Problem with 6L6 Monoblock

[sorenj07]

well, another "false alarm". turns out the sovtek tube i had in the left mono in the direct-coupled stage was hummy. subbed out each mono's Sov at this position with an OS RCA coin-base 6SN7GTB and all is good :)

learned an important lesson - ALWAYS have spare tubes, preferably of a different brand. can't wait to try different 6L6's. anyone want to happen to try my Sovtek 6L6WXT+'s in exchange for some other brand? be warned, i'm kicking the pants out of them in this amp: 500VB+, 55mA per tube (27.5W dissipation). way above the GC's screen rating but they're doing OK and most decent tubes should live.

tweaks like NFB can wait for vacation - something I want to fine-tune, probably. thanks everyone!

[sorenj07]

Another problem:

I connected some NFB on one of the monoblocks and got a very painful oscillation once the amp heated all the way up. It sounded like a square wave at around 1-2KHz. I used a 22K 1W resistor from the 8-ohm speaker jack bypassed by a 120pf cap (had it lying around) and connected to the cathode side of the first 470 ohm resistor. What did I do wrong?

[mesherm]

Swap the OT plate and UL leads on the 6L6s and that will change the feedback from positive to negative. To clarify, swap plate leads then swap UL leads. That will change the output signal 180 degrees and your amp shouldn't oscillate.
If its easier to do, you can swap the inputs from the PI stage. That will accomplish the same thing.

[Shannon Parks]

Also, since this is a Williamson (ie extra stage -> extra pole) you might even get motorboating (ie LF instability). This is after you get your phases correct as Mesherm states. So be on the look out for it. If you don't want to fiddle with it too much, you could just put in a 100k or 50k resistor in there for starters. It will lower your noise floor considerably but be safe.

[sorenj07]

Swap the OT plate and UL leads on the 6L6s and that will change the feedback from positive to negative. To clarify, swap plate leads then swap UL leads. That will change the output signal 180 degrees and your amp shouldn't oscillate.
If its easier to do, you can swap the inputs from the PI stage. That will accomplish the same thing.

I'll give it a try. I take it that switching the OPT's secondaries doesn't make a difference?

[mesherm]

Swapping the feedback and ground wires on the speaker leads will also invert the phase. Its usually not done that way but it will provide a quick test to see if phasing is the problem.

[sorenj07]

I have my macbook hooked up to the monos in my dorm room but unfortunately one channel hums like the dickens. the two amps, when cables are hooked to them and they're on, hum a bit (especially the right channel) and when I rattle the cables, the cable noise carries to the speakers. It's eerie.

When I wave the ends of the cables near a florescent light fixture, crazy buzzing fills both channels. I use an adapter to hook the RCA's to the headphone out jack on a macbook and when I plug it in, the right channel has some audible hum. I built both monoblocks pretty much identically so I have no clue what the problem is. I do know this: When I plugged the same cable into a different (IBM) laptop, there was no hum problem.

What could I try doing? is it an impedance problem? I kind of have to play my music louder than usual to get past the hum and during quiet passages there's no helping it. anyway, thanks for the advice.

[kheper]

I have my macbook hooked up to the
monos in my dorm room

I am assuming that the macbook has a
stereo sub-mini output jack and that you
are using a sub-mini to dual rca adaptor.

The sub-mini male part of the adaptor may
not be making proper contact with one channel
inside the sub-mini jack. You can wiggle the
adaptor in the jack to test, but this may
cause very loud pops in your speakers.

Better yet, reverse the rca cables. The hum
should then be coming out of the opposite
channel.

Get or make a new adaptor.

[sorenj07]

reversing the cables made no difference...

[sorenj07]

chased down the problem by unplugging various things from my laptop. i think i got a nasty ground loop to just one of the monoblocks, somehow, through the following path:

Right channel amp -> RCA cable -> headphone in on Macbook -> mini-DVI to DVI cable to external monitor -> grounded monitor plug -> amp ground

in any case, the monitor doesn't seem to really need a ground so I used some pliers to snap the pin off. It's a plastic-covered 19" that really shouldn't pose much of a threat. now, both amps are quiet as a baby. I'm happy!

