Just Lost a KT-88 in the ST-120

The ST-120 blew this morning.  Listening at low volume and all of sudden the left channel screamed with hum and the front left KT-88 glowing cherry red.   Within seconds flipped the power button but it smells like something burned.  Pulled the ST-120 and the front left 10 ohm cathode socket resistor looks a little charred but measured 10.0 ohms and the nose test tells me yes it got hot.  So will replace it nonetheless.   Nothing on the driver board looks stressed, coupling caps OK, and the left output transformer ohms out OK.  Think I will be good after replacing the cathode resistor. 

I cannot recommend the Shuguang Natural Sound KT-88-T as this is the 2nd one to go with about a month of run time.  The Psvanes, also built by Shuguang, have been much better and survived for over 6 months before exchanging.  After repair think I will return to the Gold Lion KT-88s which also have about 6 months of run time.   Been a while since I heard those tubes.

Life would be boring without a little drama but I almost spewed coffee on the keyboard when the tube went south.

arledgsc wrote:
I cannot recommend the Shuguang Natural Sound KT-88-T as this is the 2nd one to go with about a month of run time.  The Psvanes, also built by Shuguang, have been much better and survived for over 6 months before exchanging.  After inspection of the amp I think I will return to the Gold Lion KT-88s which also have about 6 months of run time.   Been a while since I heard those tubes.

"The Psvanes...survived for over 6 months"? Does this mean they went bad somehow?

I'm curious, because 6 months doesn't seem like a long time for power tubes to live. Unless you're one of those fortunate people who can listen to music almost all the time

dougmon wrote:I'm curious, because 6 months doesn't seem like a long time for power tubes to live.

The Psvanes have been very reliable and swapped just to try something different in a grand but expensive output tube rolling trial.  New Psvanes take several hundred hours to bloom so you have bake them for many months before final judgement.  I'll get back to them and sound very nice in the ST-120.  I just haven't heard the Gold Lions in over six months so eager to try them again.

Of the four output tubes tried in the ST-120 I've only had issue with Shuguang Natural Sound KT-88Ts.   The fourth tube type is the Tung Sol KT-120.  I'll get back to those eventually as well.

The big advantage of the KT-120 is the fact that it is being run very conservatively in the ST-120 so they should have a very long life.

I recommend RAM labs for properly matched tubes. I don't work for them. Just a satisfied customer.

The bias resistors seem to be a fairly common failure point. Should we be considering a mod or upgrade for this circuit? Maybe socket them and go with a fuseable version instead?

If nothing else, they do seem to go fast enough to protect the rest of the amp when a tube goes south. I expect that falls under the heading of "good thing" ...

Knock on wood - I've got a couple hundred hours on the KT120's now with no issues. I just recently picked up a Mullard GZ37 too, so I should be good there.

PS ... waddaya running for bias? I understand the ST120 is good from 35-55mA, and some run as high as 60mA on the KT120's. Also could be a rectifier issue? Apparently a 5AR4 is borderline in these amps, and the recommendation is either a GZ68 copper top or GZ33 or GZ37.

I still like the idea of socketing the bias resistors for easy replacement. Maybe next time I get under the hood. I found these breakaway socket strips that look like they'd do the job nicely ...



AMP 510-AG90D-10 (Mouser)


Basically just replace the leads on the current resistors with solid wire, attach the sockets, and slide the resistors into the sockets.

I'll put it on my "to do" list with a question mark.

The ST-120 is repaired and back in service.  Cost me 15 cents for a new 2W 10 ohm resistor from the local electronic junk store.  Sounds great with the broken in Gold Lion KT-88s.  These tubes can throw a big soundstage.  Little hot on the top but are better overall than I remember.  Kept me listening for over an hour last night and that is what it is all about.

SkiZo wrote:PS ... waddaya running for bias? I understand the ST120 is good from 35-55mA, and some run as high as 60mA on the KT120's

Speaking of Gold Lions I ran those tubes hot (55mA) with the copper top WZ68 in the newly built ST-120.  Then one day I figure with 495V plate voltage the bias operating point is set at 78% of the max 35 watt rating.  Now I run tubes around 51ma to 52mA which is still 72-73% max.   With a tube rectifier and slightly lower plate voltage you may need a higher cathode current to get the bias right.   With KT-88s I really don't notice much difference in sound with the bias set between 50mA and 55mA.

Well i been using tubes for more than 60 years.......Never had minny tube fale like you guys!
New are old tubes.............gesssss...
One thing i have alway used a varack on all tube setups........you can get one for $50. on line...i got about 10....i paid as littel as $10 at sales....lot less cash than tubes....i find i get more output of the amps if i hit the sweet spot....it at about 110-15 AC.....
ARC amps sweet spot B+ 440....if thay need more power that just add more output tubes.....

Back in  60s the frist time i saw jimi Hendrix in a small hall.....paid $5. to get in....this was when he was seting his gitar on fire..........those day...the stag was low....maybe 3' off the ground...an i saw jimi with a varack on his tube amps....an he would use it to get diff tones.....an i have been doing the same thing for a long time ...........works great on preamp....i have found the heaters are to  always to high..droping the small input just a littel alway sounds better ...well to me..
when i got my frist hhscott 299c with all stock 60 years old tubes... i had to find out haw thay got them to last so long.....some of best sound ever ....thay ran there small input tubes at 5.5.....

as with all things just one mans o-pine


thanks for the site Bob...........

Does biasing the amp for a lower voltage change the sound and prolong the life of the tubes?

Does biasing the amp for a lower voltage change the sound and prolong the life of the tubes?

