@zackthedog,
@trobbins,
@Pio1980
Hey Folks,
It has been quite a while since I've updated this thread. About 5 weeks ago, I received NOS 12AU7s and some new KT66s for each amplifier. Initially, they sounded very good, and after some burn-in, they sound amazing. I've been in audio bliss for a while now.
But....
The right channel amplifier used to emit (though the speakers) a sort of Pfffftt sound about 7 seconds after power up. Then it worked fine. This annoyed me a little, but I figured I could live with it. Well, over the ensuing weeks, that amplifier has developed some low-level noise. That noise sounds much like the sound of a record player after the music is done and it's running in the blank groove at the end. Often it takes about 20 minutes of running the amp before the noise sets in, but once it's there, it doesn't stop. This noise is NOT a hum. It's random, slightly scratchy, slightly poppy, and only noticeable between tracks or in low-level passages of classical music. If I listened only to rock, I'd probably not notice it. But since I listen to a ton of classical music, it's very annoying.
Today I put the amp on the bench and started to look for faults. (By the way, since I last wrote, I bought a new DVM, a Klein Tools MM600, so my measurements are more reliable.) First, I swapped all the signal tubes with the other amp. This made no difference, which is not surprising as all the tubes are new. The Heathkit manual suggests that noise could be a bad resistor or a bad soldering job. Since almost all the resistors are new (I kept the old wire-wound resistors, but replaced everything else), a bad resistor seems unlikely. But I did look closely at my solder joints and re-flowed a couple of them. No change. However, I noticed that the 47K ohm resistor which sits between the power supply (capacitor G half-circle) and V1 P7&P1 (plates of the two sides of V1 a 12AU7) looked like it had been hot. I measured its temperature and it was running at 117°C. If I'm reading the data sheet correctly, it should be fine up to 155° C, and it has a rated operating voltage of 500 V. In any case, I temporarily swapped the old resistor back into its place, but that made no difference to the noise. However, the old resistor quickly got hot too. This is a 1 watt resistor and I'm wondering if it'd be smart to replace it with a 2W resistor. By the way, this resistor is seeing about 107 VDC when it really should be seeing closer to 88. Some of the other resistors also seem over-warm to me, but I'm not experienced enough to know how normal this is. The other resistors are much cooler than the one mentioned, but I did not measure their temperatures.
As a side note, even with new tubes, the over-voltage problems I had earlier are still there more or less. A smidgen might be line voltage as my house current measures 123 VAC today, which is about 7% higher than the old standard of 115VAC. Interestingly, I think I may have a bad rectifier tube as it puts out 466 VDC on pin 2. I swapped it with the rectifier from the other amp, which puts out 509 VDC. The swap made no impact on the noise. I guess I'll get another rectifier tube, but I always thought that rectifiers were pretty much either functional or dead. In any case, aside from the weirdness of having a low-output rectifier combined with over-voltages elsewhere, I don't think this is the problem as the amp sounded the same with either rectifier tube. And the other amp has the same over-voltages, mainly on V1, but I'll consider that a problem for another day.
To troubleshoot noise problems, the Heathkit manual suggested removing tubes, one at a time, and powering up to see if the sound would go away. So starting with V1, I removed tubes. Without V1, there's no noise, though there is a little hum without V1. So I measured the resistors around V1, but all were on spec. I also measured the resistance between pin 3 of V1 and ground with V1 removed and got a figure of 427 ohms, which seems about right. There's a 470 ohm resistor from V1 P3 to ground plus a 4.7K ohm resistor that connects to V1 P3 and is grounded on the other end via the secondary winding of the OPT. So based on my calculations, the 427 ohms resistance is pretty much on the money. And since removing V1 got rid of the noise, I didn't repeat the experiment with other tubes.
I also "chopsticked" the amp, basically poking and prodding solder joints, tube pins, wires, etc, while running the amp. Nothing changed the sound of the noise, so I'm tending to rule out more bad solder joints. And reflowing the questionable ones didn't make any difference. While I suppose it's possible I've gotten a bad resistor, it seems unlikely. I ordered low-tolerance, metal-film resistors (1% mostly). And the electronics industry seems to have quality control down to a very precise science these days.
So I'm at a bit of a loss as to what my next step should be. I have a cranky, old, Textronix oscilloscope which could be brought to bear. But frankly, I don't really know how to use it beyond the most basic idea of tapping into the signal path at various points to see what I can see. My main concern is not blowing up the scope via tapping into something at an intolerably high voltage.
Anyway, I remain indebted to those of you who have helped along the way, and any suggestions about how to solve my noise problem are very much welcome.
Thank you,
Kim G
Boston, MA
Where a stay-at-home advisory has given plenty of time for projects.
P.S. I'm wondering if my over-voltages could be due to the fact that I rebuilt this amp with a capacitor board with modern, low ESR capacitors that replaced the old can capacitor, which presumably has higher ESR. Thoughts?
