I occasionally see the use of redundant backup diodes on a rectifier tube. I see many pros and few cons with this approach:
Possible pros...
1) Keeping the rectifier tube at all still gives you a slow ramp up of B+
2) Protection in some cases of the power transformer getting blown on a tube short should the mains fuse be late
3) Prevent inrush arching in some rectifier tubes
4) Protection downstream should the rectifier tube get a plate to cathode short, the circuit would still be getting DC instead of raw AC because the diodes are there. Blown filter cap.
5) Allows the use of a larger first filter capacitor than the RCA manual recommends (? is this true within reason?)
What are the cons? (besides cost)
My main interests are for #1 and #2 right now protecting the power transformer in multiple ways and also retaining a slow start to the B+
I have a project that uses a pretty costly One-Electron (BFT-1B) power transformer (I will fuse the secondary to protect it as well) so I dont want to see a blown power transformer secondary, ever. And slow ramp of B+ for tube life, I really don't like the use of semiconductor timers to delay B+.
One Electron Transformers
Are there any big Cons to using backup diodes on a rectifier tube I should be aware of? Any sonic implications? I heard rumor that many Dynaco ST70's had to be modded this way to prevent rectifier arching problems.
Possible pros...
1) Keeping the rectifier tube at all still gives you a slow ramp up of B+
2) Protection in some cases of the power transformer getting blown on a tube short should the mains fuse be late
3) Prevent inrush arching in some rectifier tubes
4) Protection downstream should the rectifier tube get a plate to cathode short, the circuit would still be getting DC instead of raw AC because the diodes are there. Blown filter cap.
5) Allows the use of a larger first filter capacitor than the RCA manual recommends (? is this true within reason?)
What are the cons? (besides cost)
My main interests are for #1 and #2 right now protecting the power transformer in multiple ways and also retaining a slow start to the B+
I have a project that uses a pretty costly One-Electron (BFT-1B) power transformer (I will fuse the secondary to protect it as well) so I dont want to see a blown power transformer secondary, ever. And slow ramp of B+ for tube life, I really don't like the use of semiconductor timers to delay B+.
One Electron Transformers
Are there any big Cons to using backup diodes on a rectifier tube I should be aware of? Any sonic implications? I heard rumor that many Dynaco ST70's had to be modded this way to prevent rectifier arching problems.
There's absolutely nothing wrong with using "backup diodes" as "insurance" with a tube rectifier.
3 amp, 1000PIV diodes will do the job nicely.
In effect, there's no downside, and don't listen to people who insist that it effects the resulting "sound" of an amplifier, because it doesn't.
The only thing it effects are the brain cells of those people.
3 amp, 1000PIV diodes will do the job nicely.
In effect, there's no downside, and don't listen to people who insist that it effects the resulting "sound" of an amplifier, because it doesn't.
The only thing it effects are the brain cells of those people.
If the OP is talking about the series SS diode tweak shown in the provided graphic, it's a good thing. The SS diodes provide some additional PIV headroom. The Sovtek 5AR4 can arc, when employed towards the top of the documented allowable PIV range. Except for that flaw, the New Sensor product is an excellent, cost effective, replacement for expensive OS 5AR4s. Roughly $1 spent on 2X UF4007s allows for trouble free operation, reasonable total expenditure, and good sound. Enjoy!
Attachments
I added the mod shown in post #3 to all Tubelab SSE boards produced after early 2010. There were too many failures seen in new production 5AR4's during this time period. I explained that any builder who didn't want the diodes in the path could simply install jumpers in the PCB holes.....nobody ever said anything bad about the added diodes, and blown 5AR4's became a thing of the past.
I am working on a new board design which like the SSE will run the rectifier close to its max specs. The diodes are already on the test board, which is in the testing phase now.
Another good thing is to install a CL-140 inrush current limiter in series with the HV center tap. This softens the hit on the rectifier tube as it tries to feed the hungry filter caps on startup. The SSE board uses this too.
I am working on a new board design which like the SSE will run the rectifier close to its max specs. The diodes are already on the test board, which is in the testing phase now.
