Mystery sagging heater voltage SOLVED

[xtian]

This is a post-job report, just FYI, because this was something I had never encountered. DUT was a Carvin Nomad 112 in very good condition. Tubes would not light up. Heater voltage was present, but very low. I pulled all tubes out, and voltage was good at 6.4vAC. As soon as I would plug in one or two preamp tubes, voltage would sag under 6 volts. Measurements were erratic.

I pulled the heater winding from the PCB. (Fast on connectors! This is a clue for later!) Winding voltage read nominal. I clipped in a 4.3R resistor, drawing over an amp, and voltage remained OK at about 6.3v.

I pulled the main PCB, retouched some solder points, but everything looked OK. When I pulled the heater's fast-on connector, I noticed some heat damage (browning) on the plastic safety sleeve of the fast-on, and I noticed the connection was a bit loose. Ah ha! So I painted all the connectors with DeOxIt, retensioned the fast-on, and reassembled everything. ALL GOOD.

So that one connection was a bit loose, expressed as heat, and making the voltage unstable.

[drewspriggs]

This is a post-job report, just FYI, because this was something I had never encountered. DUT was a Carvin Nomad 112 in very good condition. Tubes would not light up. Heater voltage was present, but very low. I pulled all tubes out, and voltage was good at 6.4vAC. As soon as I would plug in one or two preamp tubes, voltage would sag under 6 volts. Measurements were erratic.

I pulled the heater winding from the PCB. (Fast on connectors! This is a clue for later!) Winding voltage read nominal. I clipped in a 4.3R resistor, drawing over an amp, and voltage remained OK at about 6.3v.

I pulled the main PCB, retouched some solder points, but everything looked OK. When I pulled the heater's fast-on connector, I noticed some heat damage (browning) on the plastic safety sleeve of the fast-on, and I noticed the connection was a bit loose. Ah ha! So I painted all the connectors with DeOxIt, retensioned the fast-on, and reassembled everything. ALL GOOD.

So that one connection was a bit loose, expressed as heat, and making the voltage unstable.

Do you mean those little crimp connectors that join to a spade? I found the biggest issue with those is people use either a pair of pliers or really shitty crimpers to join them onto wire, which can cause heat build up/joint expansion over time that loosens.

Using them in automotive stuff, once they were crimped we'd slide the insulator off, fill them full of solder, then put the insulator back on - seemed to reduce the rate of them failing significantly.

[xtian]

This is a Carvin amp, and their build quality is good. The crimp to wire was solid. It just had a loose grip on the spade.

[maxkracht]

I've seen similar things in many amps, sometimes the scorch marks are way more apparent. These days I default to clipping the press on bit and soldering wires direct to the board or a tube socket. Makes it slightly more annoying to work on, but should be way more reliable if done right. Probably fine to retention, but I worry about metal fatigue. I wonder what the current rating and how many connect/disconnect cycles most of those are rated for.

Edit: to be clear, I only think this is necessary for heaters due to the high current. I've seen this failure most often in higher powered amps.

I got curious and looked for datasheets. Couldn't find any info on number of connection cycles or current rating, and only some have a voltage rating.