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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 10:54 am
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Definitely.

I can't see a down side at all.

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 1:27 pm
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I've read through quite a few posts in the forum that BMW provided the link to. I have not been able to find any real explanation as to why Dynacos would have more of a problem with GZ34/5AR4 rectifiers than any other tube amp. Also, the references I was able to find regarding JJs to be more problematic than NOS tubes seem to be about 2 to 3 years old.

The diode mod does seem to be one that really causes no harm and can in fact be of benefit. It is easy enough to do that I might look into it for my 5F6A after I solve my panel resonance problem (gonna go look at that after I finish here).

I am puzzled and a little concerned by this paragraph in the Premier Guitar article that shimmilou referenced:

"After the mod, the B+ will be 0.7V smaller – an insignificant amount. If the rectifier tube just wears out and quits conducting, the amp still quits working; that’s what replacement tubes are for. But if it shorts, the solid-state diodes now take over the load. In fact, the amp simply keeps playing if the rectifier tube shorts. The high voltage supply actually rises by maybe 50V, so the amp gets a touch louder. You can keep playing until the show is over and you have time to put in a new rectifier. "

If the tube shorts but the amp continues to play, how will one ever know that the rectifier has failed? What are the consequences of not realizing the tube has failed and not replacing it in a timely manner? What are the consequences of running the amp with the B+ 50 volts higher than it should be? What is the impact on power tube bias? Any thought on this?

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 2:04 pm
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Bluesky, Good question. If the tube DIES BUT NOT SHORT and the soild-state diodes take over, you will lose the slow warm-up character of the GZ34. Now, if this is a 5U4GB you are bypassing, you won't have a slow start-up to begin with. But, the amp will become stouter with full load on the solid-state diodes, and lose the "sag" character of the 5U4GB.

Important to note the difference. What these ss diodes due is reduce the stress on the tube rectifier... BUT NOT prevent the rectifier from shorting out, due to failure in the tube's design. It's why I still like old Mullards.

FWIW, I have not had a tube rectifier die with the added UF4007. This is after several ST-70 fixes. Like I said before, if you have no issues with whatever current tube rectifier you are using, let sleeping dogs lie. If however you have some failures, you can consider trying this cheap mod. Cheaper than looking for NOS or good used Mullard or Amperex GZ34. Or NOS Sylvania or GE 5AR4.

Now, as for the tone... YES the ss diodes will change the tone. Apparent in 5U4GB amps, especially. You'll will get kinda a cross between ss rectification, alone and 5U4GB rectification, alone. That's why I recommend this mod in amps that have failure issues with modern tube rectifiers. Like vintage Vox 30's. Which push the four EL84 to nearly Class "A." With curent draw and lack of adequate cooling (this easily fixed, but lose original layout of the amp)--- these amps are noted for early GZ34 failure with new issued rectifiers.

Why? Because many modern "GZ34" are not built to original Mullard/Philips standards. In fact many are not true GZ34. But 5Z4 or some such rectifier, relabeled. The 5Z4 were originally designed for constant voltage/constant current use in Chinese and Russian military components. And not the dynamic use in hi-fi or guitar amps.

Or the tooling & material used of the newly made "GZ34" is not to the tight tolerances of the Mulllard. Leading to loosening of the guts of the rectifier, esp in combo amps. This plus excessive dynamic voltage and/or currents demands on these kinda tubes leads to arc-over and sometimes catastrophic failure. Tube rectifiers are, by far, one of the more vulnerable to failure in any amp. It's why good used or NOS Mullards command such high price. Even good NOS Sylvania or GE are going up in price.

I personally, have tried to use good testing Blackburn, UK Mullard made GZ34 or Sylvania or GE 5AR4 in vintage amps (they are just so much more reliable and predictable in tone). Because I don't like having these things coming back with fried PS caps and/or trannies. But, I have used this diode trick on some amps in the past. And they seem to all be working fine.

The last blown Sovtek GZ34 I worked on was a AB764 PR. This short caused the cap can to overheat and pop its seal. Which lead to electrolytic fluid all over the chassis. A major mess, but fixable. The secondary on the PT showed signs of short on one half of the winding (to CT). So after a new can, new tranny, various new resistor and coupling caps sprayed by the 'lytic fluid. And thorough cleaning of the chassis--- the amp was fine. Oh I did change the output tubes, as they biased very low.

Sorry, I couldn't find the specific Latino post on the issues with modern rectifiers and the ST-70. It's just so well known, that most owners have taken this issue for granted over the last 20 years. Not that you can't find a good new GZ34/5AR4. It's just their QA/QC is often questionable. Which may lead to early failure issues. FWIW....


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 2:35 pm
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BMW,

Great discussion but it still doesn't address my question regarding the rectifier shorting out and the diodes taking over. The PG article makes it sound like the amp will keep going, so how do you know the tube has shorted and the diodes have taken over? Seems like running with the B+ increased by 50 volts for any length of time would not be a good thing.

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 2:52 pm
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Bluesky, the ss diode will NOT take over, if the tube rectifier SHORTS TO GROUND. Only, if it wears out (I used the word "die") or shorts from cathode to anode-- but is still conducting. What these ss-diodes do is reduce the voltage/current stress demands that can lead to shorts in the rectifier.

