Vintage Asylum

Posted by 1973shovel (A) on May 08, 2017 at 08:25:56

In Reply to: RE: Dynaco SCA-35 replacement boards problem... posted by Eli Duttman on May 07, 2017 at 12:01:38

In general, you certainly may be right, Eli. But as noted in Gillespie's installation manual (copied and pasted below), that may not be the case with the SCA-35. That's one reason I suggested the original poster refer to Gillespie's guide when installing his boards.

"10K resistors - why are these necessary?

While, in many amplifiers, a 6GH8A or 6U8A will perform well as a replacement for the 7199, this is not always the case with the SCA-35. The bias and feedback circuit is somewhat unique, and specifically suited to "original" 7199 tubes. The 10K resistors are installed in place of jumper wires on the bottom side of the PC-10A. These are necessary because the 6GH8A or 6U8A are not exact replacements for the original 7199. The resistors will prevent instability issues which may occur as the amplifier is just driven into clipping. As well, additional phase shift components are included on the PC-10A to prevent spurious oscillations which may otherwise appear in some cases when using a 6GH8A or 6U8A."

End quote.

Whether or not these "additional phase shift components" on Gillespie's boards are installed on the original poster's Chinese boards is something he should have investigated in advance, not to mention the likelihood of counterfeit components being used on his ebay boards.

I wasn't aware that Dave had these boards available for the SCA-35, so I thank the original poster for inadvertently making me aware of that. That way I'll have two more 7199 tubes to add to the precious few I have to keep on one of my stock ST-70s running a little longer.

• Dave's Installation guide   (Open in New Window)