SVT-CL power tube board question

[gjb1956]

Kind of in the weeds, but...

Working on a made in China CL, so newer than the US ones I've worked on before. The power tube board is fitted with all the components marked, but update TIB0003 and later schematics I have seen say "DO NOT STUFF" or to remove the 1N4007 diodes across the 220 ohm Screen resistors (which used to be 22 ohm). As this amp is newer than any schematics I have, I need to ask if this has changed. Why are these diodes back in the circuit, or why were they removed previously? I am inclined to leave them alone, but it makes me curious.

Thanks for any insight.

 

[Wasnex]

Ref: see attached Service Bulletin:

AFAIK the higher value resistors are intended to extend tube life.

The diodes were intended to protect the resistors, however the result may be burned solder traces. Better to let the resistors act as fuses IMHO.

See attached datasheet for resistors:

 

[beans-on-toast]

Your amp looks like it was (poorly) modified to add the diodes. I’ve seem CL’s from China and Vietnam, they didn’t have these diodes in place. I would remove the. Current Ampeg amps that have the 6X6550 configuration all show the diodes on the schematic but indicate do not place. Early revision had them. As far as I know, once removed, they didn’t come back.

 

[agedhorse]

Your amp looks like it was (poorly) modified to add the diodes. I’ve seem CL’s from China and Vietnam, they didn’t have these diodes in place. I would remove the. Current Ampeg amps that have the 6X6550 configuration all show the diodes on the schematic but indicate do not place. Early revision had them. As far as I know, once removed, they didn’t come back.

Yeah, that's a real hack job...

 

[JuanB]

Yeah, that's a real hack job...

That's generous.

 

[gjb1956]

Gents,

Thanks for affirming my suspicions. I appreciate your observations that the modification somehow doesn't seem up to par. I also agree that the resistor should be more than adequate as a "protection device" in an untimely event.

Much appreciated and thanks again,

 

[beans-on-toast]

The switch to 220 ohm 1/2W flameproof resistor is intended to help protect the tubes. Don’t use a higher wattage. It will blow to protect the tube if necessary. Flameproof is important.