Suhr Custom Audio PT50 problems

P

poeticprizm

New member
Hey folks, first time on this forum!

My Suhr Custom Audio PT 50 has started giving me problems:
- A loud pop in the speakers whenever I try to turn on, or turn off my amp
- A sudden droop in volume after about 1 min.

I initially thought it might be the power tubes, and put in new ones (JJ EL34s). Same issues. It's something else.

I had bought this amp used very recently, and it had been working great for about a month.
I had always used the amp in a 4CM configuration.

Just this one time, I decided to plug in my guitar directly to the amp, but unfortunately I had left the "in active" switch in the back engaged (i.e. I ended up enabling the effects loop without having anything connected). Right after that, things went bad.

I'm starting to freak out - this is not looking good. Anyone else experienced such issues? Did I blow my transformers?

Thanks much!
 
LP - you were right - it was one of the preamp tubes. I think one of the tubes wasn't seated properly. I pulled out each preamp tube, examined them (they all looked fine), and after reseating them - problem solved! Thanks!

One interesting thing I noticed as part of this experiment -
The JJ's produce a more creamy sound as compared to the Valve Art tubes (channel 2). But, they are less reactive to the guitar tone knob, at lower volumes. After the volume is cranked up above 4 or 5, they start to react normally.

This is the first time I've actually been able to do a A/B with power tubes, and the difference is noticeable!
 
Yep, everything in the chain will make a difference. Some more than others. Just curious, did you bias the amp when you changed power tubes? That is essential for the amp to operate properly.
 
Hi LP, no, I didn't even know about this biasing. Sounds easy enough to do, but I can't seem to find any info on the recommended range.. would you happen to have that info?
Thanks again.
 
You first need to know the plate voltage before you can set the bias. I'd recommend taking it it a tech if you aren't familiar with biasing an amp. There are lethal voltages that you don't want to come across. If you feel confident I'd recommend picking up a bias meter like the Weber Bias Rite.

http://www.tedweber.com/bias-rite-instructions/
 
LP Freak":3exucweb said:
You first need to know the plate voltage before you can set the bias. I'd recommend taking it it a tech if you aren't familiar with biasing an amp. There are lethal voltages that you don't want to come across. If you feel confident I'd recommend picking up a bias meter like the Weber Bias Rite.

http://www.tedweber.com/bias-rite-instructions/
Click to expand...
Absolutely no need, it is built in to all our amps
You just need a DC Voltmeter.
Measure .034VDC approx. across the smaller 1 ohm resistors on the power tube sockets. Adjust the bias for that within .002VDC
Be careful, use alligator probe clips on the sockets and don't touch anything with your hands or drop a screwdriver in there. If in doubt see a tech, it is a 15 minute job including removing head.
 
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