Possible Valve Problems?

[CAlbs_Plexi50]

Hi All,

First time poster, but long time Metroamps lover. I built a Plexi 50 Kit with my late Pops about 10 years ago and have enjoyed this amp every time I have turned it on.

I had my amp in some storage for the last couple/few years as I'm going through that phase in life where children have been brought into the equation and priorities shifted. Now that I am getting some time back, I turned it on and noticed a few little quirks that were not there before. The Amp ran and powered on fine, but there was a few of pops and crackles when playing and even after pulling out the input socket, there was some residual noise that came out the speakers almost as if the signal was working its way through the amp. I tried to clean the input sockets and ended up damaging one connector on the high treble channel (bonehead move, I know), but ive done and bought a replacement for this. Hoping these pops and crackles are just dirty connections.

When the above happened and due to the sentimental value of this amp, I turned it off and immediately started to educate myself on what could be wrong with it. I put the amp on a bench, poked around with a chopstick on the high treble input and it seemed to quiet down when holding the connector closed, but until I change this input out I wont really know for sure if the pops and crackles go away. I am hoping this issue is from dirty connectors, but this will be TBD I guess.

I also tried moving the preamp tubes around, looking for microphonic tubes and V1 seems a little microphonic but i also read this is somewhat normal for the Plexi 50's. Is this true? I'm not sure if this could impact things, but once i have the ability to i will replace these just to be sure.

Aside from the above, I found the original instructions for the Lead Plexi 50 Kit and started taking various resistance/voltage readings. Prior to plugging things into the wall, I checked all of the resistors all of these seemed are okay. I plugged into the wall and all tubes were removed, I took voltage readings across the heater wires and other various pre and power amp pins and I believe things to be pretty close to what the specs in the instructions.

Once I insert tubes, again voltage readings change by 5% (or maybe 8-10%) but again I believe these are okay, if anything the voltages were even close with tubes in vs. tubes out. But this is where things go a little beyond my knowledge and hoping its something as simple as the power tubes not being fully matched. When I try to bias the power tubes, one is at 28mV while the other is at 40mV. When I switch the tubes and put V4 into V5 and V5 into V4, the bias reading moves with the tube, so it makes me feel like the tubes are just not matched anymore.

If the two power valves are out by 12 mV when biasing, is this considered problematic or is this still within range of "matched tubes"? To me this seems a little beyond matched and maybe time for new tubes. My other question is how can I ensure my Output Transformer is operating correctly? Could a fault O.T cause the difference I see when Biasing? This is overkill i think because the tubes carry the bias difference, but i love this amp and just want to make sure i wont destroy anything.

I know there is a wealth of knowledge to say the least on this forum, so if i am asking anything that would be considered elementary to others, i do apologize.

Cheers,
C

[danman]

If the amp has sat in storage for more than a couple of years, there is a good chance that one or more of the electrolytic filter caps have dried up and failed. They are the large blue cylinders mounted on top of the chassis and they serve to filter the power supply and keep the amp quiet. They will deteriorate with age if they sit with no power applied. If they go completely bad while you are playing the amp, they can destroy the power transformer and blow up inside the chassis causing a real mess. The pots are another item that may cause some noise if they haven't been used in awhile. A shot of contact cleaner inside and a few turns should clean them out and quiet them down.

If the power tubes have drifted further apart over the years, they can cause some hum in the amp once they get far enough apart. Some folks like the sound of mismatched tubes while others prefer them to be a closer match. 12ma is pretty far apart so you might want to consider a fresh set.

[CAlbs_Plexi50]

Thanks Danman for the reply.

Now that you have my attention with the filter caps, is there a way for me to test them? I only have a digital volt meter and not so sure on how to go about testing them to see if they are okay. Visibly, they are not leaking or bulging or anything but for piece of mind I wouldn't mind ruling these out and swapping if needed.

As for the tubes, its just a matter of time until i grab some new ones because I agree, to me they are just a little too far apart. Thank you for confirming this for me

As for the Pots and other contacts, once I get this thing up and running without any issues I do plan on cleaning things with Deoxit or Isopropyl alcohol (any preference?).

I did swapped out the high treble input on channel 1 which I messed up myself and then I cleaned all the other inputs to the best of my current abilities without any cleaner, and a lot of the noises I was experiencing have cleared up. This makes me happy.

I feel like there might be a grounding issue on my guitar side though. When nothing is plugged into the Amp, its nice and quiet, no hum or hiss, however as soon as I add a guitar to the inputs it sounds as if the guitar ground is terrible. This is the next thing I will need to solve because its kind of annoying. (Any suggestions here?) My thought if the input circuit is closed before any input and the amp is quiet, the addition to the circuit (the guitar being plugged into the amp) is now what is causing the noise.

Appreciate the support

Cheers

[danman]

There are testers that can test E-caps at that rated working voltage but they are rather expensive. Most manufacturers suggest replacing them at 15-20 years of service. Some will last longer but it's a crap shoot as to when they will fail and cause damage to the amp. If you intend on playing the amp, changing them out would be wise. The sound can also be affected by old caps causing the low end to get flubby sounding and the amp will lose it's punch.

The input jacks have a shunt switch that grounds the v1 grid when the cable is removed. The fact that yours is quiet with the cable inserted means that the input jacks are functioning properly. I would try a different guitar and cable to rule out those possibilities first. If the amp is noisy with multiple guitars, it's probably gonna need a checkup with a tech to find the issue.

[CAlbs_Plexi50]

Thanks danman, I really do appreciate your help.

I was able to swap guitars, still had this annoying hiss or buzz. I then kept the guitar plugged in with the amp on low volume and used the guitar as an antenna and probed around my basement to see what made my noise worse. It turns out that the location of my basement where i am working is situated directly under a pot light and there is some major electrical interference happen once the coils in the guitar are plugged in. As soon as I turn the lights in my basement off, the amp quiets down and the noise is back to the normal neutral hum/white noise you hear from these amps. Thank God.

And thank you again for helping outline the potential of bad filter caps. I plan on taking my amp to a local store in time to have it properly assessed to ensure its in fine order. I don't plan on playing it until this time, more of a piece of mind check to ensure all is working as it should.

I have to say, the old Kit amps Metro Amps sold were and still are amazing. Thanks to knowledgeable people like you who continue to help people like me out, its a nice feeling to have knowing these amps will continue to rock for more years to come.

[danman]

Glad it was an easy fix. Light fixtures, light dimmers, computer equipment cellphones and other appliances in the house can cause noise that is picked up by the guitar or the amp itself. If you live in an electrically noisy area, it can be a real pain to eliminate the problem.

Since your amp is only ten years old and the noise level is minimal, the filter caps are most likely just fine. If you notice that the amp begins to lose it's punch, sounds weak and becomes noisy in the future, then it would be wise to have them checked.