Crack maybe killing tubes, weird sounds/hums in left channel [resolved]

[ohshitgorillas]

I recently moved cross-country and my Crack developed some loose wiring and issues while on the move. I recently finished rebuilding it, and things have been going great... except when they're not.

For full documentation/reference, the mods that I've done (even though probably irrelevant to the problem):

• Replaced RCA jacks
• Replaced volume knob with Alps Blue Velvet 100k
• Replaced diodes with Schottky types on PCB
• Added wima 2.2uF PP film bypasses to first two power caps
• Added Triad C-7X choke in place of 2nd R in power supply
• Replaced last power cap with 100 uF Panasonic film cap, bypassed with 1 uF PIO cap
• Replaced output coupling caps with Dayton 100uF film caps, both bypassed with 1 uF PIO cap

It's hard for me to sum up all the symptoms I'm hearing from my Crack because they are so intermittent. Most of the time, the amp works and sounds fantastic. I LOVE this amp.

For example, recently I was running with a 5998/Raytheon tube combo. I started hearing static in the left channel and microphonics, so I lamented the death of my 5998. I should also note that unplugging/plugging the headphones back in caused a change in the levels of the static. I couldn't find a place on the case that responded really consistently to tapping or thwacking, although if I had to guess it would be somewhere between the output tube and the headphone jack. In my grieving, I switched out to a GE power tube... this combo only output a low-freq hum in the left channel. I swapped out the preamp tube, but only heard the same hum. I switched to a Shuguang power tube and things worked great for days. Today, I come back from a weekend out of town, and my Shuguang tube is now the one outputting a low-freq hum. Unplugging and plugging the headphones back in can sometimes have an effect on this, but overall the result is an unusable amplifier (and a couple heart attacks thinking the Crack had ruined my fancy headphones, although it just turned out to be a DAC that had glitched out and needed a power cycle...).

I am going to flip the plate over tonight and go through the power tube/output stage with a fine-toothed comb, looking for places with cold joints. If anyone here has any suggestions as to where to look, or has heard of a similar issue, please point me in that direction. I am no EE but I have a decent grasp on what's going on inside of the amp. Maybe I'll finally order one of those fancy Neutrik jacks, although I like that the stock jack drains to ground when nothing is plugged in and am not quite sure how I'd do the casework... in any case, I'm getting off topic now.

[Doc B.]

Reflow every solder joint. Takes all of 15 minutes.

[ohshitgorillas]

As someone who's taken months to rebuild this amp (because I had to squeeze in time for working on it between a full time job and finishing a dissertation, and because I'd get halfway through a rebuild and decide something like "I'm going to use green wires for the B+..." and tear everything out again), I heartily disagree at that time estimate.

That said, you're probably right about re-flowing everything. I was very very very careful to make sure every joint in the amp was beautiful and shiny, but it's possible I missed something.

In the meantime, is it normal for tubes to just drop out and produce a low-frequency hum? For previously non-microphonic tubes to sound aggressively microphonic with static in the background?  Do you think my tubes are alright or is that their death knell?

[Doc B.]

I'm never berating someone when I say this. It's simply the most direct approach to solving the problem. I'm pretty sure it doesn't take much more than 15 minutes to simply reflow every joint, because we do that on almost every Crack amp that comes in for repair. We're not taking out wires and changing colors or rebuilding the amp. We're simply reflowing every solder joint. Whether they shine or not. Once this is done one of the major causes of odd noises is addressed. If they are gone, job complete. If not, at least you know it's not due to a bad solder joint that you previously missed and you can move on to other possible causes.

Regarding tubes, they can get more microphonic and/or noisy with age. But a bad contact can be microphonic too, and if sound is cutting out that tends to indicate the latter condition. If a tube stops playing music and just makes a low hum there is often a bad connection in the signal wiring. A tube that has a grid short can also exhibit this behavior. Hence my suggestion to make sure it's not a bad connection before moving on to checking the tube.

[Paul Birkeland]

If you installed an Alps Blue Velvet and didn't use one of the PC boards that goes with it, the connections on your pot may be intermittent and damaging your amplifier.

[ohshitgorillas]

Re: Doc B, no worries, I didn't feel berated or anything. It probably takes y'all less time than it would take me because this is what you do for a living, but point taken that it shouldn't take long. I didn't get around to it last night but I will try to make some time tonight.

Re: PB, Could you elaborate a bit on how intermittent connections on the pot could damage the amplifier? That concept is new to me. I actually ordered the PCB for the Blue Velvet, but it was rendered useless when one of the solder pads came off and stopped making a good connection. So, the wires are soldered directly onto the pot's leads.

[Paul Birkeland]

If any of the 3 pins on each level fractures, this will allow the grid voltage on half of the 12AU7 to rise, which will throw off operation of one channel completely.  This will at least damage the 12AU7, and potentially also the 6080.  You will also hear hum on one channel when this happens.

[ohshitgorillas]

Well, I went in to reflow stuff and I think PB called it... the pot's right channel output wire was barely clinging to the pin. On the other hand, the loud hum that wrecked my tubes--oh, and it did wreck my tubes--was most prominent on the left channel. And my 6AS7s seem to have fared far worse than my 6SN7s... so maybe there was another problem elsewhere. In any case, I put it back together and it seems to be doing well. I listened for an hour last night with no issues whatsoever--except, of course, f'ed up tubes.

I now need to replace my favorite tubes (TS 5998, Raytheon VT231), not a cheap mistake... RIP.

Edit: It looks like there are blue velvets on ebay with solder tabs now! might just go with one of those rather than wait another 5 weeks for the PCB to arrive from Bulgaria

[Paul Birkeland]

Edit: It looks like there are blue velvets on ebay with solder tabs now! might just go with one of those rather than wait another 5 weeks for the PCB to arrive from Bulgaria

These sellers also probably have $10 Rolex watches and $20 Louis Vuitton purses.  There are no genuine Alps RK27 pots with solder lugs.

[ohshitgorillas]

I was kinda afraid of that... another PCB it is. Thanks for the tip.