Another good thing is to install a CL-140 inrush current limiter in series with the HV center tap. This softens the hit on the rectifier tube as it tries to feed the hungry filter caps on startup. The SSE board uses this too.
Windcrest, what cases of protection were you referring to in (2) ?
Also, where did you come across (5), as the added ss diode has no influence on the capacitor sizing imho?
You indicated there were a few 'cons', but I can't see you identifying any.
The only other deployment advise I can offer is that the PIV of the added ss diode(s) should be rated for the working PIV of the specific amp. Given generous margins and tolerances, that would indicate using 2x 1N4007 in series when the transformer secondary is above about 300-0-300V, and 3x in series above about 450-0-450V.
You would also need to appreciate how beefy a diode is appropriate. Given reasonable deratings and temperature environment, a 1N4007 may only be good for up to 0.45A average, whereas a UF4007 would be less at about 0.3A average (but mindful that you need to know what 'average' is in your amp's power supply configuration and use).
Also, where did you come across (5), as the added ss diode has no influence on the capacitor sizing imho?
You indicated there were a few 'cons', but I can't see you identifying any.
The only other deployment advise I can offer is that the PIV of the added ss diode(s) should be rated for the working PIV of the specific amp. Given generous margins and tolerances, that would indicate using 2x 1N4007 in series when the transformer secondary is above about 300-0-300V, and 3x in series above about 450-0-450V.
You would also need to appreciate how beefy a diode is appropriate. Given reasonable deratings and temperature environment, a 1N4007 may only be good for up to 0.45A average, whereas a UF4007 would be less at about 0.3A average (but mindful that you need to know what 'average' is in your amp's power supply configuration and use).
At 40 cents each, the MCC version of the UF4007 is the lowest in cost. It's rated for 1 A. continuous average and a good deal more pulsed.
Amphenol mentions for their thermistor series an allowed bulk capacitance of the device to be protected. Does this property refer to just the first PSU cap or to the total of all caps, including RC and LC filters?
The main reason I started putting the UF4007's and CL-140 in my amps was poor quality of the new production rectifier tubes at the time. It was somewhat common for a brand new tube to "spark out" on first power up.
Sometimes two or three would be needed to find a good one, which would then live for years. Granted the SSE design, as many users built it runs the rectifier tube hard. It used a power transformer of 720 to 750 VCT to feed a pair of KT88's, 6L7GC's, or EL34's which could be biased as hot as 100 mA each. The first filter cap could be up to 47 uF, and many builders increased the second cap into the 200 uF region.
I smashed open several suspect tubes and found two common defects that caused this issue. The coating on the cathode was not always a uniform thickness and the cathode rod was not always straight or perfectly centered in the plate circle.
The rectifier tube has a tough job. It gets awakened from a cold sleep and starts conducting as its cathode begins to warm. It needs to fill the empty filter caps and feed all the tubes in the amp as they warm up. Sometimes the output tubes will start to draw current before the rectifier tube is fully heated.
If the cathode coating has any thin spots, they will reach emission temperature before the rest of the cathode and need to support ALL of the current necessary to fill the hungry caps......sparks happen.
If the cathode is not perfectly centered inside the plate circle more current will flow in a small area of the tube than in other areas. This can cause the tube to spark out on first power up, or it can cause a localized hot spot that will lead to a weakened cathode coating, warped parts, or both. Sparks will eventually happen.
The older directly heated rectifiers like the 5U4 can spark, recover, and live a long life. Once a modern 5AR4 sparks, it will usually do it again.
This is a common failure in poorly made output tubes as well. It is manifested as a small localized red spot on the tube plate if the tube is pushed hard. A well made tube will heat the plate evenly when pushed hard.
Adding the diodes or the CL-140 will not protect the power transformer from failure under all conditions. It will extend the life of a less than perfect rectifier tube and thus prevent a common failure mechanism.
Fusing the transformer secondary can be problematic if the typical 250 volt glass fuse is used. They may shatter if they blow, and high voltage fuses in small current sizes are not common. Never put a glass fuse on the DC line. If a short causes the fuse to blow, it will arc internally and EXPLODE. I learned this the hard way.
