PRS MT-15 v1 amp + PRS 1x12 MT cab

[AlbertA]

I got the PRS MT-15 v1 about a month ago ($for 400) and I've been looking for a speaker cabinet to go with it.
This past week a PRS MT cab popped up for $190 so I snatched it up. It's a 1x12 with a Celestion V30 16Ω.

And wow this amp was made for a V30 - when I tried my Archon Stealth cab (Celestion v-type) the amp was too "spitty".

It gets loud real soon though - I can barely touch that master - The V2 seems better in this respect so I will look into modifying that.
It has a half-power switch that does drop volume but I don't like what it did to the tone - Full power for me.

Here's a short demo with a Ibanez JS1200:

 

[AlbertA]

The PRS MT 1x12 Cab





Here's the impedance curve I measured, nice peak at 100Hz.

 

[AlbertA]

PRS MT-15 Amp gut shots


JJ 5881 Power tubes
The amp was biased at 24mA, a little too cold for me - I increased it to 29mA.
The amp runs the plate voltage at ~425V.



Looks like an soldering iron burn - I guess someone's been around here.


And this is probably why I didn't like what the half-power switch did to the tone - I believe it switches this resistor together with the connected speaker load.

 

[AlbertA]

Loop Noise

Yeah it's a bit noisy - probably the first thing I will address.

• 00:00 - Amp off - just a background noise to give you a reference
• 00:11 - Amp on - standby on
• 00:37 - Standby off - amp is now fully on, nothing on the loop
• 00:53 - Connecting to an Axe-Fx 3 using a humbuster cable for the return.

 

[AlbertA]

I tried a 12AU7 preamp tube at V5 which drives the FX loop- no change to the amount of hum when using the FX loop.

You can hear its 120Hz hum.

Same deal as before:

• 00:00 - Amp off - just a background noise to give you a reference
• 00:10 - Amp on - standby on
• 00:40- Standby off - amp is now fully on, nothing on the loop
• 01:03 - Connecting to an Axe-Fx 3 using a humbuster cable for the return.

 

[Alex Kenivel]

Cool, kinda. Sucks about the noise. Mine doesn't seem to have that issue. Do you have a date in yours? IIRC the build date in mine is April 2021.

 

[AlbertA]

Cool, kinda. Sucks about the noise. Mine doesn't seem to have that issue. Do you have a date in yours? IIRC the build date in mine is April 2021.

Based on the serial number sticker on the back, looks like 03/22

I'm going to replace the wire harness that goes to the FX loop PCB with some shielded cables and report back.

 

[AlbertA]

Great - looks like whoever was here broke those traces for some reason.

V5 is the socket for the tube that drives the loop.

 

[Alex Kenivel]

Great - looks like whoever was here broke those traces for some reason.

V5 is the socket for the tube that drives the loop.


View attachment 51348

Lame, but fixable? I've broken traces and made jumpers to fix it. Maybe some dabs of solder?

 

[AlbertA]

Lame, but fixable? I've broken traces and made jumpers to fix it. Maybe some dabs of solder?

Actually after some reading looks like they did this rework at the factory to address grounding issues.

So back to diagnosing.

 

[CorrosiveMatter]

I saw a video on the tubes where a guy rerouted stuff and made the amp silent, if you have not seen it and want I can find it and link it.

 

[AlbertA]

I saw a video on the tubes where a guy rerouted stuff and made the amp silent, if you have not seen it and want I can find it and link it.

Thanks! I did watch it actually and I have replaced the FX loop cables that go from the mainboard to the FX loop PCB with shielded ones but no change to the 120Hz hum.

I did notice inserting an open cable to the return jack is not muting the incoming preamp at all and it's supposed to be a serial loop. So there's definitely something going on there - I'm gonna have to trace the circuit to see what's up.