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The nice amp I got from eBay last week would not come down to the proper bias point. 1.75v was as close as I could get with a set of GE 6CA7's and 5AR4. I had a 5U4G that I substituted to troubleshoot with, and the bias came down into the proper range. The problem wasn't ''fixed'', but I had a clue that things weren't out of control. Learned some things about voltage drop through different rectifier tubes that way. I also ran a 5R4GYB in it and that really has a large voltage drop.

Came to understand the relationship between capacitor input and choke input and why it's necessary when running a rectifier near the upper edge of its voltage range to have low capacitor and or high choke filtering to keep the 'ON' surge to the capacitor from arcing across the plates. The 5AR4 with its super low voltage drop must have plates close together making it vulnerable to arcing when run near its max voltage with high levels of capacitor input.

To fix this one, after checking all the resistors were on the numbers, I first replaced the bias filters one at a time, but still got no change in the adjustment range.

I replaced the selenium rectifier with a good diode and BINGO!!

Fixin' stuff is fun, m'Kay?

Good job! Feels good, doesn't it?

Shannon

Hello, Mr. P. Yessireebobcatail.! Playin' with this '70 has been way more fun than golf.

Tim

I replaced a pair of 5U4GBs with 6DE4/6CQ4s and the voltage went from 460 to 473. Should be hard to arc a 6DE4, with its 5500 volt rating. But having a capacitor input is a mystery, since it isn't supposed to be a power supply rectifier.

Do 2 half-wave rectifiers equal 2 full-wave rectifiers in your amp? Were the 5U4's not being used for half-cycles each?

20to20 wrote:Do 2 half-wave rectifiers equal 2 full-wave rectifiers in your amp? Were the 5U4's not being used for half-cycles each?

Yep they are in a full wave rectifier, each were doing a half wave. The anodes of the 5U4s were tied together. I could have used 1 5U4, but I wanted to make sure it had enough current handling capacity.

With the "bad" selenium rectifier you did not have enough negative voltage to the grids of the output tubes. Whenever I work on any tube amp, the selenium is the first to go.

Sal

Sal, do you also find your 375v. runs high after that for the same biasing, like 380-385v. When I changed the 50mf's to 100mf's I neglected to take a test point reading before I changed the SR so now I don't have a data point for future reference on just a cap swap.

20to20 wrote:Do you also find your 375v. runs high after that for the same biasing, like 380-385v.

Is something kaput in there for the B+ to be that low? My ST-70 with a 5U4 was getting 425V....

Cheers!

Geek, the manual has 375v. at eyelet #20, direct wire from stock cap lug "B", the triangle. That 375v. TP also comes out on the preamp power socket pin 5.