All good but the tube that redplated, right rear, was the same position that redplated before.
Questions are: where to begin, what to look for and, perhaps, it may have been just a bad tube?
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sKiZo wrote:First thing I'd check is the bias resistor to ground on that socket. Maybe a weak solder connection that occasionally goes fubar ... I had one that looked perfectly fine but had gone open when I wasn't looking. Replaced it, and no problems since.
It's what Bob L told me to do after I fried a coupla tubes in the same hole.audiobill wrote:This is great advice!!!!
deepee99 wrote:Andy Bowman said, which to paraphrase, "You guys spend thousands of dollars on cabling but you don't clean your tube pins and tube sockets."
Bob Latino wrote:Red plating on an output tube ..
Most often caused by a bad tube but can be the fault of bad contact (as others have said) or a problem in the bias system of the amp that causes the bias for that socket not to be present.
1. If that same tube red plates IN ANOTHER socket socket while another tube does not red plate in that socket, nearly always the tube is bad. Replace the tube .. Another clue is that this tube may bias properly and the amp may play OK
2. If ALL tubes red plate in that socket then ..
A. There may be a contact issue on one of the pins as others have said > Clean or retension the socket pins a little. (bend in a little the sides of the pins with needle nose pliers)
B. Check the value of the 10 ohm resistor (on VTA amps) on that tube socket that goes from pins 1 and 8 to chassis ground.
C. Check the value of the 1000 ohm 1 watt resistor that is between pins 5 and 6 on that tube socket
D. Check the 270K (or 150K in the VTA ST-120) resistor on the driver board that feeds bias to that tube (R29 - 32 depending on the tube socket)
E. Check the Russian PIO cap that feeds that tube socket (it should have no continuity across the cap)
Under #2 above > A is the most likely problem. Also check on B, C, D and E above that the part mentioned is soldered properly on to the driver board..
mhardyman wrote:Fired up my ST-70 this morning and let it settle in for a few hours. The KT-88 has stayed stable and I'm trying a new EH 5AR4 to see how it holds up. I get about 6 months on the Shuguang rectifiers and hope to do better. My track record with NOS rectifiers is terrible so I gave up.
Jim McShane wrote:mhardyman wrote:Fired up my ST-70 this morning and let it settle in for a few hours. The KT-88 has stayed stable and I'm trying a new EH 5AR4 to see how it holds up. I get about 6 months on the Shuguang rectifiers and hope to do better. My track record with NOS rectifiers is terrible so I gave up.
There is no such thing as a 5AR4 EH, just so you know. Did you mean Sovtek?
Also, the reissue U77/GZ34 Genalex and the 5AR4/GZ34 Tung-Sol reissue are proving to be VERY fine tubes, and very reliable. If the NOS scares you the new stuff is a great alternative at a fraction of the price.
mhardyman wrote:AAARRGGGG! My bad. I had a small set of the Sovtek 5AR4 tubes to try out. So far so good. The Genalex line is of interest to me. The quad of KT-88s I bought from you have been superb!