VTA ST 120 needs surgery!

Jlskey wrote:I built a vta st 120 from a Mr Latino kit a year and a half ago.  Great build experience and a fantastic listening experience!

It’s been running:  4 sovtek 6550s, a tung sol 5ar7 and electro harmonix 12au7 as drivers.

Amp was running great until a few weeks ago.  Turned the record, went to grab a beer and when I came back the lights were out on all tubes and wouldn’t light back up.  I powered it off for a bit and then powered it back on.  No lights.  A fuse was blown and it wouldn’t take any bias.  Multimeter was showing 000.

Later on I replaced the fuse and Figured it was the rectifier.  I went back into the amp and switched the voltage line since my voltage was reading more consistently at 122 over the course of a few days.  Removed black and white wire from the first prong of the on switch and soldered on the black and yellow.  After closing her up I followed the original build instructions and put the driver tubes in and powered up.  They lit up so on to next step, powered off and put in the rectifier and the left channel power tubes and powered on after a few minutes and everything lit up.  Started to bias the left channel and whereas it’s took a bias adjustment it was a little harder and it varied a lot more than normal.  Turned amp off, put in right channel tubes and after a bit powered it on.  Started to try to bias it and it read 000, amp started making some noises and then I was treated to a light show. The right channel ones seemed to be more red and the rectifier glowed bright white before I heard a popping sound from
It and then everything went dark like the first time.

I’m already planning on opening her back up and going over all the solder  joints; replacing all of the tubes.  But I also wanted to see if there was anything else I should be looking for. Maybe the bias pots?  Resistors?  I’m all ears!  Fairly new to these matters so please feel free to mention anything you all think would help.

LeGrace wrote:Not familiar with 5ar7 rectifier, do you actually mean 5ar4?

LeGrace wrote:5ar4 failing is common issue. Replace or even better replace with WZ68 copper top.

peterh wrote:

Jlskey wrote:I built a vta st 120 from a Mr Latino kit a year and a half ago.  Great build experience and a fantastic listening experience!

It’s been running:  4 sovtek 6550s, a tung sol 5ar7 and electro harmonix 12au7 as drivers.

Amp was running great until a few weeks ago.  Turned the record, went to grab a beer and when I came back the lights were out on all tubes and wouldn’t light back up.  I powered it off for a bit and then powered it back on.  No lights.  A fuse was blown and it wouldn’t take any bias.  Multimeter was showing 000.

Later on I replaced the fuse and Figured it was the rectifier.  I went back into the amp and switched the voltage line since my voltage was reading more consistently at 122 over the course of a few days.  Removed black and white wire from the first prong of the on switch and soldered on the black and yellow.  After closing her up I followed the original build instructions and put the driver tubes in and powered up.  They lit up so on to next step, powered off and put in the rectifier and the left channel power tubes and powered on after a few minutes and everything lit up.  Started to bias the left channel and whereas it’s took a bias adjustment it was a little harder and it varied a lot more than normal.  Turned amp off, put in right channel tubes and after a bit powered it on.  Started to try to bias it and it read 000, amp started making some noises and then I was treated to a light show. The right channel ones seemed to be more red and the rectifier glowed bright white before I heard a popping sound from
It and then everything went dark like the first time.

I’m already planning on opening her back up and going over all the solder  joints; replacing all of the tubes.  But I also wanted to see if there was anything else I should be looking for. Maybe the bias pots?  Resistors?  I’m all ears!  Fairly new to these matters so please feel free to mention anything you all think would help.

Check the 10ohm resistors between 6550 cathode and ground. A tube that has
been "flashing" will easily take the resistor out ( this is deliberately to save the transformer)
If bad resistor found, replace with 10 ohm 1w film resistor and junk the tube that was in
that place ( you probably will junk both tubes on that side and replace with a matched pair)

The rectifier is junk, once flashed it it no longer reliable. When at it, add the "yellow sheet mod" where two 1n4007 diodes are connected in series with the anodes,

If no 10 ohm resistors are bad the original cause could be the rectifier itself ( which you replace)
or the can cap . If you cannot test the can cap and the fuse blows again you should
replace the can cap too.

Jlskey wrote:

peterh wrote:

Jlskey wrote:I built a vta st 120 from a Mr Latino kit a year and a half ago.  Great build experience and a fantastic listening experience!

It’s been running:  4 sovtek 6550s, a tung sol 5ar7 and electro harmonix 12au7 as drivers.

Amp was running great until a few weeks ago.  Turned the record, went to grab a beer and when I came back the lights were out on all tubes and wouldn’t light back up.  I powered it off for a bit and then powered it back on.  No lights.  A fuse was blown and it wouldn’t take any bias.  Multimeter was showing 000.

