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In Reply to: RE: Ceramic IF Filters and Tube Type Tuners posted by Eli Duttman on May 13, 2011 at 18:43:08
Eli;
Thanks again for this information. I downloaded the two articles and read them. I looked up the data on the 6AQ8 and the 6DJ8 in two tube manuals I have. The 6AQ8 is described as a "high-mu dual triode" and the 6DJ8 is described as a "medium mu dual triode". Internal electrode capacitances are quite similar. However the 6DJ8 is a later design and the transconductance figures shown in the charts in the back of the later RCA tube manual show that the 6DJ8 produces almost twice the gain of the 6AQ8. From statements made in the articles the 6DJ8 also has a lower noise figure than the 6AQ8, so it is better in two ways.
The changes required to use the 6DJ8 in the FM-3 are relatively easy to do and I might give that a try. My FM-3 still works well and still has original tubes except for just one that was changed.
I agree with the statement made in the article that the tubes run hot. I always used a cooling fan with the unit to keep things cooler. That may be one reason why I never had trouble with the cheap ceramic trimmer capacitor in the front end.
This website is a goldmine of information on modifications to improve various home electonic entertainment equipment.
I grew up with Heathkits, Eico kits, Knight kits and even a few Lafayette, Olson Radio and Burstine-Applebee items over the years. I even did some modifications to surplus military radios for SW listening.
It all brings back pleasant memories.
Joe
Follow Ups:
"Buddha" used a cascoded 6DJ8 family tube in the TRF front end. A cascode is effectively a low noise "synthetic" pentode. Gain is roughly gm of the "bottom" triode multiplied by the net load impedance. The 6AQ8 (similar to the 12AT7) has a gm slightly less than 1/2 that of the 6DJ8. Forget about μ in this situation, as gm rules the roost.
When I read that article a few years ago, seeing the frame grid tube used as Philips originally intended (cascode VHF front end) made me smile.
Eli D.
It is interesting to learn that Phillips developed the 6DJ8. That probably explains why RCA did not use it in their designs. The cascode RF design was quite popular in both television and FM tuners for many years. RCA was always partial to the nuvistor, which was their own design.
As you said, the gain of the 6DJ8 is double that of the 6AQ8 and at no additional current. Add the lower noise figure of the tube and you have a winning combination.
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