SV20h doesn't sound right - help, please

[Khalkin G. Nomonhan]

I was playing my SV20h last night at practice. It sounded as usual (great) for a few minutes while we were playing. All of a sudden, the amp lost volume and a lot of bottom end. I checked cables, etc... plugging straight in. I was able to turn it up to rehearsal volume, but it is more distorted than before, but not greatly so. I am thinking one of the EL34s?

Thoughts from the assembled sentient database here on the forum?

 

[marshallmellowed]

I was playing my SV20h last night at practice. It sounded as usual (great) for a few minutes while we were playing. All of a sudden, the amp lost volume and a lot of bottom end. I checked cables, etc... plugging straight in. I was able to turn it up to rehearsal volume, but it is more distorted than before, but not greatly so. I am thinking one of the EL34s?

Thoughts from the assembled sentient database here on the forum?

Yes, I'd say either the PI tube, or one of the output tubes are failing.

 

[Old Punker]

Tubes are the first thing I would check too, at least after checking your connections as you've already done.

 

[Khalkin G. Nomonhan]

Which one of the three ECC83s is the phase inverter?

Thanks.

 

[Dogs of Doom]

Which one of the three ECC83s is the phase inverter?

Thanks.

 

[Khalkin G. Nomonhan]

View attachment 127979

Thank you!
I popped a new GT 12ax7 into the PI position. Not only did the strange sound disappear, but the bottom end appears tighter sounding. Over all, the sound is cleaner than it was before, which leads me to believe that this tube has been troublesome all along.
Thanks again, everyone, for your help.

 

[Dogs of Doom]

Thank you!
I popped a new GT 12ax7 into the PI position. Not only did the strange sound disappear, but the bottom end appears tighter sounding. Over all, the sound is cleaner than it was before, which leads me to believe that this tube has been troublesome all along.
Thanks again, everyone, for your help.

typically, you follow the signal path.

Starting w/ where you plug in the guitar, the tube closest to the input, is V1. V1 usually has a metal sleeve to eliminate electronic interference. The next in line is V2, V3, etc...

The one closest to the power tubes, would be the PI (phase inverter). This is because the PI is actually, the input of the power amp section (it's a power tube). The PI can be in-line w/ the other pre-amp tubes, but, also can be either offset, or even not in-line at all w/ the pre-amp tubes. Also being close to the power tubes, it's also typically closest to the output transformer.

In this case, V3 is the PI.

There are exceptions, such as the JVM, which has a weird sequence, but no need to worry about that for this amp, or the SC, etc...