Fusing the primary is required for safety reasons. Choosing a good value for the fuse is another issue. When I lived in Florida where lightning was common, the fuse needed to be large enough to prevent it blowing with evvery power surge, but not too large to be useful. A fuse that was large enough to blow on a big surge, maybe once every few months, will not protect against an output tube runaway event, which is another common failure with a new production output tube that is of less than perfect construction.
Sometimes two or three would be needed to find a good one, which would then live for years. Granted the SSE design, as many users built it runs the rectifier tube hard. It used a power transformer of 720 to 750 VCT to feed a pair of KT88's, 6L7GC's, or EL34's which could be biased as hot as 100 mA each. The first filter cap could be up to 47 uF, and many builders increased the second cap into the 200 uF region.
I smashed open several suspect tubes and found two common defects that caused this issue. The coating on the cathode was not always a uniform thickness and the cathode rod was not always straight or perfectly centered in the plate circle.
The rectifier tube has a tough job. It gets awakened from a cold sleep and starts conducting as its cathode begins to warm. It needs to fill the empty filter caps and feed all the tubes in the amp as they warm up. Sometimes the output tubes will start to draw current before the rectifier tube is fully heated.
If the cathode coating has any thin spots, they will reach emission temperature before the rest of the cathode and need to support ALL of the current necessary to fill the hungry caps......sparks happen.
If the cathode is not perfectly centered inside the plate circle more current will flow in a small area of the tube than in other areas. This can cause the tube to spark out on first power up, or it can cause a localized hot spot that will lead to a weakened cathode coating, warped parts, or both. Sparks will eventually happen.
The older directly heated rectifiers like the 5U4 can spark, recover, and live a long life. Once a modern 5AR4 sparks, it will usually do it again.
This is a common failure in poorly made output tubes as well. It is manifested as a small localized red spot on the tube plate if the tube is pushed hard. A well made tube will heat the plate evenly when pushed hard.
Adding the diodes or the CL-140 will not protect the power transformer from failure under all conditions. It will extend the life of a less than perfect rectifier tube and thus prevent a common failure mechanism.
Fusing the transformer secondary can be problematic if the typical 250 volt glass fuse is used. They may shatter if they blow, and high voltage fuses in small current sizes are not common. Never put a glass fuse on the DC line. If a short causes the fuse to blow, it will arc internally and EXPLODE. I learned this the hard way.
Fusing the primary is required for safety reasons. Choosing a good value for the fuse is another issue. When I lived in Florida where lightning was common, the fuse needed to be large enough to prevent it blowing with evvery power surge, but not too large to be useful. A fuse that was large enough to blow on a big surge, maybe once every few months, will not protect against an output tube runaway event, which is another common failure with a new production output tube that is of less than perfect construction.
Doing that is like puting a full-time 5R resistor under the first filter cap in series to ground, if not all the filters. That would be a 5R ESR for that cap. Isn't that breaking the high ESR taboo?
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The CL-140 is effectively in series with the power transformer's DCR. Adding 5 ohms to a transformer winding which is already 50 to 150 ohms isn't going to change much. Modern production transformers have a lower DCR then their 50 year old counterparts and are somewhat harder on the rectifier than their older brothers.
Most rectifier tubes have a minimum plate circuit resistance spec which is routinely violated by many amp designers. The CL-140 is a good compromise since it raises this resistance into the hundred ohm or more region for the few seconds that matter the most in a rectifier's life.
The added 5 ohms is only in the filter cap's charging circuit loop. The discharge path remains unchanged.
Most rectifier tubes have a minimum plate circuit resistance spec which is routinely violated by many amp designers. The CL-140 is a good compromise since it raises this resistance into the hundred ohm or more region for the few seconds that matter the most in a rectifier's life.
The added 5 ohms is only in the filter cap's charging circuit loop. The discharge path remains unchanged.
The discharge and charge is the same as it goes to the PT CT. The ground point of the CT is the beginning of the coil, not the chassis or internal DC resistance, for AC which is what the filter is there to remove. Draw a line between the ground symbol of the filter and the ground symbol of the thermistor and you have resistance between the PT coil and the (-) of the caps.
