ST 70 Runaway Bias

[kingmatt]

Over the past 2 years I've had my Triode ST 70, I've very much enjoyed its (mostly) trouble-free operation. Ever since I moved a couple of months ago, I'd get the occasional bias flare up when I would leave the amp on for many hours at a time (I finally found a local radio station that plays some decent jazz and like to have it on more or less all day). Up until today, I'd notice that the bias voltage would drift slightly depending on how long the unit had been on. So if I checked bias within a minute or two of firing it up, I'd get a somewhat high reading relative to what it should be (1.10-1.18v), so thinking something was up, I'd back off the bias slightly and get it back down to spec (1.00v). After a couple hours of playing, everything would naturally be warm, and I'd get a slightly low bias (.89-.92v). If I readjusted to meet spec, I'd get a low voltage when it was warm again. I'm not sure if that is normal for component values to drift that much, or if it is even a problem.

That brings me to a few minutes ago. I had the unit on for a couple of hours, all was going just fine, then I glanced over and noted that 1 of the tubes was having some serious red plating (it got hot enough to melt the paint off the tube). I immediately killed power and dialed back both bias trim pots to fully CCW, waited a minute or two, and fired it back up. I now read .75v across both bias test points.

Right now I'm not quite sure what to think, though I reckon I've got two separate problems (one being the heat related fluctuation of the bias, and the other being bias flare-ups). Does anyone have any advice on what to look for going forward? And is the tube that went red plate garbage now? Thanks guys!

[Ty_Bower]

You didn't mention the brand of power tubes you are currently running. I'd also check to make sure you've got reasonable values for the grid leak resistors.

[kingmatt]

I'm running Electro Harmonix 6CA7s. All the resistors are stock values as directed by Triode. I checked the resistance of the 10 ohm bias resistor on the tube socket for that side cold and it was in spec, so I pulled out my mini torch and heated it up slightly, and the resistance dropped to 8.1 ohms. I've replaced it and everything seems OK for now, but I won't leave it unattended lest it flare up again. I suppose it is possible to have a tube ready to pack it up...

[Geek]

How old are the tubes?

One should retube every 2 years (or more often if a daily runner), as cathode gunk sticks to the grid and interferes with its ability to control electron flow.

Cheers!

[20to20]

And is the tube that went red plate garbage now? Thanks guys!

Could simply be the tube, itself. Try a channel swap and monitor the voltage again to see if the problem moves with the tube.

20

[TomMcNally]

Shouldn't the bias be set to about 500 MV across the 10 ohm resistor?
If it's at 1 volt, you're running those tubes really hot.
Or am I missing something ?

... tom

[EWBrown]

Depends if the 10 ohm "bias" resistors are for the individualltubes (should read 500 mv for 50 ma) or for each channel pair of EL34s (should read 1.00V for the two tubes, each drawing 50 mA). The latter assumes that the two EL34s in the channel are reasonably well matched.

/ed B in NC

[kheper]

http://site.triodestore.com/NEWST70Instructions.pdf

"*Since the Bias Pots interact with each other, you will need to adjust back and forth between the Bias Test Points until you achieve a reading of 1.0VDC on each Test Point Jack."

1.0V is the correct voltage.

When my tubes get old, the bias drifts. When I rebuilt the bias circuit, the voltages drifted a little for a few days (or so).

Looks like at least one of your tubes is shot. Before installing new tubes, you may want to rebuild your bias circuit (or at least verify that every component in it is sound).

[TomMcNally]

Thanks Joe - I couldn't find that on the Triode site, although I did see
pictures of my ST-70 and MK-III's

I built one of their ST-70 kits for a friend a few years ago, he had
a cathode resistor burn up, but after he replaced it things have
been fine, tubes were OK. Not sure what happened there,
music too loud ? haha

... tom

[kingmatt]

First off, thank you very much for all of your help. This place rocks!

To answer some questions: the tubes are just under 2 years old, and have a fair amount of hours on them. I've rechecked the resistance for the entire bias circuit and aside from the resistance drift when I heated up the one resistor, it has been all within spec. I replaced that 10 ohm resistor.

I've come to the conclusion that my tubes (or just one tube) is nearing the end of its useful life. They have a lot of hours on them (a lot of HARD hours on them as I really enjoy loud music). Until I can round up the cash to replace the entire quartet of tubes, I've dialed the bias all the way back and haven't gotten any flare ups for now.

Are any brand EL34s/6CA7 tubes better or worse than any other one as far as longevity goes? I like the EH tubes but had hoped for a bit longer life than these have given me...

[kheper]

Are any brand EL34s/6CA7 tubes better or worse than any other one as far as longevity goes? I like the EH tubes but had hoped for a bit longer life than these have given me...

Regarding the longevity of tubes, I think it may be the luck of the draw, but the JJ EL34 tubes I've used fail more rapidly than others.

You're not restricted to EL34s. I like the Tunsol KT66s in my ST-70.

[Geek]

Are any brand EL34s/6CA7 tubes better or worse than any other one as far as longevity goes? I like the EH tubes but had hoped for a bit longer life than these have given me...

Consensus is J/J makes the shortest lived ones (avoid the KT77's too)

Cheers!

[Ty_Bower]

I like the ElectroHarmonix 6CA7EH, but have noted that in some samples the screen construction is not perfect. They might have a few turns out of alignment, and they might visibly display excessive screen dissipation (i.e., they start to glow). I've also personally observed audible mechanical vibration coming from the tube itself. I assume some part of one of the grid structures is rattling about a tiny bit. This does not go away as the tube reaches fully warmed up temperatures.

I've used a set of JJ 7027A, which I would assume is the same exact plate/grids/cathode structure as their 6L6GC tube, only with a different pinout. I'm impressed with this particular sample, both in the quality of the physical construction and its sonics. I wish JJ had a better reputation for their current state of quality control. It would be nice to think the next set of JJ octal power tubes will be as good as these, but the evidence suggests the odds are against me.

I've also run some cheap (like, $1 each - seriously!) Chinese 6L6 tubes. I can't see the grids because there are no holes in the plates, but surprisingly these dirt cheap tubes actually sound pretty darn good. I haven't put enough hours on these guys to have even the slightest idea how they will hold up in the long haul. Once again, you spins the wheel and you takes your chances.

Of course, the most conservative answer is, "Go with old stock Mullards or Tesla." But, you'll pay a pretty penny for those.

[Thermion]

Consensus is J/J makes the shortest lived ones (avoid the KT77's too)

I wouldn't call it consensus. After a previous post questioning the lifetime of JJ tubes, I googled it and read what the guitar heads are writing. Seems more like controversy than consensus. For every post that says they're crap, there's another that says they're great.

I have never had to replace a JJ EL34 or KT88 yet after many hours of use.

JT

[Geek]

I have never had to replace a JJ EL34 or KT88 yet after many hours of use.

Oh my, you're a lucky one!

I've had zero issues with their 6L6GC's in guitar heads, but I have even received en-mass-recall advisories from wholesalers on the KT77 and a DynaMutt made real short work of a set of their EL34's.

Cheers!