Hi folks, I've owned my beloved R4B since the mid-70's. From about 1985 until 2000, it was stored while I got busy with job/family. I used it again for several months in 2002 and 2003, and I didn't notice anything unusual. Now I am active again and want to use it full time, but I am hearing audio distortion after about 5 minutes of operation.
Poking around, I observe the cathode of V7 is at higher voltage than expected, and the grid has more than 2 volts, where 0 is expected. Coupling cap C185? No, lift one side, and no change. R154 has the correct value. Cathode resistor? R44 has the correct value, and I tried a different one anyway, in case it was thermally unstable, with no change. I've replaced C176 (along with the other small electrolytics in the RX), no difference. I haven't tackled replacing the big electrolytic can yet, but voltages and ripple seem pretty much OK on all terminals of that unit (e.g. 7mv at C90A, and 35mv at C90D, 75mv at C90C). At power up, the V7 grid is slightly more than 0V, cathode voltage nominal. As time goes on, these creep slowly upward. After 5-6 minutes, the grid is approaching 3V. 30 seconds after power up: V7 pin 1: 4.8 pin 2,5: .006 pin 6: 134 pin 7: 146 5-6 minutes after power up: V7 pin 1: 6.2 pin 2,5: 2.9 pin 6: 129 pin 7: 139 I note screen and plate voltages have fallen off a little. If they were a little high to begin with, it may be because my line voltage is sometimes a bit high: I measured 122VAC today. I presume the drop in these values reflects increased current through V7. I get the same basic behavior when I have the receiver on a variac and control the line voltage to something less than 120VAC. The audio sounds good for the first several minutes, but it becomes harsh sounding at some point. I don't know what the output waveform looks like. So I could feel better about V7 running hard while I poked around, I put a fan on it, above the chassis ... and lo, the grid/cathode voltages started to drop. The grid stabilizes at about 0.3V with a healthy amount of forced air on the 6EH5, and the audio sounds good. Conductive socket material when hot? I don't think so: I put the tube on a 1.5-inch extender to reduce heat at the socket, with no change in behavior. Bad tube? No, two others behave the same way. I've researched other R4B audio distortion discussions, some on this list. None seem helpful. I'm stumped... any ideas? Thanks and 73, Randy WB4SPB _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
