One channel quiet and muffled

[rt60]

When I first built the BeePre, all of the voltages checked out fine, and it's been sounding beautiful for the last month or so. But then suddenly the left channel became very quiet and muffled. I tried switching around the tubes and inputs, but the problem stayed in the left channel.

I did a few voltage checks, and most notable is the left filament regulator board has kreg=0.75V and +reg=94.5, while the right side regulator board has values of kreg=4.3V and +reg=146V. I checked the voltages coming out of the transformers, and they were the same on both the left and the right. And like I said, switching tubes did not significantly alter the voltage measurements.

Has something gone wrong with my left regulator board, or could the problem be somewhere else? I haven't gone through and tested every terminal, but I can if that would help.

[Paul Birkeland]

Pull the EL84 on the muffled channel, power it up, then see how the voltages look (the 94.5V should pop up to ~180V).  You can run the preamp like this for a fair amount of time without concern for damage.

-PB

[rt60]

I removed the EL84 from the "bad" channel, and that channel's regulator board had readings of +reg=192V and kreg=0.57V. I then did the same test on the "good" channel and measured +reg=192V and kreg=1.16V. Does that answer anything?

[Paul Birkeland]

Oh, that is actually very, very helpful.

Next, see if the low voltage follows one of the EL84's.  Also, listen to the preamp w/o the EL84's to be sure the muffling is gone (it should be).

If the low voltage doesn't follow one of the EL84's, but stays on one side, I would start pointing fingers at the TL431.

-PB

[rt60]

The low voltage is not following one of the EL84s. I swapped them around, but they both produced approximately the same measurements. The low voltage always stays on the left side. And when I removed both EL84s and listened to the preamp, the muffling seemed to be gone.

[Paul Birkeland]

I am about 90% confident that the TL-431 on the lower voltage channel needs to be replaced.  We can mail one to you if you like.

-PB

[rt60]

Ok. I'll try replacing it. Should I email Eileen for the part?

[Paul Birkeland]

Ok. I'll try replacing it. Should I email Eileen for the part?

E-mail replacementparts (At) bottlehead (dot) com

-PB

[rt60]

It was the TL431. I was able to find a local electronics supplier to get the part. Once I replaced it, the left channel went back to normal. Thanks for the help!

[4krow]

And that, folks is what is so good about this site! There are a few too many 'hit and run' dealers out there. Here, you will find knowledge and support. I feel compelled to mention this occasionally on this site for those who are new here.

[adamct]

The more time I spend here, the more I am impressed by the time, patience, insight, helpfulness and dedication of the Bottlehead team. It is astonishing and inspiring.

Thanks!
Adam

[marco08]

oops, I think I have a similar issue.

two quirks : every hour or so of playing time, I have a 30 second burst of static (very annoying, continues even with volume turned down to zero), I have to turn off the amp for a couple minutes.

the second one is the left channel going almost mute from time to time (sometimes 30 minutes, sometimes after a couple hours of uptime). tried swapping the tubes to no avail.

which voltage should I check, and any thoughts on the static/crackle issue ?

thanks

[Doc B.]

Always start with checking all of your solder joints. Noise that happens after the amp warms up is often due to thermal expansion in a solder joint that isn't 100%.

[4krow]

 It's a good point. I had the same trouble with one of my Quickie projects. "Ah, it COULDN"T be my soldering." Well,  in the volume control set up that I was using, there a lot of joints, but all of them looked great. They weren't, and after rewetting them all, heaven returned.

[marco08]

thanks guys...

i went and checked everything yesterday, and lo, my trusted Nakamichi PA7 my dad gave me 30 years ago has a channel that goes dead after warming up a while.... the 'Pre is blameless for the channel going mute !!!

for the static noise, still sound's like the pre is the culprit. all solder joints look good, I'll give them a go anyway, but it's a very tubey noise (similar to the one all tube thingies do when they startup, except it happens from time to time even if it's warm). one of the tubes being weak / starting to go ?