[sorenj07]

Unfortunate new development: the right channel monoblock has always hummed a bit more, but recently, after letting it drop back down a little too quickly after looking underneath it, I saw the 5U4GB rectifier flash a bit of white from the tiny holes for the cathode at the top. Hum began to steadily increase until I switched it off. "Damn", I said. (loose translation). I switched out the 5U4GB from the other working amp and tried again. The amp started up OK, except I saw continual tiny sparks through the mica in what would be the region between plate and cathode. It didn't look healthy so I shut it down again.

At this point I should remark that the rectifier in this right channel amp has been making kind of a mechanical vibratey noise for a while.

What do I do? I ordered a couple NOS matched 5R4GYB's (straight glass envelope) to replace these 5U4GB's but I suspect that the problem is with the amp and not the rectifier. I built both pretty much exactly the same so I'm having real problems figuring out what's the matter. Even worse, I have neither a soldering iron nor a DMM at the moment, but I DID notice some soft, cloudy bluish glow inside the 6L6WXT's of the right amp. I haven't looked inside the left. If my 6L6's are gassy somehow (maybe the bumpy car trip to college did it), would it mess with the rectifier in this way? I'm hesitant to sub the left channel's 6L6's in because I don't have any tools to bias them at the 55mA point.

I will bug my parents to send my DMM and soldering iron (maybe there's a cold solder joint somewhere?) and maybe I can take a few resistor measurements. I worry that maybe my choke shorted and now the 5U4 is getting beaten up having to deal with the 100uF cap right after it. Help!

[sorenj07]

another note: I popped the rectifier which had been in the left (working) channel back into the right channel and powered up that monoblock. Not as much sparking but there was definitely a lot of blue cloudiness in the rectifier which I hadn't seen when it was running in the left amp. The 6L6s' glow also was a lot more pronounced, with more bluish stuff inside the plate structure, and the blue on the inside of the envelope had a much more purplish tinge. I'm a bit confused. There was also a lot more hum. Could drifting bias cause these conditions?

edit: I'm thinking of picking a quad of new JJ KT77's to sub in, I think it might be worth it to run the output tubes a little easier/within their specs... 500VB+ 55mA per KT77 into a 6.6K a-a load should be all right, right?

[sorenj07]

Update - some good news, some weird. I put the other pair of 6L6WXT+'s into the right channel amp and they exhibited similar darker purplish glow as opposed to blue glow (this is on the envelope) and a bit of cloudy blue glow inside the plate structure.

KT77's on the way. 600V plate + grid max voltage is encouraging, and the extra filament current might help the heater voltage drop to a more reasonable level. While one of the amps is in the "sick bay" it's a good excuse to start changing some things, mainly with a 6SF5 input tube and 6SN7 LTP.

[EWBrown]

As I just mentioned in the ST35 forum, if a rectifier tube has an internal glow (other than the filament), it is bad... (unless it is a MV rectifier).

A blue, purple or even greenish glow inside the glass of a power tube (triodes, pentodes or Beam Power Tubes) is OK, as long as the glow is only on the inner surface of the glass.

If the glow is inside (or closely surrounding) the plate, then that is bad - as in a "glowing" rectifier, it indicates gas inside the tube, either from an incomplete vacuum, a bad getter or a slow leak (the most likely scenario, caused by a bad or damaged pin-to-glass seal).

You mentioned fluorescent light fixtures. They can put out a lot of EMI and RFI, keep them far away as possible from audio gear. :o

/ed B in NH

[sorenj07]

Indeed, I see some glow inside the 6L6 tubes. I assume they're in dire straits, however, this glow effect is almost nothing in the "non-weird" monoblock (left channel). What gives? Gassy in one amp, not gassy in another?