I Think I'm Done with Tubes. - AVS Forum
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post #1 of 13 Old 11-26-2013, 01:24 PM - Thread Starter
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Once again I have a KT88 power tube failure after only 90 days of use - out of warranty of course. And this batch were the more expensive Genalex Gold Lions. And, you should always replace output tubes as a pair. I think my all tube LCR system is getting too expensive and time consuming to maintain at peak operating condition. Tubes, especially power tubes, have limited life and require monthly bias adjustments to keep at optimum operating conditions. Note that we here demand peak performance of out HTs all the time don't we? And then there's the idea that the amps are constantly degrading as the tubes age over a year.

The Chinese and Russian tubes are not of the legacy 1960s quality from the old American factories. And keeping up my system that uses 12 power tubes and 15 small signal tubes is a bit much. Rack Space, waste heat from the filaments and the required fan cooling, and the intermittent crackles and pops in the speakers that are normal with tube amps. And yes, my tubes are run within spec and not over driven in any way. I even installed AC filament regulating transformers to keep that parameter in spec.

As many here know tube amps have a particular good sound to some people but they are not sonically accurate. I have mine down to 0.1% distortion which is as good as many transistor amps but the frequency response is not ruler flat and changes with power output. This is quite common when using output transformers which are mandatory with tubes (OTL amps excluded).

It was a fun run since 2006 when I built them but as I have quite a few pro amps laying around from various jobs I think it's time for a change. And in light of my new all digital audio system rework, perhaps it's also time for different power amps as well.

Comments appreciated.

NOTE: The picture still shows the tube crossover at the top. I ditched that two years ago in favor of a Behringer UltraDrive unit. Not as bad as some people think of Behringer but it was a bit noisy at low levels.

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post #2 of 13 Old 11-26-2013, 03:11 PM
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Originally Posted by Glimmie View Post



Once again I have a KT88 power tube failure after only 90 days of use - out of warranty of course. And this batch were the more expensive Genalex Gold Lions. And, you should always replace output tubes as a pair. I think my all tube LCR system is getting too expensive and time consuming to maintain at peak operating condition. Tubes, especially power tubes, have limited life and require monthly bias adjustments to keep at optimum operating conditions. Note that we here demand peak performance of out HTs all the time don't we? And then there's the idea that the amps are constantly degrading as the tubes age over a year.

The Chinese and Russian tubes are not of the legacy 1960s quality from the old American factories. And keeping up my system that uses 12 power tubes and 15 small signal tubes is a bit much. Rack Space, waste heat from the filaments and the required fan cooling, and the intermittent crackles and pops in the speakers that are normal with tube amps. And yes, my tubes are run within spec and not over driven in any way. I even installed AC filament regulating transformers to keep that parameter in spec.

As many here know tube amps have a particular good sound to some people but they are not sonically accurate. I have mine down to 0.1% distortion which is as good as many transistor amps but the frequency response is not ruler flat and changes with power output. This is quite common when using output transformers which are mandatory with tubes (OTL amps excluded).

It was a fun run since 2006 when I built them but as I have quite a few pro amps laying around from various jobs I think it's time for a change. And in light of my new all digital audio system rework, perhaps it's also time for different power amps as well.

Comments appreciated.

NOTE: The picture still shows the tube crossover at the top. I ditched that two years ago in favor of a Behringer UltraDrive unit. Not as bad as some people think of Behringer but it was a bit noisy at low levels.

I spectacularly blew up a Graaf 200 OTL tube poweramp, and have been avoiding tube poweramps like the plague aver since. Preamps never had a tube issue ever. Also had a tube DAC once with no problems.
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post #3 of 13 Old 11-26-2013, 03:25 PM - Thread Starter
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I spectacularly blew up a Graaf 200 OTL tube poweramp, and have been avoiding tube poweramps like the plague aver since. Preamps never had a tube issue ever. Also had a tube DAC once with no problems.

Yeah, you can get 20,000 hours or more out of preamp tubes. Outside of a microphonic tube every now and then they do last.

And yes OTL tube amps in particular have a bad reputation for reliability. In addition to that, the OTL nature of the design will often take your speakers out along with the amp failure - just like unprotected 1970s SS amps did. There is one vendor that disputes this claim but most OTL stories I have read were not good.

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post #4 of 13 Old 11-26-2013, 07:40 PM
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Way back when I first got interested in audio, it was all tubes. Transistors were few and far between, cost a fortune and didn't work well at all. I had my fun with tube amplifiers too, but moved onto solid state in the 70s. As I recall, 6550s were the unreliable device and everyone was replacing them with Mullard KT88s. Welcome back to reality!
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post #5 of 13 Old 11-27-2013, 09:56 AM
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Your amps are gorgeous Glimmie! Do you know Steve Fryette?www.sfdamp.com ? He's here in the southland and builds a lot of guitar amplifiers using the KT-88, he might be able to steer you to a more robust source. You probably know the ARS folks, they sort pretty well and are local. Hate to see you give up on the bottles. I have a pair of monoblocks that my deceased father in law built in the sixties. They have 6l6 complements. Pretty lucious sound with a turntable in particular. I have quite a few guitar amps with tubes and get a lot of hours out of them, but they may be biased more conservatively than your rigs. Best of luck! Happy thanksgivikka!

