Express Build

[ampbldr2]

I got to try out the amp with a 8ohm cab today and it is very hissy like it needs lead dress attention. I also get a lot of hum out the speaker in standby.

Now that i know I have the ot/pt wires correct I was thinking I will cut to size. I also twisted the two CT wires on the PT which may have not been the right thing to do. On a scope everything looks clean so I am not going to find it probing around.

[romberg]

I got to try out the amp with a 8ohm cab today and it is very hissy like it needs lead dress attention. I also get a lot of hum out the speaker in standby.

The hiss you may not be able to do a whole lot to eliminate. This is a high gain amp and some/most of this hiss is due to thermal noise and is normal in a high gain amp. The hum is another matter.

Does it hum with no chord plugged into the amp? If the hum is gone when you unplug from the amp then the problem is outside the amp. Otherwise it is inside. Always start with the simple things first. Some 12ax7s hum. They have poor insulation between the heaters and cathodes. So, they can inject hum into the amp all on their own. Try replacing/swapping pre-amp tubes (especially V1) and see if that does any good. If not, it is probably wiring.

The only thing that jumps out at me in your wiring is the buss soldered directly to the back of the pots. Doing this is not the best setup for a high gain amp as it creates many little ground loops which can pickup all kinds of noise (like hum). If a tube swap does not cure your hum I might take a look there.

Mike

[aceofbones]

Hey ampbldr2,
I also have transformers from RJ and wasn't sure which way around the primaries went. I left them long so I could switch them if I have a problem. Do you have red to V5 and brown to V4 or did you switch them so brown is V5 and red is V4. Let me know which way is working for you and I'll change mine to match.
Thanks

[Colossal]

As Mike said, hiss is normal with these amps. They are very gainy and everything is right at the brink of instability. However, you should not have hum and especially in Standby. These amps are VERY sensitive to field effects and will sniff out and couple with your pickups from a distance. The amp should be stable on the bench with the shield plate off, but you might put a shield plate on to try to rule out that your room is the source of the hum (fluorescent lamps, etc). An interesting feature of the Express is the power supply compression. You will hear the hiss drop and rise back up when you smack a chord. The hiss will disappear into the noise floor at playing volume. You may need to roll preamp tubes through your amp to find those that give you the performance and tone you want.

[ampbldr2]

Hey ampbldr2,
I also have transformers from RJ and wasn't sure which way around the primaries went. I left them long so I could switch them if I have a problem. Do you have red to V5 and brown to V4 or did you switch them so brown is V5 and red is V4. Let me know which way is working for you and I'll change mine to match.
Thanks

The correct Primary wiring for RJ's "Pacific Wreck OT" is Red to V4 and Brown to V5. CT is blue to the Standby.
That is what worked for me.

http://www.rjguitars.net/product/pacifi ... j-pac-415/

[ampbldr2]

I reached out to RJ about the hum. This post explains https://ampgarage.com/forum/viewtopic.php?t=17505
I unbolted the OT and Power on / stby off with no tubes installed and can move the OT closer into the Magnetic Lines of the PT and the hum is greatly reduced.

I messed up not doing the headphone trick to find proper placement. I will try to sort out next few days.

[aceofbones]

Awesome, thanks very much. I'll have to swap my red and brown. Cheers.