VTA-ST120 - Low bias on all four power tubes

Introduction:

• Under normal conditions, with bias pots at the 11 o'clock position, bias is at recommended 0.5 volts
  
• Bias is suddenly very low on all four power tubes
  

Issue:

• With pots at 11 0'clock position, bias on all four power tubes is now very low, at 0.19 volts, instead of the 0.5 v recommended.
  
• I now have to turn the bias pots to 4 o'clock to achieve recommended bias.
  
• There is also a hum from both speakers
  

Question:

• I presume I have to replace the quad cap?
  
• What else must be done?
  

Observations:

• It appears that the amplifier was turned on and off multiple times in quick succession during the brownout - see timeline, Monday
  
• Quad cap looks damaged
  

Quad cap:   Expected  Actual

• Semi-circle: 80          87.4
  
• Square:       40          4.66
  
• Triangle:      30          130.04
  
• Dash:          20          87.6
  

Bias resistors:

• Not blown. All at 10.1,   10.1,   10.3,   10.7 ohms
  

Timeline:

Monday 10/9 @ 19:00 :

• Electricity supply brown out in my home: twice, 2 second duration, at two seconds apart.
  
• Lights flickering, loud buzz from speakers.
  
• GFCI did not disconnect power, amp remained on after brownouts stopped
  

Tuesday 10/10:

• Noticed the issue with the low bias
  
• I replaced the Weber WZ68 rectifier with the original solid state rectifier ("Dynaco VTA ST 70 120 Mono Block Tube Amp Rectifier Kit")
  
• Issue persists: low bias and hum from speakers
  

Configuration:

• Amplifier: VTA-ST120
  
• Purchased: 3/7/2021
  
• Built from kit
  
  
• Power Tubes: matched quad Tungsol KT120
  
• Driver Tubes: Left, Right: RCA 12AU7A ECC82 Clear Top; Middle: Brimar CV4003
  
  
• Rectifier: Weber WZ68 solid state rectifier, installed: 4/4/22
  
• VTA-TDR Time Delay Relay Board installed
  
  
• GFCI Adapter: Yellow Jacket 2762, 120-Volt, 15-Amps, 1800-Watts Single Outlet
  https://www.amazon.com/gp/product/B000MM3Z6C/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
  
  
• Variac: Circuit Specialists, Model TDGC3-0.5D, 5 amp max output 0-130v, 0-5a
  https://www.circuitspecialists.com/products/tdgc3-0-5d-variac-variable-transformer?_pos=1&_sid=f241e8fac&_ss=r
  
  
• Digital bias meters, installed 06/11/23
  https://dynacotubeaudio.forumotion.com/t5050-digital-bias-meters
  
  
• Speakers: Klipsch Cornwall IV
  
  

[/list]

Alex, your GFCI does not appear to latch down (off) on a power flick. Look at TRC14650006-6 on Amazon. This has a manual reset. Dale

Hi Bob,

Thanks for the confirmation. I've ordered a replacement quad cap and I'll be doing quad-cap surgery as soon as it arrives...

@Dale Stevens - as it happens, I do have one of the TRC 14650006-6 bought off Amazon in my spare parts collection, so I'll be using that going forward.

I repaired my VTA-ST120 amp  Saturday 10/14 : replaced the quad cap and had glorious use of my amplifier for 48 hours.

Now there is a more severe problem.
Details in a new post at:

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