Lead channel not working, not valve problem?

[keir118]

Hi all. I recently sold a PRS MT15 and the buyer recieved it with an issue.

Firstly the head came with a big hole in the box and the 2 metal hinges on the front had snapped off. This would suggest it has taken a rather hard knock during postage.

When plugged in the clean channel works perfectly, however the lead channel is extremely quiet and sounds fizzy, as if the amp was turned to 0.

What he has tried: different cabs
different guitars
different cables
new power tubes
swapped the pre amp tubes about

He is currently waiting on replacement preamp tubes to test them, however no matter the order of the pre-existing pre amp tubes the lead channel would not work right.

The amp does get slightly louder when the master is turned to full but still quiet enough to talk over, and he noticed that the treble knob seems to have certain positions where the volume goes back to normal working levels but the tone remains fizzy.

Just wondering if anyone else has any ideas on what to check for before handing it over to a tech?

Many thanks!

[g1]

he noticed that the treble knob seems to have certain positions where the volume goes back to normal working levels but the tone remains fizzy.

This amp has separate tone controls for each channel, yes?
The impact may have cracked or damaged one or more of the controls.

[Jazz P Bass]

Here is a 'tube chart'.
I agree with G1 that a control took a hit.

[Mick Bailey]

If the amp has a regular FMV tone stack all of the premp signal is taken off the wiper of the treble pot so what you are describing is consistent with a bad pot, connection or solder joint. A flat-earth model of the treble pot that makes it easier to understand is that you have a resistive track and one end of the track gets fed with the bass and mids and the other end gets the treble. The wiper can sweep anywhere between the two and connects to the next preamp stage, but If the bass side of the track gets disconnected you get a thin, fizzy sound because you now only have the treble content. Maybe the pot is connecting intermittently but when it does, only on the treble side.

With a high-gain channel you can usually still hear a ghost sound even of the pot is completely disconnected because the signal is often coupled by the capacitance of either a circuit board or wiring, but it will be very low volume and fizzy.

[glebert]

Did you insure it? Seems like a claim is in order if you did.

[keir118]

If the amp has a regular FMV tone stack all of the premp signal is taken off the wiper of the treble pot so what you are describing is consistent with a bad pot, connection or solder joint. A flat-earth model of the treble pot that makes it easier to understand is that you have a resistive track and one end of the track gets fed with the bass and mids and the other end gets the treble. The wiper can sweep anywhere between the two and connects to the next preamp stage, but If the bass side of the track gets disconnected you get a thin, fizzy sound because you now only have the treble content. Maybe the pot is connecting intermittently but when it does, only on the treble side.

With a high-gain channel you can usually still hear a ghost sound even of the pot is completely disconnected because the signal is often coupled by the capacitance of either a circuit board or wiring, but it will be very low volume and fizzy.

I understand what you are saying, thanks very much for that

[J M Fahey]

Why didnīt buyer reject unit as damaged (it is) and claim insurance?