07-26-2008, 11:52 AM
Ron Ramirez Wrote:If you compare the schematics of the 37-690 and 38-690, you will see that the tweeters are driven by the output tubes in the 37-690, as is the woofer. This is the reason the tweeters are so loud in the 37 model. (Too loud, in my opinion; even taking the NRSC pre-emphasis curve into consideration.
The whole problem lies in the size of the "tweeters". I agree with you on the upper limit frequency of around 10Khz. I think the Philco engineerz designed this model to be "impressive" hence the illogical size of the tweeters (one would have be sufficient AFAIK). 4" tweeters would offer a better frequency response and also be a better match to the woofer. In such a design, the engineer should have taken into account the response curve and sound pressure of the woofer then match the tweeters with a simple resistor/capacitor network, something they obviously didn't do.
Interesting that Philco adopted such a complicated solution in adding a special winding to the driver transformer instead of adding a simple resistor/cap network to the tweeters and keeping them in parallel with the woofer. I guess they were radio engineers, not audio engineers...
By looking at the schematic, a 6F6 used as a driver (equivalent to an SE amp) I figured 1-2Watts to be enough to drive both tweeters. I guessed wrong apparently. My suggestion in this case would be to add a small value power resistor (say 4ohms) in series with a 4uF cap. The whole in parallel with the _woofer_. This should do it. Idea is to match the total db level of "tweeters vs the woofer".
I would also add a small "whizzer" to the tweeters. Yeah I know, it's hot-rodding, but it is reversible and will definitely enhance the upper frequencies. I did it with my top of the line Nothern console's 12 inches -in a ported enclosure mind you- and it did a very noticeable difference (I had to recone the woofer too so it wasn't complicated to do). I have plenty of whizzers on hand so this was an easy fix, still it is pretty simple to make one using cardstock. Definitely worth it.
Just a thought.
Syl
