But then it's a more static sounding effect than just a real even hiss.

Take any other amp that has an all tube signal path and set it up as loud with the same gain and brightness as your amp. I'll bet you get about the same level of hiss.

The hum issues could be grounding or layout. Which are nearly impossible to change on your amp.

Sorry, I know this isn't very helpful.

Chuck