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current hogging in a 2 tube rectifier ps

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Posted on July 27, 2014 at 12:46:43
DAK
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 I almost finished with my new power supply which will use 2 paralleled 5ar4. I recently read an article which brought up the possibility of one tube passing most of the current. Should I worry about one tube shouldering most of the work in a 2 tube rectifier circuit. I have checked out many schematics of 2 tube rectified power supplies and don't recall any additional components to address this. Any insights will be appreciated. cheers, Dak

 

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Read about balancing resistors, posted on July 27, 2014 at 14:25:00
Chip647
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 Most rectifiers have a minimum resistance. If your winding does not have enough resistance, all tube rectifiers specify putting in resistors in series with the plates. If you balance with 100 ohm resistors, you should be fine from a current hogging perspecitve.

 

RE: current hogging in a 2 tube rectifier ps, posted on July 27, 2014 at 15:18:21
Triode_Kingdom
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 "I have checked out many schematics of 2 tube rectified power supplies"

I always thought this was considered poor practice due to the relatively high values of balancing resistors needed. Would it be possible to post a link or two to the schematics you've found?

 

RE: current hogging in a 2 tube rectifier ps, posted on July 27, 2014 at 22:36:48
DAK
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 I was just looking at the Scott 296 schematic. It has a 33 ohm resistor after the CT from the heater windings. I thought it use was just for voltage dropping. Also the LK 150 has a pair of 10w 80 ohm resistors after the CT of the heater. The Fisher SA300 has 2 15 ohm 20w in series. I thought those resistors was for dropping the voltage. cheers, Dak

 

RE: current hogging in a 2 tube rectifier ps, posted on July 28, 2014 at 07:13:27
Triode_Kingdom
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 I looked at the Scott 296; the two rectifier tubes are not wired in parallel. Each tube is 1/2 of the full wave power supply. Is that what you're building, or have you actually wired two rectifier tubes in parallel? The 33 ohm resistor in the Scott is probably intended to limit inrush current when the amplifier is first powered on.

 

RE: current hogging in a 2 tube rectifier ps, posted on July 28, 2014 at 09:05:33
DAK
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Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 My rectifier circuit is copied from an earlier post which should be a parallel arrangement. Is there any advantage or disadvantage for hooking up the 2 rectifiers in parallel vs the way in the Scott? My PT doesn't have a ct. regards, Dak

 

RE: Read about balancing resistors, posted on July 28, 2014 at 09:31:56
DAK
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 Is it possible to match the rectifiers so that each output is the same? What about using the circuit in the Scott 296? cheers, Dak

 

RE: current hogging in a 2 tube rectifier ps, posted on July 28, 2014 at 10:57:54
Triode_Kingdom
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 "Is there any advantage or disadvantage for hooking up the 2 rectifiers in parallel vs the way in the Scott?"

The rectifiers conduct alternately in the Scott arrangement, so current imbalance isn't an issue. How are you wiring this if your xfmr has no CT? That requires a bridge with four rectifiers.

 

RE: current hogging in a 2 tube rectifier ps, posted on July 28, 2014 at 11:40:21
DAK
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 The high voltage is ct. The 5v heater is not unlike the scott.

 

RE: current hogging in a 2 tube rectifier ps, posted on July 28, 2014 at 13:05:28
Triode_Kingdom
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 OK, that's not too important. Most of the 5V heater windings I've seen have not been center-tapped. What are you doing with the rectifiers - two parallel tubes on each leg of the HV winding? Maybe you could post a sketch.

 

RE: current hogging in a 2 tube rectifier ps, posted on July 28, 2014 at 13:25:41
DAK
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 Hi TK, I copied the schematic from the post on July 12, in this section. It is under the title "what is the best way to hook up 2 rectifiers" and under sub section "tom I am not sure what you mean by that".

 

RE: current hogging in a 2 tube rectifier ps, posted on July 28, 2014 at 16:19:28
Triode_Kingdom
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Posts: 10045
Location: Central Texas
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OK, I see that what you're doing is identical to the Scott, other than the CT for the filament winding and parallel filaments. There's no problem paralleling the anodes within a single envelope. You don't need balancing resistors, they'll just degrade supply dynamics. If it were me, I'd consider copying the Scott schematic in terms of the 30 ohm resistor and first two filter caps. Increasing the second cap to 40uF or so might be beneficial, but only if the power supply sags on extended low-frequency passages.

 

RE: current hogging in a 2 tube rectifier ps, posted on July 31, 2014 at 16:15:14
Tre'
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Location: So. Cal.
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 "I was just looking at the Scott 296 schematic. It has a 33 ohm resistor after the CT from the heater windings. "

The 33 ohms resistor is part of the CRC filter (C217-R221-C216) that feeds the output stage.

The following resistors and caps are more filtering (and voltage drop) for the other stages in the amplifier.

There would be a lot more ripple filtering if R221 was a choke instead of just a resistor.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: current hogging in a 2 tube rectifier ps, posted on July 31, 2014 at 19:09:50
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I rebuilt a 296 where I used a CLCLC filter with small 1H x 30 ohm and the sound was much improved. cheers, Dak

 

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