800B FM Sensitivity Thoughts

[Joseph W Strickland]

I live in a fringe area for most of the Dallas and Ft. Worth stations that I like to listen to. The original RF amplifier tube that E H Scott used for the 800B is a Sharp Cut-off Pentode 6AG5 for the RF amplifier circuit. This is a decent RF amplifier for the TV and FM band and saw a lot of use by various manufacturers. Its gain ranges from 4500-5100 micro-mhos. There are some plug-in compatible tubes which are also quite good in the same frequency range and have higher gain. One is the Sharp Cut-off Pentode 6BC5 which has a gain range of 5700-6100 micro-mhos. Then there is the Sharp Cut-off Pentode 6CE5 which has a gain of 7600 micro-mhos in the same frequency range.

If having any issues with weak signals on FM and have verified the 6AG5 your set is using in the FM RF amplifier stage has good emission, you may try the 6BC5 or the 6CE5 and see if it allows your set deliver a more noise free signal on FM.

E H Scott made an excellent choice in the FM IF strip by using the Sharp Cut-off Pentode 6AC7 which has a gain of 9000 micro-mhos. Even the Sharp Cut-off Pentode 6AU6 commonly used in many more recent designs has a lower gain of 3100-5200 micro-mhos.

Joe

[Norman S Braithwaite]

Joe:

Have you tried these other tubes against a good testing 6AG5?  Unless a tube is weak, the actual amplification of the circuit is almost always limited by the circuit to a reasonable maximum for the application, not the amplification factor of the tube.  If this were not the case, the receiver performance would start deteriorating as soon as the mutual conductance of the tube started to reduce rather than when the mutual conductance of the tube drops below a minimum acceptable value.

Norman

[Joseph W Strickland]

Norman;

No I have not been able to check for possible improvement of sensitivity yet by using the above listed tube types. However, as soon as I am able to do so I will. One aspect of the other tubes mentioned above is that they are pin for pin compatible with the OEM tube. They will work with no damage to the radio circuitry, so they can be used as substitutes in case a 6AG5 is not available.

My most recent acquisition is closer to actual use than the first unit I acquired. I am still waiting for some10-pin plugs to connect them to a speaker. I ordered plugs in December and something went wrong with the order. I may have to look for another source. The company I ordered from is not being very helpful.

Joe

[Joseph W Strickland]

As part of a follow-up, I checked with the company I had ordered the 10-pin Cinch Jones connector from and asked them to cancel the order and refund the money I had paid.

I checked for availability of the P-310-CCT Cinch Jones 10-pin plug at Allied Radio. Allied Radio has the part in stock, so I ordered two of them, one for each 800B I have. I don't know why I did not think of ordering from Allied Radio in the first place. I will have to remember to do so in the future.

Joe