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post #6 of 13 Old 11-27-2013, 05:09 PM
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Dave O'Brien of McIntosh Test Clinic fame once told me that in his opinion, the most reliable, stable output tube was the 6L6-GC. McIntosh originally was using Tung Sol 6550s in the MC275, but the cathodes were stripping and they switched to KT88. Dave didn't seem to be all that impressed with the KT's either, and that was more than 30 years ago.
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post #7 of 13 Old 11-28-2013, 12:39 PM
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Glimmie, here a few thoughts on solving your tube failure problem.

Try another brand of tube. I needed to buy a couple of replacement 6CG7s for my headphone amplifier a couple years ago. After doing research on the various brand names, I came up with Electro Harmonix as being the better brand. They offer both 6550s and KT88s. 6550s of today may not suffer the problems of the past.

Try reducing output stage idling current. McIntosh ran the KT88 output stage in the original MC275 class AB2. Idling current was low enough that you could actually touch the tube envelopes without incinerating your finger. Class AB2 tubes draw grid current on signal peaks, so they had to drive the output tube grids directly from a 12AZ7 cathode follower stage. In the MC2000 they directly drive the output tube grids from a transistor darlington stage bootstrapped to the output tube cathodes.

Have fun!
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post #8 of 13 Old 12-01-2013, 01:29 PM - Thread Starter
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Originally Posted by fastl View Post

Glimmie, here a few thoughts on solving your tube failure problem.

Try another brand of tube. I needed to buy a couple of replacement 6CG7s for my headphone amplifier a couple years ago. After doing research on the various brand names, I came up with Electro Harmonix as being the better brand. They offer both 6550s and KT88s. 6550s of today may not suffer the problems of the past.

Try reducing output stage idling current. McIntosh ran the KT88 output stage in the original MC275 class AB2. Idling current was low enough that you could actually touch the tube envelopes without incinerating your finger. Class AB2 tubes draw grid current on signal peaks, so they had to drive the output tube grids directly from a 12AZ7 cathode follower stage. In the MC2000 they directly drive the output tube grids from a transistor darlington stage bootstrapped to the output tube cathodes.

Have fun!

Appreciate the help but I have been there. Remember these amps were built in 2006 and the tube operating parameters are well within guidelines. I always biased the KT88's and 6V6's on the low side to help life. I have tried several brands. It think it's just par for the course with tube amps in this duty cycle. I still have my tube music system which has been very reliable - but then only gets a few hours use per month.

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post #9 of 13 Old 12-01-2013, 05:12 PM
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Well I remember frequently re-tubing Dynaco MK IIIs, amongst others. Don't miss the ritual. I suppose you could use them as space heaters.
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post #10 of 13 Old 12-01-2013, 06:58 PM
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Glimmie,

I had to combine my love of home theater with my love of music/audio in one room. I love tubes for music amplification, but wanted the reliability of SS for home theater stuff. So I just have two separate systems. Or..kind of. In my case, I have two separate runs of speaker cable from my L/R speakers back to my equipment room. On pair connects to the AV receiver for movies. But when I feel like listening to tube amplification, I just insert the other pair of cables which are attached to my 2 channel gear - Conrad Johnson tube mono-blocks (and sometimes my Eico HF-81 integrated tube amp), tube pre-amp, DAC, turntable etc.
It's as close to best of both worlds as I could manage and generally works great.

Do you have a separate system for listening to music?
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post #11 of 13 Old 12-01-2013, 09:40 PM - Thread Starter
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Do you have a separate system for listening to music?

Yes. It shares with the family room system. There are two 10amp relays on the main left and right speakers that engage when the tube amp is on. Otherwise a common Denon AV receiver is inline feeding the main L&R.

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post #12 of 13 Old 12-02-2013, 12:11 AM
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The KT-120 is a new ultra stable , ultra life, higher output tube designed just for audio. It is a beefy replacement for 6550s and KT-88s. It is the basis for all of Audio Research's new Ref series amplifiers. I would like to see Glimmie design an amp around these tubes. It might change his mind on tubes. Norm
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post #13 of 13 Old 12-02-2013, 05:12 PM
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It might change his mind on tubes

I get the impression that the honeymoon is over! I'll have to go check out those KT-120s. I've seen them around for awhile, but never looked any closer.
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