VTA-ST120: variac fuse blows when the TDR switches on the "power circuits".

The variac fuse blows when the TDR switches on the "power circuits".
This is likely a cascading failure from the brown-out I mentioned in my previous post

What must I measure to confirm if the power transformer is destroyed?
What else should I be looking for?

Timeline:

• Saturday 10/14 : replaced the quad cap and had use of my amplifier for 48 hours.
  
• Monday 10/16 : there was a crackling sound and driver tubes and power tubes were no longer lit up.
  
• The fuse on the variac had blown.
  
• Fuse on the ST-120 had not blown.
  

Per the test sequence in the "Initial Startup" in the construction manual:

2) Test of the driver board, no rectifier or power tubes, is successful: the 3 x 12AU7 tubes light up, and remain on through the switch-on of the Time Delay Relay (TDR)
3) Test of the power tubes fails on both sides:

   Install rectifier
   Install 2 x 6550 tubes (LEFT SIDE)
   Turn on amplifier
   Driver board turns on: 3 x 12AU7 tubes remain lit for 17 seconds
   TDR switches on
   Variac fuse blows
   Fuse on the ST-120 did not blow
   Driver board turns off: 3 x 12AU7 tubes turn off


   Install rectifier
   Install 2 x 6550 tubes (RIGHT SIDE)
   Turn on amplifier
   Driver board turns on: 3 x 12AU7 tubes remain lit for 17 seconds
   TDR switches on
   Variac fuse blows
   Fuse on the ST-120 did not blow
   Driver board turns off: 3 x 12AU7 tubes turn off

First time this happened, the power transformer was hot to the touch, at "usual" temperature; outut transformers were stone cold.

Configuration:
4 x 6550, or 4 x JJ KT88
3 x 12AU7

Brand new quad cap
Solid state rectifiers under test:
WZ68 Copper Cap
Dynaco VTA ST 70 120 Mono Block Tube Amp Rectifier Kit

All four bias resistors intact at 10 ohms
4.7K ohm 3 watt resistor between 20 MFd and 30 Mfs lugs is brown on 20 MFd end, but still measures 4.7k ohm

the power transformer should never run hot.
Check your quad cap, it's probably bad, and will burn out rectifier tubes and blow fuses.
Make sure once you get a good quad cap and rectifier in there to adjust the bias correctly.
A better way to use the TDR is not to connect the B+ thru the relay, but to connect the 5v supply to the rectifier tube thru the relay.
If you do that, you can change the timing resistor to about 10-15 seconds instead of 15-20 second which is standard

Out of interest, what are some of the issues that would cause the power transformer to run hot?

An update: my VTA-ST120 am is back up and running again.

First failure:
Electricity supply brown-out took out the quad cap.
I replaced the quad-cap
Amp played for 16 hours

Second failure:
Discovered the lead from pin 8 of the rectifier was attached but not soldered to the 40uf terminal of the quad cap.
The 40uf portion of the 525v quad cap had blown.
Under these conditions, the variac fuse blew when the TDR turned on.
I had neglected to solder that lead on both ends.

Success:
After replacing the quad capacitor and solid state rectifier, and soldering that joint, my VTA-ST120 am is back up and running again.

Thank you Dan and Bob for your advice that helped me resolve this issue.

My ST-120 has been unused since april 2023..... I started it up a few weeks ago and it sounded perfect ....HOWEVER, when I checked the bias, 3 tubes (KT90) had perfect bias of 0,5 VDC....BUT, the right back tube maxed my MM totally out.... even at 10VDC scale....I didn't have the nerve to try the 50 VDC scale.... Where do I start ?

MarcVBelgium wrote:My ST-120 has been unused since april 2023..... I started it up a few weeks ago and it sounded perfect ....HOWEVER, when I checked the bias, 3 tubes (KT90) had perfect bias of 0,5 VDC....BUT, the right back tube maxed my MM totally out.... even at 10VDC scale....I didn't have the nerve to try the 50 VDC scale.... Where do I start ?

It sounds like the 10ohm resistor is open. Maybe a flashing tube broke it.