Opinions on Tube Amp Problem?

I have a pair of ARC monoblocks that were trouble-free, but had the original tubes (I have no idea when they were purchased as I am the second owner), so I retubed them about 18 months ago, and all was well. Then about 4 months ago an output tube (a SED "Winged C" 6550) in one of the amps got microphonic, and started buzzing and putting nasty noises into the speaker. I replaced the matched pair, and all was well again in tube heaven. Then last night, on turning on the same amplifier, I saw a spark emanating from the tube, with the tube turning cherry red, and a loud crackle from the speaker. I quickly switched it off.

I am going to switch back to the original tubes to see if all is well, but was wondering if any tube gurus here have opinions on what is wrong. Is it the tubes (coincidentally the same in both cases) or the amplifier that has (to borrow a phrase from my friend MiTT) "gone kaka"?

No idea Bernard. Sorry. Just make sure you disconnect the speaker before trying again!!

I have had a few issues with preamp valves over the years, but replacing them has always fixed the issue - I'm sorry power valves aren't quite as easy (and cheap!) to replace.

Do you know if the Winged "C" valves were supplied by ARC? I have heard these particular ones have slightly different electrical characteristics (they're probably fine, but maybe you can try some more generic 6550s if there are ongoing issues).

Tubes go bad. It is an unfortunate event but it does happen, that said the SED is a pretty tough tube and should be better. Maybe a bias resistor or a cap is out of spec and generating nasties. The red glow is usually a sign of a bad tube or bias resistor. Have the amp checked.

1st off don't panic - it probably is the tube. Since they are monos, just pull the tubes from the other amp and give it a try again. It should be okay with luck. Power it up and keep a good eye on it for a while -switch off immediately at the 1st sign of trouble.

I'd use KT88s - the 6550 is just an American "feeling hard done by" copy of a great British tube

Joking aside, Russian tubes are usually very reliable, so I am a bit surprised. I've seen power tubes go, and the most common way seems to be the heaters either just stop working or they go bright red, and overheat other parts of the tube until they glow bright red. Time to switch the amp off!

The power source for the heaters comes from the transformer taps. 6.3V for a KT88/6550. Pretty unlikely there is something wrong with the transformers, but it is possible.

Risabet's advice is the safest. I've had quite a few KT88s go... usually Tesla Vrsovice. I just plug new ones in and see what happens... but I am less cautious than most I guess.

In the many years I have spent with tube amps, it has always been the tubes and never the circuits (actually a small cap did go in my Bill Beard once). Maybe I have been lucky...

Additionally, I have never ever had any preamp stage tube fail on me ever, apart from in my guitar amps. Loads of EL84s have gone on the Bill Beard (see my system) - JJs and Golden Dragon (terrible for EL84s, can't vouch for their KT88s).

At first blush it would appear the tube had "brewed up", but the cherry red color of the tube (plate structure specifically) suggests taking a look at the bias supply. Are the tubes individually biased, or are they "batch biased" in that are they all seeing the same bias supply? Check this out. Once a diagnosis is made, I'd suggest getting a matched set of the new Gold Lion KT88s if that's within your budget. IMO the best sounding tubes in that class. I use them in my Citation II and they're wonderful.

Your ARC has only one bias pot per channel, so it's more important to have them as closely matched as possible. look at the ARCdb.tv to see if they have biasing instructions, otherwise you can get them from ARC if you don't already know how...

That's the only downside to that amp, one tube goes and you really need to get four new ones. Unless you are a real tube geek and have a way of matching them yourself.

amey01 said:

No idea Bernard. Sorry. Just make sure you disconnect the speaker before trying again!!

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You shouldn't operate a tube amp without speakers connected. It's supposed to be bad to run a tube amp without a speaker load. Check your manual.

Craig said:

You shouldn't operate a tube amp without speakers connected. It's supposed to be bad to run a tube amp without a speaker load. Check your manual.

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Thanks for that Craig - connect some crappy ones then!!

Correct, you shouldnt operate most tube amps without the speakers being connected. The output transformer is loaded down by the speakers, and that load is 'reflected' back to the tube circuit's output stage......hence the speaker is part of the amps design, so to speak.

Jeff is also correct, as my ARC buddies pointed out. In these amps, tubes should be closely matched, and changing tubes sometimes requires a resistor change (tube rolling stuff), somewhat difficult as the resistors are soldered into the circuit boards.

Tj

Craig said:

You shouldn't operate a tube amp without speakers connected. It's supposed to be bad to run a tube amp without a speaker load. Check your manual.

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Some tube amps (mine in particular) can be powered up w/o a load attached but they should have no input signal applied in that state. YMMV

Some tube amps are OK without a load, but if you always connect one, you never need worry I guess.

You can also connect a resistor across the output terminals if your worried about speaker damage. A 4-16 Ohm will be just fine i.e. typical loudspeaker.

