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    Dynaco and the 7199



    Posts : 269
    Join date : 2011-04-04

    Dynaco and the 7199

    Post by sailor on Thu Sep 26, 2013 1:22 pm

    The other day I posted the following under a different thread but decided it really applied to all 7199 amps.:
    The other day I was trying to find some info on the 7199 when I stumbled onto this article written by RCA. There are a lot of things in the article I want to comment on but I will need to wait until tomorrow. There is one thing in the article you need to click on it's under heading [Applications] marked [Fig. 1] which shows the 7199 being used without positive feedback. Basically a similar design to the one I proposed. They claim it to be very linear and stable. Ignore the output stage as it uses a different tube.
    I fined the Fig. 4 chart very interesting as Dynaco uses a 270K pentode plate resistor RCA recommends a 220K read why. More to come.
    If you look at the tube voltage measurements for the 7199 in the manuals for the Stereo 70 and the SCA35 you will see something interesting. In the 70 manual Dynaco states "Measurements at these points vary from tube to tube and do not indicate whether performance is normal" But on the SCA 35 there are measurements for the 7199. I can think of only one reason for this. Because of the high gain, low neg. feedback and instability of the SCA35 Dynaco cherry picked the tubes used in this model which gave all of the 7199 used in this model the same characteristics. It also the reason why so many 7199 are to noisy to use in the SCA35 but work OK in the 70. My guess is they were low current tubes see Fig.4 in the above article. This chart also shows that Dynaco use of a 270K pentode plate resistor in both amps shows that they had to weed out the high current tubes to keep the noise levels down. I don't have the test equipment to prove it but I bet if that resistor was changed to a 220K many of the marginal NOS 7199 and all of the Sovtek 7199, 6u8 and 6GH8 would be more stable and less noisy making for a better sounding amp.

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