Later on I replaced the fuse and Figured it was the rectifier.  I went back into the amp and switched the voltage line since my voltage was reading more consistently at 122 over the course of a few days.  Removed black and white wire from the first prong of the on switch and soldered on the black and yellow.  After closing her up I followed the original build instructions and put the driver tubes in and powered up.  They lit up so on to next step, powered off and put in the rectifier and the left channel power tubes and powered on after a few minutes and everything lit up.  Started to bias the left channel and whereas it’s took a bias adjustment it was a little harder and it varied a lot more than normal.  Turned amp off, put in right channel tubes and after a bit powered it on.  Started to try to bias it and it read 000, amp started making some noises and then I was treated to a light show. The right channel ones seemed to be more red and the rectifier glowed bright white before I heard a popping sound from
It and then everything went dark like the first time.

I’m already planning on opening her back up and going over all the solder  joints; replacing all of the tubes.  But I also wanted to see if there was anything else I should be looking for. Maybe the bias pots?  Resistors?  I’m all ears!  Fairly new to these matters so please feel free to mention anything you all think would help.

Check the 10ohm resistors between 6550 cathode and ground. A tube that has
been "flashing" will easily take the resistor out ( this is deliberately to save the transformer)
If bad resistor found, replace with 10 ohm 1w film resistor and junk the tube that was in
that place ( you probably will junk both tubes on that side and replace with a matched pair)

The rectifier is junk, once flashed it it no longer reliable. When at it, add the "yellow sheet mod" where two 1n4007 diodes are connected in series with the anodes,

If no 10 ohm resistors are bad the original cause could be the rectifier itself ( which you replace)
or the can cap . If you cannot test the can cap and the fuse blows again you should
replace the can cap too.

I think I see what you mean by can cap.  Are you referring to the  550 volt Authenticap that I believe Bob calls the ‘quad cap’.  My command of the industry lingo is a little weak.

peterh wrote:After replacing 5ar4 and any repairs of resistors changing power tubs etc turn all 4 bias
adjustment pots fully anti-clockwise ( minimum current on all tubes ) . Reason for this is
that the 5ar4 is operating on the brink of it's capacity, 2 6550 tubes drawing excess current
can easily destroy the 5ar4. A 5ar4 that has "flashed" is broken and cannot be trusted.
When powered on and warmed up, slowly start adjusting bias to normal level according to the
build description.

Jlskey wrote:

peterh wrote:After replacing 5ar4 and any repairs of resistors changing power tubs etc turn all 4 bias
adjustment pots fully anti-clockwise ( minimum current on all tubes ) . Reason for this is
that the 5ar4 is operating on the brink of it's capacity, 2 6550 tubes drawing excess current
can easily destroy the 5ar4. A 5ar4 that has "flashed" is broken and cannot be trusted.
When powered on and warmed up, slowly start adjusting bias to normal level according to the
build description.

Sounds good.  Will certainly replace the rectifier tube, also the power tubes on the right channel.  But what are your thoughts on the driver tubes and the power tubes on the left channel?  Should I just suck it up and get a matched quad?  I reckon the left channel probably has had similar problems and I won’t be able to get the same
Brand of tubes for the right channel.  Would probably get electro harmonix to pair with the remaining sov tek 6550.  Don’t really like mixing brands.

markeby wrote:I have been using a Telefunken GZ34-TK without any problems.  I stayed away for the 5U4GB because it has a 3 amp filament vs the 2 amp on the GZ34-TK (and also 5AR4).  I believe my instructions indicated a 5AR4.  Both the 5AR4 and GZ34-TK are exactly the same.  The 5AR$ is the U.S. designation and the GZ34-TK is the British designation.

markeby wrote:I have been using a Telefunken GZ34-TK without any problems.  I stayed away for the 5U4GB because it has a 3 amp filament vs the 2 amp on the GZ34-TK (and also 5AR4).  I believe my instructions indicated a 5AR4.  Both the 5AR4 and GZ34-TK are exactly the same.  The 5AR$ is the U.S. designation and the GZ34-TK is the British designation.

Tubes4ever wrote:Just an informational note, the VTA 120 transformer is designed for up to 4 amps on the 5V winding. You can use a 5u4GB with no problems.  I got this answer directly from Bob Latino.

peterh wrote:

Jlskey wrote:

peterh wrote:

Jlskey wrote:I built a vta st 120 from a Mr Latino kit a year and a half ago.  Great build experience and a fantastic listening experience!

It’s been running:  4 sovtek 6550s, a tung sol 5ar7 and electro harmonix 12au7 as drivers.

Amp was running great until a few weeks ago.  Turned the record, went to grab a beer and when I came back the lights were out on all tubes and wouldn’t light back up.  I powered it off for a bit and then powered it back on.  No lights.  A fuse was blown and it wouldn’t take any bias.  Multimeter was showing 000.

Later on I replaced the fuse and Figured it was the rectifier.  I went back into the amp and switched the voltage line since my voltage was reading more consistently at 122 over the course of a few days.  Removed black and white wire from the first prong of the on switch and soldered on the black and yellow.  After closing her up I followed the original build instructions and put the driver tubes in and powered up.  They lit up so on to next step, powered off and put in the rectifier and the left channel power tubes and powered on after a few minutes and everything lit up.  Started to bias the left channel and whereas it’s took a bias adjustment it was a little harder and it varied a lot more than normal.  Turned amp off, put in right channel tubes and after a bit powered it on.  Started to try to bias it and it read 000, amp started making some noises and then I was treated to a light show. The right channel ones seemed to be more red and the rectifier glowed bright white before I heard a popping sound from
It and then everything went dark like the first time.

