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RE: Schematic of Properly Designed Amp

Posted by drlowmu (M) on September 3, 2008 at 07:43:39

In Reply to: Schematic of Properly Designed Amp posted by MikeT on September 3, 2008 at 07:37:33:

Hi Mike T,

Nope, I wonder if any exist, probably so.

Fix what you own. You can simply change op points of your 300B DRD and get it to be really nicer sounding and long term reliable. Here is a good way to salvage what you bought:

Three steps will help it tremendously: Under $15 in parts cost.

1) Add a CL-80 thermistor in series to either one of the power transformer's primary legs. This is a mandatory first step for reliability.

2) Remove and replace the two Rs coming off the H.V. FW bridge, ( 2.7 ohms and 47 ohms) with one Triad C-40X choke (Allied, $10.79)

3) Readjust the current in the 300B to read between 0.5 to 0.7 VDC across ten ohms, and NOT 0.9 VDC . Use your ear to determine the sweetest bias point in that range. Readjust, if necessary, the second slider that sets the 220 VDC.

This will considerably reduce your 300B's plate dissipation to give you tens of thousands of non-degraded hours of use from the tube, and it will sound better to boot - a lot more free, less stressed, due to lower voltages across the tube. The B+ will be quieter by having a choke as the solid state rectifier's input filter, rather than a cap input filter, which is always an audio mis-design, a no-no.

Jeff Medwin

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