Good quality rectifier tube for Bob's 120

First time post for me here. I built my 120 and preamp under the supervision of my retired EE dad with assistance from Bob about 2 years ago. Bob received way too many phone calls from us, but glad he took them! My dad built Dynaco kits in the 60s along with amateur radio equipment so he had a very critical eye over this project.

But the Important thing is I love it! I'm 36 and when people my age or younger see my setup they look completely puzzled. It's great. Been running the unit into a pair of Cerwin Vega 12" floor speakers sitting in custom cradles that pitch the speakers up about 10 degrees. Spinning some Steely Dan as I write this...

Now to the question: rectifiers. I had the Russian one, I think it was a Sovtek GZ34 or similar and it only last for about 6 months (came with the kit). I replaced it with a Mullard GZ-34...the new ones from Russia. It just crapped out on me and I can see the failure point - got a nice light show as it died. I think I got about 14-18 months out of it. My dad is just bewildered that it already crapped out and thinks it's the shoddy manufacturing. We all know things aren't built as good as they used to - part of keeping business flowing....

Would a NOS Mullard be better? I've read on guitar amp sites that the originals last way longer - decades he says (my dad agrees they should last a LOT longer). This writer suggests the new stuff only lasts hours to a couple years max and doesn't compare to the old stuff regarding quality of build . Obviously it depends on how much one person uses their system...

The unit sits in an open audio rack designed for turn tables, spacing is good for cooling and I have a small computer fan suspended above it to move air over the unit. All is plugged into a variac running around 113-115V.

Thanks,
Mark

Hi Mark --- Welcome to the Forum.

Your Dad is correct, most of the modern manufactured tubes just don't last as long as tubes manufactured in 40's,50's,60's or 70's.

I have had very good luck (duration of use) with US Made GZ34's (Sylvania/GE/RCA/etc) and Mullard/Amperex Types. There are other manufacturer's GZ34's that are also good to use.

The GZ34's found in Hammond or Conn Organs were usually made by Mullard and are good.

Another possiblity is to use 5U4G type rectifiers (which was a TV rectifier) but are also usable in the Dynacos.  I have used them in the original ST-70 and in Mk IIIs and Bob says they can be used in the VTA-70.  They drop the B+ by about 35 volts or so.   If you do a Forum Search for 5U4G you can get an idea of the use.  Probably, also in the VTA ST-120 .... I would check with Bob on this.

Also, take a look at the e-site and see what is available GZ34-wise and the relative cost for the older tubes.

Any questions, just ask.

Hope this info helps.
Stew

My experience:

Chinese 5ar4: light show after 1 week
NOS Sovtek 5ar4: light show after 1 month  
NOS Mullard GZ37: still counting, 9 months

Not to mention the GZ37 aesthetically is such a cool tube:

I'm using a Sovtek 5AR4 well over a year in my ST-120 and I listen to my system almost every day and have around 500 hours on it with no issues
Have you done the yellow sheet upgrade and or do you have the time delay board installed?
I have both since I wanted the option to use tube rectification and a Weber Copper cap

The Sovtek rectifier I got with mine lasted only a short time also. If I remember one of the Sovtek KT-88's also died at the time also. I put a Webber solid state rectifier in next to use with my Tung Sol KT-120's and the Webber is fine but one of the KT-120's went bad after about a year and a half. Next up is my Genelex Gold Lion GZ34 and KT-88's. They've been in about 4 months and so far so good.

I've been running the same RCA 5V3A since October 2015. Not sure how many hours are on it but it's gotta be several hundred now.

I can scarcely believe it, but of all the ST-120 rectifiers I've used, including Mullard GZ-37, Ruby, Sovtek and Gold Lion GZ-34s, Weber SS, and several others, the ONLY one that has lasted more than 5 months is Shuguang 5AR4. The ST-120 is hard on rectifiers.
I believe that the Chinese factories are improving dramatically. One should be aware that there may be dealers selling off old or untested stock. This is my source: http://www.ebay.com/itm/NEW-Shuguang-5AR4-GZ34-Rectifier-Vacuum-Tube-2PCS-5U4-/182274000535?hash=item2a70608697:m:m13aZEhOdOSqm5r926wi9xg

Not only is the price right, but this is the very tube that Bob Latino is shipping with his newer kits and builds.

