tube rectifier "yellow sheet" diode mod

Just got in a Mullard re-issue 5AR4, one of the New Sensor variables out there along with that Tung Sol and Genelex.   Other than pricing schedules I doubt there's much difference between them as they're all made at the same factory.  There does seem to be a difference between this Mullard/New Sensor Russian specimen and the Chinese variants I've tried, and also the JJ, both of which cannot take more than one arc.  

Since placing the diode mod on the ST70s I have I can now use these less expensive tubes as none have failed coupled with the two diodes.
Of course nothing beats a vintage Mullard for longevity, and they easily handle the voltage swing even without the MOD.  
However, this MOD is a no brainer no matter what mfg. rectifier being used and at this point it should be standard in any build.

Thank you Mr. McShane,

You and your tubes have been highly recommended at forums that I frequented before I found this one.

So, you say that reproduction 5AR4's failed at a PIV of 620 volts. That was not something I considered. I figured some L di/dt pulse killed the tube rectifier and that the tube met spec. If 620V PIV killed a poorly made repo, I can see why the mod would help. Thanks for taking the time to educate me and for being kind. Still not sure that I understand the exact failure mode of a tube rectifier but I did have a Sovtek flash over. I will be in touch for some KT88's. Thanks.

First off it is NOT the reissue 5AR4s that failed. The new Tung-Sol and Genalex have been bulletproof. It's the Sovtek and Shuguang/Chinese that have failed

Second, I just went back and edited my post to use more real world numbers. The AC voltage of 170 volts in my example was too low - senior moment I guess!  

Please re-read the post now as it is now more "real world" in the voltages I used. The difference is more typically 800 volts or so, not 620. My apologies for the error. The concept of how the diodes work was right on the money as I described it - but my example needed improvement!

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Jim McShane wrote:First off it is NOT the reissue 5AR4s that failed. The new Tung-Sol and Genalex have been bulletproof. It's the Sovtek and Shuguang/Chinese that have failed

Second, I just went back and edited my post to use more real world numbers. The AC voltage of 170 volts in my example was too low - senior moment I guess!  

Please re-read the post now as it is now more "real world" in the voltages I used. The difference is more typically 800 volts or so, not 620. My apologies for the error. The concept of how the diodes work was right on the money as I described it - but my example needed improvement!

Hi Jim,

Do you (or anyone else) see any issues using the BY448GP-E3/54 diodes for the mod? I like the PIV. And I have some.

http://www.vishay.com/docs/88543/by448gp.pdf

I've got some 1N4007 diodes in transit. After they're installed, what should the voltage readings be on pins 4 and 6?

Didn't want to start a new thread... First, after three good weeks of running the 5V3A in the ST 120, it is flawless. It doesn't light up as cool as a 5AR4, but that's okay. I'm thinking of adding the series diodes (1N4007) to pins 4 and six of the rectifier tube, then tying pins 4 and 6 together. I've read the AK threads on the subject but haven't heard of anybody actually tying the pins together and flipping the switch. The other benefit besides increased current capability appears to be a drop in voltage reduction by half. For the 5V3A, that would cut it from 44 to 22, just 5 volts more than a 5AR4. I doubt my 66-year old ears could hear the difference in the higher B+, but some likely could. If those diodes ever failed, it would probably be a spectacular light show......

That .05 uf capacitor is the ceramic disc type, correct?

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I've re-read this thread and it seems that the advantage of the diode mod is to protect the 5AR4 in case of power on/off blips.
I have a VTA ST-70 and I use a Sylvania 5AR4 and the entire system in plugged into one of the Skizo recommended non-latching GFI gizmos. It kicks out when there is a power interruption and I have to manually reset it.
So, am I right in thinking I don't have to worry about this diode mod?

Hi Dave,
yes, the diode mod is for protection, it doesn't do much of anything else. Seems that with your Skizo mod you're likely accomplishing the same protection.
But for most people 40c worth of diodes is cheap protection for a $20 rectiifier, let alone any other tubes downstream . . .

tubes4hifi wrote:this is a common topic and search, so it'll be easier to find now
this mod should be done to all tube amps using tube rectifiers, to reduce the chance of damaging the tube rectifier

If I might ask a question.
The diode mod for the rectifier tube is in addition to the diode to replace the selenium rectifier, not in lieu of?

the diode mod for the rectifier tube (B+ circuit) has nothing at all to do with the diode replacing the old selenium rectifier for the bias circuit.

