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Iain M Churches
 
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Default 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   Report Post  
Steve Robertson
 
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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   Report Post  
Robert Casey
 
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Default

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   Report Post  
Andy Evans
 
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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   Report Post  
Choky
 
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Default

?????????????????????

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.





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Tom Bavis
 
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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   Report Post  
Mark
 
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Default

"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   Report Post  
Robert Casey
 
Posts: n/a
Default

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   Report Post  
Ned Carlson
 
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Default

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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