Ruby 5U4GC Red Plating

Good morning everyone... I hope all of you are having a good day....

Slowly, but surely, I have been able to finally get around to completing my build on my VTA ST-120... Sadly I have been financially strapped, due to the inflation and the economy and I have been unable to complete it until now...

Everything looks pretty good and every voltage on this amp seems to measure normally and consistently... but I'm having a problem with the 5U4GC rectifier tube, red plating...

Once I turn the unit on everything appears to be normal and then in about 10 to 20 seconds the rectifier tube starts growing red hot and I have to shut it down... Has anyone ran into this problem before and does anyone know how I can fix this problem?.... I have looked around all over the chassis to see if I have a short somewhere, but there doesn't seem to be a problem in that area... I have two of these Ruby 5u4GC tubes, and both of them are red plating, and they are both brand new... Still need to put a yellow led light in the front and paint the red oak wood frame with a satin clear coat and do A little clean up work...  

Here is some pictures down below of my build...

Have a good day everyone.

One more thing... When I pull all 4 power tubes (New KT88's That Check Good) The red plating on the 5U4GC stops red plating... also when I install just 1 KT88 the red plating begins to happen.

I'm not a tech.

Don't these amps call for a 5AR4?

Dave_in_Va wrote:I'm not a tech.

Don't these amps call for a 5AR4?

It calls for ether a 5AR4 or a 5U4...

"The optional tube set includes three 12AU7 driver tubes and a matched quad of 6550 tubes, along with a 5U4 rectifier tube"

I have not been able to find a spec sheet to see if the 5U4GC is different than the 5U4/5U4G or the 5U4GB.... I assume they are close to the same in values ?

What is the voltage on the bias test points ? It should be in the neigborhood of 0.5V
for all 4 output tubes.

peterh wrote:What is the voltage on the bias test points ?  It should be in the neigborhood of 0.5V
for all 4 output tubes.

That is my next step.... the problem is the 5U4 starts red plating before I have a chance to measure it.... I do however have the pots turned counter clockwise all the way down... If that makes a difference ?

peterh wrote:What is the voltage on the bias test points ?  It should be in the neigborhood of 0.5V
for all 4 output tubes.

Also... when I pull all 4 power tubes (the KT88) the voltages on the sockets are all nominal or within tolerance.

I found the problem... One of the brown 150uf 350V caps is testing bad and it's shorted... I'm going to replace both of them with a higher voltage cap... I suspect that is the reason the 5U4 tube is red plating. Last year when I was building the cap rail I installed one of the caps backwards by mistake and Bob Latino spotted and called it out in my post... So I de-soldered and removed the cap and re-installed it correctly... I believe the double heating from the soldering iron was to much for it and shorted it ?

Welcome back Tom. Nice build and it goes with your shirt! 
It looks like you have the time delay board in there- you could keep a solid state rectifier as a backup. Less $$ than a tube at this point.

New2Tubez wrote:Welcome back Tom. Nice build and it goes with your shirt! 
It looks like you have the time delay board in there- you could keep a solid state rectifier as a backup. Less $$ than a tube at this point.

Thanks... :-)

Yes, I did put the time delay board in there... I know there is talk about this board being hard on the tube rectifier, but I live in an area in Texas where we have a lot of thunderstorms and power outages, sometimes daily... I would rather have the time delay board in there to save and protect my other tubes and I can always replace the tube rectifier if I need to...

I don't know about the delay board being hard on the rectifier as I don't have this board. 

All I meant was the delay board would make using an s/s rectifier an easier option. My ST120 has the auto bias board and it has about a 50 second delay so I took advantage of this to use a Weber WS-1t in my amp. w/o the delay an s/s may be harder on the tubes. That's all.

Glad you got it fixed.

New2Tubez wrote:Welcome back Tom. Nice build and it goes with your shirt! 
It looks like you have the time delay board in there- you could keep a solid state rectifier as a backup. Less $$ than a tube at this point.

