Quick, but stupid, Valve Amp Question

B

benjaturner

New member
I hate to ask because it's going ot make me sound really stupid. :smack: I keep blowing fuses in my amp. I replaced one today after less than two hours use.

The only thing I can think of that could be cusing the amp to falter like this is the way I'm turning the thing on and off.

Could someone please explain how you're supposed to turn valve amps on and off, the whole warming up thing. :duh:

Thanks,
Ben
 
Re: Quick, but stupid, Valve Amp Question

Turn on in standby mode
Wait 10-20 seconds
Turn amp On (standby mode off)
Play

to turn off, you simply turn off
some people put it into standby first, but theres no need.
 
Re: Quick, but stupid, Valve Amp Question

A bad output tube or tubes is normally the cause of blown fuses,after that it's filter caps or a bad transformer..
 
Re: Quick, but stupid, Valve Amp Question

So when I start playing, I turn the standby off? I've nevre done that before that must be where my problems are coming from. Thanks
 
Re: Quick, but stupid, Valve Amp Question

StratD, I hope it's not the output valves. They're only about 4 months old. They're are cheapo crappy ones though. Electro Harmonix ones.
 
Re: Quick, but stupid, Valve Amp Question

Electro harmonix are crap, and they sound it, get some JJ's in there, cheap, reliable and sound really good, id get them tested from eurotubes.
 
Re: Quick, but stupid, Valve Amp Question

BigBazz, could you please explain the process of turning the amp on again. I'm not sure I understand exactly what you meant.

How I do it is to turn the power on for about 1 minute, then turn the standby switch on. I leave the standby switch on while playing.

When turning off I turn off the power switch off first. After 1 min I then turn the standby switch off. I then immediately turn the plug off at the wall.

Please could someone either confirm I'm doing this right or tell me what I should be doing.

Thanks again,
Ben
 
Re: Quick, but stupid, Valve Amp Question

well you are doing it correctly.
standby mode is when the amp is just warming up, and no guitar is going through, when you turn the amp "on" the guitar comes through, so you are turning Standby mode Off.

dont worry, you are already doing things right, though an amp tech that built one of my amps said that theres no need to let the amp cool down in standby mode, you can just turn it off.
 
Re: Quick, but stupid, Valve Amp Question

I have a vox valvetronix AD30VT. Should i be going thru this whole warming up process too?
 
Re: Quick, but stupid, Valve Amp Question

I always turn on the power with standby engaged and let it warm up in standby for 1-2 minutes.

I always turn off by putting it in standby for 1-2 minutes and then turn off the power.

I'm not that busy that I wouldn't have time with waiting 1-2 minutes before and after playing. I also read in the manual, something like that if I remember right.
 
Re: Quick, but stupid, Valve Amp Question

Thanks for the help with this. I guess there must be something else up woth my amp then. Or more likely, it's just rubbish, he he. It happened again today, the fuse being blown when I turned the amp on. I went through the same process as I mentioned before.

Everytime I change the fuse I have to open the whole thing up. I noticed today that some components had melted a little bit. I think they're capacitors. Big cylindical blue things. My amp is one of those crappy 'tubes mounts soldered to actual circuit board' things. The capacitor/thingy that had melted is directly above the power amp tubes so it's probably that. The amp still works, once I but the new fuse in (almost a daily occurence now). But it can't be healthy.

I think I'm going to sell it. I love the amp but I just can't be bothered with all the hassle it causes. Any british guys know what it's worth? It's a Marshall JTM60 with three 10' speakers.

Thanks,
Benja
 
Re: Quick, but stupid, Valve Amp Question

The JTM60 series runs hot, and is know for cold solder joints.

That being said, I have the 622 model, a 2x12, and I added a fan to take the hot air out of the chassis an off the tubes. This is a must have due to the design, IMO. In the JCM 600's they added a vent by the handle.

It sounds like you have some other issues now, so if you take it to a tech have them add a little fan for you. Might be a keeper after that.

I am taking it for granted you biased those EH tubes, which aren't junk.
 
Re: Quick, but stupid, Valve Amp Question

No, I didn't do anything to the valves before putting them in. I didn't know I was supposed to. I bought them matched though, through a reputable online seller. I thought you only needed to do this if you were putting different valve types in. Please could you explain the process of biasing to an electronics beginner. Thanks
 
Re: Quick, but stupid, Valve Amp Question

Biasing sets the "idle" of the amp. Bias too cold, you can get down into crossover distortion, bias too hot, and you will shorten the life of your tubes. Also biasing too hot will add alot of heat, something that amp doesn't need.

That amp does need to be biased when you change tubes. Even the same brand tubes will likely run at different voltages.

That is a hard amp to work on, the chassis is built in an odd way.

This process involves getting the plate voltage with a multimeter, and likely purchasing a bias tool.

Your bias pot is a little blue pot, only viewable with the chassis out, looking directly at the chassis from the side.

In getting the plate voltage, you will be dealing with voltages of at least 450 volts, enough to liquify your fillings, so you may want to think about this abit.
 
Re: Quick, but stupid, Valve Amp Question

Take it to a tech. Nuff said.

Dangerous voltages inside. They can kill you.

I learned something the other day, Off Topic, but that 5AR4 rectifiers really don't require a standby mode since they take a bit longer to ramp up than other rectifier tubes. Isn't that bizarre?
 
Re: Quick, but stupid, Valve Amp Question

You can smell hair burning in this one huh Scott.

You little tidbit on the rectifier has been stuck away for future reference.
 
Re: Quick, but stupid, Valve Amp Question

So how do more serious minded guitarists change your valves? Do you own a multimeter and do all this stuff yourself? When I put new valves in my amp I started a thread on here asking how to do it properly and everyone said just whack 'em in. As long as the power-amp valves are matched you've got no problems. I guess this isn't the case.
 
Re: Quick, but stupid, Valve Amp Question

www.webervst.com sells a little gizmo called a Bias Rite. It goes between your power tube(s) and the tube socket(s). You can just measure one at a time or buy a pair. I then plug them into my multimeter on DC Volts. Then I bias to taste (my ears), but normally for a 6L6 amp, somewhere between 30-40 mV seem to work pretty well. For 6V6's, I try to stay a bit under that.

If you know what you are doing, there is a nice "callculator" at this link. http://www.webervst.com/vstbbs/bbs.html

If you really want to learn about this stuff, there are reams of info on the net. Start with the safety stuff located here.

http://www.geofex.com/tubeampfaq/taffram.htm

and here...

http://www.drifteramps.com/safety.html
 
Re: Quick, but stupid, Valve Amp Question

Hey Ben,

Don't get down on this. Matched tubes are needed yes, but the rule of thumb is to think an amp needs to be biased rather than not. Even on amps that are cathode biased (fixed bias, or not easily biased), I check the bias. I have seen tubes run way too hot out of the box. Always best to check.

The amp needs to go to a tech anyway, you said you have some other issues, so just have it biased.

You are on a better track now. Think positive.
 
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