WZ-68 blew up upon start up & lots of noise from right channel

HI, (AND HELP!) I started up my st-120 this morning and it cracked and popped and then blew the fuse .SO, I removed my penta labs kt-88 's and replaced with a set of ruby tubes and all I had was a JJ 5AR4 and 2 sovtek's so i put in the jj which burnt up too ( I have never had any luck with the JJ rectifier's so that was not a big surprise) so i dropped in a sovtek and it is working so far . My Question is would(or could) a bad or shorted or ? Output tube cause this ? or could something else be wrong . Or maby the webber just reched the end of its life and this is normal. ( I just don't trust the JJ as a reliable trouble shooting tube) I Guess I should have the penta labs kt-88's tested? Where is best place to get a webber wz-68? Any advice would be more than welcome....

I've got an ST120. I've had what you described happen twice but it was while I was running GZ34's and running the amp loud (too Loud).

If this happened while running the WZ-68 I'd say it is odd or rather agree with you it was odd. I don't think these blow as they are pretty overbuilt.

I would say it is 99% chance that the tubes are suspect. You could try putting in the WZ-68 back in with no KT88s... See if a fuse goes.. Or just leave the amp for now as it is running... Get the KT88s tested.

New copper cap:
http://www.webervst.com/ccap.html

http://www.webervst.com/ccap.html

I'd probably just run the amp as you have it now and wait for Bob to chime in on this.. You may want to look at the diode mod Stickey on this forum... Think about it. Easy to undo this mod if you decide it isn't for you. I've got this mod in my ST120 and I run my ST120 pretty loud... I'm running cheap Sovteck GZ34 and running er hard and no issues..

I had the same type of problem a year or so ago. I actually re-built two of my Weber WZ68's and used 3 watt, 1000 volt diodes because the power here is so bad but the point is that they can malfunction. When I had the problems I also had a burnt out bias resistor so there may be a connection there as well.
Mine burnt one of the big wire wound resistors once and a diode once I'm not 100% sure what would cause a fairly robust component to burn up like that but since I re-built mine they have survived a lot of power anomalies and at least 2000 hours of duty.
Bob mentioned that they are already quite robust and had given good service in his own amps I'm wondering if he has ever changed them out.

I have high voltage here also usally 125-130 volts. So I put in a wire wound power resister in the line between the on off switch and the transformer. I have a 200 watt 10 ohm one on my st120. It seems to work just fine. need to put the resister in a place where it can get good cooling. Its amazing how much cooler things run from just a few volts. I put resisters on on all my tube equiptment because this stuff was made to be run at 110-115 volts not 130. Might have to experoment with a few diffrent resisters to find a voltage your amp likes.

Should i take this into a tech? because it arced again on the last rectifier tube I have .It did not blow but it arced so I shut it down .Any thing I should tell my tech? I just know it ran fine for a month then it started this problem and I have no idea of what to do .I have a new set of pre tubes ( I am using 2-electro harmonix 12BH7's and one Psvane 12AU7) .With the attenuator and a CD player and no pre amp..Thx Scott

And it ran absolutely perfectly for 7 months pretty much every day before you received it.

HI, Bob I bought this amp from jeff hoak last month and yes he used the low gain board . So I did not build it. Jeff built it last june I believe .Yeah I guess I will take it to my Tech .I am just an audiophile, not a tech .And he told me about the 12au7 (in the center)&12bh7's flanking it . And i was not turning it up too loud .I had just turned it on .I have been very gentle on it .I dont need to drive it hard as the LaScala I am using are in a room about 12'x12' so I have never turned it up past about 10 oclock position .I will take it in ... I have used it for about a month so it has been working perfect till now .I thank you for your time,Scott

I just had one last question. Could an output or rectifier tube cause all this? I ask because I have used two st-70's on this power source for 3 1/2 years . and this st-120 ran flawless for a month.And as for running it too loud this happened at start up .( I have been warming it up and checking the bias at about 5 minutes,and then playing one disc [45 minutes ?] and then checking it again. Now this morning all the tubes are glowing and I am getting no bias and no sound at all. does this indicate anything very telling ? I am just worried that something big has gone wrong . I am taking it in to the shop now .I will take all my tubes too.

