Voltage Issues LEDS

[Mesz77]

Hello all,

This is my very first post to the forum and I am a brand new member to the tube amp community. Built my one and only tube amp (Reduction) about a month ago, but then left it at home since I live up at school (college student here). Been looking forward to spring break to finally use the preamp, but I've run into a bit of a problem. I was listening to some records and turned the volume down only to discover a good bit of scratching, crackling, and popping in the right channel. I flipped the board over and noticed that the LED attached to A3 was solid for the most part, but would flicker from time to time. As for the LED attached from A8 to A9, it's not coming on at all. I checked the voltages in the right channel and found that terminals 7,13,14, and 15 were all in the upper 40s instead of 70-90. Any help is very much appreciated and keep in mind you're dealing with newbie here!

[Paul Joppa]

Those low voltages suggest the relevant LED is shorted; the flickering suggests a poor solder joint making intermittent contact. PB, who does most of the repair work and thus knows the most about fixing problems, will probably chime in soon, but meanwhile see if you can get clear photos of the tube sockets, showing the individual pins - especially the LEDs. That will give him a leg up on diagnosis.

[Mesz77]

Thanks for the quick reply and heads up on the photos!

[Paul Birkeland]

Did this Reduction pass its initial voltage checks?

A flickering LED, as PJ mentioned, indicates flaky connections or an LED that's actually damaged. 

You could conceivably have some glowing LEDs and low plate voltages from flaky solder joints elsewhere in the circuit causing the bias to not be set by the LED alone.

How are your resistance checks? 

[Mesz77]

The preamp did pass the initial voltage and resistance checks when I built it about a month ago. Everything checked out and the LEDS were all solid. After building it, I played it for about 30 min total without any problem and then I had to leave it for school. I checked all the resistances today and they were all good except for terminals 10,33,and 37. Terminal 10 was fluctuating a good bit; it would spit out 300s through 700s and sometimes would go over 1M and 2M. As for 33 an 37, the manual says that they should be around 0 when the tubes are in, but I'm getting a reading of 25M on both of them.

[Paul Birkeland]

Can you post a few more photos of your build?

What DC resistance do you get at 36?  How about 34?

[Mesz77]

I got 0 for both 34 and 36. Sometimes when I checked them they would fluctuate under 10 mV, but for the most part they would both read 0. I've noticed today that the LED attached to A3 has been solid. Haven't seen it flicker at all. Let me know if you need a more specific pic of something. Heading to work soon so won't be able to reply until later tonight. Thank you for your help so far!

[Doc B.]

One possible scenario that could cause an intermittent connection is oxidation occurring when the amp sat unused. It certainly wouldn't hurt to clean the tube pins. Even pulling the tubes and reinserting them two or three times can often clean up an oxidized pin.

[Paul Birkeland]

Terminals 33-37 are badly in need of more solder on the upper lugs and more heat on the lower lugs.

[Mesz77]

Went around a resoldered 33-37 and quite a few other components that didn't look great. Did some voltage readings and now I'm getting 35V for terminals 7 and 13-15

[Paul Birkeland]

Are those your only voltages that are off?

[Mesz77]

Other than getting 35V for 7 and 13-15, the only other problems with voltages that I came across are some fluctuations with terminals that are suppose to be 0. Terminals 10, 11, 23, 27 hover above zero with values in mV that go no higher than 20mV. I don't know much about electricity, but I have a hunch this isn't a big issue. Where it gets funky is when I touch terminals 12, 21, 28, and 30; they rise up to 300mV and then go back to zero. Only other thing is terminals 46 and 47 being in the high 40s mV.

Thanks for the tip, Doc. What do you suggest I use as a cleaning agent on the tube pins?

[Paul Birkeland]

37 should give you an incredibly reliable reading.  If it's not 6V, the Reduction isn't going to work.

[Mesz77]

Yup 37 is on the dot with 6V

[Paul Birkeland]

I would recommend resoldering all the black wires that attach to the ground post, then follow them and keep reheating all the joints as the black wires fan out.

When voltages don't make sense, bad ground connections are nearly always to blame.

-PB