Left 5e3 on overnight

hpharley90

Silver Supporting Member
Messages
1,297
So I left my mojotone 5e3 that I put together
on all night still plugged into guitar.
It used to be dead quiet when just idling (no guitar signal).
Now there is a loud audible hum without any signal.
Can anyone with knowledge tell me where to look to fix.
One other thing when I found It on this morning it had a crackling sound coming from it and I immediately shut off.
It works but that loud hum has to go.
 

2L man

Member
Messages
701
Sometimes new tubes break but leaving amp On should not break it. If it was bad tube measure new tubes biases just in case so that they do not idle too hot!
 

hpharley90

Silver Supporting Member
Messages
1,297
I took out the Tube Amp doctor 6v6 tubes and replaced with JJ. It still had loud hum .
So I replaced both 12ax7 and 12ay7 tubes and that was the ticket.
Quite like it used to be.
And It sounds better too!
I don't know what preamp tube stopped hum
but I don't care.
 

tmac

Goldmember
Platinum Supporting Member
Messages
2,340
Good preamp tubes along with closely matched power tubes will keep noise down in a cathode bias amp like yours. Leaving on overnight is no big deal. It wasn't working to amplify anything while it was on and idling. Worst case should 9nly be very slight wear on tubes if everything else in the amp is in order and was well ventilated.
 

Guitar Dave T

Member
Messages
11,861
Ok cathode bias. No adjustment.
I Thought no adjustment was fixed bias.
My marshall 2204 had a bias potentiometer to adjust.

Yeah, the terminology can be counterintuitive, but the short version is Fixed Bias = adjustable bias, either with a pot or by swapping out resistors; Cathode Biased = does not need to be adjusted.

Cathode biased circuits like the 5e3 are generally more touch-sensitive, easier to compress and capable of breaking up at lower volumes.
 

hpharley90

Silver Supporting Member
Messages
1,297
Good preamp tubes along with closely matched power tubes will keep noise down in a cathode bias amp like yours. Leaving on overnight is no big deal. It wasn't working to amplify anything while it was on and idling. Worst case should 9nly be very slight wear on tubes if everything else in the amp is in order and was well ventilated.
I try not to keep on when I'm not using because it gets so hot.
Area around power switch and lamp get so hot you can't touch after a while.
 

Guitar Dave T

Member
Messages
11,861
I try not to keep on when I'm not using because it gets so hot.
Area around power switch and lamp get so hot you can't touch after a while.

Another characteristic of cathode-biased amps is they tend to run hot.

Blowing a 6" USB-powered desktop fan on the back of the amp, just enough to get some air circulating, helps immensely.
 

2L man

Member
Messages
701
Ok cathode bias. No adjustment.
I Thought no adjustment was fixed bias.
My marshall 2204 had a bias potentiometer to adjust.
Fixed bias means there is no bias adjust potentiometer and "fixing" is needed to bias and this cover Grid and Cathode biases. So resistor(s) have to be changed to change bias. Cathode bias is often fixed but it does not mean that It don't benefit of "fixing"
 

hogy

Member
Messages
14,840
No, that’s not how it works. In “fixed bias” a steady bias voltage is applied to the grids of the power tubes. This voltage is determined by a resistor in the bias supply circuit. That resistor can also be a potentiometer, which is nothing but an adjustable resistor. The cathodes of the output tubes in this system are tied to ground.

In “cathode bias” there is no fixed bias voltage at the grids. Instead the tube self biases via a cathode resistor (over a certain range). This generally means that you can plug in power tubes without adjustment.
 

telewired

Silver Supporting Member
Messages
448
So tell us how do you like this Mojotone 5e3 amp? Could someone who has never built an amp put one together. I build guitars some..Went to Galloups Guitar Building and Repair School so I can solder decent enough.
 

hpharley90

Silver Supporting Member
Messages
1,297
I love the amp .
I bought complete kit from mojotone and studied instructions before hand.
I was in no rush.
Took me a few hours to put together but when I fired it there was no smoke or smells.
Been working great ever since.
Over 2 years now.
I did look at alot of pictures of other builds and watched video for project on mojotone website.
I never did anything like this before.
 




Trending Topics

Top Bottom