Wednesday, May 11, 2011
Vintage Parts Excursion Crashes and Burns
I went to an auction Saturday and bought four cabinets full of vintage capacitors-Mallory Gems and Sprague Black Beauties, so I thought it would be nice to outfit the Redhead with a suite of cool old vintage capacitors and see how it sounded.
Well. It barely worked at all. No volume until about half way up and then it let out a strangled squawk-or maybe it was a burp.
I removed the chassis from Brownie and laid them out side by side on the workbench and proceeded to retrofit the Redhead with exactly the same values in the same places, adding and removing nice new parts-yellow polys as required.
In doing so I also took the opportunity to validate my board layout and bring the Redhead back to Brownie configuration. At that point I also found that what voiced the second channel in the original Redhead configuration was a 150k resistor in series with the first grid on the second preamp tube, and a small silver mica capacitor bridging the channels. Both items were noted and removed. One .1 uf capacitor was added in the same place it exists on Brownie.
The result? It sounds just like Brownie, only a little heftier because of the bigger output iron.
The Bad Betty speaker sounds pretty good as well.
So the board layout is complete, noted and checked for accuracy, and I can now concentrate on the connections to the tubes and controls. At that point I can proceed to start a schematic.
I guess what I learned is to check stuff carefully before installation, especially if it's old stock.