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**Enzo** · 02-26-2014, 02:52 AM

So Artist, not Artist VT?

If you have tubes that gently glow blue, it is normal for them to pulse or blink when sound impulses pass. Watch them as you play.

If it really is thermal, then a heat gun is made for this. Literally. So good you went there.

When the chassis is in the amp, it hangs. The transformers hang, everything hangs. On your bench usually the transformers sit on the bench with the chassis pressing down on them. yes? no?

Make sure the mounting hardware is snug on the transformers. Loose mounting bolts can in fact cause crackle noises. That is one possible difference out of the cab.

Look at the phase inverter, the schematic has voltages all around those transistors, any way off?

Since you can't force it, look at potential spots: look at the 400 ohm (390) screen node resistor at the stdby switch. and the switch itself and connections. The bias pot is a suspect.

If it semi-reliably fails in the cab, try salting the mine. Tack a good filter cap to ground from the bias split, where the two 220k come together. That will soften noises coming down the bias supply. Or tack a largish cap - 0.1uf comes to mind - from grid to grid of the tubes. In other words cap used to shunt the ends of the 220k resistors together. That would kill any differential signal to the tubes. If the noise remains it must be in the power tubes. You have the two flyback diodes, one could be breaking down. Hit it with freezit or heat to see, or just unsolder one end of each and lift them. Noise still there with them out?

The phase inverter runs off 300v, except the bases which are biased by the +24 from the preamp. That preamp supply could be at fault. Before I realized what I was doing I was going to suggest disconnecting the power from the preamp. Lower left on preamp page, lifting the diode or the 100 ohm, but then I saw we needed that for the phase inverter. You could tack a wire to the +24 amd trail it out when the chassis goes into the cab, then watch it to see if it carries the noise. Or disconnect the supply and feed +24 from a bench supply into the phase inverter, leaving the preamp unpowered.

Even if the symptom won;t evidence itself pout of the cab, we are not powerless to probe it.

**bsco** · 02-26-2014, 07:46 AM

Hi Enzo. Thanks for replying...It is an Artist VT....I have the chassis hanging up-side down the same way it would be if it was installed in the cabinet....and yes, the transformers are what is supporting the chassis..I have a couple of blocks underneath them to allow some space between the bench surface and the tubes..I will check the mounting hardware to make sure it is good and tight...I'll check the voltages on the phase inverter......but as long as the amp is working fine won't the voltages be pretty much normal...I tried the heat gun but never tried the freeze compound.....I was thinking it was related to heat...so I figured it wasn't hot enough which led me to use the heat gun.....I'll check a few things out and let you know what I find....It is 4 AM here now...couldn't sleep...I might go downstairs at check it out for awhile....I did notice that when the amp did make the noise, I had switched it back into stand-by...and when I switched it out of stand-by, this noise was still there...it could be in stand-by for awhile and when you switch it back to play mode the noise re-appears, however, if the amp is turned off and allowed to sort of cool down, then the noise is not there until it heats up enough again...this is why I was thinking it was a heat related problem while being installed inside the cabinet.....I'll go through your suggestions here and follow along on the schematic.....I'll let you know what I find out. Should be a good challenge!!
Cheers,
Bernie

**Enzo** · 02-26-2014, 08:12 AM

It may not be what is wrong, but understand that sitting on the transformers - as we all do it - means they are not "hanging" they are simply under it, the amp sits on them instead of them hanging from the chassis.

The voltages on the phase inverter may be OK, but by bringing out a wire from that 24v for example, we might find that when your noise occurs, that 24v goes nuts. This is scope work, your volt meter would never catch noise unless the supply is just collapsing.

I have no idea what is making your noise, we are trying to isolate it to some subsection of the amp.

I'll look at the VT then, I was working from the Artist schematic before. They are different. There is no 24v supply on the VT, it does not have the transiostor preamp of the Artist. The Artist VT has an op amp preamp.

The VT also has loop jacks. PLug something into the PA IN jack, that will disconnect the preamp. Does the noise continue or not - that tells us which half the noise is in. The preamp runs on +15 and -15, the phase inverter uses the +15 instead of the +24 in the older unit.

A thermal related problem is one that shows up under certain temperture conditions. That is some are there until it wrms up and some are there after it warms up. But APPLYING heat and cold is a way to apply thermal shock. EVen things that are not necessarily thermally sensitive may still react to a sudden chill. It is just another tool in the kit.

**bsco** · 02-26-2014, 09:01 AM

Yeh...the VT has plus and minus 15 volts...I noticed that....I had jumped out the loop jack with a jumper wire....it still happened...but I will just plug in a jack to that input like you said....to break the connection....I will run two wires from the 15 volt supplies and then scope them to see if the noise is associated to the power supply....I might have to lay the chassis back inside the cabinet to get it to act up.....thanks for the tips,
Cheers,
Bernie
Just a note...when this noise does happen it is pretty loud....My wife and I were upstairs when it first happened and it scared the daylights out of her.......

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