No the CT leg is just in the charging current pulse circuit paths, not the capacitor discharging path - well not for a normal well made power supply where the CT leg goes directly to the first filter cap neg leg (at a star connection point where the downstream load/filter circuitry is then connected).
Good points, Tubelab, regarding rectifier tubes! I'll never understand and follow the wide-spread argument that an indirectly heated rectifier tube provides some soft start function to save the final tubes' cathodes, 'cause the arcing simply swaps over to the rectifier's cathode and nothing, zero, nada is gained. One might try it, but with cheap rectifiers only, such as TV dampers.
As for fuses: There are sand filled ones which don't show arcing and explosions when they are tripped, even at higher DC voltages.
Best regards!
As for fuses: There are sand filled ones which don't show arcing and explosions when they are tripped, even at higher DC voltages.
Best regards!
OK, point taken as the DC power returns to the filter directly from where it's used. But my point is related to the ESR for filtering the AC. The PT coil is normally short coupled to the filters with nothing in between. The thermistor is adding filtering ESR. It's a Taboo.... right? Can't have it, says so many. Or maybe it just isn't a practical issue at all...? Wait, I'm going to add a second guess here about the discharge path being outside the PT. I believe it is part of the path as the PT magnetic flux collapses on half of the coil there is current from the power section moving in to fill the collapse, that completes the power flow, not just a negative refill of the cap.
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I've used solid state diodes in series with the rectifier plates for decades.
I fitted an HH Scott 299C amplifier with these protection diodes about 30 years ago.
The amplifier still has it's original 5AR4 rectifier tube which shows no signs of old-age.
I fitted an HH Scott 299C amplifier with these protection diodes about 30 years ago.
The amplifier still has it's original 5AR4 rectifier tube which shows no signs of old-age.
That is such an elegant solution to getting slow B+ application! Thanks! I'm a newbie but I have literally read thousands of amplifier schematics in the past year and I dont remember seeing this trick. I always see the the thermistor on the primary side (which would slow down the filament too making it useless). Or I see elaborate delay mechanisms, or I see delay relays which they dont make anymore. Or I see two transformers. I was planning on a fuse in the secondary CT already, the thermistor also gives a whole lot of tube life protection, all for a few bucks and no complexity. Amazing.
20to20, the charging pulse shape and magnitude depends on the ESR of the winding and reflected primary ESR, and series resistance of the diode, and the amount of filter capacitance and loading across it. Each charging pulse occurs near the top of the winding voltage half-sine, and has a relatively short conduction time (or angle with respect to the mains frequency) - there is a relatively large time between conduction pulses through each half winding/diode.
There are advantages to having that higher effective resistance for the charging pulses - the charging pulse has a lower magnitude and wider conduction time, and hence lower level of mains harmonics that have to be filtered out from the B+ supply, and lower level of waveform distortion on heater voltage.
There are advantages to having that higher effective resistance for the charging pulses - the charging pulse has a lower magnitude and wider conduction time, and hence lower level of mains harmonics that have to be filtered out from the B+ supply, and lower level of waveform distortion on heater voltage.
Thanks, I listed no cons because I'm hoping to ask for them here!
I always tend to oversize things so would most likely being conservative on the diodes.
I recently blew the B+ power transformer in my Heathkit IP-17 bench supply, the rectifier tube was fried so I suspect that was the cause, but I dont have time to test out the whole circuit. If anyone wants an IP-17 with a blown transformer but all new caps and resistors, its theirs.
Being able to upsize the first cap was wishful thinking on my part, thats why I presented it as a question! I see if the tube is no longer having to rectify much, it still has that inrush.
Thanks for this info. I see now it is impossible to find high voltage, non-exploding fuses in the mA range. In the plant I work at we use an opto-coupler set to glow at some pre-determined current, if the current is reached a latching relay disconnects the motor until someone comes along to reset it or find out why it drew too much current. This sounds like a useful little module to have here, it would just have to be adapted to sense current at the higher voltage.
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