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update to B+ dilemma....

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Posted on July 21, 2014 at 15:27:05
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 In the last episode of our saga, our intrepid audio adventurer had received many helpful suggestions from his stalwart companions on how to slay the twin dragons "B+ too high" and "loud hum" that were plaguing the phono pre he was attempting to construct.
Having decided that building the power supply instead of bread-boarding it might reduce some of his troubles, he embarked on that quest and found the holy grail in the early morning hours of today. Whatever demons had possessed the bread-boarded psu where vanquished when it was carefully reconstructed in a permanent fashion, for now there is only a token of hum that he hopes can be banished from the realm when he builds the permanent front end on a separate chassis. Furthermore, the sound now coming from the unit is most pleasing to his ears.
Only one thing still troubles our protagonist; to slay the "B+ too high" dragon required using a 5.5k bleeder resistor across the final cap. He has played it that way form some time with no ill vapors appearing, but his knowledge of these things is limited and he hopes that his trustworthy companions can once again aid him and set his mind at ease. Do any of them know of any curse that might be brought to life with such an act?
He also wishes to thank them for their most appreciated help previously and now.
Further episodes will follow as the saga continues.

 

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RE: update to B+ dilemma...., posted on July 21, 2014 at 18:41:22
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015
 5.5K is low for a bleeder. Don't know the voltage, but it is pulling some real current. Keep check on heat from the power xfmr in case it's not up to it.

 

300 volts across a 5.5K resistor is 16 Watts & 55mA, that is mad., posted on July 21, 2014 at 19:57:41
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 Too much B+ is a function of design flaws, not dragons.

 

RE: 300 volts across a 5.5K resistor is 16 Watts & 55mA, that is mad., posted on July 22, 2014 at 01:45:41
RC Daniel
Audiophile

Posts: 1922
Location: Brisbane
Joined: November 3, 2002
 I have not been reading the ongoing trials and tribulations of this build - apologies if I have missed something...

Is an "extra" 55mA being drawn and dumped to ground simply to increase the current draw across PS series R, thereby dropping the B+? Is that correct? I'd be checking the ratings of any upstream resistors in the PS circuit, if there are any. Seems excessive... esp for a phono amp.

I am guessing I am missing something and should perhaps just not post...

Cheers.

"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki

 

so we are back to were we started, minus one dragon, posted on July 22, 2014 at 03:46:34
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Ok, one down and one to go. I will have to go back through the previous thread and see which idea seems like the best approach to slay the remaining dragon.

 

RE: 300 volts across a 5.5K resistor is 16 Watts & 55mA, that is mad., posted on July 22, 2014 at 03:48:47
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Ah come on Chip, where is the magic or adventure in "design flaws"? Life is full of the mundane and everyday; we need to let our imaginations have a little fun once in a while. : )

 

Burn baby Burn, posted on July 22, 2014 at 05:19:21
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 Just make sure that your transformer can handle the current and your resistor is rated +50 watts or it will catch fire. If you are using one of those aluminum cased resistors, they do need heatsinks. You will also need lots of ventilation as you have now built an easy-bake oven.

 

RE: so we are back to were we started, minus one dragon, posted on July 22, 2014 at 06:12:56
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 There are several simple techniques to resolve this. Let's hope someone whose opinion you respect happens to think of them.

 

RE: so we are back to were we started, minus one dragon, posted on July 22, 2014 at 07:35:58
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 What's wrong with using VR tubes?

 

RE: 300 volts across a 5.5K resistor is 16 Watts & 55mA, that is mad., posted on July 22, 2014 at 09:18:20
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 You are correct, but this line of effort is being abandoned.

 

RE: Burn baby Burn, posted on July 22, 2014 at 09:20:37
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Not too worry Chip, I am abandoning this solution after reading your post and several others like it. That's why I asked the question.

 

Another possisblity......, posted on July 22, 2014 at 09:26:22
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 How about I save the current tranny I am trying to use for another project and buy one that will give me the voltage I need for this one?
Might be easier than any other solution and not too much more expensive. I do want to study up on the VR idea a bit first since several have noted that it a good thing to have for phono pres anyway. I just don't know much about them and will have to do some web crawling.

 

RE: update to B+ dilemma...., posted on July 22, 2014 at 09:28:46
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Giving up on this method for the reasons you and others have noted.

 

RE: so we are back to were we started, minus one dragon, posted on July 22, 2014 at 09:31:04
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I am going to look into this as it has been suggested by several people now. I just don't know squat about it so I don't know yet if it would be the best solution for me or not. Going to cruise the web a bit and see if I can get a basic understand about it and how it could be implemented.

 

RE: so we are back to were we started, minus one dragon, posted on July 22, 2014 at 11:43:28
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 You might also try the Radiotron Designer's Handbook and/or the Morgan Jones book.

 

Yep, getting the right $40 transformer will save $$$ and trouble, posted on July 22, 2014 at 13:57:34
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 The other solution is to use your current transformer and get a $40 choke, a 10H 200mA Hammond choke will allow you to choke load your power supply giving you DC at about 90% of the AC voltage rating. 300 vAC choke loaded will give you 265 vDC. As a bonus, you get a choke loaded power supply which is a very good thing.

 

I may have found the solution...., posted on July 22, 2014 at 15:32:31
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I was reviewing the schematic once again to see if I had missed anything and I think I did.
The situation is that I have a B+ of 272 vdc after filtering and I need 250 for the phono pre. I think I may have found an easy solution, but I am gun shy since I have missed on a couple others and am hoping you guys can tell me if I have it right this time......
On the schem for the phono stage (see link) it shows the B+ basically going to R9 and R4 which are 30k each. I looked at the RCA tube manual and it says that standard operating plate voltage is 90 vdc for the 6dj8, so I assume the designer of the circuit was shooting for something near that. I had some nice wire wound 5w 39k resistors in my bins so I stuck a couple of those in for R4 and R9. I had previously measured the voltage at the plate side of those resistors as 112 vdc (about 22 over the desired) and when I measured with the new resistors it was 93 vdc. Close enough I think.
Looking at the schem it doesn't look to me like that one change will effect any of the other down stream voltages adversely as they all were probably proportionally too high with the 270 B+. Also seems like it should be ok for the caps too, or do I have it wrong?

 

RE: 300 volts across a 5.5K resistor is 16 Watts & 55mA, that is mad., posted on July 23, 2014 at 01:58:07
RC Daniel
Audiophile

Posts: 1922
Location: Brisbane
Joined: November 3, 2002
 Hi Vinnie,

Hey, it worked, but it is just a very aggressive, perhaps wasteful and potentially problematic way of achieving your goal.

Good luck - I hope you can get it sorted to your satisfaction.

Cheers.

"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki

 

RE: 300 volts across a 5.5K resistor is 16 Watts & 55mA, that is mad., posted on July 23, 2014 at 03:43:37
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Look at my post up top; I think i found a better solution.

 

No comments?, posted on July 24, 2014 at 07:51:36
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Does everyone think it will work, or do you not want to hurt my feelings by telling me where I have it all wrong again? : )

 

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