Ampeg SVT-ii Pro No output w/EQ on

[ca7922303]

Swapped 3 for 3 6550s and red/green bias lights are now at bias control 2. With the output being normal and strong like it is, how do I weed out the bad power tube? Won't be able to see the bias lights without 6 tubes in?

[ca7922303]

I've got 5-GE 6550As and 1-Shaugang 6550B(china). Is this mismatch causing the bias light to stay red)green. Also, I believe it sounds better without the 6550B in there?

[nevetslab]

Usually I can spot the troublesome 6550 power tube by it's cathode current. Did you go thru the process of checking one pair of power tubes at a time as suggested? Set the first pair up for 22mA each (220mV across the 10 ohm cathode resistors on the main Power amp board. You need to stand the amp up on it's side, with the bottom cover off so you can measure it between Ground (the four resistor leads of V1, V2, V3 & V5 cathode resistors along the edge of the PCB is ground. Other side of those resistors is where you measure the voltage drop. The reason I try to preset the bias voltage, (which will take a few passes to get it there) to then reveal just how far out of balance the tube's transconductance is. If you're lucky, you can get the cathode currents of all six to be relatively close.

As far as if one or more are prematurely distorting, you can also measure the voltage across the cathode resistors under drive. In this mode, you can see the current sharing of all six tubes, and the 'bad' tubes can be seen in that mode.

I normally will mount a solder lug at the corner of the main PCB support screw to hang my DMM and scope probe on, as getting a place to hang your probe ground is otherwise a PITA.

Let me see if I can add a link or two of some previous threads I've posted in this sort of servicing.

[nevetslab]

Here's a previous post I made Ampeg SVT Classic - Music Electronics Forum (music-electronics-forum.com)

Here's another....this one shows using the scope probe across the cathode resistors together with displaying the output. The scope probe will show upper or lower half-sine wave,....to reveal current sharing of the six power tubes. Ampeg SVTCL - Music Electronics Forum (music-electronics-forum.com) I had tack-soldered small lead-wires formed into test loops at each 10 ohm cathode resistor of the power tubes.

[ca7922303]

Here's a previous post I made Ampeg SVT Classic - Music Electronics Forum (music-electronics-forum.com)

Here's another....this one shows using the scope probe across the cathode resistors together with displaying the output. The scope probe will show upper or lower half-sine wave,....to reveal current sharing of the six power tubes. Ampeg SVTCL - Music Electronics Forum (music-electronics-forum.com) I had tack-soldered small lead-wires formed into test loops at each 10 ohm cathode resistor of the power tubes.

I was able to move the 6550B to a different socket and now have green lights on both bias controls. Thanks for all the help. Appreciate it.