I’m already planning on opening her back up and going over all the solder  joints; replacing all of the tubes.  But I also wanted to see if there was anything else I should be looking for. Maybe the bias pots?  Resistors?  I’m all ears!  Fairly new to these matters so please feel free to mention anything you all think would help.

Check the 10ohm resistors between 6550 cathode and ground. A tube that has
been "flashing" will easily take the resistor out ( this is deliberately to save the transformer)
If bad resistor found, replace with 10 ohm 1w film resistor and junk the tube that was in
that place ( you probably will junk both tubes on that side and replace with a matched pair)

The rectifier is junk, once flashed it it no longer reliable. When at it, add the "yellow sheet mod" where two 1n4007 diodes are connected in series with the anodes,

If no 10 ohm resistors are bad the original cause could be the rectifier itself ( which you replace)
or the can cap . If you cannot test the can cap and the fuse blows again you should
replace the can cap too.

I think I see what you mean by can cap.  Are you referring to the  550 volt Authenticap that I believe Bob calls the ‘quad cap’.  My command of the industry lingo is a little weak.

Yes, that is what i was referring to.

Jlskey wrote:

peterh wrote:

Jlskey wrote:

peterh wrote:

Jlskey wrote:I built a vta st 120 from a Mr Latino kit a year and a half ago.  Great build experience and a fantastic listening experience!

It’s been running:  4 sovtek 6550s, a tung sol 5ar7 and electro harmonix 12au7 as drivers.

Amp was running great until a few weeks ago.  Turned the record, went to grab a beer and when I came back the lights were out on all tubes and wouldn’t light back up.  I powered it off for a bit and then powered it back on.  No lights.  A fuse was blown and it wouldn’t take any bias.  Multimeter was showing 000.

Later on I replaced the fuse and Figured it was the rectifier.  I went back into the amp and switched the voltage line since my voltage was reading more consistently at 122 over the course of a few days.  Removed black and white wire from the first prong of the on switch and soldered on the black and yellow.  After closing her up I followed the original build instructions and put the driver tubes in and powered up.  They lit up so on to next step, powered off and put in the rectifier and the left channel power tubes and powered on after a few minutes and everything lit up.  Started to bias the left channel and whereas it’s took a bias adjustment it was a little harder and it varied a lot more than normal.  Turned amp off, put in right channel tubes and after a bit powered it on.  Started to try to bias it and it read 000, amp started making some noises and then I was treated to a light show. The right channel ones seemed to be more red and the rectifier glowed bright white before I heard a popping sound from
It and then everything went dark like the first time.

I’m already planning on opening her back up and going over all the solder  joints; replacing all of the tubes.  But I also wanted to see if there was anything else I should be looking for. Maybe the bias pots?  Resistors?  I’m all ears!  Fairly new to these matters so please feel free to mention anything you all think would help.

Check the 10ohm resistors between 6550 cathode and ground. A tube that has
been "flashing" will easily take the resistor out ( this is deliberately to save the transformer)
If bad resistor found, replace with 10 ohm 1w film resistor and junk the tube that was in
that place ( you probably will junk both tubes on that side and replace with a matched pair)

The rectifier is junk, once flashed it it no longer reliable. When at it, add the "yellow sheet mod" where two 1n4007 diodes are connected in series with the anodes,

If no 10 ohm resistors are bad the original cause could be the rectifier itself ( which you replace)
or the can cap . If you cannot test the can cap and the fuse blows again you should
replace the can cap too.

I think I see what you mean by can cap.  Are you referring to the  550 volt Authenticap that I believe Bob calls the ‘quad cap’.  My command of the industry lingo is a little weak.

Yes, that is what i was referring to.

In my continued quest for reviving this puppy I’m going to test the quad cap.  I know I need to get a cap meter (on the way), and that I need to remove it from the circuit (remove all other connections), but beyond that I’m lost.

Jlskey wrote:
Used my newly obtained cap meter to measure the quad cap and it was in working order.  Still curious if anyone knows anything about my post below on the red and black power transformer cord to the pcb board.  Thanks everyone!!

Bob Latino wrote:

Jlskey wrote:
Used my newly obtained cap meter to measure the quad cap and it was in working order.  Still curious if anyone knows anything about my post below on the red and black power transformer cord to the pcb board.  Thanks everyone!!

When the amp is on > the eyelet where the red/black wire connects to the driver board should measure about 50 to 60 volts AC to chassis ground. Note - that red/black wire from the transformer is part of the bias system. The 55 volts AC is then sent to the banded side if the diode. What comes out of the OTHER side of the diode is about -75 volts DC. This negative DC voltage is then padded down with the 4 bias pots to get a lower negative DC voltage which is injected on to pin # 6 on each output tube and then through a 1000 ohm resistor to pin # 5 to bias all four output tubes. This negative DC voltage keeps the 4 output tubes under control.

Bob