Crackling and popping on my left channel

[Shannon Parks]

Hi Mongo,

Your tube switching was a good idea. I had a EL84 go bad over the winter and it made those types of noises.

I would look at all the solder joints. I have sometimes experienced a 'crackly' channel in a new board that disappeared after I went over all the solder joints on that side and touched up any dodgy ones. Also double check all the flying wires for solid connection. Finally, take a look at the socket pins themselves to see if you might be getting a poor connection on any of the tube pins, particularly the left side EL84 sockets. All these items mentioned are subject to the heating and cooling found in a power amp.

Shannon

[Shannon Parks]

I went over all the solder joints on the left side channel, know everything works fine and smooth, no cracks. Thanks! Yellow_Light_Colorz_PDT_15

Thanks for the update, Mongo!

Just out of curiosity, what type of solder do you use? Is lead based solder available anymore in the EU?

Shannon

[Shannon Parks]

Hi Mongo,

You aren't able to get me a close up photo of the suspect side, are you? That would be very handy.

Shannon

[EWBrown]

Did you use the gold-plated contact tube sockets? sometimes, the process of soldering, the solder will stick and flow very nicely to the gold, but then the plating can lift off the metal pin and cause this symptom.

While you're conducting the forensic investigation, look for any cracked resistors, they can do some pretty weird and annoying things.

Lastly, pull out the 12AX7 and 12AU7, just leave in the EL84s, and listen for any suspicious noises, this will help isolate the problem even further. Since the EL84s are capacitor coupled (thus DC isolated) the tubes should just silently idle at their present current (approx 35 mA).

If just the 12AX7 is pulled, and the 12AU7 lis eft in, the amp will oscillate loudly, due to the PFB from the UL/SG tap (been there, done that).


A good trick to isolate a possible bad component or solder connection, is to gently tap each component with an insulated (plastic or dry wooden) rod or dowel, anything that is bad, microphonic, or poorly connected will show up readily


In the "X-files" or "twilight zone" category of problems , I did have one that I may have posted here last year, thhe left channel had some strange intermittent noises and distortions, and the biasing on one of the tubes would become very unstable. I suspected a high (ultrasonic or low RF) frequency oscillation was screwing things up. Upon further investigation, I found a low positive voltage on the offending tube's G1 grid (they should be zero or just a few millivolts). I tried changing tubes, even changed the coupling caps, suspecting a leaky one, none of this cured the problem.

My last ditch effort was to disconnect the UL taps on that side, and try it in "triode" mode - problem solved. There was something strange with the OPT (It was a Chinese made "DARED" brand OPT) and even though the UL tap showed the correct voltage ratio of 43%, the winding may have been reversed internally. So, the permanent fix was just to change the amp over to triode mode on both channels -something I wanted to try anyway, and I'm very happy with it. The power level is closer to 5WPC rather than 17, but it sounds truly excellent! For triode mode, just disconenct the UL taps, and connect a 100 ohm to 1K 1Watt resistor between the plate and SG/UL connection at each of the four terminal strips. And remove that little 15-20 pF cap, as it is no longer needed, and will introduce other strange situations... The bias may or not have to be re-tweaked, but that is quick and simple to checkk.

.

HTH

/ed B in NH

[Shannon Parks]

As Ed was mentioning, the first step in times like this is to recheck all the voltages, AC and DC. Some small DC offset can wreak havoc. I like the wood stick tapping trick. Yellow_Light_Colorz_PDT_15 You can also get a can of inert Freeze Spray - this is handy for isolating these problems, too.

Shannon

[Shannon Parks]

Hi Mongo,

In addition to swapping the power tubes like you already did, try swapping the input source RCA cables and the speakers, too. Then you can feel 100% confident it's the amp.

With the amp off, you can measure the cathode resistor network to ground from pin 3 of each power tube. I would first examine all of these, making sure their ranges are the same by turning the pot all the counter-clockwise and then clockwise, taking measurements with your DMM and writing them down. I would then set all of them to something like 390 ohms, then leave them like that for the remainder of your testing. You can also measure the capacitance to ground at each of these locations, to see if any of your cathode bypass caps have started to fail. Make sure none were put in backwards, too.

We have seen a similar problem with dodgy contacts on a socket. This one is a bit harder to troubleshoot without just replacing the whole socket. Are these the two-piece ceramic 9-pin sockets?

Lastly, you say you've have this identical problem with a second ST35?

Some close up digipix would be great. Just email them to me and I'll post them.

Shannon

[EWBrown]

Any where from 10 to 15 volts on Pin 3 (cathode) above ground can be considered normal, I found that JJs seem to run around 14V (35 mA into 400 ohms), where some of the Russian 6P14Ps and old vintage 6BQ5s can run closer to 11-12 VDC above ground.

This persistent "scratchy" problem sounds like an intermitent socket pin contact, it could be a loose fit, or there is some flux or other residue on the contact. The Chinese white ceramic sockets usually have a very tight initial fit, but then sometimes they can loosen up too much.

Lastly, check the wiring and soldering at the terminal strips for the noisy side, there could be cold solder, a poor crimp on a ring lug, or some corrosion betwen the wire and the crimp.

With my initial ST35 builds I used the terminal strips and ring lugs, later on I dispensed with them, and just soldered the OPT wires directly to the appropriate pad locations. Both for "cheapness" and it also eliminates another few wiring junctions in the signal paths. It does make troubleshooting, if required, a little more challenging, but I found that when the amp is up and working, it seems to last and last forever, maybe a tube needs replacing but I haven't run into any board-related problems using this approach.

/ed B in NH

[erichayes]

Hi All,

One thing that is sometimes overlooked is board surface contamination from crappy flux or other foreign substances. I use a stiff toothbrush and acetone (FLAMMABLE, VERY LOW FLASHPOINT!) to clean all the gorp off the board after I'm through soldering, and it does make a difference a lot of times.

Just a thought . . .