Help! Amp problems ..

Chat about Equipment Info

Help! Amp problems ..

Postby monroe » Wed Dec 31, 2008 9:23 am

So a few months ago I bought a used Fender Hot Rod Deluxe. Not the greatest amp in the world, but it gets the job done. Anyway, about 2 weeks ago, I was practicing at home when the amp made a horrible horrible sound and then went dead. I checked and sure enough ... blown fuse. So I replaced the fuse and went on my merry way. I thought it was a little odd that the fuse blew at such a low volume (I had it on like 2), but it was easy enough to get a new fuse so I didn't really worry about it. The amp has been working fine for the last couple of weeks. Until a few days ago. When I fire it up, it works great for about 30 minutes to an hour ... Then it starts making a nasty crackling, rumbling, hissing sound. I'm new to all this wired up, electrofied stuff, so I'm not sure what's going on, and some help would be appreciated.

When the crackling starts, if I put the amp into standby, the problem goes away. That leads me to believe it could just be a case of a worn out tube(s). However, (and this is where my lack of knowledge and experience may come shinning through) the blown fuse from a few weeks ago concerns me, and makes me wonder if the problem I'm having isn't something else .. something electrical.

Finally, some tube questions, in case it does turn out to be just a case of bad tubes. If I change out the preamp tubes, should I (or do I have to) replace both of them, or is it acceptable to just replace the one offending tube? I know changing out the main tubes requires re-biasing and such, which is a task that I'm not up to myself. But if I understand correctly, I can just change out the pre-amp tubes on my own with no further work required. Is that right?

Thanks in advance for all your help!
monroe
Blues for Allah
Blues for Allah
 
Posts: 69
Joined: Tue Aug 19, 2008 6:17 pm

Re: Help! Amp problems ..

Postby jeffm725 » Wed Dec 31, 2008 9:49 am

Blown fuses are an indicator of a Power Tube issue (if it is a Tube issue at all)

The first thing you should do is replace the Power tubes.

If you still have an issue after that, it could be any of a number of things. Be very careful though, you can do output tranny damage if this has a real problem.

I would take it in.
.......................................................have you heard the one about the yellow dog?
jeffm725
Senior Member
 
Posts: 1072
Joined: Mon Jun 09, 2008 1:16 pm
Location: Chester, CT.

Re: Help! Amp problems ..

Postby waldo041 » Wed Dec 31, 2008 9:52 am

"Tone is in the instruments. Technique in the hands. Do what you will." ~ quote from some guy at the TGP forum
User avatar
waldo041
Senior Member
 
Posts: 2817
Joined: Thu May 19, 2005 10:58 am
Location: Indiana

Re: Help! Amp problems ..

Postby tigerstrat » Wed Dec 31, 2008 11:59 am

monroe wrote: If I change out the preamp tubes, should I (or do I have to) replace both of them, or is it acceptable to just replace the one offending tube? I know changing out the main tubes requires re-biasing and such, which is a task that I'm not up to myself. But if I understand correctly, I can just change out the pre-amp tubes on my own with no further work required.


Jeff is most likely correct that it is a power tube issue, however to respond to this part of your request, YES you can swap preamp tubes around ALL you want, in perfect safety to you and your amp. In fact you I would encourage anyone that owns ANY tube amp, to purchase 1 or 2 high quality 12ax7/7025's and experiment with swapping them into different preamp sockets...
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia
User avatar
tigerstrat
Senior Member
 
Posts: 4628
Joined: Wed Nov 30, 2005 1:20 pm
Location: Portland,OR

Re: Help! Amp problems ..

Postby jonarobb » Wed Dec 31, 2008 4:04 pm

Thought I'd chime in here...

In a Fender amp where the fuse is only on the primary side of the power transformer, anything down the line from that is fair game for pulling enough current to blow that fuse. Although power tubes are suspect it should not be said that to blow a fuse is solely a tube issue.

In an amp that has the HT secondary fused on the output transformer, such as a Marshall, Orange, Traynor, etc... when that fuse blows it's 9 times out of 10 a power tube or a failed screen grid resistor.

Back to the Hot Rod, any of these could pull that fuse:

Bad center tap ground connect
Bad rectifier diode(s)
Shorted or bad B+ Electrolytics (particulaly right after the SS rectifier on a Hot Rod)
Low Voltage winding on the PT secondary(this has been coming up alot lately. The filament winding over heats because of the poor wave soldering job on the hot rods. It's only 6.3v but when it arcs it'll cook out the power transformer)
Bad screen resistors
Cold solder joints on power tube board
PI Plate load resistor open
Loose fuse bucket
Bad AC switch
The list goes on....

Hopefully it is just bad power tubes. If you still have the Groove Tubes MIG fighter jet tubes in there, you might want to yank those.

A quick test to see if the power tubes are the source of the noisy rumbling and other problems, is to pull them altogther, just the 2 power tubes. Turn the amp on with the standby off, as if you were going to play. With no guitar plugged in. Just let the amp saute' for a half hour or so. If you're noise does not reappear at all then you know it's a problem between the PT and the OT.
jonarobb
 

Re: Help! Amp problems ..

Postby Jon S. » Fri Jan 02, 2009 8:01 am

monroe wrote:When I fire it up, it works great for about 30 minutes to an hour ... Then it starts making a nasty crackling, rumbling, hissing sound. I'm new to all this wired up, electrofied stuff, so I'm not sure what's going on, and some help would be appreciated.

When the crackling starts, if I put the amp into standby, the problem goes away.

Some good news: these describe a classic cold solder joint issue on an amp line well known to be prone to them. The intermittent nature of the problem is due to needing to heat the board up sufficiently to flex and stress the joint. If so, it's an easy, cheap fix (<1 hr. bench time). Advise having your tech, while he's got the chassis out, check and reflow any and all joints that look even the least bit questionable to prevent future time bombs.
User avatar
Jon S.
Senior Member
 
Posts: 1395
Joined: Wed Apr 30, 2008 12:34 pm

Re: Help! Amp problems ..

Postby monroe » Fri Jan 09, 2009 1:58 pm

Sorry for the delayed response, and thanks for all the info.

Luckily, I live near one of the preeminent amp repairmen in all the land (or so I'm told) in Doc Electro. I took my amp over to him to get checked out. Sure enough the problem was the power amp tubes. Replaced those and it's smooth sailing.

Thanks!
monroe
Blues for Allah
Blues for Allah
 
Posts: 69
Joined: Tue Aug 19, 2008 6:17 pm


Return to Grateful Dead Equipment Discussion

Who is online

Users browsing this forum: No registered users and 2 guests