Problem with 6L6 Monoblock

the thermionic watercooler

Postby EWBrown » Mon Sep 24, 2007 5:29 am

The blue glow in the 6L6s is caused a form of "secondary emission" and is normal, provided it's on the inner surface of the glass . If the glow is inside of the plates, then that indicates some "gas" problems, and may or may not be a real problem. If it's really bad,then the plate current and biasing will be unstable, or worse.


Easiest way to check, is to swap the 6L6s between the two channels, and see if the problem follows them.

If there is a blue or other strange color "glow" inside the rectifier then it's a real problem...

/ed B in NH
Real Radios Glow in the Dark
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Postby sorenj07 » Wed Sep 26, 2007 7:40 pm

the problem does indeed follow them. i just got a big box of gear from home, including my 50W adjustable Weller station, a bunch of solder, and a ton of baggies of resistors and caps and whatnot, so i'll be able to have a look very soon.
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Postby sorenj07 » Thu Sep 27, 2007 8:10 pm

Just found out the problem with my amp, now that I have a DMM. My negative bias supply is shot, leaving the grids are at -3V or so. Cathode current is around 180mA idle per tube and as a result the B+ has dropped to 360V. What a disaster! The little trimpots as specified in the DiyTube Eiclone shorted out. I shudder to think of what's been happening to my Sovtek 6L6WXT+'s with that amount of current. I'll probably buy some 2W 25K chassis-mount models and find a safe place to drill a couple holes.
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Postby sorenj07 » Sat Sep 29, 2007 12:53 am

Until I get these pots and a drill to mount them on my chassis, I cut the 100K bias connection resistors and brought them to ground. I cut out the little biasing boards and brought each 6L6's cathode to ground with a 470 ohm 5W resistor bypassed by a 470uF 63V electrolytic. Took out the 5U4 tubes and put two 1N4007's in series for each leg of the HV winding. Not my intended "final" fix but for now it's working OK. There's some hum but I think it's from ground loops in my crappy dorm wiring. For example, connecting my powered USB hub to my macbook makes hum increase a TON.
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Postby Ty_Bower » Sat Sep 29, 2007 5:14 am

sorenj07 wrote:My negative bias supply is shot, leaving the grids are at -3V or so. Cathode current is around 180mA idle per tube...

Your poor rectifier! Sorry to hear about the trouble.

Isn't there something that can be done to help protect about loss of bias voltage? I thought some people put a fuse on the OPT center tap in case of such an event.
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Postby sorenj07 » Sat Sep 29, 2007 2:28 pm

I guess so... either that or a 150mA fuse on each tube's cathode. It's been suggested to bypass each fuse with a .22uF 1KV film cap, I'm not sure why but maybe it's that the fuse doesn't conduct quite the same as a copper wire.

Unfortunately, my temporary cathode-biased setup hums/buzzes a bit, even with the new KT77's. I'll have to figure out what's up, and if it's a ground loop (ouch) or something else I can try to fix.
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Postby sorenj07 » Wed Oct 03, 2007 7:17 pm

First thing - reconnect the chassis/bus ground to earth. I'd cut it but measured 10-15VAC between the two, and touching the amps increased hum a bit. This is my left monoblock by the way, the one that had buzz on top of the hum and noise. I've since installed RCA brown-base 5R4GYB's.

Then I measured plate voltage on all the tubes and found a stupid wiring mistake, where the 47K plate resistors in the driver stage weren't in parallel - I had 47K going to one plate, then 47K off that PLATE to the plate of the other tube - in effect, one tube had 94K. I'm surprised the amp worked at all the way it was set up. I fixed this but didn't notice much difference in buzz.

Then, another "doh!" moment. It turns out, that I stupidly had left the negative bias tap off the B+ connected, thinking that with the tubes disconnected, no current would be drawn. Of course, there's a bleeder resistor after the rectifier to ground. One of the capacitors was only rated for 160V and was seeing 300V on it, and was bulging. I disconnected the coupling cap from B+ and let that wreckage sit on the ground bus - can't do any harm the way it is. I had hoped that this would have fixed my problem, but I think it only decreased buzz a bit.

I'd been using a portable CD player to test the amp but when I connected it back to my laptop, I had the most horrible hashy noise on top of the buzz. I tried cutting earth ground, which made my laptop have only as much buzz as with the CD player, but had 15VAC on the chassis again!

Damn, this ain't very easy. The only nice news is that with a B+ of 420V, and a 30V drop on my 470 ohm bias resistors for the KT77's, I'm dissipating a comfortable 25W (64mA per). I'm considering just keeping cathode bias because this lower plate voltage/higher current seems a bit closer to class A - am I right?

Anyway I have work to do but I'll keep plugging away at this amp (the left one) until I get it shipshape. I wish I had my workshop - I'm soldering on the floor in my dorm hallway!
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Postby Shannon Parks » Thu Oct 04, 2007 5:34 am

sorenj07 wrote:I wish I had my workshop - I'm soldering on the floor in my dorm hallway!


Don't let the RA catch you! ;)

One roomate and I shared a huge toaster oven that was perfect for baking those $1 frozen pizzas. Dunno what happened (puff? puff?), but he pulled it out of the oven once and laid it on the carpet in our dorm room to cool. Doh! Ended up with a huge black, melted plastic spot on the floor. Thank god for throw rugs.
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Postby sorenj07 » Thu Oct 04, 2007 8:19 am

Heh. those wouldn't be Stouffer's French Bread Pizzas would they? I have an unhealthy obsession for those things... any thoughts on these f$#$@!g monoblocks by the way? Yellow_Light_Colorz_PDT_07
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Postby TomMcNally » Thu Oct 04, 2007 8:30 am

Hi Soren -

Sometimes it's hard to keep buzzzzz out of amps connected to
devices (like your laptop) that have a path to ground.

I would troubleshoot this way ... see what the amp (or amps)
do with a shorted RCA connected to the input. If you have
buzz then, work on the amp itself. If you only have buzz
when connected to another device, check all of the grounds.

I am thinking your RCA jacks aren't the same - maybe one is
insulated from ground and the other isn't ?? Do a good visual
check.

Isolating a computer or device connected to TV cable can be
difficult - many people give up and resort to isolation
transformers. The one Radio Shack sells for $ 16.95 is
pretty limited in frequency response, but works well.
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Postby sorenj07 » Mon Oct 08, 2007 2:36 pm

Using a Discman, I still get buzz in this one channel.. With my laptop grounded and both amps grounded, I get a CRAZY rush of buzz/screaming noise that I really suspect is a ground loop.

My situation? I think my amps need work on grounding (though they USED to work just fine at my house) and general wiring, and I need to figure out how to break the ground loop. I'm tempted to change the Williamson design to one that uses fully differential topologies i.e. a LTP direct-coupled to some GC drivers, so that filament heater hum might be cancelled out.

The problem doesn't follow the preamp tubes, but one of them is microphonic.

I'm very, very, very stymied.
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Postby TomMcNally » Tue Oct 09, 2007 7:14 am

Soren -

A couple of thoughts ...

1) 1/8" stereo mini plugs are terrible. Often the ground in the
jack is a tiny little piece of corroded metal that barely touches
ground, if at all. It often leads to missing channels, hum,
an out of phase channel, etc. Also, often the case of the plug
is too big to fully insert in the "hole" where the jack is, making
it worse.

2) Grounded computers + grounded amps = HUMMMMMMMM
It's hard to get rid of. Sometimes, isolation transformers
are the only way.

3) Unbalanced audio in general is tricky. You often have to
juggle grounded and ungrounded power cords, etc to get
around it.

... tom
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