Warning

 

Close

Confirm Action

Are you sure you wish to do this?

Confirm Cancel
Member Login
Posted: 2/2/2006 9:21:40 AM EDT
I have a sick guitar amp. It is a peavey classic 30.

Symptoms: Plays fine at low volume and high notes, however when a low note is played or the amp is even slightly vibrated, the amp crackles very loud. It will also crack once in a while without any vibration. Also, the last EL84 tube in line is getting extremely hot. The entire EL84 glows, and gets hot enough that if I left it on it would burn the vyinl.

My thoughts, Power is going somewhere through the tube it is not supposed to, causing the heating, and the overheated tube is causing the crackling. It might be a socket problem, but I am not sure. I have read online that the sockets can someitmes be like a distributor on a car, in that cracks or carbon trails can cause arcing. I will post pics of the head unit in a few minutes.

Any thoughts are welcome, I am not looking forward to diving into the head with a flukemeter & soldering iron.

Link Posted: 2/2/2006 9:26:03 AM EDT

im going to add a question too, if you don't mind. i have a tubed 9" sony triniton display with an analog board that i'd like to convert to accept vga input. ice scoured the web and it is possible, but i cant figure out how.

any help would be great
Link Posted: 2/2/2006 9:33:49 AM EDT




Link Posted: 2/2/2006 9:37:11 AM EDT
Put a new set of power tubes in it and see if that fixes it. If it doesn't fix the problem you'll have a spare set of power tubes that you'll need someday anyways. On my tube amps I'll allways start by swaping in my spare tubes as its a easy thing to check. But then I don't have a tube tester, if you have a tube tester that you can use, test the tubes.
Link Posted: 2/2/2006 9:38:36 AM EDT

Originally Posted By DVCAPI:
Put a new set of power tubes in it and see if that fixes it. If it doesn't fix the problem you'll have a spare set of power tubes that you'll need someday anyways. On my tube amps I'll allways start by swaping in my spare tubes as its a easy thing to check. But then I don't have a tube tester, if you have a tube tester that you can use, test the tubes.



I already did that. No change.
Link Posted: 2/2/2006 9:45:22 AM EDT
Then I quess the next step is to start looking at all the traces on the pc board. For tube amps I prefer the older hand wired fenders. I think they sound better than the pc board tube amps.
Link Posted: 2/2/2006 9:49:26 AM EDT
FIRST OFF, I LOVE THESE AMPS!!! These were perfect alternatives to Fender tweed amps and cost a LOT less while delivering essentially the same tone. So the end tube is getting really hot? Sounds like a bias resistor went bad. Could also be a bad socket. Ceramic ones are the best and its the first replacement I make on any tube amp I've ever bought. Word of caution tho. If you've never worked on tube gear before, be SUPER careful! Some of those caps in there are holding onto 450 to 600 volts! One wrong move and you WILL toast yourself. If you're unsure, take it to a tech to get fixed, its worth it in the long run. If you're into doing it yourself, get some books on the subject, like Aspen Pittmans(Groove Tubes) The tube Amp Book. TONS of schematics as well as a good beginners tutorial on how to diasgnose your own tube amp problems.

First, turn on the amp, and with nothing plugged into the input turn the volume up all the way, amp off standby(amp is on and ready) and tap on the tubes with a pencil or similar object. If its coming through the speaker, then that tube is a suspect bad tube. Replace and recheck.

With power off, unplug tubes and spray Blue Shower electronics cleaner into all tube pin sockets. Insert and remove tubes at least 5 times apiece and that should clean any carbon traces from the sockets causing arcing problems. Hope that helps a bit.
Link Posted: 2/2/2006 9:55:19 AM EDT
Thanks, I will try that. I will look for those books. Which bias resistor?
Link Posted: 2/2/2006 10:19:36 AM EDT
[Last Edit: 2/2/2006 10:20:53 AM EDT by Green_Canoe]
I've built my own using help from AX84.com. These guys and gals are gurus of tube amps with far more knowledge than I. AX84.com is like the AR15.com of the DIY tube guitar amp world. I would recomend you post on their "other than ax84 amps" forum at the bottom of their forum list. Sounds like the tube is "red plating" where the current travels out of control through the tube. First thing you will need is a schematic, which you may be able to get off the net if you don't have one already. Other wise you will never find the bias resistor. (Well you might, if you really know what you are doing.)

Another place to go is 18watt.com but they seem to be oriented more closely the the Marshal 18 Watt build.

Here's a couple pics of my project:





Kent

Link Posted: 2/2/2006 10:25:48 AM EDT
If you've swapped tubes and the same socket is causing the tube to glow cherry red, then I suggest following the leads back to the components that feed that socket. Measure all resistors, and replace anything that is 15% or more out of spec. If the problem was in the bias supply, I would expect both tubes to be running hot. If the socket has arced, replace it.

As always, please remember that tube amps contain lethal voltages, even after they have been unplugged for hours. If you're not 100% confident in your ability to work safely on the amp, take it to a qualified technician.

Link Posted: 2/2/2006 10:44:33 AM EDT

Originally Posted By Blaculo:
If you've swapped tubes and the same socket is causing the tube to glow cherry red, then I suggest following the leads back to the components that feed that socket. Measure all resistors, and replace anything that is 15% or more out of spec. If the problem was in the bias supply, I would expect both tubes to be running hot. If the socket has arced, replace it.