I understood what you were saying... and thanks... just to let everyone know I am not putting down the time delay board... all these companies and manufacturers that make a similar board have this inherit problem.... I may look around and see if there is another option for a board that is similar, but it also includes some type of buffer mechanism that will allow the high voltage, after the relay has clicked in, to allow the high voltage to come up slowly to where it doesn't damage the rectifier tube with a sudden jolt of high voltage.

If anyone knows of a better option please let me know in the comment section.... one of the problems that I'm having is we have a Lot of thunderstorms where I live and the power sometimes goes off and on quickly and sometimes it happens when these thunderstorms are far off in the distance and it's not even raining here where I'm located... the power will just simply go off and immediately come right back on and it's not good on the tubes... This is also the reason why I added the "Yellow Sheet Diodes" in my build.

Have a good day everyone and thanks New2Tubez.

Tom, welcome back. Be sure you have your amp plugged in to a LATCHING DOWN GROUND FAULT INTERUPTER. Has to be manually reset each time there is any slight power outage. Trust me , it works. Just search here - several posts on it. Dale

Dale Stevens wrote:Tom, welcome back.  Be sure you have your amp plugged in to  a LATCHING DOWN GROUND FAULT INTERUPTER. Has to be manually reset each time there is any slight power outage. Trust me , it works. Just search here - several posts on it.  Dale

Awww yes.... Actually, I have one of those outside.... I think I will look into that and buy one of those for the amp inside... That's a very good idea.... Thanks Dale Stevens

I got one of those GFI plugs but I also use a variac and the instructions said not to use with the GFI so I don't. Not exactly sure why though.

Hi Tom. I had an st70. Clone from Bob, which I modified in the following after 2 different rectifier went redplate and nearly caused a fire. Both were russian or chinese tubes, in the 5AR 4 family. Bob makes a great kit, but it runs rectifier at nearly 100% of its rated output. A good NOS USA rectifier , or vintage strong European rectifier will probably survive, but forget abouf chinese/russian made tubes .

I also have a VT-120, but I made it much more resistant to burning rectifiers. My solution was to make these changes:

1. Eliminate the quad cap can
2. Relocate all power supply and filter caps under the chassis
3. Upgrade all caps to at least 550.volts capacity
4. Remove the triode/ pentode switch ( pentode mode 100% of timd.
5. Run two parallel wired rectifiers. You can mount the 2nd rectifier in the quad cap spot
6. I choose to run two Emission Labs 274Bb rectifiers.
7. Some minor changes are: eliminate soft start circuit ( it becomes unnecessary )
8. Two parallel wired 274b mesh plate rectifier give many advantages
9 Mesh plate tubes are slower to warm up, so you don’t slam cold possr tubes with 500 + V of B+
10. Parallel wired rectifiers give you double the current capacity of single rectifier. The rectifiers don’t work as hard, and last longer.
11. 2 parallel rectifiers cut the voltage drop by 50%! This makes the amp much mode responsive , but maintains the smooth tube tone.
12. I also run KT88 power tubes. Emissions Labs 274B easily provide the current to run these power tubes at their full potential, no worry about redplating, or sparks in rectifiers.
13. They only need 2 amps each fi
For heaters, so power tranny can handle them
14. They can handle between 550 and 600 V on plates, which would fry any Chinese or Russian tube. Mine have lasted over 10 years. Still strong!
15. Just be careful with capacitors , don’t exceed the max specified by tube manufacturer. My tubes state that first cap should be max of 4uf. But I contacted EML. , and they said I could double that to 8uf when running 2 rectifiers in parallel.
There are a few other mods required to make it work ( add resistors to inputs of rectifiers, and be careful wiring the rectifiers. But it works!

the 274B is a fantastic tube, as long as you keep that first cap down to 4uF (8uF if using two in parallel).
You can also run any 5AR4 or GZ34 two in parallel, just keep the first cap down to 33uF.
You can run as much capacitance as you want AFTER going thru the choke.
The Tungsol 5AR4 is a good rectifier tube for around $45-50
The 274Bs are $350 each !! http://www.tubesusa.com/ELtubes.html