Little Help Needed

[Structo]

This morning I changed something on the tone stack of my D'Lite 44 (TAD 6L6) and then plugged it in.
I checked the bias and it was set at 33mv on the test jacks.

Plugged the guitar in, no sound. wtf?
I glanced down and the outside power tube is red plating!
I quickly checked the bias and it was over 220mv.
The other one still at 33mv.
I have noticed that the one that red plated was always about 1.5 to 2mv less than the inboard tube.

Now is this just a coincidence that a power tube went out?

I pulled all the tubes and plugged in my light bulb current limiter, no shorts and the voltages check out.

So what do you all think?
Did this power tube short out?

Guess I haven't ever had a tube do that before so I'm kind of lost here.
I don't have any spare 6L6 to try either.

Thanks for any help you can suggest.

[burnt fingers]

The one and only time I bought groove tubes 6v6 I plugged it in, turned it on ant the plate glowed red.

I picked up the tube 100 miles away when I was on business so I figured I'd just call GT. The guy argued with me over the specs of the replacement OT that I put in the amp and finally conceded when I talked through the impedance and voltages. Oh, and its cathode biased.
I thought it was funny how the jerkoff automatically assumed I did something wrong.

Anyhow - I just remembered that grief. The plate glowed bright red and the other side was fine. I did not check the current but I'm sure it was high like yours.

Check the wiring, make sure you did not bump the grid to ground. Pop another tube in there with your VM plugged in. Kick on the standby and check the current.

There's not much that can go wrong here.

[ampdork]

Are you getting bias voltage on the power tube grids?

[Structo]

Yes I had adjusted the bias earlier then I set about doing a mod on the tone stack so, no I didn't mess with anything in the power section.
But I remember that tube always biased a little lower than the other one.
Like about 1.5 mv on the bias jacks.

I swapped the power tubes and the same one red plates...
So I guess that means the tube went bad, huh?

Trouble is I don't have that many hours on these.

Now I have to decide what brand 6L6 to go with.
Any suggestions?

[burnt fingers]

I've always liked the SED winged C. I admit that I haven't tried many. Guess I read the reviews on tubestore.com, tried the tubes and never had a desire to try something different. I did try various el34s and found their reviews to be fairly accurate.

Do yourself a favor and get the tungsol 12ax7 while your at it paying for the shipping. They're a little bright but always win the v1 position with every amp I put them in.
I tend to like long plates in the OD sections of my amps.

I have some "matched pairs" power tubes that have a couple mA difference in my dlite. I haven't had one give up the ghost but they don't seem to sound as good as more perfectly matched (DC) tubes so they come out of the amp quickly.

[Normster]

I had EXACTLY the same experience with a TAD short bottle. First tube I've ever had that red-plated after working fine for months. I wonder if that's common for TADs?

[hot_tube_tone]

Swap the tubes around and see if the glowing plate goes with the tube or stays with the socket.

[ampgeek]

Megabummer on the wigged out tube. I HATE it when that happens!

I tried a pair of Sovtek 5881 WXT's in my homegrown D'Lite a week or so ago and was very pleasantly surprised with the tone.

I have been running these in Ampeg V4's (hellishly high plate voltage!) for many years with zero issues. Nice sounding bottle, extremely robust for short coin.

Check your tube headroom though. They are pretty tall. In fact, I was bummed that they wouldn't fit into my head cab (Bogen CHB-35 metal enclosure)

Good luck!
Dave O.

[Structo]

Swap the tubes around and see if the glowing plate goes with the tube or stays with the socket.

I swapped the power tubes and the same one red plates...
So I guess that means the tube went bad, huh?

Yep, it's the tube.

Moss, the great guy he is, is sending me some SED Winged C's to try.

The TAD's he included in my kite, were some he had burned in on another amp so I don't know how many hours they had on them.

This head cab has lots of room so I don't think there isn't any tube I can't run.

I ordered some JJ 6L6's as well from Bob at Eurotubes for some cheap backups.
Anybody have anything to say about the JJ's?

[Structo]

Also, what was strange is when I turned on the amp and before I noticed the tube red plating, I took the guitar cord that was plugged into the amp and put my thumb on the tip of the cord, you know to see if I got the hum through the amp.
I have a DPDT switch for my OT primaries (4K & 6K ) and there was a arc inside that switch. I saw it and thought that the switch was bad.
So I ran all over town to find a better switch with a higher current rating.
It was after installing the new switch that I noticed the tube red plating.

So now I am wondering about the OT. whether it got hurt.
With the tubes out and the amp off, I measured the DC resistance of the OT to make sure it isn't fried and I got something like 345 ohm and 245 ohms on the primaries.
Does that sound right?

I also measured the voltage with tubes out on pin3 of the power tube sockets and was getting, I think 430v.

[JimiB]

thats not good. they should be closer than that. did you disconnect the primary leads? Is this a stock dlite ot?

[Structo]

Well I have the power tubes out so there is no connection other than the center tap.
242 ohms on the 4K leads and 344 ohms on the 6K leads.

Can't really measure the secondaries because they are connected to the impedance selector and through that to the presence circuit.

[JimiB]

so you get roughly the same measurement out of each side and the different measurements are the 4K and the 8K primarys?

[Structo]

Yes on the two primary windings.
I had a guy measure his OT on the Brown Note forum and that's roughly what he measured as well.

[JimiB]

as long as both sides measure roughly the same you are probably fine. Sounds like you just had a tube die. You might want to replace that screen resistor if it got hot.