Need some help with VTA ST-120 issues

I built it, and it was fine from the beginning and ran without issues for a couple years.  Then the Weber rectifier with the big resistor in it failed, taking out the fuse.  New fuse and I built a rectifier using Bob's sketch with a couple diodes.  That worked fine for about 6 months, then blew the fuse again.  I tried a WS-1 from Webber, which is what it has now.  I have the time delay, the Russian caps, and the output potentiometer installed.  I like the sound but the unreliability is getting annoying.  It is stock but I added a pilot light several years ago since it being "on" in a sunny room is not otherwise obvious.

I have noticed that the bias voltages have been wandering on all 4 tubes.   Not a lot.  Set at .496, for example, it might wonder up to 51 or so.  Sometimes they are down in the .47 range, even though they were set at .50 previously.  This will happen in a day or two of operation, running 4 to 6 hours.  I check the bias after the amp has been on for at least an hour.

I was out of town for 10 days and my wife reported she was listening and heard a loud pop or bang.  She shut it off.

I got home and turned it on. It sounds OK.  But one of the output tubes now is biased at 0.62 and the range of adjustment won't let me get it any lower.

Any of your comments or suggestions would be appreciated.  I'm a wiz with a soldering iron and can follow directions but I don't have a great deal of electronics knowledge.

Thanks!

And I'll add that I just swapped the power tubes on the side of the amp with the issue. Now the different tube in the socket that read too high is reading 18. Not 0.18, but 18.0. I shut it off.

Thank you Bob, I appreciate it. I'll get to it this morning.

The three sockets which are not acting up too much are 10.0, 9.9 and 9.9. The one which was measures 0.0 ohms. So being the electronics genius that I am, I think I should replace that resistor.

I really appreciate your advice Bob!

I soldered in a new 2 watt 10 ohm resistor. Turned it on and it sounded fine for about 15 minutes, then all silent with a blown fuse. What should I try now?

And I'll add that the bias readings were all ok, well under 0.50, and slowly climbing as it warmed up. I hadn't adjusted them yet.

I will check. But I deliberately put a different tube in that socket before I powered it up, as you suggested earlier. Thanks.

The resistor I replaced is still good. How do I know it is the rectifier? If it is, why am I going through these ss rectifiers so quickly and over and over again? This would be #3 in two months.

Strongly suggest you install the dual rectifier board upgrade:

http://www.tubes4hifi.com/DR.htm

How do I test one of those copper top ss rectifiers to see if it really is bad? When fuses went before I replaced the rectifiers since that seemed like the standard advice on here. This one (WS1) has really only been in service 25 hours or less before the fuse blew this time.

My only concern about the dual rectifier kit was the quality of the instructions. I need good step by step instructions. I emailed tubes4hifi about that and did not get a response.

And let me add, since I don't think I mentioned it previously, the amp is running a pair of Cornwall speakers. So I might be using perhaps 2 watts of my 60. I thought the diode failure was related to running the amp hard. I'm sure not doing that.

Both the VTA 120 and M125 designs run the rectification section really hard. Makes no difference how much the amps are drawing. SS offers a bit more margin then a 5ar4, but folks still can run into issues. I went with AB boards to address frequent failures on my M125's, was losing tubes, 10 ohm resistors, and fuses left and right to the point I was ready to give up on tubes. 5 years post upgrade amps remain totally stable. When I look at whats involved installing this board appears much simpler to me. Strange you didnt get a response, but there was an ownership change.

Well, I solved the mystery of the wandering bias readings. Here in the PNW, it is quite common for homes to be electrically heated. Out house, built in 1994, has eight baseboard heaters throughout the house. Depending on which one(s) click on and when, the line voltage does drift up and down by a few volts. Could be 115, or it could be 119V, or anywhere in between. From what I can tell, that has a surprising (to me, anyway) effect on the bias readings.

As to other issues and the blown fuse, I moved the amp out to the garage and hooked it up to two smaller Klipsch speakers. I looked at the blown resistor I replaced and it looked fine and tested good. With a new 5 amp slow blow fuse, I powered it up. It ran fine and loud for two hours in two different sessions. Obviously the WS1 rectifier is still working fine. At start up, before the TDR kicks in, the amp draws 0.65 amps. When it does kick in it draws a rock steady 1.65 amps.

So why would the fuse have blown after I put in the new resistor?

I agree with LeGrace about adding the DR upgrade.  I am currently working on assembling an ST-120 with a Duel rectifier upgrade. It is not difficult to install.  My personal ST-120 has a 5u4gb, and those are rated for 250ma.  There are possible issues with how fast a 5u4gb warms up, but I have had no issues to date. It also will lower B+ some which is needed anyway if your line voltage is approaching 120vac. Either solution should get you very reliable performance from your amp. Of course you will need to re-bias after either of these mods, and FWIW I run output tubes at 45ma in that amp.

If I go with a 5U4GB, do I have to bypass the time delay option?

I would disconnect it, I am not running a time delay, and I would not recommend hitting the rectifier with full current load all at once after all other tubes are warm.   I had a discussion with friend about using the time delay to control heater voltage to the rectifier and delay rectifier warm up, but I have not tried this in practice to see if there are any issues. I would also recommend getting an NOS US made 5u4GB. Although, I will say I have many hours on a new production EH 5u4GB in my amp.  Mileage may vary with new production tubes though.   For clarification on what is going on with warm up, a 5u4 variant will warm up and start conducting in about 4 seconds, a 5ar4/gz34 takes ~12 seconds which is about how long all the other tubes take.  This allows for a "soft start".  With a 5u4, B+ is up much sooner than the other tubes cathodes are warm, and many argue that this will damage tubes by what is called "cathode stripping".   I don't want to go down a rabbit hole here, but this does not seem to be an issue with small signal tubes operated at relatively low voltage.  I have designed and built many amps with solid state rectifiers that have full B+ almost instantly, and not had any issues over thousands of hours of use. Again, my personal experience.

Another note on this. A 5u4 pulls ~3 amps of heater current, the ST120 power xfmr is rated for 4 amps so not an issue. Original Dynaco ST70 xfmr was only rated for 2 amps.

These amps sound great! and with this mod, you should be able to enjoy it for years.

I will say my VTA-120 has been running so much better and more stable with Dual Rectifier board. I was wondering if my tube/resister issues were related to the large 4 in 1 cap.