EDIT: removed my KT88/6550 advice since I think my experience is based on a different amp far removed from an AR.

User211 said:

Some tube amps are OK without a load, but if you always connect one, you never need worry I guess.

You can also connect a resistor across the output terminals if your worried about speaker damage. A 4-16 Ohm will be just fine i.e. typical loudspeaker.

If you decide to change brand for what ever reason (like you think SED are unreliable), expect your amps to sound markedly different, Bernard.

I found the Sovtek KT88, JJ KT88 and the original Tung-Sol 6550 very reliable - listed in order of increasing SQ. My instinct would be to try some re-issue Gold Lions these days, though.

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No tube amps are ok without a load. Whenever you fire it up without a load, you risk output transformer damage. Use the resistor (get at least a five watt one, preferrably higher if you can) or just get a cheap speaker.

Which ARC monos do you have....

Something like the Classic 120's or 150's?

Funny I read recently that some are... I'll try and find the quote to see if we can establish whether it's BS or not. Risabet seems to think it's alright on his amp too, with caveats.

Looks like you caught the tube advice I edited away...

Worst case scenario: could be a plate or cathode resistor. I had one go in my VT 100 III. I had it repaired by an independent tech for about 125 bucks and now all is well. He claimed that some of the original parts spec'd by ARC were under-rated.

Hm. Sometimes I wonder why we post on these forums. They are a mass of conflicting advice sometimes.

Just who do you believe? I think posts are usually made with the best intent, though.

Not saying anyone is wrong here. Could be a number of things listed so far.

Still trying to find that quote...

I can't find it. I was sure I read it in a magazine I have, and I thought it was a bit odd. Risabet? Am I guilty of talking out of my ar$e? Possibly, but not intentionally.

See quote below, anyway.

"The AC current and voltage on the primary and secondary sides of the output transformer are directly affected by the load on the secondary side. The optimum conditions exist when the secondary (or output) side of the transformer is properly loaded. Some tube amps use shorting jacks for the speaker outputs. When there is no load connected to the speaker jack, the output of the transformer is shorted to ground. Now while this is far from an ideal condition, it will, for a short duration, prevent damage from occurring in the output transformer. It will, however, cause damage to most transformers if this condition is sustained for any extended period of time because too much current is being drawn from the transformer."

Does anyone know what happens with an unloaded OTL? Sorry Bernard, but I am interested in this, as I am sure others will be.

Some tube amps have a standby, such as a Lumley ST70 I once had. Looking at another quote below, for a guitar amp, but the principle should be the same:

" sure, you can run an amplifier in standby without a load. In standard standby circuits there is no high voltage in any part of the circuit so nothing at all will happen. In the JVM the standby is a bit different but there is no current flowing through the output transformer and nothing will be damaged.

I don't know if you are aware but the JVM has a "silent recording" mode and that's with the Standby switch OFF so the whole amp is working but the power amplifier section isn't. In such case, and as written in the manual, it is perfectly safe to have the amplifier running without any load attached.

Going further, running signal through the amplifier without load doesn't necessarily result in an immediate output transformer failure and usually such failures are "helped" by output stages biased way too hot or specially by intermittent/faulty speaker connections. It may even happen that the output transformer will not blow up after long periods without load but in any case and to be on the safe side never switch the standby ON without a load connected to any valve amplifier.

There is a lot that can be talked about output power transformers and what kind of stress they will suffer if a load is not properly attached but basically never switch the amplifier out of standby mode without a load and be very careful with faulty speaker connections. "

and supporting Risabet:

"You can have your amp on standby with no load connected, this is perfectly safe. You can also have your amp fully turned on with no load connected as long as there is no signal passing through it ie. guitar plugged in and played by somebody. The potential for damage is when the OP Tx tries to drive signal into an open circuit (no load). "

Bingo - think I have found the answer.

http://209.85.229.132/search?q=cache:Y9CZEOfJtWEJ:www.davidberning.com/support/zh270+OTL+tube+amplifier+with+no+load&hl=en&ct=clnk&cd=49&gl=uk

See link. Looks like OTLs, because they have no output transformer, can be operated without a load. At least, this one certainly can.

tonepub said:

Which ARC monos do you have....

Something like the Classic 120's or 150's?

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I have the VTM120's.

I contacted PartsConnexion (the Sonic Frontiers guys), the guys from whom I bought the tubes and explained the whole situation. The tubes are out of warranty, but they offered to replace that one tube, if I send them the specs off the box, at a reduced price. I find these guys really good to deal with; they go out of their way to please the customer. I will take them up on their offer and monitor the situation closely.

User211 said:

Does anyone know what happens with an unloaded OTL? Sorry Bernard, but I am interested in this, as I am sure others will be.

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No problem, Justin. This is the forum for this kind of stuff.