VT-120 Tubes won't bias

Built the VT-120 about 18 months ago. Worked flawlessly until about 3 months ago when I had to replace the rectifier tube - no issues with the replacement. Then same problem came up (I would hear a pop or other noise during warmup) last week. Replaced with JJ GZ34, and my troubles really began. Very hard to bias and then a bright flash at the base of the GZ34. Replaced the rectifier, and I replaced the power tubes. Same deal - bias all over the place and then a flash at the GZ34 base. I may have screwed up the biasing (mixed up the pot and the test point), but dang it's sensitive if that was the case.
Ordered some Tung-Sol 5AR4's, and my guess is that I will need to replace the KT-88s. I am partially convinced that I got a bad batch of JJ GZ34s - I have one good one in my VT-70, hard to bias but I got it in. A few noises on startup, but otherwise working so far (4 days).
Question:
If the rectifier blows (bright flash), does this automatically mean my power tubes are shot?
If I put in a good rectifier, my guess is that I should see some bias voltage. If it is zero, then I assume that the KT-88s are gone?
My concern is that there is something else wrong with my amp - quad cap, driver board? My 10 ohm resistors are good, and no obvious signs of damage inside. Oh yeah, the fuses blow, which is probably a good thing.

My older VT-70 built about 4 years ago is still going strong, so my workmanship can't be that crappy... LOL

subman wrote:Built the VT-120 about 18 months ago.  Worked flawlessly until about 3 months ago when I had to replace the rectifier tube - no issues with the replacement.  Then same problem came up (I would hear a pop or other noise during warmup) last week.  Replaced with JJ GZ34, and my troubles really began.  Very hard to bias and then a bright flash at the base of the GZ34.  Replaced the rectifier, and I replaced the power tubes.  Same deal - bias all over the place and then a flash at the GZ34 base.  I may have screwed up the biasing (mixed up the pot and the test point), but dang it's sensitive if that was the case.
Ordered some Tung-Sol 5AR4's, and my guess is that I will need to replace the KT-88s.  I am partially convinced that I got a bad batch of JJ GZ34s -  I have one good one in my VT-70, hard to bias but I got it in.  A few noises on startup, but otherwise working so far (4 days).
Question:
If the rectifier blows (bright flash), does this automatically mean my power tubes are shot?
If I put in a good rectifier, my guess is that I should see some bias voltage.  If it is zero, then I assume that the KT-88s are gone?
My concern is that there is something else wrong with my amp - quad cap, driver board?  My 10 ohm resistors are good, and no obvious signs of damage inside.  Oh yeah, the fuses blow, which is probably a good thing.

My older VT-70 built about 4 years ago is still going strong, so my workmanship can't be that crappy... LOL

Rectifier flashing during start is a sign that it's too loaded. ( bias to high or to large 1:st cap )
Reduce bias on all tubes and retry.

Sometimes the "yellow sheet mod" will save tube rectifier, but i also understand that
solid state rectifier is the way to go with st-120.

Rectifier flash is not normally negatively affecting power tubes. Only red plate episode normally should lead to worry. You do not indicate what brand is first rectifier tube but 15 months is quite good. Reputation wise Tung Sol is one of the better for lifetime, whereas JJ tend to be one of the shortest, just days or few weeks commonly reported.

I would strongly suspect something is wrong with the filter capacitors in the filter can. Pull the power tubes and power on with just the rectifier tube in place. See if things settle down and measure the B+ voltage and see if it is steady. and within range of what's stated in the assembly manual.

check the quad cap, that often goes bad if a rectifier tube or output tube goes bad. With no power and cap discharged, all sections should read greater than 2 Megohm to ground, if not, replace.
Also, when you start up the amp, turn ALL bias pots down (CCW) about 1/4 turn, then reset bias slowly and make sure it holds, if one tube needs much more bias than the others it is going bad.

Guys, thanks for all the input. Checked the quad cap, 34M ohms on all tabs so that was good. Paid real close attention to my biasing and dug out some 5AR4s, and it worked! And it turns out that my old KT-88s are still fine. You guys were awesome with your sound input.
Lesson is to never panic.

2004-2011 vta board.
Clockwise on the pots lower bias for me.

subman wrote:Guys, thanks for all the input.  Checked the quad cap, 34M ohms on all tabs so that was good.  Paid real close attention to my biasing and dug out some 5AR4s, and it worked!  And it turns out that my old KT-88s are still fine.  You guys were awesome with your sound input.
Lesson is to never panic.

I know you fixed this with a different Rectifier tube. The JJ's are terrible with the VTA-120. I had one flash on me when I did my first power up after building my VTA-120. I installed a Mullard GZ37 and never had a problem again. The older NOS GZ34 are better but the SS rectifier is the probably the best in the log run.

Drummerboy2 wrote:

subman wrote:Guys, thanks for all the input.  Checked the quad cap, 34M ohms on all tabs so that was good.  Paid real close attention to my biasing and dug out some 5AR4s, and it worked!  And it turns out that my old KT-88s are still fine.  You guys were awesome with your sound input.
Lesson is to never panic.

I know you fixed this with a different Rectifier tube. The JJ's are terrible with the VTA-120. I had one flash on me when I did my first power up after building my VTA-120. I installed a Mullard GZ37 and never had a problem again. The older NOS GZ34 are better but the SS rectifier is the probably the best in the log run.

Or use a 5u4GB or 5V3. Both very reliable. NOS can be had for a reasonable price.