As always, please remember that tube amps contain lethal voltages, even after they have been unplugged for hours. If you're not 100% confident in your ability to work safely on the amp, take it to a qualified technician.




Easier said than done. In order to get a good measurement many of the components will have to be disconnected at one end. It could also be a coupling capacitor which, if bad, would throw B+ voltage (240 volts DC on up) onto the grid of the tube. W/O a capacitor checker he can't, well... check the capacitors.

I will guess that it is most likely it is an issue related to the bias circuit affecting only one leg of the bias circuit feeding the affected tube. Something seriously wrong, not just something drifting out of spec. Short? Bad resistor? Tubes are quite robust. It will take a major problem to make one glow red. (Other than the heater glow.)

Kent

Kent
Link Posted: 2/2/2006 10:59:23 AM EDT
Sounds like you need a new vertical hold tube.

Just take the tube out and take it down the the local drug store and put it on their tube tester.

Oh sorry, this is 2006 and not 1966. My bad!
Link Posted: 2/2/2006 11:02:18 AM EDT
Looks like you did a nice job on your amp Green_Canoe.
Link Posted: 2/2/2006 11:04:43 AM EDT
I found a schematic. If I had to take a guess being a non expert. I'd look in the areas of R40,44,45,49,52,55 and C32 and C35. If all the tubes had problems I'd look at teh bias power supply in the bottom left corner of this drawing. If you can Isolate which tube and the relatively few components feed that tube you might get in the ballpark rather quickly.

Kent
Link Posted: 2/2/2006 11:11:35 AM EDT
I will give that a shot over this weekend.
Link Posted: 2/2/2006 11:20:35 AM EDT

Originally Posted By CSM:
Looks like you did a nice job on your amp Green_Canoe.



Thanks! I'm pretty proud of it, if you hadn't noticed. I'm currently working on the cabinets (head and speaker box) for the amp. This part of the project is turning out to be much more work that the electronic part.

Kent
Link Posted: 2/2/2006 11:23:59 AM EDT
One more thing I didn't think of it could be a broken trace or solder joint gone bad leaving an open curcuit on the grid of the tube.
Link Posted: 2/2/2006 11:24:36 AM EDT

the amp is even slightly vibrated, the amp crackles very loud.

I'd hit each soldered connection with a soldering iron. The problem could be something as simple as vibration, whether from playing loud or from moving the amp, knocking a wire loose. With most of the guitar amps I've worked on, the problems were cause by physical abuse.

Also, make sure that you never touch tubes to see if they're hot. That deposits oil on them which makes them get hotter. Dozens of times I've seen someone that kept touching a tube eventually cause a problem with the tube. It's a self-fulling prophesy.z
Link Posted: 2/2/2006 6:53:18 PM EDT
First, you have to mark everything so you know which tube goes in what socket.

Then put all the tubes in a brown paper sack. If you use a plastic bag it will mess up the tubes as plastic grocery sacks did not exist when tubes were in their heyday.

Put the sack in the basket on your bike and ride down to the 7-11, which used to have a tube testing machine (and they sold tubes out of the cabinet in the bottom of the machine).

Or at least that's how I did it when I was a kid.

Unfortunately for you, 7-11 has not had tube testing machines for many many years.

You are screwed, dude.
Link Posted: 2/2/2006 7:24:43 PM EDT

Originally Posted By Green_Canoe:
One more thing I didn't think of it could be a broken trace or solder joint gone bad leaving an open curcuit on the grid of the tube.



That's what it sounds like to me, check the 220k (red/red/yellow ) or 47k (yellow/violet/orange)resistors feeding that tube along with the wiring that connects them to the tube and the bias power supply. It sounds like the tube has no bias voltage allowing it to pass all the current that it can, not good for the tube...
Link Posted: 2/3/2006 2:23:02 AM EDT
Link Posted: 2/3/2006 8:02:15 AM EDT

Originally Posted By -brass-:
Is it Valve 4 / V4 / Tube4 on the circuit board?

I would take a wild SWAG at replacing C56 (1pF cap) due to a short inside the cap (small ceramic would be my guess, they don't often fail).

This is due to the fact high frequencies are not affected, and new tubes do not help. Change that cap in the finals.

If there are other caps, they should be warm, if you have a tube getting that hot, you have other components that are supposed to limit that current getting hot (power supply or filters). Try to find the hot point (other than the tube) without shocking yourself, and then look at potential failed components in the immediate area.

A Semiconductor/transistor trick is to spray freon onto suspect transistors, but this will also work for caps. It will either make them work, or fail entirely. Give it a go.



Brass, what is the purpose of the freon? I don't understand what it does or how to use it. Is the freon cooling the transistors? Could I use something else, like dry ice vapor or nitrogen vapor, as freon is hard to come by?
I understand basic circuits, bridge circuits, filters, op amps...
Link Posted: 2/3/2006 10:49:04 AM EDT
Link Posted: 2/3/2006 2:28:18 PM EDT
I would definitely look at C32 and C35. It's very common for a coupling cap like these to break down and supply DC current to the grid and drive the tube into saturation. And remember it's really Hi-Z so we're only talking microamps here, if even that. A megohm or less DCR can cause your plates to glow red.
Link Posted: 2/15/2006 6:24:09 PM EDT
No updates or more questions yet. I still haven't gotten to fix it. I might have time this weekend, I am not just blowing your advise off.
Link Posted: 2/16/2006 10:19:29 AM EDT
Top Top