Classic 50 12AX7 voltages

[Danlampton]

How can I find out what the voltages should be for these tubes?

V1 and V2 are pretty much the same but V3 is quite abit higher all around.

Thanks.

[Chuck H]

Enzo or one of the other really busy repair guys here may already have the info. It never surprises me anymore. But you could just call Peavey. Nice bunch, I think they'll just put you on hold for a minute while they get the info for you. I've called them before and that's about how it usually goes. You could save them a little time and just email them if your not in a hurry.

Why do you need to know this? Is the amp misbehaving or sounding different? Or are you just chomping to see if it's supposed to be how you've measured it?

[Danlampton]

Thanks Chuck H. The Peavey people have been great at sending schematics and parts. I appreciate their service.

I have a Classic 50 in that has some nasty crackling sounds going on in the preamp section of the clean channel.
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.

Trying to find out how they should measure.

[nashvillebill]

I'm quickly glancing at the schematic for mine. If the noise is ONLY occurring in the Normal channel, then we must consider what's unique to that channel. The amp uses a relay to switch in Normal or Lead. Normal appears to use V1B, whereas the other channel does not (i'm hastily looking at this while waiting on my old lady to finish getting ready to eat).

So that would mean the noise is coming from any of these:
-the relay contacts K1
-the solder at the relay on the Normal side
-V1B socket (pin connections as well as the solder joints)
-the Molex connector that joins the boards there
-the components around V1B

Hope that helps.

[Chuck H]

Try replacing the plate load resistors for that preamp tube. That's a rough guess but it works often enough that it could save additional time. I don't know what the rating is on those resistors but in new PCB amps it seems MFG's often use 1/4 watt rated. This is barely up to the job for this part and they can fail. The early reissue Fender Bassman had this problem often.

Since you replaced the tubes that sort of rules out dirty tube sockets. Usually unplugging and plugging in tubes will temporarily fix or at least change the problem. But you might go ahead and spray a little contact cleaner on a 12ax7 tube pins and gently rock it in a circular motion in the sockets. It goes quick enough and it's nice to be sure.

Over exposure to moisture has caused this kind of thing before too. Keep the amp chassis close to a wall heater or exit vent between testing and this will help dry it out if this is the problem.

I don't know enough about the nature of transistor failures in these amps to make any guesses about how they might be involved.

[nashvillebill]

The only transistor in these is the op-amp for the reverb driver, so that's likely not an issue.

Oh, I forgot to mention the volume pot for the Normal channel, is it noisy when you turn it? That's only for the Normal channel as well, so it's another component that could contribute to popping in that channel only.

[Billy R aka DynaFreak]

assuming the amp works normally and all voltages are normal, nasty crackling is often the tube, the plate resistor, or the (dirty) socket. those are the 3 I check first before digging in to other possible problems.

[Chuck H]

Does anyone even read these threads before posting anymore???

He replaced the tubes, we eliminated dirty sockets and I told the OP about the plate resistors. Myself and Bill had a simulpost and we covered other possibilities too.

[Billy R aka DynaFreak]

You are correct sir, that my post was entirely redundant and unnecessary (athough somewhat more succinct) and failed to give credit to the suggestions that you and nashvillebill provided. Yawl rock. My apologies for attempting to contribute. It won't happen again.

[Chuck H]

Please... Contribute. It just happened to me on another post on that same day and I had a twitchy moment.

[Billy R aka DynaFreak]

OK, thanks, and in the spirit of Christmas I will make a New Year's resolution to actually read a post before I chime in...