Ampeg SVT Classic Bias Light issue

[mac1amps]

I've also found it to be worthwhile to upgrade the 12AU7's to 12BH7's (used in the original "classic" SVT's). I've serviced a number of the newer SVT's and found the 12AU7's to fail more often that the 6550's, whereas the 12BH7's hold-up much longer.

[TimmyP1955]

www.mac4amps.com no workie (05/04/2015 at 5:35pm eastern)

[tal35]

Well, I finally got a chance to check bias, all tubes were within 6ma of each other which is pretty good. I did try to move the tubes around to match them as close to each other. No difference. The red bias light is on on the right side, I have 2 of the ampegs SVT Classics that do this, both the same channel. Ecc82's and 83 in proper sockets, also new EH brand and mullard brand 6550's. the other amp has Ruby drivers and Sovtek 6550EH I believe. They both sound good, just not sure why the red light will not go out , stays on even with the green bias light on.

[TimmyP1955]

If swapping the tube triplets side-to-side changes nothing, I'd have a look at the screen resistors (on the socket board) and the plate resistors (on the main board).

[tal35]

Ampeg

Changing the tubes around had no effect, the 220 ohm resistors are all new on the tube board, what value are the ones on the main board? 47k 3 watt or so? If so, I had checked them and they were ok. Sorry I dont have the schematic in front of me.

[TimmyP1955]

The ones on the main board are 10 Ohm.

[tal35]

classic

The ones on the main board are 10 Ohm.

Are they the 2 block resistors that are 5 watt?

[TimmyP1955]

I believe there's a group of four physically in parallel with each other, and a group of two.

[nevetslab]

Actually they are grouped in a cluster of four and two, but are discrete...one cathode resistor per tube...NOT in parallel with each other. I marked up one of the photos earlier in this thread to show them.

The two cement style Block resistors are on the SVT-VR amp...NOT the SVT CL, AV or Heritage amps. Common cathode resistors per half....with each tube having a 5W Plate resistor.

[nevetslab]

Sounds like you have circuit issues with the High Side of the window comparator that drives the RED LED on that one side. You could pull the power tubes (mark them first) and feed low DC voltage (mV range) to the sense circuit....comes off each of the 10 ohm cathode resistors. GRN range is 147mV thru 294mV. Above 294mV, you're into the RED LED range. Below 147mV, NO LED's should be lit.

[TimmyP1955]

I meant physically in parallel, not electrically in parallel :-)