I've have not heard of a GZ34 wearing out with one of these ss-diode mods. But, prolly because most owners of amps with this rectifier change out the tube periodically. And not leave them in for decades, like in the old days. People tend to be a bit more anal about this than before, when tubes were readily available and a whole lot cheaper.

Actually, what is going on with this arrangement, is you have TWO rectifiers in series. So, the ultimate rectification character is the combined character of two diodes (solid state and tube). Each one contributing to the DC power. Because ss-diode react so much faster than tube, they will contribute most of the needed transient response to the amp. Which alleviates some of the issues with leads to tube rectifier wear and/or failure.

And ss-diodes last longer than tube rectifiers. So, as the tube starts to wear out, the ss-diodes character becomes more apparent, as they take over. You would start losing some "sag" and slow-startup quality as the tube's aging continued.

All the tube rectifier is doing is kinda cruising along adding its slow startup and "sag" character to the amp. But, not having to do much of the sudden current or voltage demands. It's kinda like ppl that put resistors inline with pure ss-diode rectifiers, to mimic some of the sag qualities.

HTH!


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 4:03 pm
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I think what the article referred to is if the tube shorted Plate to filament (Cathode in this case), it would be like connecting the Cathode of the diode(s) to the output of the tube, in which case the diode(s) would indeed become the rectifier. The diodes would also prevent the AC through the shorted tube, protecting the filter caps. If the tube "opens", then the amp would still just die normally, just the same as without the diodes. Open tube = no output.

The rectifier shorting is the only way that the diodes would take over, becoming the rectifiers for the amp. Other than a short in the tube, the diodes have virtually zero role in the rectification, and are simply for protection, effectively bolstering the tube. You still have the same tube sag just as without the diodes. The 0.7 volt drop on each diode is insignificant, and apparently no other change in the tube rectifier operation. The diodes play no role in rectification if the tube gets weak, they can't do anything to change that. To keep it simple, the diodes and the tube are just like one-way valves. If you have a weak valve (tube), putting stronger valve in front (diode) can't change the weakness of the tube.

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 4:36 pm
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I think shimmi has it, but the statment that says the B+ will be 50 volts higher if the tube shorts and the diodes take over, still bothers me. How does one know that the tube has shorted and the diodes have taken over so corrective measures can be taken?

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 5:02 pm
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The sag (voltage drop) of the tube would no longer be part of the equation, resulting in higher DC out when the diodes "take over".

As far as knowing when this happens, it might only be noticeable at higher volumes, when the loss of sag becomes apparent. Otherwise, might be difficult to detect.

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 5:25 pm
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shimmilou wrote:
The sag (voltage drop) of the tube would no longer be part of the equation, resulting in higher DC out when the diodes "take over".

As far as knowing when this happens, it might only be noticeable at higher volumes, when the loss of sag becomes apparent. Otherwise, might be difficult to detect.


Sag is a transient effect, though, right? It occurs when the guitar is hit hard, the power supply voltage drops, and then it recovers. So depending on what is being played, sag might not always be detectable regardless of the average volume level, correct? I understand the theory and math behind sag, but I don't really have a physical appreciation for it. I have cranked my 5F6A up to 10 and 12 and hit a power chord hard. I honestly cannot say that I can detect any sag. I can honestly say, though, that the amp is loud as all get out. :shock: :lol:

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 5:37 pm
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Bluesky, for the GZ34, it's easy. You'd lose the slow startup of an IDH rectifier.


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 5:57 pm
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I overlooked that, good point BMW2002Ti.

And then, that leads to the use of a standby switch, in which case the slow start-up wouldn't matter as much.

Not sure if a rectifier tube with a higher voltage drop would also sag more than one with a lower voltage drop, but it makes sense that it would. There is some sag on any rectifier tube, and I agree that it certainly sags more when hitting a chord hard and/or at higher volume. The normal voltage drop of the tube would be there regardless though, and not present with diode rectifier.

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 6:17 pm
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Shimmy, usually with a good, newly manufactured diodes UF diodes, when they replace the amp, it's easy to hear the difference. Esp between a tired or worn out tube rectifier. Like putting a strong tube rectifier into an amp with a tired one.

You need to rebias the amp, if you add the solid-state rectifier mod.


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sun Jun 10, 2012 12:23 am
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As long as we're talkin' about rectifiers, I'd like to throw out a question for your opinions. I use freds in my SS rectified Bassman and Reverb unit. Been using thermistors in the B+line. My thought was a slower inrush. In the Bassman, the first cap bank gets fully charged. When the standby is closed, the next PF cap gets a full slam. Do you know what I mean? Art

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sun Jun 10, 2012 12:41 am
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Art, Yeah a thermistor or in-rush limiter (like a CL-80) is a good way to allow slower cap filling. Also, a big choke after the rectifier helps to limit the damage due to rapid in-rush--- esp with big caps after the rectifier. An elegant, but expensive alternative (as it would need to be big to handle the current needed at this point).


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sun Jun 10, 2012 1:05 am
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Right, I didn't mention the choke. But yeah it's ahead of the choke. "Elegance", gonna have to consider the amps like women!!! Art

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