DIY Decware ZKIT thread

[bjlefebvre]

Ugh. So it passes the dim bulb test - no shorts. But with the dim bulb tester in the chain, the test points are way too low on voltages - like the first test point shows ~220V where it's supposed to show ~340V. It starts off ok then rapidly declines as the tubes warm up. If I take the tester out of the chain, it's the opposite problem -test point shows voltage going into the 400s (and something starts to hum) before I turn it off. I wonder if I wired any of the tube sockets to the PCB board incorrectly....

EDIT: A little later, and I tried making measurements without the tester in place. Tubes warmed up...and then something started smoking. Not sure what it was. Immediately cut power and now scratching my head. Everything is laid out according to the PCB board.

 

[jdsalinger]

My 6.8K resistor started smoking and I used the one that Decware specified.

I ended up using a 25W heat-sink type power resistor. 2 watts (or 6 watts depending on schematic) is way too low IME.

Double check every component to make sure caps and diodes are positioned properly (I ended up using a bridge rectifier in lieu of the dual diodes...my voltage was low I think. Bridge rectifier worked great).

 

[bjlefebvre]

Yup, diodes and polarized caps are all pointing the correct way. I triple checked that the wires connecting the tube sockets are soldered to the correct PCB board points. With speakers hooked up and volume down to zero, voltage at check point A is ~240V DC, about 100V below where it’s supposed to be. With no speakers hooked up, its more like 440V - way too high. Either way, the left side 651P tube heats up something fierce -way more than the other two tubes and much too hot in genera - and if power is left on too long, the area around the tube starts to smoke.

EDIT: Took @jdsalinger's advice and replaced the 2 watt resistor with a 1k 50w . That helped bring down the heat and seems to have helped bring the voltages closer to where they need to be, though not yet perfect (see below).

 

[bjlefebvre]

After hooking that back up and cleaning a little more of the tube-socket-to-PCB wires up, I'm pulling voltage at the test points that are getting much closer to spec -

A: 368 VDC (spec: 346vdc +/- 10 vdc)
B: 315 VDC (spec: 309vdc +/- 10 vdc)
C: 8 VDC (spec: 10.25vdc +/- 2 vdc)
D: 2.5 VDC (spec: 2.6vdc +/- 0.5 vdc)

Obviously that point A has to come down another smidgen, while points C and D are actually a tiny too low. I'll keep cleaning the solder joints connecting the chassis mount tube socket pins to the PCB board to see if that helps. The problem is the wire I used for that part of the build - the 24awg black coating stuff was really hard to strip. I'm actually wondering if the smoke I'd been seeing was coming from any of smudges of that plastic coating that may have been left on the ends of the wire at the soldering joints burning off. I'm definitely not going to use this wire again.

EDIT: I hadn't plugged in speakers for that last test, but did so after posting the above info. I got basically the same voltages as above. But it still doesn't seem quite right. The left SV83 still gets much hotter than the other two tubes and there's the smell of something burning if I leave the amp on for more than a minute or two.

It just hit me that the speakers I use in my work room most likely won't help here - they're NHT Super Ones, with a sensitivity of 86db. I hooked up my phone to to the amp and started streaming music via Spotify, but nothing came out of the speakers. Even jamming the volume all the way up in my phone and the amp didn't produce any sound out the speakers. Oddly enough, I did hear a really quiet, tinny version of the music coming from what seemed like the amplifier itself - like when you play a record with the sound off and you just hear the needle itself reading the groove. That was odd.

 

[century tek]

Are your output tubes in the right spots?

The sounds like you heard coming from the amp itself was probably an output transformer. This is what they will do if you don't have speakers connected to them. So it could be a faulty speaker connector or broken wire or something you're just going to have to look for. Ohm meter?

 

[jdsalinger]

Did you swap tubes to make sure it was the circuit and not the tube that's the root cause of the hot tube?

I suggest figuring out the tube issue first, then if you still have a high B+, you could replace the 1K 50w resistor with something closer to 1.3K ohm 50w.
I think there's a chance your tube issue is causing the voltage issue.

 

[bjlefebvre]

For closure: Well, looks like what we have here is an example of the importance of good fundamentals. I spent today cleaning up some of the less-than-stellar solder joints that were on the board and that - oddly enough or maybe not - did the trick. Problem solved. This thing is ready for speakers.

 

[bjlefebvre]

This amp is sounding veeeeeeery nice. I think it's actually a better fit in my main system than what's in there now.

 

[jdsalinger]

Ben, Merry Christmas! Hows the SE84 doing? I have a pair of Pilot power transformers in my junk box that are begging to be used in a project. I'm considering another SE84 or DGSE-1. I just need to find some Decware-approved OPT.

 

[bjlefebvre]

Merry Christmas! The SE84 is still rocking - got it in my workroom system. I have Edcor transformers on mine - I think Steve used those in his earlier builds, or at least had those on his ZKIT parts list.

One of these days I need to open it up again and add some Loctite or something to the RCA interconnects - they swivel a little bit.I was also thinking at some point I may redo the chassis - right now mine is just wood frame with an aluminum top plate, but I think I want to eventually make it an all-metal chassis with wood veneer on the sides.

Either way, go for it - this amp sounds great.

 

[djnagle]

I ran that amp driving Omega 3R speakers on my desk for 5 years. What a great combination.