VTA ST-70 problem

Hi All,

My VTA ST-70 has performed faultlessly since I built it in 2018. Earlier today it did something strange. I switched it on, sent it some music but no sound came from the speakers. I looked at the valves and noticed one wasn't glowing:



I'm not sure whether the image of my amp will upload, but of the four EL34s it's the front right one that didn't glow.

So I shut down and swapped the front two EL34s. Still the right one didn't glow, so I knew It wasn't the valve.

Also it seemed the centre and right-hand valves of the driver board were cold to the touch while the left one was (very) warm!!

So I sat down to write this and while doing so (about five minutes from initial power-up) the sound came on, the front right EL34 began to glow and now all three valves on the driver board are very warm to the touch.

So obviously I'm pleased and relieved but being naturally curious I wonder whether anyone can offer an explanation as to why it did that?

Best Wishes to all,

Chris

I hope that rectifier can be saved - it is a nice one. I had mine redplate really bad a while back - but so far it is working just fine GZ37.

Dear Bob,

Thanks for taking the time to reply. All has become good again so I'm leaving well alone!

Seeing as I'm here I would like to share my thoughts about this amplifier. I use it without a dedicated preamp, Front end is a Marantz CD6006 (UK). I also send the Marantz high bitrate MP3, Flac, WAV and the Marantz's DAC sends direct to to the VTA ST70. Speakers are Klipsch Heresy III.

1. The kit came with excellent instructions and was a joy to build. I was sad when it was over!
2. This amplifier is absolutely marvelous.
3. Super quiet when nothing is fed to it. All it does it take whatever signal it's given and amplifies it. It removes or adds nothing.
4. It is absolutely ruthless! If you give it rubbish (poor or low quality front end, 'bad' or low-fi recordings) you will get amplified rubbish out. But oh, if if you give it good quality signal with well-produced recordings it just rewards and rewards. Imaging is superb.
5. The rating at 35W a side seems conservative; I rarely have it up full volume.

I've owned a lot of high-end kit in my time (Linn, Cyrus, Quad amps & electrostatic speakers, Tannoy Yorks (15" Monitor Golds), ATC SCM100 (which I regret selling!). I'm done swapping, chopping and changing (well, maybe I'll get into some so-called 'tube-rolling'). This current setup will probably see me out.

Best wishes to all, Chris

Hi

I suggest you also check the 10ohm? Resistor to ground.

Hi Gabriel,

[b]I suggest you also check the 10ohm? Resistor to ground.[b]

Please could you give a bit more info please?

Regards,

Chris

Highly agree with your comments on the VTA ST-70. I absolutely love mine. In fact built one for my best friend and he loves it too. No trying to convince him to look into Fritz speakers -- this is a great combination. My GL KT-66's are original (coming up on 9 years) and still work well. Have gone through a few rectifiers though. Ordered up some Nesstone 6550's to swap out based on the comments of replacing output tubes every 3-5 years. (GL KT-66's not available now due to the crisis) Guess I'm listening on borrowed time today! Will be interested to hear the differences. Believe I need to re-bias to .500V too.

seeing as how you said it started working about five minutes later, it should be very obvious you have a bad connection on that right front tube, most likely a cold solder joint.
That's also were the filament connections to the right half of the driver board come from. Resolder everything on that socket, and check the amp for any other possible cold joints.

"Seeing as how you said it started working about five minutes later, it should be very obvious you have a bad connection on that right front tube, most likely a cold solder joint.
That's also were the filament connections to the right half of the driver board come from.   Resolder everything on that socket, and check the amp for any other possible cold joints."

+1 Exactly

If some of your filaments went dark, and now they light again, that suggests to me a cold or dry solder joint. I'm not familiar with your exact model, but check the filament chain--with octal tubes you generally have twisted pairs of wires connecting together all the pins 1 and 8, and the filament transformer winding. Grab those wires and wiggle them. If the ends of the wires on the socket lugs wiggle, you have a bad joint. Depending on where in the chain the driver boards filament supply is taken it can also be affected.

With multiple connections on a lug, it's very easy for the joint on top to look nice and shiny and wicked onto all the visible surfaces, but not quite make it to the wires below. Such a connection may perform OK for a while, until it doesn't. If you find such a thing, that should be incentive, now that you've had a chance to enjoy the fruits of your labor for a while, to take some extra time and recheck all your work. It could be that in your zeal to finish the project you got a little careless. Nothing to be ashamed of, it happens to most of us.

First things first, as in KISS. Swap out to another tube (check) swap to a spare rectifier (you DO have a spare, doncha?), then clean the sockets and pins, and re-tension all the sockets.

PS - even just sliding each tube (carefully) up and down in each socket can work wonders at breaking oxydation that could result in this sort of intermittent problem.

And ya. Hide the soldering iron if you have one and don't know what to do with it ... ;-}

Chris A wrote:Hi Gabriel,

[b]I suggest you also check the 10ohm? Resistor to ground.[b]

Please could you give a bit more info please?

Regards,

Chris

The 10ohm resistor attached to pins 1 and 8 to ground chassis, the four output tubes.
Check resistance values.