Thanks much Garey, that explains too the magic marker markings on the two Sylvanias.
Chuck On Dec 9, 2007 5:15 PM, Garey Barrell <[EMAIL PROTECTED]> wrote: > > Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist > gang > ---------------------------------------------------------------------- > Chuck - > > Those numbers are about what you see on a TV-7/D tester and are > equivalent to transconductance, or "gain" of the tubes. > > The procedure used by Drake was to plug them into a test set that > duplicated the circuit in the T-4X final, set the bias for 35 mA, and > then plug in one at a time however many they wanted to test/match. > Their numbers, not related to any quantitative measurement, were > typically in the 35 - 40 range and were marked on the top of each tube > with a felt marker. Neither method has any direct relationship to their > performance at RF! > > Matched tubes are not really _necessary_, but mismatched tubes can have > 3rd order distortion products considerably higher than a matched set. > This can vary from a few percent up to tens of percent depending upon > the degree of mismatch. If you consider a pair of tubes in parallel, > say that one tube draws 30 mA at a given bias voltage, and the other > draws 40 mA. The plate meter shows 70 mA total, but one tube is > considerably different from the other. When drive is applied, one tube > will also draw more plate current than the other, so one tube may be > providing 60W of the output and the other 70W. Distortion increases > rapidly when tubes are driven past the optimum, which Drake measured at > 175 mA per tube. So if one tube is drawing 150 mA and the other is > drawing the other 190 mA, distortion can be pretty bad! Your nearest > neighbor ham probably won't appreciate it...... Of course it gets MUCH > worse if you are driving an amp, since the amp contributes its own > distortion on top of that from the transmitter. This tends to widen > your circle of "fans"! :-) > > Mixing "brands" of tubes is also not prudent, even if "matched" DC wise, > because different internal construction can make it difficult to > neutralize the final over all bands. It works ok a lot of the time, but > if the neutralization seems hard to optimize, or better on one band than > another, this could be the cause. Drake used both Sylvania and RCA, but > always paired, or tripleted(?) them by brand. Sylvania was the only > sweep tube manufacturer that published data for linear RF use. > > 73, Garey - K4OAH > Glen Allen, VA > > Drake 2-B, 4-B & C-Line Service Supplement CDs > <www.k4oah.com> > > > > Chuck Grandgent wrote: > > looks like I found a "TR4 matched set" of 6JB6s in the garage that I > > got back in '99, looks like from WB2LHP. Two Sylvania's and an RCA. > > Slips of paper for each, two are marked 109, one is marked 110. > > > > Those numbers are exactly what ? What is the typical variation seen > > among NON-matched 6JB6's ? What bad stuff might one see when not > > using matched sets ? > > > > Would be fun to know what I paid for them back then. Of course those > > dollars were worth more then than now. > > BTW RFParts didn't have any Sylvania/RCA last week, but did have GE's. > > > > Chuck, K1OM > > > > > > ---------------------------------------------------------------------- > Submissions: [EMAIL PROTECTED] > Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body > Hopelessly Lost: [EMAIL PROTECTED] - help in body of message > Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net > ---------------------------------------------------------------------- >
