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SVT low wattage problem!

1/11/2000 4:53 AM
Mark Abbott	SVT low wattage problem! I would appreciate any help offered to help me get my brother's early 80's SVT performing correctly. The problem is the amp puts out 288 watts RMS (96 volts P to P across the 4 ohm load resistor, no visable clipping from 1Khz sine wave). Is this normal, or should the amp deliver the 350 watts mentioned on the chassis? All tubes have been replaced in the output stage (power amp), Svetlana SV6550c (plat match from New Sensor), NOS 12bh7's Sylvania's, and the 12AX7 is a Sovtek WXT+ ps. Mods done to the amp are listed below. Basically, I have heard the early 80's SVTs weren't as good as the earlier models, I replaced the coupling caps with Orange drops 715 (I received a reply from Groove tubes saying the Jap SVTs used inferior components). I replaced the bias and balence pots with ten turn pots (other techs who had worked on the amp prior to myself used silastic or glue to keep the pots from moving and bias drifting). I carried out Kevin O'Connors mod where he replaces the 22 ohm resistor, and diode going to the screen grids with a 1K 5watt resistor. K O'C claims this increases tube life. I did think the clamping diodes at the input of the power stage (driver) was causing the clipping, but I removed them and the output was the same. I also tried setting the amp up at different bias currents (50 mA, 60mA, "72mA", and 85mA), once again no significant increase in output (2 to 3v pp). All advice welcome. Much thanks Mark.

1/11/2000 4:53 AM

Mark Abbott

SVT low wattage problem!

I would appreciate any help offered to help me get my brother's early 80's SVT performing correctly.

The problem is the amp puts out 288 watts RMS (96 volts P to P across the 4 ohm load resistor, no visable clipping from 1Khz sine wave). Is this normal, or should the amp deliver the 350 watts mentioned on the chassis?

All tubes have been replaced in the output stage (power amp), Svetlana SV6550c (plat match from New Sensor), NOS 12bh7's Sylvania's, and the 12AX7 is a Sovtek WXT+ ps.

Mods done to the amp are listed below.

Basically, I have heard the early 80's SVTs weren't as good as the earlier models, I replaced the coupling caps with Orange drops 715 (I received a reply from Groove tubes saying the Jap SVTs used inferior components).

I replaced the bias and balence pots with ten turn pots (other techs who had worked on the amp prior to myself used silastic or glue to keep the pots from moving and bias drifting).

I carried out Kevin O'Connors mod where he replaces the 22 ohm resistor, and diode going to the screen grids with a 1K 5watt resistor. K O'C claims this increases tube life.

I did think the clamping diodes at the input of the power stage (driver) was causing the clipping, but I removed them and the output was the same.

I also tried setting the amp up at different bias currents (50 mA, 60mA, "72mA", and 85mA), once again no significant increase in output (2 to 3v pp).

All advice welcome.

Much thanks

Mark.

Replies:

Psycho Bass Guy What are the power supply filter ca... -- 1/11/2000 7:33 AM
Bruce I've only worked on one of the newe... -- 1/11/2000 7:53 AM
Uncle Ned >The problem is the amp put... -- 1/14/2000 3:08 AM
JB I remember an old Ken Fischer artic... -- 1/14/2000 7:22 PM