My Sovtek 5ar4 that came with the ST120 kit didn't last long. The replacement Genalex GZ34 was also short lived. Since I have been using both a JJ GZ34 and a Weber Copper Cap WZ68. I recently added a TDR unit to the ST120 and the JJ arced when the relay sent high voltage to the rectifier. So, I'm back to the WZ68 which works well and can pass a lot of current. I've modified the Copper Cap by drilling 5 holes in the copper case and it now is running very cool and I think my power transformer is also running cooler not having to work the tube rectifier heaters. So for now, it is the WZ68.

I have been using the Weber WS1 rectifier in both my ST120 and VTA-70. I run my ST120 every day from 8am to 8pm, and the WS1 runs flawlessly with KT-120s. The WS1 runs very cool, without the sagging resistor. It also bumps up the B+ pretty high, but well within the capacitor limits. The added B+ puts the KT120's in a nice part of the curve! I also use Cerwin Vega speakers like the OP. I have both the 12" series (RS30), and the 15" AT15's. The AT15s have a sensitivity of 104, and can be driven to concert level SPL with the ST120! The bass from a 15" woofer is incredible.

Mike

Kently,
Surprised those Gold Lions gave up the ghost so quickly. Never had any luck with Sovteks; two did the out-of-the-box light show. The GLs seem to loaf in an M-125. Also surprised at the Mullard. Somebody in CCCP is making ersatz Mullards, so I have read; maybe you got one of those. Or did you get that GZ-37 from me? Again, talking apples and oranges ST-120s v. M-125s and I am not that familiar with the 120's architecture.
Never messed with the Weber WS-1 but we used to put basically the same thing in our old Collins S-Line ham gear to replace the pwr supply tubes. After the mod I blew four good 6146A output tubes to smithereens within seconds of applying the B+ -- this was my first real-world lesson in "correct biasing."
There is some correlation between rectifier stress and speaker load/impedance I do not understand but have experienced and mentioned elsewhere. Running a big pair of Maggies (4-ohm) strapped into a pair of 4-ohm Heils (being too lazy to re-wire the blown tweeter windings on the Maggies) would chew up a Weber WX-68 after a couple of weeks - the resulting impedance down there in the <2-ohm range. This was before Troy came up with his cooling hole mod. Not to mention the Maggies aren't very efficient at 86 db/watt. I infer from other postings that I'm not the only one to have experienced this same phenomenon.
Gawd, I miss the old RCA 6146As. They'd sure look cool in a tube amp . . .

I haven't blown a single rectifier tube in 3 years.  I continue to read these stories and look back on my own bad rectifier tube experiences of the past and feel even better about my decision to use UF4007 diodes in my ST120.  

Agree 100% I haven't had a rectifier die on me yet install the diodes and be done with it.

Maintarget wrote:Agree 100% I haven't had a rectifier die on me yet install the diodes and be done with it.

I thought the diodes were only for protecting other downstream components when they do go nuclear.

Having said that I have not lost a single rectifier since performing the yellow sheet mod.

LeGrace wrote:

Maintarget wrote:Agree 100% I haven't had a rectifier die on me yet install the diodes and be done with it.

I thought the diodes were only for protecting other downstream components when they do go nuclear.

Having said that I have not lost a single rectifier since performing the yellow sheet mod.

The diodes from the yellow sheet mod are doing the actual rectification, not the 5AR4.  The diodes keep the tube from seeing the high reverse voltage during the part of the cycle when the plate would be negative in relation to the cathode.  All the tube is doing is acting as a voltage dropping device.  For tube purists, the yellow sheet mod is the same as using a Weber.

Tubes4ever, I respectfully refer you to the sticky posts titled "tube rectifier diode mod" for a long discussion of this application. I believe you to be in error mode.
In short, the rectifier still rectifies, but is protected in the event of sudden discharge of the quad cap {or equivalent} which might cause downstream failures of sundry sorts. If the tube doesn't fail rapidly, nor the mains fuse, there might be trubble in pair-a-dice.

Kentley wrote:Tubes4ever, I respectfully refer you to the sticky posts titled "tube rectifier diode mod" for a long discussion of this application. I believe you to be in error mode.
In short, the rectifier still rectifies, but is protected in the event of sudden discharge of the quad cap {or equivalent} which might cause downstream failures of sundry sorts. If the tube doesn't fail rapidly, nor the mains fuse, there might be trubble in pair-a-dice.