Yes, it does affect sound as well as life of the tubes perhaps.  When you are tweaking the bias voltage you are really adjusting the tube's idle power which is traditionally 70% of the output tube's maximum rating.   By design this 70% idle power should yield optimum sound but sometimes though you get better sound quality with a higher idle power.  The original Dynaco amps adjusted them fairly high for this purpose.  But with higher power you get more heat and stress so there is a trade off.  Everyone keeps saying these expensive modern tubes aren't as robust as NOS so I felt I was pushing my luck at constant higher than normal bias point.  Through experimentation by adjusting bias down (55ma->54ma->53ma etc) I really could not hear any difference is sound quality at low volume levels.  Again I am using the WZ68 SS rectifier which yields a higher plate voltage than a GZ34 so it makes sense the cathode current is less for the same power.  This is my situation and others may disagree but trust your ears.  And Bob has the most experience so by all means go by his recommendations if you are unsure.

Google Class AB amplifiers for more information about how these push-pull amps operate.

But when you lower the bias..... you get more B+ on the plates....
If you have a Varick...you get better sound an amp run cooler... new tube last for ever....it a win..win...
You can drop the B+ an then set your bias....an youll hear a diff....
Look were at 120-125 AC coming from the wall.....in the day or nite......
I have found with UL output trnx setups.....or any output tube setup.... any thing above 500 B+ well kill tubes fast...an can get hard sounding...
460-80 on the B+ is the best sound....
If tube amps i have owned over the years eat tubes like your guys....i would not Have the best sound going...i would be lost with Sand amps............not good!
When i sale a tube amp......for sure if the amps old stock....Dyanco..hhsott...Fisher...thay get a Varick with the amp...........an No dead tubes..... new are old........good luck...have fun

thanks for the site Bob...........

I'm going to have to play with a Variac one of these days to regulate the line voltage as it fluctuates. Can you recommend a Variac with a nice voltage meter so you can just walk by every so often and adjust the output voltage easily and accurately without connecting a voltmeter?

Seems a Variac would be necessary equipment for vintage gear. 40-50 years ago 110-115V was common at the wall plugs and the equipment of the day designed accordingly. Thanks!

Funny....i have none with meter,s........but like i say night is when AC risers the most here .....what is you B+ at any time??
I bet it well over 500dcV.....get  down to....460-80 let ride... then you mark the Varick... i stay at 110 AC...if i drive the amps had may 115AC...
Youll love the sound even more.....when go to anyone house to hear are re-tube a amp i always take a Varick..
Most cant beleve it....drop the AC a littel an you have a new Amp-preamp are you name it...
Even the Krell amps an pres an DAs sound better with less AC....but SS amps pulls a lot of AMP from the AC....so to run SS amps you need a big Varick....tubes pull low ma,s....work great!

Variac would be necessary equipment for vintage gear....the amps star to make sound at about 70-ACV.....
I say any tube setups Varick are must!......Any Varick is bettter than NO Varick!!!!
Look 2 dead tubes.....thats more than a Varick....less you have BIG $$....to me it not right to eat tubes .... 1 year the tubes are not eve brok in.....I have a pr of rudy 6550s here from 1990...
an i have used them for amp testing an have run them red...from time to time an was just playing them for fun to see if that still sounded good....shocker...thay sound all most as good as my 60 year old GE 6550s.....test strong........have fun with tubes...


Thanks for the site Bob....

You are correct but feel a lot of my tube woes of late are simply the Shuguangs I purchased are crap.  They came from China, cheaper price than most, and were probably first quality line rejects.  Shuguangs about a year ago or so had tube weld issues.  But thanks for the information and will have to pick up a Variac one of these days.

I have the Tung Sol KT-120s in the amp today.  Biased at 55ma and they really sing.  Wow!  These are nice tubes even with a few hours of run time.  Don't remember them sounding this good in the new ST-120.  So for new amp owners wait at least six months before heavy tube rolling experiments.

Double yup on the KT120's. I switch back to the Psvane KT88's every now and then just to exercise them, but it's just not the same. +1 on the 55mA also. Another thing I notice is that I have to tweak the bias fairly regularly on the Psvane's where the KT120's are rock steady.

Your comment on rolling is appreciated, but it's very difficult if not impossible for some of us to hold off. At least I'm at the point where everything I've got is broken in. I've even settled (more or less) on my favorites.

I did make one last tweak of the week ... I swear! ... Been running the KT120's with a Mullard GZ37, 1940's RCA foil getter 12AU7's on the channels, and an RCA clear top in the middle. The tweak? I've got three of the clear tops and switched to one of the spares for the middle.

wOw ... BIG difference. So, moral of the story ... even identical tubes can have differences, so if you got extras, try them. I liked what I was hearing before, but this one made for some serious improvements.

PS ... your source for the Chinese tubes could very well be the problem. I received several stern warnings when looking at amps to be very sure and use factory authorized sellers. Apparently a lot of seconds floating about, and even rumors of street kids taking over the factories on the graveyard to slam out some clones. You could get lucky, but anything goes wrong, your chances for support or remediation are slim to none.

I agree with sKiZo it is hard to resist tube rolling, IMHO the VTA ST-120 with just over 200 hours and all upgrades offered connected to Klipsch Heresy speakers sounds incredible with 12BH7s

I agree with sKiZo it is hard to resist tube rolling

ha-ha! I've been tube rolling from day 1 and will keep doing so.  I am though going to have to go back and reassess my original tube screenings but it will be interesting.  The KT-120s originally sounded OK with the bass accentuated and not much on the top (darker) as compared to Gold Lion or Psvanes.  Now they sound flatter with excellent bass authority and clear crisp trebles.  Glad I switched over to the KT-120s and they only have a few days of total run time.  The KT-120s should last a long time, available everywhere, and cost about 1/2 the price of tweaky, hand selected tubes from exotic locations.