Especially when you're a tube seller.If they caused a difference in "resolution" and "treble," then the amp is bizarrely incompetent in design and/or construction. And I've never experienced an amp THAT badly designed.
Here here! You beat me to it SY.Apparently idiots abound............
AFAIK, 1.2 KV. is the current max. for Schottkys. You can wire them is series, but voltage equalizing resistors are essential to avoid cascading part failure. Cree part # C4D02120A is rated for 2 A. and should be quite satisfactory in new builds.
1N4007s produce hideous amounts of switching noise. IMO, 1Nnnnn parts have no place in HIFI equipment, either tubed or SS. You can come darned close to, if not completely match, Schottky performance by carefully snubbing UF4007s. AAMOF, simply replacing 1Nnnnn parts with the corresponding UFnnnn parts greatly reduces the switching noise level.
If you're so afraid about H/K isolation failure inside a damper tube, juste connect one side of the heater transformer to ground.
When a leakage will occur, your HV source will flow directly to ground, blowing the HV fuse.
No risk at all !
Is this necessary if the heater circuit has a center tap?
Not if you ground the CT.
For those who want their heaters elevated, add a big FAT zener diode from the heater circuit to ground. It should have a zener voltage higher than the elevation potential so that it is out of the circuit under normal operation, and large enough (at least 5 watts) to blow the HV fuse in case of a H/K short. I use the 1.5KE series Transorbs.
My experience in TV repair was limited to fixing old junk or ripping it apart to make tube amps as a teenager. In high school I got a job in a TV repair shop (1968 to 1970.) The shop was a Philco authorized shop and about 75% of the TV's we did were modern (for 1970) Philco TV's. Damper (Compactrons) failures were rare. Common problems were IF strip oscillations (6EJ7 and 6EH7), Vertical foldover (6LU8) caused by bad cathode bypass cap, HOT (6KD6) failure, and the usual HV arcing caused by dust and humidity. We also had a few bad yokes. Philco TV's were sold at Firestone tire stores, among other places. The Firestone stores offered credit to anyone, so a lot of those TV's went into non air-conditioned homes, some of which were quite dirty, and roach or ant infested. A roach in the HV box can blow a damper tube, and fry the flyback! The roach will carbonize and conduct.
We also did quite a few GE color TV's. These would also get the usual HV arc, but the Lexan internals in the HV box would carbon track, leading to fried parts and more than a few fires!
There were a few metal cased Sylvanias. Big, heavy, and ugly, but stone reliable. Heat, humidity, and bugs were the usual causes of death.
My experience may be different than others, since south Florida's heat, humidity, and bugs have factored in the failure modes.....and I didn't even mention beer + electronics, that would be my next job. I ran the service department of an Olson Electronics store near the campus of the University of Miami. The store was at the end of Frat House row. Frat parties can kill TV's and stereos.
For those who want their heaters elevated, add a big FAT zener diode from the heater circuit to ground. It should have a zener voltage higher than the elevation potential so that it is out of the circuit under normal operation, and large enough (at least 5 watts) to blow the HV fuse in case of a H/K short. I use the 1.5KE series Transorbs.
My experience in TV repair was limited to fixing old junk or ripping it apart to make tube amps as a teenager. In high school I got a job in a TV repair shop (1968 to 1970.) The shop was a Philco authorized shop and about 75% of the TV's we did were modern (for 1970) Philco TV's. Damper (Compactrons) failures were rare. Common problems were IF strip oscillations (6EJ7 and 6EH7), Vertical foldover (6LU8) caused by bad cathode bypass cap, HOT (6KD6) failure, and the usual HV arcing caused by dust and humidity. We also had a few bad yokes. Philco TV's were sold at Firestone tire stores, among other places. The Firestone stores offered credit to anyone, so a lot of those TV's went into non air-conditioned homes, some of which were quite dirty, and roach or ant infested. A roach in the HV box can blow a damper tube, and fry the flyback! The roach will carbonize and conduct.
We also did quite a few GE color TV's. These would also get the usual HV arc, but the Lexan internals in the HV box would carbon track, leading to fried parts and more than a few fires!
There were a few metal cased Sylvanias. Big, heavy, and ugly, but stone reliable. Heat, humidity, and bugs were the usual causes of death.
My experience may be different than others, since south Florida's heat, humidity, and bugs have factored in the failure modes.....and I didn't even mention beer + electronics, that would be my next job. I ran the service department of an Olson Electronics store near the campus of the University of Miami. The store was at the end of Frat House row. Frat parties can kill TV's and stereos.
I have a Scott 200 integrated amplifier that I bought for $150 in well-working condition in 1998. It uses an 5AR4 - it appears to be original. Well, I decided to remove it and hold it as the back-up, and use a Sovtek. After 3 years of use the cathode coating began flying off and creating flashes, as well as sending thumps through the system. I pulled it before failure (thank goodness). Put the original 5AR4 back in and it still works to this day. Had the amp in for service after the power supply cap began failing and i started having motorboating - service was by one of the former Conrad Johnson in-house techs (I live in the DC area). He tested the 5AR4 and said it still had a lot of life. Whatever you may say about snakeoil claims of NOS GZ34/5AR$, my personal experience is that the 5AR4 is a rugged tube and new tubes do not match current production. This has been seconded by tube vendors who would be more than willing to sell me $250 NOS tubes, which says much about their integrity and their knowledge, IMHO.
