SS diodes in front of rectifier tubes: fad or good idea?

[Diablo]

How about sticking a meter on the back of the amp that reads out the B+ voltage?
That would tell you if the rectifier shorted out, when the voltage jumped up.

[pdf64]

My Weber Copper Cap WZ34 uses an 82 ohm sag resistor.
The effect of that is that at high current demand, the output voltage seems to drop rather more than occurs with a real GZ34.
An example of the effect this has is that with a WZ34, the 'max square wave' power output (for a 2x6L6 type amp) is about 55 watts but with a GZ34 it's nearly 70 watts (measurements taken whilst monitoring the line voltage and used a load resistor with a true rms meter).

I acknowledge that's only a 2dB difference but it seemed to result in a noticeable tone change, however I haven't undertaken a direct AB comparison.

So, my current feeling is that a copper cap / sag resistor isn't an adequate equivalent to a real tube rectifier, for a GZ34 at least.

With regard to the original post, I'm an advocate of Si diodes in series with a GZ34 tube rectifier but worry that the VB+ rise in the case of a failed 'higher sag' rectifier, eg 5Y3, may be too much of a problem.
Pete

[Enzo]

Of course it will change things if the tube blows. It is not intended to be transparent after tube failure. It is there to protect the amp from the tube, and allow the amp to at least function.

Each individual diode will cover its own plate to cathode tube short. If the tube shorts plate to plate, that puts the two tube sections in parallel, but they are after the diodes, so unless I am missing something, the amp continues to operate. In fact, the tube sag even remains with a plate to plate short.

[R.G.]

My Weber Copper Cap WZ34 uses an 82 ohm sag resistor.
The effect of that is that at high current demand, the output voltage seems to drop rather more than occurs with a real GZ34.
An example of the effect this has is that with a WZ34, the 'max square wave' power output (for a 2x6L6 type amp) is about 55 watts but with a GZ34 it's nearly 70 watts (measurements taken whilst monitoring the line voltage and used a load resistor with a true rms meter).

I acknowledge that's only a 2dB difference but it seemed to result in a noticeable tone change, however I haven't undertaken a direct AB comparison.

So, my current feeling is that a copper cap / sag resistor isn't an adequate equivalent to a real tube rectifier, for a GZ34 at least.

Of course, it's not exactly the same in all cases. But it may be acceptable, and many people don't hear the difference. YMMV.

With regard to the original post, I'm an advocate of Si diodes in series with a GZ34 tube rectifier but worry that the VB+ rise in the case of a failed 'higher sag' rectifier, eg 5Y3, may be too much of a problem.

Yeah, I was always more worried about the voltage rise. The worst problem would be if the amp had preamp tubes or bypass caps on the hairy edge of failing at the normal B+. That could make a cap fail. But it's the same problem that a high line surge would give.

On the other hand, having B+ go a bit high without the corresponding rise in heater voltage that would accompany a high line surge might be less disastrous.

The point in protective devices and circuits is that what they do is less of a problem that if they were not there, not that they're necessarily perfect. Although "perfect" is something to look at and strive for. Air bag don't prevent car-body damage, but they make the car occupants hurt less.

I'm pretty sure that a 50V rise in B+ is less of an issue than having a shorted rectifier tube dump high voltage AC onto the first filter cap, for instance.

[NateS]

If I'm not mistaken, most vacuum tubes already contain an indicator light, though it may be hard to see due to other structures inside the tube.

[g1]

Dang! Now we need another indicator light to tell us when the rectifier has failed?
Seriously though, if it shorts are you going to notice the difference in the sound?

Of course it will change things if the tube blows. It is not intended to be transparent after tube failure. It is there to protect the amp from the tube, and allow the amp to at least function.

If I'm not mistaken, most vacuum tubes already contain an indicator light, though it may be hard to see due to other structures inside the tube.

My point was not whether there is an actual difference, but whether it is noticeable enough for "joe average" user to notice. And for those who do not notice, how much damage is being done to the amp from continued use.
I've had people bring in amps that have one of a push-pull pair laying in the bottom of the cab. Not being clean players, they "thought it sounded a little off about a month ago but wasn't sure". Is somebody like that going to notice their B+ went up a bit?

I'm pretty sure that a 50V rise in B+ is less of an issue than having a shorted rectifier tube dump high voltage AC onto the first filter cap, for instance.

I'd like to agree, but for a couple 2 cent diodes, why aren't manufacturers installing them?

[pdf64]

'for a couple 2 cent diodes, why aren't manufacturers installing them?'

Fender do on many models equipped with tube rectifiers, eg http://support.fender.com/schematics..._schematic.pdf that particular model supports their use with 5Y3.
Pete

[trobbins]

If I'm not mistaken, most vacuum tubes already contain an indicator light, though it may be hard to see due to other structures inside the tube.

If the comment relates to a rectifier tube with a shorted anode-cathode, then that fault condition doesn't directly change the tube's heater 'light' output compared to a normally operating tube. If the comment relates to using an IR camera to inspect the diode tube, then yes that would be an excellent (although possibly somewhat exotic for many repair people) way to quickly check that the diode (eg. both diode anodes in a typical dual-diode tube) is correctly passing power.

[NateS]

If the comment relates to a rectifier tube with a shorted anode-cathode, then that fault condition doesn't directly change the tube's heater 'light' output compared to a normally operating tube. If the comment relates to using an IR camera to inspect the diode tube, then yes that would be an excellent (although possibly somewhat exotic for many repair people) way to quickly check that the diode (eg. both diode anodes in a typical dual-diode tube) is correctly passing power.

Yeah, it was mostly smart aleck, but I was a bit mistaken. I figured a directly heated cathode would gravitate to one extreme or the other in the failure condition. Perhaps even letting out all its light at once.

[Gingertube]

My understanding is that the series diode mod was something amp techs started doing in response to a whole heap of modern production rectifier tubes which were prone to arcing (GZ34 / 5AR4 mostly). The SS diodes handled the Peak Inverse Voltage and prevented the hollow state diodes from arcing.
Those modern production problems have mostly been sorted but I would still put the SS series diodes in a new build - "belt and braces" design is often a good idea and nowhere moreso than in a tube guitar amp.
Cheers,
Ian