Svt 3pro loss of power

bugsy · #1 03-09-2011, 05:09 AM

After changing my tubes to jjs, I have now a sudden massive loss of power!
My clipping light doesn't come on at all even with everything maxed
Something is wrong. Anyone help?

bstringrandy · #2 03-09-2011, 05:23 AM

I'd say power it up with the top removed and check that the tubes light up. If you still have the original tubes, you can then swap them in one at a time to see if you can isolate to a bad tube.

If that doesn't work, it sounds like you might need a MOSFET bias adjustment. It's a sensitive procedure that I would leave up a tech unless you're comfortable around electronics!

Good luck!

Jerrold Tiers · #3 03-09-2011, 06:50 AM

Bias is not affected by changing tubes.... and it causes distortion, not loss of power, in general.

Possibly the 12AU7 went into the wrong spot..... the 12AU7 is a rather different part to the 12AX7, and will not produce the same performance if substituted, particularly in the preamp.

Does it work right if the OLD tubes are put back?

If not, then quite possibly some weak solder joint was broken by the wiggling and pulling associated with a tube change.

bugsy · #4 03-09-2011, 02:02 PM

thanks guys
will do a swap of tubes back, but its a bit disappointing as i just brought these JJs
I will swap them one at a time

RickenBoogie · #5 03-09-2011, 05:28 PM

It's not necessary to change preamp tubes in a hybrid amp unless they are broken. Obviously, something went wrong when you swapped the tubes. A set of preamp tubes can last for decades.