Convert RCA tuner/pre to high perf AM only - diyAudio
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Old 3rd May 2008, 09:25 PM   #1
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Join Date: Jun 2004
Location: Indiana
Default Convert RCA tuner/pre to high perf AM only

For those of you who don't participate in the "funwithtubes" mailing list I posted this query regarding the RCA there but would welcome input from you guys too.

I happened upon this RCA console stereo (FM is mono) from the late tube era which being far below the level of collectible I have elected to part out for other projects. The power output module will be used either for the basis of a PP keyboard amp high frequency section or as a hi-fi treble amp with active xover.

This post regards the tuner preamp section. If you go to my site at http://tubbythetubie.embarqspace.com and select "RCA Console Find" you will see a button that you can press to view the PDF of a three page service manual for this unit. Mine does not have the PB300 stereo demux board (thus the mono FM). I have been told by several knowledgeable folks that RCA cut quite a few corners with this tuner and my informal testing indicates that the performance is only average. Thus I would like to convert it to an AM only tuner of much higher performance.

I am only beginning to learn about tuner design and would like to solicit your help in understanding this thing so that there might be some hope of modifying it for better performance specifically in the area of sensitivity and selectivity.

So let me explain what I think I see and ask a few questions along the way.

The first 6BA6 (V201) looks like a fairly straight forward RF amplifier transformer coupled to the next stage. One section of the tuning cap appears to be used to tune the secondary of the coupling xformer. It looks like the input to the next stage is taken off of a tap part way down the secondary rather than from the top of it. Why is that?

The next stage is the 6BE6 Heptode (V202) used as an oscillator/mixer to convert the RF signal to an IF signal for further processing. Here is where it starts to get confusing. For some reason the IF output from the secondary of the coupling xformer is passed through the secondary of the FM converter output before going on to the second 6BA6 (V203) which is I take it an IF amplifier stage. What is going on with that?

Since I will not be using the FM section could one just remove the FM board and jumper the output of the AM converter directly to the input of V203 or even better insert another IF amplifier stage at that point? Alternately is the 6C9 of the FM section an appropriate tube to be used as a dual IF amplifier or is a remote cutoff pentode required here?

Now V203 is, I take it an, IF amplifier for the AM section with a tuned load provided by T204 but what is T205 for? Now the notes on the schematic seem to indicate that V204 is an FM IF amplifier and the AM detector. But how can this be when the AM audio signal is taken off to the input selector before the grid of V204?

Now could V204 be configured as strictly an additional IF amplifier and either the 6AU6 (which has no AM function) or one section of the 12AX7 used as a detector? I have another use for the 6AU6 for another project if the 12AX7 could be used. Say one half of the AX7 for detector and the other half for amplification of the audio output.

And one more question... If I end up not using the 6C9 in this project what do you think of its applicability as an audio cathode follower for preamp output line driver or active filters?

mike
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