Thanks

I may be wrong, but here is my thinking.

The specified peak reverse voltage of a high quality vintage 5AR4 is 1500v,

https://drtube.com/datasheets/5ar4-amperex1958.pdf

but I've heard it said that the new copies by Chinese and Sovetek might have much lover PIV limits before they self destruct.

It seems that in normal operation the tube will have  PIV equal to the difference between the B+ DC in the filter caps and the negative peaks in the transformer secondary. So this is about 800v right? I guess the thinking is that some of the newer tubes even 800v might be unhealthy.

The addition of the diodes removes the negative from the plates so the tubes don't see a high peak inverse voltage.

Yes, the diodes alone would work just fine. The tube is mostly there to slow the rise time of the B+.

Just break down and buy a genuine, NOS Mullard GZ-33. Your problems will be over. Did everything I could to ruin mine; nothing worked.

Is the GZ-33 a drop-in substitution for 5AR4?

I have been sitting on my fingers, and I will continue to do so. But the nature of filaments, hot and cold, clearly needs to be better understood.

rjpjnk wrote:Is the GZ-33 a drop-in substitution for 5AR4?

I discovered that it is. I have an ST70. I substituted a NOS GZ-33 in place of my 5AR4 and I didn't even need to rebias the tubes! That's how close the voltage drops of both tubes are. The GZ-33 is definitely a much more robust tube and looks much better!

Tubes4ever wrote:

rjpjnk wrote:Is the GZ-33 a drop-in substitution for 5AR4?

I discovered that it is.  I have an ST70.  I substituted a NOS GZ-33 in place of my 5AR4 and I didn't even need to rebias the tubes! That's how close the voltage drops of both tubes are.  The GZ-33 is definitely a much more robust tube and looks much better!

It's the "studliest" tube for audio applications since the Great War! And for our purposes, the pin-out's the same as a 5AR4. Light a pair of genuine GZ-33s up in a dark room and pretty and otherwise reserved women will disrobe at the sight.

Don't know why I'm chiming in on this subject now, but I guess that little 'teaser' thing that pops up on email is directing me here........so.....here's my comment: I've been using this diode mod for years. As stated in prior posts, the Inverse Voltage protection aspect seems to be a reality, as I don't believe I've had one episode of an arcing rectifier tube since. I guess it's pretty safe to say: It works at some level. Is it infallible? Probably not! But, does is it worth a few cents and a few minutes of time of protect a tube rectifier? Yes! I know when this thread was started the figure of $30 was mentioned as 'cost' of a rectifier tube. Since I see some 5AR4/GZ34 tubes now selling for upwards of $100 or more......seems like a couple of $0.15 diodes is worth the investment. So, I do this mod on virtually every tube amp I own. One thing I didn't read in all this was the fact that when doing this 'diode mod', the tube rectifier itself still acts like it did without the diode mod. By that, what I'm referring to is the fact that it still has to warm up prior to passing current. So, not only does the diode mod protect the tube, but the tube still acts to provide the 'slow start' function it was designed for. I consider that a 'win-win'.

At the risk of flogging this dead horse.. This is a good choice for a diode also. Yes, it's overkill but that's the mode we're in anyway. Same forward voltage drop as the 1N4007 so it will dissipate the same power but because the case is larger the temp rise will be slightly lower. I plan on using them in a M-125.

https://is.gd/qsjSYX

Regards,
Ken

I'm not sure what happens either if the power is lost then restored within a few seconds but this relay trick will make sure it doesn't happen.  When the Power switch is turned
ON (or AC is restored with the power switch in the ON position) the relay is not energized until the momentary Start button is pushed.  When pushed, the relay switches and latches itself ON and supplies AC out.  When power is lost for more then about a cycle or two, the relay opens.  Considering that we're running non-critical equipment such as stereo amplifiers this seems to be a good solution.