PRS MT15 Lunch box amplifier by Paul Reed Smith.
This little amplifier came in via a long standing customer. I don’t mean he stands up for extended periods, but I have known him for a long time.
The fault was a little odd and was associated with the CLEAN channel. Customer commented that the channel had a strange sound and when the volume control was returned to zero, there was still an output.
With the bottom cover removed, we could see a very nice PCB with components facing you. Always a good thing unlike some makers who shall not be mentioned as I am currently in a good mood. This sort of problem can be caused by a broken potentiometer or cracks in PCBs etc.. However, this was not the case here. Every potentiometer measured ok and worked correctly. With a signal connected and a scope looking at the output, you could see a signal output with the volume control set to zero. As the control was advanced, the output reduced a little and then gained and shifted phase. Nothing was obvious. So, with begging bowl out, I sent an email to PRS in the USA, asking if there was a chance of a diagram, as nothing was available on the national web service. Normally, manufacturers are quite reserved in providing information unless you are an authorized dealer, as I am for companies like Victory, Mesa and others. With my expectations low, a couple of hours later, an email arrived from a very helpful guy at PRS with a schematic attached. I promised to keep it for my eyes only.
The circuit is quite straight forward all very nice. Now we could do some proper testing. There seemed to be some odd things happening around V2 which is in the CLEAN channel’s tone stack and buffer circuit. There seemed to be a bleed of signal between the 2 sections of the Triode valve, almost too much gain. Matters were made significantly worse by pulling the boost gain control. I then noticed that some resistors in this circuit may have been changed. Well, a long head scratching session, the resistors were replaced with correct types and the 12AX7 replaced with a 12AT7. This resulted in the channel working correctly, boost or no boost enabled. A very nice sounding amplifier.
So gentlemen, for what it’s worth, this little MT-15 wins the Zeta services award for the best amp of the Week. It is nicely made, the main PCB is good, the components used are very decent and the resistors are all well rated, unlike some of the silly tiny things that seemed to be favoured today. Well done Mr. Paul Reed Smith and a thank you to the support department for their help.
