Hey all,
Now just very curious to what the failure mode of a bad rectifier tube is internally.
Last night while getting around to playing with my tube amp design station (just a wooded board with PT', turret strips screwed in,)
I have a canister with a handful of NOS tubes from a deceased engineers stash (Scott Kent). One of the handful of Rect tubes I got was a very good looking Mullard, one of those that cost your car if its good. So, after measuring the voltages for a new/good JJ GZ34 I popped the NOS Mullard in. Bright flash from the transformer, I thought I took out the primary but was happy to see I did screw in a fuse holder with a 2A fuse installed.
Following the "surprise" flash I pulled the tube and ohmed out all the pins to see how the bad rect tube would differ from the new JJ GZ34. There was no difference and to my surprise, the filament seemed to still be intact (measured a couple of ohms or so).
I was hoping to hear any comment on what are the things in a rect tube that go wrong, and in last nights case, an obvious internal short. Does anyone know what elements of the rect tube actually short?
Thanks, now I'm afraid to try out the three 5Y3 ANOS tubes that I have.
Best,
Phil D.
Geez, that vintage GZ34 looked like it was good.
Moderators: pompeiisneaks, Colossal
Geez, that vintage GZ34 looked like it was good.
I’m only one person (most of the time)
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Geez, that vintage GZ34 looked like it was good.
Well first you need to look at the recto tube pin out itself.
Some recto’s are directly heated and some are not.
What’s meant by that is a recto like a GZ34 / 5AR4.
has a separate heater and cathode, in a recto like a 5y3 the heater and cathode are one.
This also means that recto’s built like a gz34 have a slow warm up time and don’t slam the preamp and output tube plates with high voltage before there heaters are fully warmed up, not to mention the filter caps getting no time to charge up slow either.
Recto tubes that are gassy will / can arc internally when they get up to peak voltage just like output tubes will and if they are bad in this way may not read a short when testing for such.
Eventually though with enough flash overs they will be shorted permanently.
Tubes like the 5y3 and 5u4 do not have the separate cathode that allows a slow rise in DC output.
Some recto’s are directly heated and some are not.
What’s meant by that is a recto like a GZ34 / 5AR4.
has a separate heater and cathode, in a recto like a 5y3 the heater and cathode are one.
This also means that recto’s built like a gz34 have a slow warm up time and don’t slam the preamp and output tube plates with high voltage before there heaters are fully warmed up, not to mention the filter caps getting no time to charge up slow either.
Recto tubes that are gassy will / can arc internally when they get up to peak voltage just like output tubes will and if they are bad in this way may not read a short when testing for such.
Eventually though with enough flash overs they will be shorted permanently.
Tubes like the 5y3 and 5u4 do not have the separate cathode that allows a slow rise in DC output.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Geez, that vintage GZ34 looked like it was good.
In the olden days when new tubes of any type were readily available, it made sense to just plug in a new tube and see if a new tube worked. Well, also, there were only tubes to use if you wanted to do any kind of protection.
Today, we can put in protection circuits that will minimize collateral damage if a bad tube slips by. For rectifier tubes, this can start with solid state backup rectifiers in series with the anodes. A bum rectifier tube can flash over or short, and the diodes will prevent damage to the PT and the filter caps. If the tube shorts, the amp still keeps working, but with a 30-40V higher B+.
Rectifier tube current needs to be limited to get the best life out of them. Here's one way:
http://www.geofex.com/Article_Folders/m ... 0Clamp.pdf
The MOSFET "saturates" to ~ 1 ohm or less in normal operation, and the current sampling resistor R2 will always be less than about 0.6V. Over 0.5 to 0.6V, the NPN steals the MOSFET's gate drive and current is clamped to no more than that value. Setting R2 lets you ensure that the rectifier tube will ...never... be larger.
Normally you would pick R2 to get just a hair more than the current pulses that are needed for full power operation. This all by itself gives a slower ramp-up when power is turned on, because the rectifier pulses at turn on are bigger.
True, the heater warm up on the rectifier tube allows for a gradual power up. The current clamp does not change this behavior. But you can do even better. You can hook the NPN transistor output of an opto isolator in parallel with the NPN in the circuit. Anything you do to turn on the LED of the opto isolator turns the MOSFET off. So you can remote-shutdown the B+ by turning on an LED in the opto. A CMOS 555 timer driving the LED can flatly hold B+ off for a while when the amp is first powered on. Sneakier use of this can help with standby issues if you really want to do a standby.
Today, we can put in protection circuits that will minimize collateral damage if a bad tube slips by. For rectifier tubes, this can start with solid state backup rectifiers in series with the anodes. A bum rectifier tube can flash over or short, and the diodes will prevent damage to the PT and the filter caps. If the tube shorts, the amp still keeps working, but with a 30-40V higher B+.
Rectifier tube current needs to be limited to get the best life out of them. Here's one way:
http://www.geofex.com/Article_Folders/m ... 0Clamp.pdf
The MOSFET "saturates" to ~ 1 ohm or less in normal operation, and the current sampling resistor R2 will always be less than about 0.6V. Over 0.5 to 0.6V, the NPN steals the MOSFET's gate drive and current is clamped to no more than that value. Setting R2 lets you ensure that the rectifier tube will ...never... be larger.
Normally you would pick R2 to get just a hair more than the current pulses that are needed for full power operation. This all by itself gives a slower ramp-up when power is turned on, because the rectifier pulses at turn on are bigger.
True, the heater warm up on the rectifier tube allows for a gradual power up. The current clamp does not change this behavior. But you can do even better. You can hook the NPN transistor output of an opto isolator in parallel with the NPN in the circuit. Anything you do to turn on the LED of the opto isolator turns the MOSFET off. So you can remote-shutdown the B+ by turning on an LED in the opto. A CMOS 555 timer driving the LED can flatly hold B+ off for a while when the amp is first powered on. Sneakier use of this can help with standby issues if you really want to do a standby.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Mark Twain
Re: Geez, that vintage GZ34 looked like it was good.
Nice little current limiter circuit there, I was able to get it while reading the article. Very nice, and not too much cash for what it may save and protect an amp from. I downloaded the article and may consider installing in any subsequent tube rectified amp. Why the heck not.
With the third build I"m on how, I came across the idea of strapping 50k or so across the standby switch to make a sort of "soft start". Plus, I added a diode after it all when it was explained that that would keep the "standby switch" out of the DC environment and put it in an AC environment as to not always be taking a full arc on switch off and on. The idea of instantly loading up and empty capacitor did come to bother me a bit so, it was good to see that the cap filled up relatively slow with the switch open (35 volts the first second). But I can see how that circuit could remove the fear factor of trying out a new (or old) rectifier tube. I have about 5 old ones that I wish worked but, no longer have the nerve to try em out. Not worth whatever damage could result, or even a big momentary electrical stress. No MOre!
Thanks for coming on by, very interesting and helpful
Best,
Phil D
With the third build I"m on how, I came across the idea of strapping 50k or so across the standby switch to make a sort of "soft start". Plus, I added a diode after it all when it was explained that that would keep the "standby switch" out of the DC environment and put it in an AC environment as to not always be taking a full arc on switch off and on. The idea of instantly loading up and empty capacitor did come to bother me a bit so, it was good to see that the cap filled up relatively slow with the switch open (35 volts the first second). But I can see how that circuit could remove the fear factor of trying out a new (or old) rectifier tube. I have about 5 old ones that I wish worked but, no longer have the nerve to try em out. Not worth whatever damage could result, or even a big momentary electrical stress. No MOre!
Thanks for coming on by, very interesting and helpful
Best,
Phil D
I’m only one person (most of the time)