Wow, that's like Deja Vu all over again. I built a ST120 late last year with the Low Gain VTA board and had a real tough time with it blowing rectifiers (5AR4) and fuses. I ultimately concluded that I had two problems. First, I have a higher than normal line voltage (125 VAC) which took out my quad cap after listening at a pretty loud volume level for close to an hour. Bob sent me a new one and that seemed to fix part of the problem, but I have to run my amp using a Variac, I now dial the line voltage down to 117 VAC. After that, I was very careful to make sure that all my voltages checked out exactly per the assembly / test instructions, I never could get the B+ voltage to measure within specification (410 to 440 VDC). After trying for almost 2 months to figure out what I did wrong I gave up and took mine to a local Tech. After looking at my amp he concluded that the 7.5K ohm resistor that connects across the 20 Mfd and 30 Mfd lugs of the quad cap was inappropriate for the Low Gain Board. He added a 2nd 7.5K ohm resistor across the same two lugs, which effectively halves the resistance. I have not had any problems since. Right now I'm listening to Michael Brecker's last recording (Pilgrimage) through a pair of new Cornwall III's and it sounds real nice.

Not sure why Bob says all the ST120s have the Standard Gain Board? I discussed my decision with him before I purchased my amp with the Low Gain Board. Based on that conversation and what I have read here and elsewhere, for high efficiency speakers the general consensus seems to support my decision. I saw no mention of the ST120 requiring the Standard Gain Board. If I had it to do over again, I would have bought the standard gain board and saved myself a lot of heartache, frustration and money.

before I even got to the 2nd post (let alone the 14th) I suspect high AC input voltage (over 125vac).
I suppose one of the reasons I have NEVER blown an amplifier in over 20 years is that I always use a variac,
it's really cheap $70 insurance (maybe $100 for a 5 or 6 amp variac, mine is 3 amps).
And a good surge protector power strip also!

OK, I want to know where to get a variac. and also do you recommend a specific model # ? And a specific model of surge protector? thx,Scott

This is the surger protector that I used with that amp:

http://www.belkin.com/IWCatProductPage.process?Product_Id=607183

Do you have Frys store near by?

http://www.frys.com/product/5422799?source=googleps

My normal line voltage here runs between 119V and 121V. I've never seen it go above 123V.

Search ebay for Variac or variable transformer. I use one like ebay item number 150776211777, which is a Statco 10 Amp model, but thats way more than you need. I paid $35 for mine (used)at a local electronics surplus store. I had to replace the cord and fuse holder, which had been removed for some reason. One of these days, when I get a little extra cash that's burning a hole in my pocket I'm going to buy a rebuilt General Radio metered unit like ebay item number 110507170826. If you like to tinker, you'll find all kind of uses for a variac. I would not worry to much about which one to buy, as long as you make sure it has enough current capability for your application, but I would stay away from the really cheap improted units. I'm quite pleased with my Statco.

this is the one I've been using for the past 5 years . . . . .
new variac

HI, I just have a hard time with the reasons that blame the wiring of the amp or the line voltage as I stated befor this amp ran perfectly for the builder for 7 plus months and I ran it for 1 month then it goes down .I cant afford to take it to a tech and put more $ into it I don't have unlimited funds.I just want it to be as dependable as my 50 year old st-70's.(I sold them both) to help pay for this st-120 .Wish i hadn't "upgraded"..

First SORRY for the "upgraded" comment .I believe that the problems I had were due to the fact that my LaScala have the AA crossover (4 OHM ) and I was also running a subwoofer with the speaker level inputs to the sub amp 4 OHM PLUS 8 OHM ) = problem , Sooo.. I had my VTA ST-120 looked at and found I had a bad KT-88 And my ST-120 was set up for 8 OHMS. I have set it up for a 4 ohm load and am using a variac and have added a McIntosh MX-114 pre-amptuner (one owner with the wood case in mint condition ,very fun !) with the sub running from what I would think was a center channel pre out into my sub. and I have to say that this has been a learning experience and it sounds so excellent.So I Have a question : Is it advisable to run KT-120 tubes in the VTA ST-120 ? I was reading that they would last longer but to check with the manufacturer .Or should I just stay with KT-88's and get some extra's?

Hi,Bob should the bias (for KT-120's) be lower? I run my KT-88's at about 5.3 volts . and I think I recall someone say the max for KT-120's should be about 4.40 volts?( I know it say's 5.5 volts for the VTA ST-120 with KT-88's but my tech said to error on the lower side Soooo, I have been setting the bias at close to 5.3 volts. Thx for your expertise.Scott

THANK YOU BOB, I want to know if the KT-120's will have a longer life . I don't have to turn this system up "loud " as it's got your VTA ST-120 driving a pair of LaScala and a JBL 2245-H 18" subwoofer with a 500 watt amp . So this is a very powerfull system in a small (12' X16" ) room. Or should I just stay with the KT-88's ? I am finally happy with the sound and never have to tax the system ,and enjoy lot's of reserve power, a nice situation to be sure.