Kentley,

I just reviewed that yellow sheet mod again and I'm correct. Assuming the diode mod has been done, try this: Remove the rectifier tube and replace it with a wire running between pin 2 and 4 and the another between pin 2 and 6. You'll find that the voltage is rectified and the amp will work properly.
It's the same thing as sticking a diode with the band towards pin 2 between pin 2 and 4 and between 2 and 6 without the diode mod being done. The tube is not allowed to rectify because the diode mod prevents the reverse current from being applied to the plates. That's what keeps the tube from arcing. I do admit that they will protect the power supply components in the event the tube shorts out. But then again, the fuse does a good job of that.

Perhaps we should refer to a Higher Power, being that I not wish to expose myself as a non-experimental toobie. However, this is fun.

Kentley wrote:Perhaps we should refer to a Higher Power, being that I not wish to expose myself as a non-experimental toobie. However, this is fun.

Yes it is fun! I do this electronics stuff for a living, so I tend to tinker a lot. I actually did stick the two diodes across the pins because I was curious what the B+ would be on my ST70. It ends up being about 460 volts with the tubes biased at 40mA.

Anyway, it doesn't really matter. The end result of the mod is that the rectifier tube will last a long time. I'm just a stickler that if I'm using a tube, I want it to do the whole job by itself.

Tim L.

I'm a sound-specific guy. Since I did the "yellow sheet mod" I've maintained one tube for five months in the slot, and I like the sound. Who could ask for anything more?

Totally agree!

Speaking of sound, I use JJ 6L6GCs in my ST70. I really enjoy the clarity of the highs over EL34s.

Kentley,
Beware what Tubes4ever says: He's from Eye-Duh-Ho, and a flat-lander to boot.

deepee99 wrote:Kentley,
Beware what Tubes4ever says: He's from Eye-Duh-Ho, and a flat-lander to boot.

Yeah, I guess you live in the hills of Eye-Duh-Ho, but a flat lander I am!

I use the Sovtek 6L6WXT+ in my VTA-70, for the same reasons as Tubes4ever, greater clarity. EL34s sound a little soggy in comparison.
Besides, I am also a "flat-lander", from the prairies of southern Minnesota.

Tubes4ever wrote:

LeGrace wrote:

Maintarget wrote:Agree 100% I haven't had a rectifier die on me yet install the diodes and be done with it.

I thought the diodes were only for protecting other downstream components when they do go nuclear.

Having said that I have not lost a single rectifier since performing the yellow sheet mod.

The diodes from the yellow sheet mod are doing the actual rectification, not the 5AR4.  The diodes keep the tube from seeing the high reverse voltage during the part of the cycle when the plate would be negative in relation to the cathode.  All the tube is doing is acting as a voltage dropping device.  For tube purists, the yellow sheet mod is the same as using a Weber.

Would love to hear comments from Bob and Roy on this....

wgallupe wrote:The diodes from the yellow sheet mod are doing the actual rectification, not the 5AR4.  The diodes keep the tube from seeing the high reverse voltage during the part of the cycle when the plate would be negative in relation to the cathode.  All the tube is doing is acting as a voltage dropping device.  For tube purists, the yellow sheet mod is the same as using a Weber.

I'm not Bob or Roy, but...

That is substantially correct. The rectifier tube sees pulsating DC as a result of the diodes having rectified the AC before it gets to the tube. This does indeed reduce the load on the tube.

However, the tubes add not only a voltage drop (which may not be the same as the voltage drop from a Weber), they also add a slow start - either just a few seconds with directly heated tubes like the 5U4GB - or longer with indirectly heated rectifiers that warm more slowly such as the 5AR4/GZ34/U77.

In many ways the diode mod gives you the best of both worlds - the higher impedance and the voltage drop of a tube rectifier along with a slow start - and the reliability of SS diodes doing the actual rectification. By using fast/soft recovery/low noise diodes (or Schottky diodes) along with the rectifier tube it is also HIGHLY unlikely that any diode noise will make its way into the audio circuit. A couple UF4007 or UF5408 PN diodes in the circuit are a really good idea IMHO.

Some of the VTA amps push tube diodes right to the limit of their capability - and even transient occurrences that put more load on the tube can cause failure of perfectly fine tubes. That's one reason why Bob always cautions about high AC line voltages and so on.

So even if you are a purist it's hard to see why you wouldn't want to add the diodes to your circuit - at least IMHO!  Do you have to? No. Would I do it if it was my amp? Hell yes!