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Old 20th January 2011, 10:38 PM   #21
tomchr is offline tomchr  United States
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This is a bit counterintuitive, but adding a resistor on the top/cold side of the inductor screws up the frequency response in the low end -- at least according to my spice sim. Basically, when the load impedance for the input tube transitions from being resistive to being inductive dominant, there's a hump in the frequency response. I think it's better if the plate load is as close to an ideal inductor as possible. One could add a cap across the resistor (R1 in my schematic below) but that gets us back to having a cap in the signal path. Though, it could be a smaller, hence, better quality cap than that used in the cathode on the 300B in my schematic earlier.

Note: I made the input cap 100 uF so it wouldn't interfere with the LF response of the circuit. In reality, it'll be smaller of course. Or completely eliminated as I do plan to use an input transformer.

~Tom
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File Type: png 300B_idea_ResTop_Gain.png (14.7 KB, 1208 views)

Last edited by tomchr; 20th January 2011 at 10:41 PM.
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Old 21st January 2011, 03:27 AM   #22
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Originally Posted by chrisb View Post
Exactly how does one contact Transcendar for list of offerings, prices, etc?
ChrisB, I actually did not contact them. I picked out some 3k single ended Transcendar transformers that were up on Ebay just before Christmas a year ago wrote down the auction number and gave it to my wife and said I would like this for Christmas. The next thing I knew was Santa Clause delivered them on Christmas day! As far as buy from them directly, I do not know that they do that. I am only familiar with seeing the products on occasion up on Ebay such as a current listing for a pair of 5k SE ended transformers listed under item number 280618272554. You could probably go to this auction and send the lister an email asking about the availability. For the money, they make a pretty darn good transformer. I am not disapointed. Mickeystan
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Old 21st January 2011, 04:29 AM   #23
tomchr is offline tomchr  United States
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Latest contemplations...

I noticed that in many designs the cathode cap on the 300B goes to the B+ supply. Does anybody know what the advantage of this is compared to letting it go to ground? Better transient response?

In the final version I'll probably use an input tube with a bit higher mu. ECC81/82/83, 6AN4, or D3A come to mind. I just haven't bothered creating a spice model for those yet.

~Tom
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Last edited by tomchr; 21st January 2011 at 04:32 AM.
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Old 21st January 2011, 09:43 AM   #24
tomchr is offline tomchr  United States
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Turns out Lynn Olson has answered my question about the cathode cap to B+: ETF Presentation, Nutshell HiFi

~Tom
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Old 21st January 2011, 01:54 PM   #25
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Originally Posted by tomchr View Post
Latest contemplations...

I noticed that in many designs the cathode cap on the 300B goes to the B+ supply. Does anybody know what the advantage of this is compared to letting it go to ground? Better transient response?

In the final version I'll probably use an input tube with a bit higher mu. ECC81/82/83, 6AN4, or D3A come to mind. I just haven't bothered creating a spice model for those yet.

~Tom
Tom, The Cathode cap of the 300B returning to B+ rather than ground topology is what is known as "Loftin White" circuit I believe. By doing this it keeps the 300B AC primary (primary amplified current) returning without going back through the power supply picking up extra noise. The incorporation of this along with the DRD in Jack's design is what attracted me to it when I decided to built my amplifiers. Mickeystan
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Old 21st January 2011, 03:25 PM   #26
kevinkr is offline kevinkr  United States
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Originally Posted by Mickeystan View Post
Tom, The Cathode cap of the 300B returning to B+ rather than ground topology is what is known as "Loftin White" circuit I believe. By doing this it keeps the 300B AC primary (primary amplified current) returning without going back through the power supply picking up extra noise. The incorporation of this along with the DRD in Jack's design is what attracted me to it when I decided to built my amplifiers. Mickeystan
Nope, what you describe is an Ultrapath or Western Electric type connection. Loftin White is a dc coupled SE topology originally depending on a tapped resistor to derive all of the different operating voltages.
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Old 21st January 2011, 03:32 PM   #27
Sheldon is offline Sheldon  United States
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Nope, what you describe is an Ultrapath or Western Electric type connection. Loftin White is a dc coupled SE topology originally depending on a tapped resistor to derive all of the different operating voltages.
See post #8 for a true Loftin-White design. Ignore the bootstrapped follower, if that makes it easier to follow the current paths.

Sheldon
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Old 21st January 2011, 04:11 PM   #28
Sheldon is offline Sheldon  United States
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This thread has more explanation: loftin white 801 amp
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Old 22nd January 2011, 02:25 AM   #29
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See post #8 for a true Loftin-White design. Ignore the bootstrapped follower, if that makes it easier to follow the current paths.

Sheldon
Kevin and Sheldon, Thanks for correcting my error. I will take another look at the posts recommendation that Sheldon supplied as I want to better understand my error. Glad you guys corrected me! Mickeystan
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Old 22nd January 2011, 03:26 AM   #30
Sheldon is offline Sheldon  United States
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No worries. Actually, it's pretty common to see pretty much any direct coupled SE amp called Loftin White. A true Loftin white uses a specific type of ripple canceling. This was novel and important at the time, because high value capacitors were not available at reasonable cost or size. Their approach takes a bit to get your head around, but is quite elegant.

Sheldon

If you can slog through this thread, you will get the full picture: Darius Loftin White explained

Last edited by Sheldon; 22nd January 2011 at 03:36 AM.
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