Diode Question

[David_NC]

If the tube needs these confounded diodes, build with a different tube. Two damper diodes will run circles around a GZ34. If the amp abuses the GZ34, fix that first...series diodes to protect a tube run over ratings is just asking for the next weakest link to break. They Dynaco stuff was well designed to its price point, so the next link is not significantly stronger, and you will be giving it an opportunity to break.
cheers,
Douglas

I think adding a couple diodes to help protect a potentially pricey rectifier tube is so simple as to be a 'why not' sort of thing, considering it doesn't cause any issues that I can find in research. It's certainly much easier than redesigning a perfectly functioning, good sounding amp. This is a Bob Latino recommended mod, not some random idea. What would you suggest doing instead, if one would like to accomplish what this accomplishes, and what do you consider the 'next weakest link' that's gonna break after I do this?

 

[David_NC]

Douglas, the rectifier tube is not being 'run over ratings' - the extra ss diodes are simply an added protection for when the rectifier tube goes gassy and arcs (with a high risk of collateral damage to the PT) - rectifier tubes, like output stage tubes, have a short enough service life to be a concern.

When mine arced due to short-cycling, it fortunately also blew the fuse. Luckily, that's all it did, and fortunately it was a cheap 5AR4.

 

[trobbins]

Even rectifier valves are not so cheap nowadays for some of us.

Although meant for protection, the ss diode mod may allow an ageing rectifier valve to have a longer service life - although the concept of rectifier tube life mainly comes down to 'the amp still works so who cares'. There is some more discussion on aged rectifier tubes and how to test them in the last section of the link below.

https://www.dalmura.com.au/static/Power supply issues for tube amps.pdf

 

[David_NC]

Even rectifier valves are not so cheap nowadays for some of us.

Although meant for protection, the ss diode mod may allow an ageing rectifier valve to have a longer service life - although the concept of rectifier tube life mainly comes down to 'the amp still works so who cares'. There is some more discussion on aged rectifier tubes and how to test them in the last section of the link below.

https://www.dalmura.com.au/static/Power supply issues for tube amps.pdf

In my case, it was cheap - a $17 JJ. The second one has lasted longer, so far. I'll have to check out that link.

 

[PakProtector]

That the PN junctions will share the voltage adequately is given. Si, PN junctions do not fail like SiC Schottky types( the domino analogy comes to mind). The SiC Schottky are not recommended for series connection to deliver larger-than-single PIV ratings without solid PIV sharing methods. I talked to an engineer at Cree when they first released the new 1200V parts nearly 20 years ago. I consider this information solid.

The PN junction types like 1N4007 come up to max PIV, and then hold there while the next one heads toward delivered inverse voltage if they don't wind up sharing equally. Once at max PIV they just sit there until the next in the string gets to max( assuming the voltage is there to reach that level ). Until you run out of poorly sharing diodes in the string, there is no issue. Get over that and all bets are of course off...

So, IFF the circuit can only expose a pair of 1N4007 to 1900V inverse, there is no risk to them not sharing the inverse voltage dose.

PN junction types do not share current well, though the SiC Schottky do. Need more current? Then || the SiC Schottky. If using PN junction types and more current delivery is needed, get a bigger diode.
cheers,
Douglas

 

[egellings]

If you can find diodes with same leakage, then ditch the resistors. If leakages vary, then that will need to be swamped out with the resistors. Or better yet, get some nice SiC 1200 volters and ditch a diode & the resistors.

 

[PakProtector]

If you can find diodes with same leakage, then ditch the resistors. If leakages vary, then that will need to be swamped out with the resistors. Or better yet, get some nice SiC 1200 volters and ditch a diode & the resistors.

For PN junction types like 1N4007, this is just not so. The sharing does *NOT* have to be equal. Though impossible, one diode after another can come up to its max PIV, and all the rest behind it will leave it there.
cheers,
Douglas

 

[trobbins]

'Measuring' ss diode leakage is almost an impossible task for diyers.

 

[PakProtector]

Douglas, the rectifier tube is not being 'run over ratings' - the extra ss diodes are simply an added protection for when the rectifier tube goes gassy and arcs (with a high risk of collateral damage to the PT) - rectifier tubes, like output stage tubes, have a short enough service life to be a concern.

Aren't run over the ratings of the tubes whose name they share maybe...
cheers,
Douglas

 

[egellings]

You need a DC voltage source and a microammeter, or the DC source, a known series resistor and a DC voltmeter. That should be doable by an interested DIYer.

 

[egellings]

Check the investigated diode's specification sheet. As for what to expect, again see the spec sheet for the required results for any diode type. For reverse leakage, the max allowable current will be listed at a given reverse DC voltage. Apply that voltage, wait a settling time, measure the current--done.

Careful with those tinfoil hats--they're highly conductive, you know. Electronikers steer clear of them for just that reason.

 

[trobbins]

The simplest 'pass' test for a 1kV ss diode is to use an insulation resistance meter that tests at 1kVdc and down to 0.5uA and is current limited - such an IR meter is common (and often cheap) 2nd hand in some countries but not in others. A typical batch of 1N4007 will not show any leakage on such a meter, as noticeable leakage kicks in typically above 1.2kV. Poor valve diodes may show up some leakage level at 1kVdc.

The simplest way I have tested to a higher voltage and measured a leakage value required a separate DCV supply to be inserted in series with the 1kVdc insulation resistance meter. The DCV supply needs some level of current liming, and both items need reverse current clamping. That works well for valve diodes, but I haven't used it for ss diodes.

There are vintage HVdc ionisation and leakage testers from BPL and AVO around (I have 2), and modern meters used by electrical manufacturers that measure insulation resistance to 5kVdc (like some Megger devices) but they are quite expensive, and using those meters requires a good level of competency unless one is aiming for a Darwin award.

If using a single HVdc supply, then it may be prudent to include current limiting, and maybe even a current trip clamp, to avoid the chance of destructive or stressed diode testing and damage to the uA meter. It's not common to find a moving coil meter with FSD down below 50uA. An alternative (for example) is to use a 4-digit cheap handheld meter with 9.999mVFS, and use a 1 k current sense resistor to get 9.999uA resolution and hope that ripple and other noise is not a problem.

 

[PakProtector]

What is the problem, if two PN junction diodes in series entirely don't share the PIV?
(in context of this discussion, this could possibly be considered a trick question).
cheers,
Douglas

 

[trobbins]

No problem - just following the discussion on from post #28, where the impression is that diode leakage can be measured, and by metering that may be common to some enthusiasts.

 

[David_NC]

I finally got the diodes installed, along with some new speaker terminals. I tested the ST-70 last night on my bench using a Marantz 1060 as the pre, and it sounded better than it did before I did the mod. It seemed there was greater bass, and upper clarity was better. I know people say that there's an audible difference to using tube rectification vs solid state. I wonder if using the diodes in this way would affect the sound in a similar fashion.