Tubes up #102 build

[andresound]

Sergio
You have a Variac on your bench.I recommend you use it (even with the limiter) when bringing your transformer online while keeping an eye on both the B+ HV as well as the - bias tap and make sure you don't have something wired wrong exceed your cap voltages and blow yourself up!

Tony

It’s Andre, and noted. Thank you for the concern. BTW I did use the variac with Limiter but too late. The variac had a broken on/off switch at the time of first fire up with just the limiter. I believe the damage was done then. I should have been more patient. I have since repaired the Variac. I usually ALWAYS employ the variac on first power ups.

[sluckey]

...I also notice that I have no negative bias. Could this also be a bad lytic? I have bias tap - 3k3 - diode - 100uf with + to ground.

The bias circuit also requires that red/yellow center tap to be connected to ground. Once you connect the CT the negative bias voltage will likely be normal unless your diode is backwards.

[martin manning]

There was 300-400V at the bias tap, but the reverse diode should have protected the bias cap.

[andresound]

Thank you Sluckey . As I said.. learning everyday . This should be an easy fix then. Will tackle on this weekend. Thank you all once again for the input.

[andresound]

There was 300-400V at the bias tap, but the reverse diode should have protected the bias cap.

Martin, there was 80v on the bias tap

[martin manning]

If you were wired as a full-wave bridge before, the center tap would be at half of the HT, so the bias tap would have been some where around there too.

[andresound]

OK.. reconnected the center tap. These are now my voltages unloaded,

B+1 528
B+2 527
B+3 513
B+4 398
B+5 392
Bias - 53v

I have a quad of new TUNGSOL 5881’s and some good vintage pre tubes to load. However, I’m a little concerned about the HT for the 5881’s?

[martin manning]

Assuming the HT will drop a bit when loaded, to around 500V say, using the Tung-Sol's would be pushing your luck. Better to go with 6L6GC.

[andresound]

Martin.. yip, plates are 500v when loaded. I do have a strange voltage of 17v on v2b!! Also, the 150k plate resistor measures strangely when in circuit as well. However v2a measures 205 ish and the 220k plate resistor measures correctly when in circuit. What am I missing here?

[martin manning]

What are the plate, grid, and cathode voltages on V2b? One of those is the 17V, I guess? With power off you should measure the nominal resistance for the plate load. If not something is wrong there.

[andresound]

What are the plate, grid, and cathode voltages on V2b? One of those is the 17V, I guess? With power off you should measure the nominal resistance for the plate load. If not something is wrong there.

Plate 17v, grid (no signal) 0v and cathode 4.5v. With power off the plate resistor starts at 40k and eventually settles at 68k. I am confused. Will have another once over in the morning. Will also pull a few sets of RCA 6L6C’s black plates I have lying around. Won’t be a matched set.. but will be safer while testing with 500v on the plates.

[andresound]

Ok its mid morning now in this part of the world. After thinking about it for a while, I decided to pull out the 220pf snubber on V2b... and voila!! The 150k plate load resistor now measures correctly. Replaced snubber with a new one (silver mica) and voltage on plate is correct.

[martin manning]

Odd. The plate end should be open circuit with power off... Do you get the same thing with the tube removed?

[andresound]

Yip. Odd indeed. The faulty cap measured as a resistor on my “all in one” component checker. Can’t remember the value right now, but think it was about 300 ohms (will ahve to dig it out my bin if you guys would really like to know).

Voltages v1 around 205, v2 around 215, PI 330v and plates 498. Still high!! Maybe replace first 2k7 with a 3k3?

[andresound]

Maybe I connected the PT primary to the wrong tap? Ie. 220v instead of 240v. I was getting 375-0-375v on the secondary. Could NOT find the color codes of the PT (Fender Bassman original L022764 - 606 - 6 -23) on the net. If anyone knows the codes, please let me have a copy.