Strange problem with Peavey Classic 30

[voodrew]

My Classic 30 sounds great for about 2-3 minutes and then all of a sudden regardless of the settings the amp starts making crackling noises. This happens everytime. I swapped my amp out so I know it is not my guitar.

[MWJB]

Power down, unplug from the wall, pull out the 12AX7 tube nearest to the EL84 power tubes. Plug into the wall & power up...still crackling?

[voodrew]

Thanks for the reply. I pulled out the 12AX7 nearest to the power tubes and now my amp will not make any noise.

[Enzo]

That just told you the power tubes are not involved. Your problem is in the small tube section.

Pulling any tube in that amp will cause the remaining tubes of that size to go dark. You can't operate with a missing tube.

Restore that tube to its home now.

Turn the reverb controll to zero. ANy help?

Turn the other controls to zero. Does the noise remain?

Does it matter if you are in one channel versus the other?

[voodrew]

Thank you fro the response. I turned down everything one at a time - Reverb, Bass, Mid, Treble, Both Channels - post and pre gain to no avail. The guitar played perfectly for 3 minutes then started crackling. I also swapped out the guitar - it is the amp.

[Enzo]

My question remains, after all you can;t play guitar through it with all the controls at zero.

SO: with all controls at zero, does the noise remain?

Does it matter which channel you are in?

Does it still make noise with nothing plugged into the amp?

And for that matter, plug the guitar into the FX return jack instead of the regular input. Does it still crackle there?


And don;t forget to try different cords as well.

[MWJB]

Also, try a known good 12AX7 in each of the 3 preamp tube positions, one by one, to eliminate the possibility that one of your 12AX7 tubes has gone noisy.

[Danlampton]

I'm having this same problem with a Classic 50.

Noise is in the "normal" channel only and is affected by the volume control.
The tone controls affect the tone of the noise but it remains.
I plugged a guitar in the effects return - there was no noise and the power amp section worked fine.
I replaced all three tubes with new JJs. One at a time and all together - this made no difference.

Where to I look next.

Thanks.

[Danlampton]

I get these measurements from the 12ax7s.

Pin v1 v2 v3
1 157Vdc 162Vdc 208Vdc
2 -.8mv .3mv 44mv
3 1.1Vdc 1.1Vdc 1.1Vdc
6 157Vdc 163Vdc 236Vdc
7 .1mv .3mv 17.3mv
8 1.26Vdc .2Vdc 44Vdc
9 -6Vdc -13Vdc -.3Vdc

[km6xz]

The only Classic 50 schematic I have, there were a couple versions which I believe were different primarily inspeaker configuration. It has been 9 years since touching any guitar amp and just now getting back into it. The preamp is used on both the bright and normal channel. The only difference is a RC filter to lower the mid and lows on the bright channel. That means the problem is before the first tube and right at the jack, R26 a 47k resistor is the only in-line component that is unique to the normal channel.
That assumes you really have the same sound as the OP, what does it should like? Hissy, any low frequency tone component in the noise? Does it vary with wiggling the input jack? Can you describe the sound it makes?

[Danlampton]

Thanks for the reply. The noise is much like the op described. A crackling kind of sound. Not constant or predicable. Frankly it sounds like a bad tube to me (I believe I have ruled that out), but I have little experience except keeping my own amps running. The sound only occurs after the tubes warmed up a bit.

Peavey assured me there were only one set of schematics for the 212 and 410 versions and they seem right to me.

Thanks again.

[Billy R aka DynaFreak]

if it is not the tube, which you believe you have ruled out, then the next most likely cause of "bad tube' noise is a noisy plate load resistor. The only plate load resistor before the volume control is R1 (150k), according to the Classic 30 schematic that I have. There are some other possible causes, next in line would be a dirty tube socket. If you wiggle the tube and you get nasty noises (with the volume control turned up somewhat) then I would clean the socket and make sure it is holding the tube snugly.

[Enzo]

I think we just hijacked voodrew's thread here.

As far as I know, there is only one circuit for the Classic 50. They did add an FX loop after about a year, but the schematic was never updated to include it. The speaker config didn;t alter the circuit.

There were also at least four versions of the much earlier Classic though. And of course a bunch of other Classic models with numbers, Classic 20, Classic 30, etc.


Here is the Classic 50 documents, and I added their FX loop add-on kit in case someone is interested.

[Paulcblais@yahoo.com]

Just ran into the same issue, crackling after about 3-5 min. Tested everything including the tubes, reflowed all solder joints, still had the issue. Turned out to be an intermittently shorted power tube. The tube tested fine in my Jackson 648 and the shorts test was inconclusive but replaces one tube at a time with known good tubes i isolated it down to one tube. That tool a while to figure out but learned something new again. Every repair i learn more about troubleshooting different malfunctions.

[Enzo]

And a good lesson there is that tube testers can tell you a tube is bad, but cannot tell you a tube is good.