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Old 9th June 2009, 05:55 PM   #1
phrarod is offline phrarod  United States
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Default Trouble choke loading tube in regulated supply

I want to choke load the rectifier tube in a supply I'm building. It has a trim pot at final output and allows me to dial in voltages. Everything works fine but now as I optimize I want to choke load the rectifier. I've done this in the past on non-regulated supplies without issue.

However, when I do it with this supply I can no longer use the trim pot. It makes the output fixed. If I remove the choke and run a wire to connect as stock the supply functions normally allowing the trim pot to adjust voltage.

What am I doing wrong? Its a 6x4 rectifier. I'll using a choke from the cathode to the first r/c which (C1/R1) is the normal way. Here's a schematic of the supply.
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Old 9th June 2009, 06:45 PM   #2
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What am I doing wrong? Its a 6x4 rectifier. I'll using a choke from the cathode to the first r/c which (C1/R1) is the normal way. Here's a schematic of the supply.
Choke input filters drop a LOT of voltage. Try putting the choke in place of R1.
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Old 9th June 2009, 07:19 PM   #3
phrarod is offline phrarod  United States
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I expected a up to a 30% drop but the supply is capable of increasing by considerably more than that to compensate. Even when I put a 200K in R8 which should allow it to climb as high as 300V it made no difference and I'm only using about 200V.

R1 is part of the pi filter and don't know how that would affect it.

BTW, born and raised in the 'burgh. Appreciate your help.
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Old 10th June 2009, 09:46 PM   #4
phrarod is offline phrarod  United States
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I've been told I need to adjust the resistor divider network but I'm at a loss which to switch. With the choke in there and a linear taper pot on R3 I dialed it up as high as 150K without any positive effect on the problem of voltage out being unadjustable and too low.

R8 and R3 are appx a 2:1 ratio of resistance (R8 100K R3 50K) which by increasing it 4:1 (R8 200K R3 50K) its supposed to increase output by +60V. Nothing I played with in this area had any effect.

I also tried putting the linear taper into R1 since this is the initial R/C . Increasing the resistance did nothing. Its has a stock value 470R in there at the moment.

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Old 11th June 2009, 05:21 AM   #5
kevinkr is offline kevinkr  United States
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Actually you are turning the rectifier circuit into a choke input and now you no longer have enough voltage for the losses in the regulator circuit and it is operating below drop out which is why you cannot adjust the voltage.

Try substituting R1 with the choke instead, and this is really what you want to do anyway as you are primarily concerned with reducing the ripple at the input of the regulator - you do not want to reduce the DC voltage there. (The choke's lower dcr should result in better performance under low line conditions and varying loads as the input voltage will not drop as much as with the resistor.)

You can also do what you were doing, but add a 22uF capacitor before the choke. Should you really want a choke input you will need to replace the transformer with one having significantly higher output voltage, and I don't think this is necessary from a performance perspective.

Incidentally I see C1 listed as a 220uF cap - this is way too high for a 6X4 and will result in a very short service life for that rectifier tube unless the winding resistance of that power transformer is unusually high. (I would use a 22uF here..)

One of my very early regulator designs was quite similar to this one, the loop gain is not very high because of the LTP, (maybe 28dB or so) but should still give you about 20dB of feedback margin. One major improvement in ripple rejection is to feed the screen grid of the 6BQ5 a well filtered source of DC, in this case you could just hang a 100uF cap off of the screen grid connection.
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Old 11th June 2009, 03:34 PM   #6
phrarod is offline phrarod  United States
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Thanks! I did some measuring and with the choke in at the position I have it I get 130VDC drop over straight choke input. Therefore no adjustment and too low an output.

C1 is actually 330uF as is C2 (it came that way as a kit). So if 220 is too high than I can imagine what you would think of 330uF. One suggestion from a friend was to replace C1 with a film cap.1uf - 5uF. Do you agree? I don't have any film caps around in those values though.

Eventually I will replace the 6x4 with a 5AR4 and adjust fil voltage according for the new rectifier.

I'll try the other mods. Thanks so much for your help. I've been stuck on this for days.
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Old 11th June 2009, 04:52 PM   #7
DougL is offline DougL  United States
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I am not aware of a common tube rectifier that is specified for more than 60 uF, some conciderably less.

"Valve Amplifiers" page 294 has a table.

6X4 maximum 16 uF and 520 ohms.

5AR4 Max 60 uF and 75 ohms.


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Old 11th June 2009, 05:08 PM   #8
phrarod is offline phrarod  United States
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Thanks Doug. I wonder why this one has such high capacitance? Even the new board which is essentially the same but wired for 5AR4 uses 330uF caps.

Well I have to lower it anyway with the choke going in. If I use a film cap what material should it be? I can't only find Metallized Polypropylene Film or Metallized Polyester Film
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Old 12th June 2009, 05:05 AM   #9
phrarod is offline phrarod  United States
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Well I went with the choke at R1 and and I replaced the 330uF cap at C1 with a 10uF cap based on the info above.

Now I have the speed and attack I was missing. Unfortunately its at the expense of low level detial, and delicacies in the music. The presentation now is very hard almost solid state sounding. Guess I'll have to figure out how to get what the choke is providing without what's its covering up.
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Old 12th June 2009, 12:49 PM   #10
kmtang is offline kmtang  Canada
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You may try to reduce the 10uF capacitor to 0.5 to 1uF and see if it can improve the sound that you like. 10uF capacitor would provide almost 95% of the full B+ supply to the regulator circuit. By reducing the value of the first capacitor, it would reduce of the amount of noise at the output.

Not sure if this is right but it worth to try.

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