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#1
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Replacing a full wave rectifier with a tube
Hi RATS,
I have a 30W push pull amp, with a faulty mains transformer. It has a SS full wave bridge rectifier. The transformer secondary is marked 345V, and according to the schematic the DC rail is 425V at 400mA I would like to replace this transformer with a centre tap unit, so that I can use a tube rectifier. I know that tube rectifiers are not so efficient as SS bridges. Question: How much voltage does the tube rectifier drop, and how much extra should I allow on the transformer secondary to compensate for this? Thanks in advance Iain |
#2
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The voltage drop of a tube rectifier depends in large part on two
things. 1) which tube you plan to use and 2) what the current draw is. Based on your specs of 425VDC at 400ma, it would appear that your amp is 170 watts! But you say it's 30 watts. So, I suspect that the 400ma rating isn't correct. Good thing because few tube rectifiers are capable of passing 400ma for any length of time without putting more than one in parallel. Tell us what the output tubes are in your amp and whether it's mono or stereo. That will let us approximate what the current draw is and hence let us know what rectifier tubes and transformer secondaries are appropriate. Best regards, Steve Robertson Iain M Churches wrote: Hi RATS, I have a 30W push pull amp, with a faulty mains transformer. It has a SS full wave bridge rectifier. The transformer secondary is marked 345V, and according to the schematic the DC rail is 425V at 400mA I would like to replace this transformer with a centre tap unit, so that I can use a tube rectifier. I know that tube rectifiers are not so efficient as SS bridges. Question: How much voltage does the tube rectifier drop, and how much extra should I allow on the transformer secondary to compensate for this? Thanks in advance Iain |
#3
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Iain M Churches wrote:
Question: How much voltage does the tube rectifier drop, and how much extra should I allow on the transformer secondary to compensate for this? Tube rectifier filaments eat a fair amount of power, and usually need a separate winding on the transformer. Don't forget that when figuring how big a transformer to buy. Figure that a tube will lose about 50V. For a full wave centertapped transformer that means add about 100V for the entire secondary winding voltage. to get 425VDC you'd need about 1200V centertapped. |
#4
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Consider a hybrid bridge - two ss diodes for the negative side to earth and a
tube for the positive side. for the tube you can use a pair of damper diodes which should be quite sturdy. They also are indirectly heated and can run off the existing 6.3v heaters if they have spare capacity, or otherwise a cheap low power transformer. Here are some possibilities: 6DT4 235mA, 12DM4A, 6DM4A 200 ma, 6AU4-GTA 190mA, 6CQ4 190mA,6DE4 180mA There's also the very handy double damper diode - the 6BY5G which I use a lot, but that's rated 175ma and probably more voltage drop than the above. === Andy Evans === Visit our Website:- http://www.artsandmedia.com Audio, music and health pages and interesting links. |
#5
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?????????????????????
for original poster : go to Duncan Monro's site ,and download PSU "something" proggy-then you'll see what you need for your amp,and you can also learn something about particular thematic..... 1000V CT for 425 volts DC??? nonsense! where is da bloody multiplier ? -- .................................................. ........................ Choky Prodanovic Aleksandar YU "don't use force, "don't use force, use a larger hammer" use a larger tube - Choky and IST" - ZM .................................................. ........................... "Robert Casey" wrote in message ... Figure that a tube will lose about 50V. For a full wave centertapped transformer that means add about 100V for the entire secondary winding voltage. to get 425VDC you'd need about 1200V centertapped. |
#6
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On Wed, 25 Aug 2004 20:01:59 +0300, "Iain M Churches"
wrote: Hi RATS, I have a 30W push pull amp, with a faulty mains transformer. It has a SS full wave bridge rectifier. The transformer secondary is marked 345V, and according to the schematic the DC rail is 425V at 400mA I would like to replace this transformer with a centre tap unit, so that I can use a tube rectifier. I know that tube rectifiers are not so efficient as SS bridges. Question: How much voltage does the tube rectifier drop, and how much extra should I allow on the transformer secondary to compensate for this? Thanks in advance Iain The transformer must be about 40% larger for a half-bridge full-wave rectifier, due to lower copper utilization (i.e. only half the winding is conducting at a time). Plus you'll need a filament winding for the rectifier, and about 20-30V extra for the rectifier drop, which will depend on the rectifier you choose. |
#7
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"Iain M Churches" wrote in message ...
Hi RATS, I have a 30W push pull amp, with a faulty mains transformer. It has a SS full wave bridge rectifier. The transformer secondary is marked 345V, and according to the schematic the DC rail is 425V at 400mA I would like to replace this transformer with a centre tap unit, so that I can use a tube rectifier. I know that tube rectifiers are not so efficient as SS bridges. Question: How much voltage does the tube rectifier drop, and how much extra should I allow on the transformer secondary to compensate for this? Thanks in advance Iain By what you say about the voltages, it is a condenser input filter. I would say a 375-0-375 volt transformer will be right. Consult a manual and look at the graph to see what voltage you need from the transformer for the output voltage you need. If it was a reactor input, the secondary would be 550-0-550 volts. A pair of 5AR4s would be the best rectifier. These have lower heater consumption than the filament types and better voltage regulation from mimimum to maximum current. It also delays the B+ so there is no cathode stripping. Cathode stripping is when there is B+ on a valve before the cathode is hot thus causing a much shorter life and cathode to grid shorts. |
#8
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Choky wrote:
????????????????????? for original poster : go to Duncan Monro's site ,and download PSU "something" proggy-then you'll see what you need for your amp,and you can also learn something about particular thematic..... 1000V CT for 425 volts DC??? nonsense! where is da bloody multiplier ? Oops, I applied the RMS to DC calculation backwards. Duh... :-) |
#9
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On Wed, 25 Aug 2004 20:01:59 +0300, Iain M Churches wrote:
Question: How much voltage does the tube rectifier drop, and how much extra should I allow on the transformer secondary to compensate for this? Depends on the tube. Most data sheets for rectifier tubes have charts showing expected DC voltage output for a given AC input and load. You can go he http://tdsl.duncanamps.com/tubesearch.php and look 'em up. -- Ned Carlson Triode Electronics Chicago,IL USA www.triodeelectronics.com |
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