There was a period of time about 3 years ago where JJ, Sovtek, and Shuguang 5AR4's would randomly spark out, sometimes on initial power up. Once a good tube was found, it would last for several years. This appeared to be batch related. I got a dozen "tested and sorted" Sovteks from a reseller who guaranteed them. They are all still alive.
I collected a bunch of the failed tubes and took them apart. The main problem was inconsistent cathode coating thickness. When the tube is first switched on the cathode heats up from the inside. The areas with thin coating heat up first. These hot spots will try to take on all the current that should be evenly distributed throughout the cathode, and the current density is exceeded. Sparks fly.
Some tubes also had alignment issues. The cathode must be centered inside a perfectly round plate structure in order for the current density to be uniform across the entire cathode surface.
Some new production tubes do not always get these things right......some old tubes, especially from the end of the vacuum tube era didn't either.
I collected a bunch of the failed tubes and took them apart. The main problem was inconsistent cathode coating thickness. When the tube is first switched on the cathode heats up from the inside. The areas with thin coating heat up first. These hot spots will try to take on all the current that should be evenly distributed throughout the cathode, and the current density is exceeded. Sparks fly.
Some tubes also had alignment issues. The cathode must be centered inside a perfectly round plate structure in order for the current density to be uniform across the entire cathode surface.
Some new production tubes do not always get these things right......some old tubes, especially from the end of the vacuum tube era didn't either.
Tubelab....Alignment would do it almost every time...same with uneven cathode coating....that goes back to quality control...especially at the beginning of a batch run....testing takes time and costs the manufacturer money..and there will be rejects..but the customer is the final inspector...
With the internet and user forums, selling crap can get a vendor a bad name fast....holds even with new equipment.
I like Eli Duttman's idea with the dampers and Schottky diodes to get around the high cost of 5AR4's in new builds....I always tie the heater to cathode, even if I have to use separate filament transformers...saves a lot of trouble in the long run...treat a 6AU4 like it was half a 5U4....
With the internet and user forums, selling crap can get a vendor a bad name fast....holds even with new equipment.
I like Eli Duttman's idea with the dampers and Schottky diodes to get around the high cost of 5AR4's in new builds....I always tie the heater to cathode, even if I have to use separate filament transformers...saves a lot of trouble in the long run...treat a 6AU4 like it was half a 5U4....
All of my past builds use either 5AR4 or the 5Y3GT. I have a whole box of vintage 5U4GB's made by Tung-Sol, Sylvania for JAN, and others that I can't remember that would like to use for something. I have some Mullard 5-20's I built that uses 5AR4 for rectification duties and the voltages are 20 volts higher than specified for the circuit, maybe I should try them here, the power transformer 5V winding is rated for 3A and the reservoir cap voltage rating is 700V, everything after that is rated for 500V.
Why does the 5U4 get no love? Is it because it draws 3A? Is it because it doesn't have the slow start up like the 5AR4?
Why does the 5U4 get no love? Is it because it draws 3A? Is it because it doesn't have the slow start up like the 5AR4?
Yes that's true, I remember now I gave a friend of mine a NOS Sylvania 5U4 for his Mesa Lonestar a few years back. I am thinking they are less favorable due to the cost of having a 3A heater winding, on a mass production scale the $$$ adds up. And we all know corporate America is after the all mighty $$$$
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5AR4
Famousmockingbird...the slow start up of the 5AR4 reduces surge voltages on capacitors, tubes, and other components. The 3 A filament on the 5U4 essentially is immaterial. The 5U4 produces dc almost immediately at power on, and high surge voltages result until the other tubes come up to temperature.
Famousmockingbird...the slow start up of the 5AR4 reduces surge voltages on capacitors, tubes, and other components. The 3 A filament on the 5U4 essentially is immaterial. The 5U4 produces dc almost immediately at power on, and high surge voltages result until the other tubes come up to temperature.
You've given some of the reasons for 5AR4 chauvanism. Add that importing tubes from Europe in 1960 frequently cost less than buying U.S. made and the panache associated with the Mullard name.
Trying the 5U4s in the 5-20s to reduce the B+ rail voltage a bit is logical. If the 5 VAC winding has a CT, use it as the B+ takeoff point. Watch the size of the 1st filter cap., as the 5U4's limit is smaller. Those TungSol 5U4GBs are regarded as being the best ever. If you need cash, selling some of your pile off will raise it.
BTW, if you want to slow the B+ rise down a bit when a directly heated vacuum rectifier is employed, install a CL-90 inrush current limiting thermistor between the rectifier and the PSU filter. FWIW, the presence of a CL-90 allows you to take a SMALL liberty in choosing the value of the 1st filter capacitor.
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