Topic: i keep blowing a fuse... (tube amp content)

[Tim]

best of luck to you mac - wish I could help. I was pretty pised when I thought one of my tube sockets had shit the bed, turned out to just be a tube.

[macdaddy]

cheshirecat - i will do all that you suggest by the weekend...

another thing i notice, but dont know if i am tricking myself...

if a tube is NOT biased properly, the sound from that channel gets kind of distorted, like a not-so-clean needle/lp combo can do... does that mean the bias is too low, or too high, or could it mean both..?

thanks - and great advice re: safety - that is one thing i do know about, but that is no joke; 30 min after unplugging should be OK to open up the case, right..?

[Genghis Cougar Mellen Khan]

Did you try swapping the el34s to the other side to see if the problem follows the tube or the socket?  That would be telling and help narrow things down.  Let us know what you find.

Have you tried this yet? 

[macdaddy]

no.

i lowered the bias, and have not had any problems yet (knock on wood). i have been really busy this week. i will get some fuses this weekend, and do the testing, and take the pics, etc. right now, i am just hoping for a few more hours of service out of the girl, so i am not touching anything until i hear or see something amiss...


but +t for the interest, i really appreciate all of the help. i most certainly will post testing results, pics of the inside, outside, etc - likely all by saturday morning...

[Genghis Cougar Mellen Khan]

no.

i lowered the bias, and have not had any problems yet (knock on wood). i have been really busy this week. i will get some fuses this weekend, and do the testing, and take the pics, etc. right now, i am just hoping for a few more hours of service out of the girl, so i am not touching anything until i hear or see something amiss...


but +t for the interest, i really appreciate all of the help. i most certainly will post testing results, pics of the inside, outside, etc - likely all by saturday morning...

The bias is a happy medium, don't think of it as turning it up/down.  You want the tubes to be biased properly or you may cause other problems.

http://www.geofex.com/ampdbug/biasing.htm
Output tubes handle probably 85% of all the power used by your guitar amp. If they are biased incorrectly or if there is a fault in one of the biasing components, it can cause a number of power supply and output section problems. A failed biasing component that lets the grid assume the same voltage as the cathode will cause an output tube to act almost shorted. Tubes which are conducting too much bias current (older tube-techs would say these are "underbiased" or "biased too hot") can cause blowing fuses, excessive power supply ripple and 120Hz hum, burned out rectifier tubes and could in the long run kill an output transformer or power transformer. They overtax in general everything in the electrical path from the AC power plug to the output transformer.[/i]

I would NOT operate the amp unless you can get the tube to bias properly.

------------------------------------------------[bias]++++++++++++++++++++++++++++

Think of where the tube should be biased as 0, less is no good, more is no good.

[pfife]

I'd follow the fearless leader/pop star's advice - if one toob is acting strange, its going to overwork the others, and cause worse damage.

[cheshirecat]

Yeah, you should definately know how to bias the amp properly... It's literally no big deal, and should be considered routine maintenance... like dusting.

If you run to Radio Shack you can pick a DMM (Digital Multi Meter) up for a few bucks.  Check the voltage at each tube's check point as compared to ground, it should be 40mV if it is not, adjust the screw until it read proprerly.  Do all 4 power tubes, once they're done go back and recheck, depending on the bias circuit, changing the bias at one tube may affect the bias at others.  Once you've done it and checked you'll see that it really is no big deal, but should be done every couple months of use, and anytime you replace tubes.

A DMM such as this should do the trick http://www.radioshack.com/product/index.jsp?productId=2103174&cp=2032058.2032235.2032305&parentPage=family

Dug this up on biasing your particular amp...

"With a voltimeter calibrated in mV DC ...measure the voltage between the measuring point of each valve (situated in the outside of the cage) and ground (The outside of the RCA inputs or the minus of the loudspeaker connectors).
Make this measure with the volume pot at minimum.
The voltage must be 40 mV DC ...if not ...adjust with the correspondent pot of bias adjustement of each valve."

[Tim]

the fearless leader/pop star

[macdaddy]

for the record - i know how to bias the amp, and have been doing it for years. but as fate would have it, i cant find the meter, so atm, i cant do the biasing. i guess i will have to bite the bullet and buy a new meter...


as for the distortion question, i was just reporting observations i have made over the years - i looks like i was correct, improperly biased tubes DO distort the sound...

like i said - i will open her up and snap some pics this weekend, and another round of T's to y'all for the interest and assistance.

[BobW]

Do you have the manual ?
I can e-mail you mine if you need it.

Definitely be careful under the hood, chesire's absolutely right about caps holding a charge.
If the circuit that bleeds the voltage off is not working, they can hold a fairly high voltage longer than 30 minutes.

You need to read your meter guide to see how it displays reverse polarity, as all are a bit different.
I still use a d'arsonval and can see the meter pin backward ! 

[Genghis Cougar Mellen Khan]

for the record - i know how to bias the amp, and have been doing it for years. but as fate would have it, i cant find the meter, so atm, i cant do the biasing. i guess i will have to bite the bullet and buy a new meter...


as for the distortion question, i was just reporting observations i have made over the years - i looks like i was correct, improperly biased tubes DO distort the sound...

like i said - i will open her up and snap some pics this weekend, and another round of T's to y'all for the interest and assistance.

I have a toy I'm borrowing and a box of tubes to test, if you can't find a tester locally I'll throw em' in the tester if you want.  Out of my 3 pairs, 2 of the pairs have a bad mate, I just ordered another set from thetubestore, best price I could find online.

[BobW]

Have you figgered it out yet ?

[Tim]

how much juice are we talking about being present in these modern tube amps?

I know it's a shitload, and nothing to fuck around (to say the least!) with but was just curious about how much we were talking about

[cheshirecat]

how much juice are we talking about being present in these modern tube amps?

I know it's a shitload, and nothing to fuck around (to say the least!) with but was just curious about how much we were talking about

]

Usually one of the HV taps will be at least 300V... so it will pack a nice punch if you're not careful... though if you know what you're doing it's not really a big deal.  Oh yeah, there will usually be a rectified DC 400V+ bugger in there as well, thats what you have to watch out for, the PS will have a bunch of capacitors, and if not bled properly will hold there charge for quite a while and give you the send-you-across-the-room jolt.

[BobW]

Definitely enough voltage to kill by way of interrupted neural synapses that control the heartbeat.
Sort of the reverse of defibrillation starting the heart, in that shock can stop it cold.
Best not to go in without training.

I always suggest to those who are new to tubes that having a mentor present is very beneficial for the first few attempts.
It's easier to have someone show you proper technique and where the danger zones are than to read it.

The Jolidas use EL-34s in the JD-302, which have as much as 800 volts on the plates and 425 volts on the screens.
More than plenty enough to cause burns, shock, and even death.
http://www.mclink.it/com/audiomatica/tubes/el34.htm

Be extremely careful under the hood !