#1
i have a peavey vk 112, i have noticed recently that is making a funny sound, it happens about every 15 seconds.

it sounds a bit like when you plug your jack into the guitar.

i spoke to somebody about it any they said it could be time for a tube change....is this correct and is it pre amp or power amp that need changing??
#3
I also own the same amp. Not that it matters, but I like to use mine for jazz.

If you're hearing a funny sound from the amp, most likely, you have a preamp tube going bad. When the power tubes begin to go bad on these amps, it will result in volume drops, muddy tone and similar issues. Replacing the preamp tubes on this amp is a bear. The tubes are covered by the aluminum shields. Even after you remove the shield, there isn't enough tube exposed for you to get a good grip. I usually end up removing the two screws from the base of the shield and then remove the base itself - you then have plenty of tube to grab onto.

Do you need to replace all the preamp tubes? Probably not. I don't have my manual in front of me, but I would start with V3, which is the phase inverter. They get quite a workout. You can eliminate the reverb tube by simply turning down the Reverb knob. If the amp is still making the noise, then have a look at replacing V1. You can also try swapping tubes to aid in troubleshooting. Swap V1 and V2, or V3 and V2 - just make sure the reverb is turned down, so it doesn't effect your testing. Also, swap the tubes with the power to the amp off and let your tubes cool for a few.
#4
i thought it may have been my reverb as i have it on constantly, but i turned it down and the problem still persists. i dont feel confident messing around with the tubes myself, i changed the speaker myself although that was easy because i had online instructions taking me through each step. i have also noticed a small volume drop......what do you think this could be??
#5
Tubes
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#6
The volume drop will happen as you're using the amp. As you play a note or a chord, the volume will drop some. This is an indication the power tubes are ready to be replaced. I'd go with V1 or V2 for now. This is something you can do yourself. While you're saving money doing it yourself, you'll also be learning how to care for your own equipment.
#7
so whats the difference between pre amp and power tubes? and how much do they cost to replace??
#8
A simple method to check if a tube is bad can be preformed. All you need is a pencil with an eraser on the end.

Take the cage of so that you can access the tubes. Turn the amp on. Plug your guitar in like you're gonna play it (this keeps the input from being grounded). Now gently tap on each tube with the eraser. If you can hear it through the speaker, that tube is definitely bad. If it doesn't make a sound, it doesn't rule it out but no harm done.

Changing preamp tubes (all 12ax7s in this amp) is very easy. Just take out the old one and put the new one in. The power tubes should be put to a guy who knows how to bias a valveking.


EDITED for some misleading info: There may be a slight thump, but that's not what I'm referring to. It'll sound like you put the thump sound through the input jack. It sometimes is a large high pitched "ping" sound. I've even came across power tubes that were very microphonic that would squeal like stabbed pig occasionally. If you'd tap on the glass you could get this nasty squeal.
Last edited by end_citizen at Mar 15, 2012,
#9
Quote by end_citizen
A simple method to check if a tube is bad can be preformed. All you need is a pencil with an eraser on the end.

Take the cage of so that you can access the tubes. Turn the amp on. Plug your guitar in like you're gonna play it (this keeps the input from being grounded). Now gently tap on each tube with the eraser. If you can hear it through the speaker, that tube is definitely bad. If it doesn't make a sound, it doesn't rule it out but no harm done.

Changing preamp tubes (all 12ax7s in this amp) is very easy. Just take out the old one and put the new one in. The power tubes should be put to a guy who knows how to bias a valveking.


If only it were that simple. This test is only valid for testing tubes for microphonics. A microphonic tube has the potential to break into oscillation. However, do keep in mind that ALL tubes are microphonic to some extent - even new ones. So, by your test, you'd throw away a lot of new tubes. The idea is to rule out tubes that are excessively microphonic AND therefore will likely cause problems.

Your method does not test tubes for emissions or anything else that a real test set would be utilized for. Most of us cannot afford tube testers, so we use our ears and other methods to determine when tubes are worn and need to be replaced. Also, keep in mind that some testers do not perform very well at indicating real world conditions, so they may give a better picture of health than actually exists - provided you can find someone with a tube tester.
#10
Quote by jamie vk
so whats the difference between pre amp and power tubes? and how much do they cost to replace??



Preamp tubes are low level, small signal tubes. They typically amplify the signal only enough to send it to the power tubes, which are responsible for amplifying the signal enough to feed the output transformer and speaker(s). Some preamp tubes have been designed into amps using them as the final stage of amplification - see the Fender Champ 600.

How much they cost to replace depends on several factors - the type of tube and whether it's NOS (New Old Stock) or modern manufacture. NOS tubes are popular with a lot of guys, because they are tubes leftover from the 50s, 60s, 70s and very early 80s. These older tubes used better construction techniques, which typically makes them sound better and more desirable. More desirable equals more money. Newer tubes can also get expensive, if they've been cryogenically treated or tested on special equipment. I realize this is more than you asked for, but hopefully you learned something. To give you a rough estimate, I would anticipate that you could retube your preamp section for $15 to $25 a tube and the power section for $20 to $50 a tube using current production tubes, in other words, new tubes.
#11
Quote by KG6_Steven
If only it were that simple. This test is only valid for testing tubes for microphonics. A microphonic tube has the potential to break into oscillation. However, do keep in mind that ALL tubes are microphonic to some extent - even new ones. So, by your test, you'd throw away a lot of new tubes. The idea is to rule out tubes that are excessively microphonic AND therefore will likely cause problems.

Your method does not test tubes for emissions or anything else that a real test set would be utilized for. Most of us cannot afford tube testers, so we use our ears and other methods to determine when tubes are worn and need to be replaced. Also, keep in mind that some testers do not perform very well at indicating real world conditions, so they may give a better picture of health than actually exists - provided you can find someone with a tube tester.



Please don't read this as me being sensitive or offended:

I am well aware that this test is purely for microphonics. That is why I said it does not rule out that the tube is bad. If this amp was brought to my bench I would definitely use my tester and at least check the emissions of the tube. Any tube exhibiting signs of overt microphonics I consider trash. I was giving just giving the OP a simple yet definitive in one direction test. *You thump it, you hear a thump, you replace it.*

Since his amp is "making a funny noise" every 15 seconds, I immediately thought "He might have a microphonic tube," and so I suggested this to him.
#12
Quote by end_citizen
If you can hear it through the speaker, that tube is definitely bad. If it doesn't make a sound, it doesn't rule it out but no harm done.

Changing preamp tubes (all 12ax7s in this amp) is very easy. Just take out the old one and put the new one in. The power tubes should be put to a guy who knows how to bias a valveking.



According to what you typed, you said "if you hear it through the speaker, the tube is definitely bad." I can almost guarantee you that nearly any preamp tube will be microphonic to a degree. That's just the nature of the beast. Using the tap test is used to separate the overly microphonic tubes from the normally microphonic tubes. Perhaps you mistyped in what I quoted above. If so, no harm, no foul.

The Peavey VK112 is fixed bias. No adjustment can be made on this amp. Normally, you just specify to the vendor that you own this amp and they send you the appropriate set of power tubes.
#13
thanks for the advice guys, some of it goes a bit over my head but its always good to learn new things