Rectifier Tube sparking - blown fuse

Greetings all - I wrote Bob about this issue directly (email copied below) but wanted to reach out to the community to see if there are any folks awake with any ideas.


I have received my kit (VTA ST70 with tube rectifier, upgraded caps) and have spent the past two nights building and I'm running into an issue. I first tested the driver circuit tubes and everything checked out - tubes lit up nicely. Then I attached my speakers and attached the speakers, rectifier and left output tubes. I was able to set the bias on these tubes.

Now the problem - once I turned off the amp and added the right output tubes the rectifier tube sparked after 10 or so seconds which in turn blew a fuse. I double checked all of the quad cap connections since I've read on the blog this is likely the cause for a slow blow.

I've swapped around the output tubes and both sides are now blowing the fuse. Additionally, I no longer read a bias voltage on the left output tubes - even with the original tubes i used when I was successful.

I've double checked the wiring and I believe everything is correct. I could attach photos if needed. Please let me know what my next steps are - bad rectifier tube? Bad capacitor?  

Thanks to you all - I've learned a great deal while surfing this forum.

Can't wait to hear how this amp sounds.

P.S. My line voltage is usually around 125V. Could this be the problem?

I guess the rectifier tube is gone. That happens. My first was also dead after a few days. I use a Sovtek now since a year. But also double check all the wireing before you try again.


Michael

make sure to add the diode mod (it's on a sticky I believe) to protect the rectifier tube,
and make sure all your bias connections on the PCB (to the four pots) and to the tubes (outputs to pins 6) are good.
If you don't have bias on each tube, it will pull way to much current, and either blow out that tube or the rectifier.

My VTA ST 70 was ordered already built. I'm running Psvane KT 88s and a NOS Mullard 5AR4. My 88's are biased at around 49-50 mA except one stays around 53-55mA. My wall voltage varies but is usually around 123-124.
Should I be worried? Everything sounds like a million bucks (w/SP-10 and PH10).

Dave_in_Va wrote:My VTA ST 70 was ordered already built. I'm running Psvane KT 88s and a NOS Mullard 5AR4. My 88's are biased at around 49-50 mA except one stays around 53-55mA. My wall voltage varies but is usually around 123-124.
Should I be worried? Everything sounds like a million bucks (w/SP-10 and PH10).

If it's a VTA-70 then it has individual bias pots. How can you then state that "one stays around 53mA " ?
Unbalanced bias will send DC across the OT, which magnetizes it and worsens the lowest frequency's.

53 is as low as it will go. I guess it's just an odd tube out of the supposedly matched quad. AND...I can't afford top send one tube 7000 miles for a replacement.
What I could do (if you guys think it necessary) is swap back in the EL 34's. (They don't sound as good as the 88's)
Is this small difference important and should I be worried about the 5AR4?
Thanks.
(Sorry for the thread hi-jack...it's kind of related to the original post.)

Dave_in_Va wrote:53 is as low as it will go. I guess it's just an odd tube out of the supposedly matched quad. AND...I can't afford top send one tube 7000 miles for a replacement.
What I could do (if you guys think it necessary) is swap back in the EL 34's. (They don't sound as good as the 88's)
Is this small difference important and should I be worried about the 5AR4?
Thanks.
(Sorry for the thread hi-jack...it's kind of related to the original post.)

Ok. It's the adjusting range that is to small.
But then you might be better off bias them all to 53mA ( or at least the two on one side )

The permanent fix is to reduce a resistor ( i don't have the schematic at hand right now, but
our friendly webmaster might suggest which resistor and what value.

try changing R39 to 2.2K or 1K, original should have been 6.8K or 7.5K

Here's what I did:

I remembered I had an extra bucking transformer (I use these for old '60's solid state Vox or Farfisa organs). This brought my wall voltage down to about 118. I've readjusted my KT 88's to aprox. 49.9mA each. Looks good.

I've got the bucking trans. going into the GFCI that SCHIZO recommended. I can use that as an on/off if I don't want the bucking transformer getting juice all of the time.

I checked with the bucking trans. builder and he said that it is rated at 3 amps so that as long as my VTA ST70 and SP10/PH10 isn't pulling more than about 300watts I'm good. What's the draw on my power and preamp?

Right now the turntable (new Music Hall) is running out of the bucking trans former but I'm going to re-plug that into the regular wall voltage.

Does this all sound copacetic??

VTA ST70 amp is 2 amps, SP10/PH10 about 1/2 amp

Okay. I think I'm set. I took the turntable and CD player off the bucking transformer circuit.
I'm reading 114 volts out of the bucking transformer now. Maybe I should drop the bias a bit to make up for when the wall voltage goes back up.

I'm pretty near "forget about it and put on some Hendrix" mode.

Something I have been scratching my head over but how does GFCI work without a ground?   In the normal configuration the ST-120 and ST-70 use a two-wire power cord and no ground.   This article implies the GFCI will work without a ground connection by comparing the current on hot to the neutral.  In normal operation what goes in must come out with two wires.  The illustration though in the article shows a no ground circuit fault but a person is in the conductive path to complete the ground circuit.  Guess you will get a brief shock with GFCI versus maybe electrocution without.

http://ecmweb.com/content/how-gfcis-work