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Chris, I think Francis is correct.

24.205.19.42

Posted by steven oda on February 13, 2002 at 08:58:53

In Reply to: Re: Improving the bass response of Williamson amps posted by Chris Spafford on February 13, 2002 at 05:55:33:

You said:

"i used 47uf/450v in series for the first section (original is 20uf/350v) and 100uf/350v (original is 50uf/350v) in series after the choke. all are the gray IC caps. tubes are 6L6GC's matched closely and still tested strong."

Francis,

When you said 47 mfd filter cap "in series"---did you mean two 47 mfd hooked up in series with each other to get a total filter of 23.5 mfd at total 700 volts rating. With these two going from the B+ to ground, hooked in series?

Also, same with the 100 mfd/350 volt caps. Two hooked together in series from B+ to ground? Total filtering would be 50 mfd at 700 volts.

This is the proper setup. And unlike what Chris said, for bass, I feel that the higher voltage rating caps are a good thing.

Choke in the middle is correct, too.

Change any resistor in this filter supply (pi-circuit) with new same value resistance and wattage rating carbon comp or carbon film babies.

Coupling caps should be changed out with similar valued units. Orange drops are fine.

Now, for those 6L6GC... Are they Svetlana? These, I feel, are the best for bass---of the new tubes. For really good bass you need a tube with a bit more headroom.

The 6550 is the king (but will require rewiring and you MUST be careful of the heater ratings). The alternative is the JAN-Philips 7581A. This tube needs to be rebiased up from the 6L6GC specs. It's heater requirement is 1amp (silimar to the 6L6GC 0.9amp). The 6550 and KT-88 require double the filament current. This will get you into big trouble. You will need to add another power transformer. Use the new one to supply the heaters of the gain stage and phase splitter tubes. And keep the original PT for heating only the 6550's.

Go up about 5-8mA per tube. (a ballpark figure). Plate voltage, strength of your power transformer, and tone will be the ultimate determinate.

You might want to try to a solid state rectifier. Some folks remove the rectifier and solder in four 1N5408 diode bridge innstead of the usual 1N4003 "plug-in" replacement rectifiers. You triple the rating from 1amp to 3amps and double the PIV from 1000 volts to 2000 volts. A lot more headroom. This will eliminate any "sag" and really tighten up the bass.

I'd only go this far. Go can tweak the tone by changing coupling cap mfd and cathode bypass caps on the gain stage tubes. But, one step at a time.

I think the heftier rectifier and the 7581A's should give you the bass you want---more of a solid state tight a** bass with good clean transient notes on songs with low bass note riffs.

The diodes can be soldered across the socket pins of the removed tube rectifier. That way, if you don't like it you can unsolder the diodes and go back to the tube rectifier fairly easily.

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