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Old 9th October 2012, 01:25 AM   #1
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Default Help with Shared Cathodes 300b

I am looking at the 300b amp of the following schematic. As you can observe the 300b stage shares both cathode bypass caps and resistors as well as dual valve diode bias.

Something tells me that this arrangement may not be optimal. What are the advantages of current schematic to more traditional arrangement - separate bias resistor/bypass cap for each valve?

I would like to add the "Ultrapath" a la WE, but at the current state of things that seems to be problematic.
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Last edited by ClefChef; 9th October 2012 at 02:31 AM.
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Old 9th October 2012, 05:18 PM   #2
palmas is offline palmas  Portugal
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Here you probably do not have a problem because you're using two separate output transformers, no doubt prepared for some DC current

If you have a singlr PP transformer you need to balance currents from both tubes. If you don't have a way to regulate each one, that means a good tube maching!
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Old 9th October 2012, 06:35 PM   #3
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Hi!

With shared cathode resistors, both 300Bs cannot bias independently.
The schematic also shows a connection between the cathodes of the driver tubes which do not have bypass caps. This seems like a design which was intentionally done to achieve a certain sounding. I would not use such a schematic as basis for a project unless you want exactly that.

If you want to use ultrapath caps, keep the cathode resistor separate.

Best regards

Thomas
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Old 9th October 2012, 07:04 PM   #4
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The cathode connection between driver tubes seems to add "lucid" effect a la Steve Deckert of Decware - it attenuates common L/R signal and therefore enhances stereo effect.

I am also interested in the opinions on the parallel damper diodes in 300b cathode bias, along with shared cathode resistors/bypass caps.
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Old 9th October 2012, 07:14 PM   #5
tomchr is offline tomchr  United States
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The damper diodes in the 300B cathodes are probably used to provide a delayed turn-on of the 300B.

If one wants the "lucid" effect you mention, why not just do the mixing/subtraction at line level.

~Tom
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Old 9th October 2012, 07:24 PM   #6
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Hi!

Quote:
Originally Posted by ClefChef View Post
I am also interested in the opinions on the parallel damper diodes in 300b cathode bias, along with shared cathode resistors/bypass caps.
It will depend a lot on personal taste if the sound of such a circuit is preferable or not. It will also greatly depend on the system. Best for such things is to try and listen. If this is your first DIY amp, build it the old school way first. Then experiment with such circuits.

I prefer cathode bias with individual resistors and ultrapath caps.

Best regards

Thomas
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Old 9th October 2012, 08:14 PM   #7
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This is LDM UltraFi 300b. Since the amp came with no schematics I had to reverse-engineer it a bit. Some of the solutions look very interesting, but being relatively new to this field I am seeking some explanations.

The sound of the amp is nothing short of fantastic - dynamic, musical. My concern is elevated (to my ears) level of 120hz buzz, which could be heard at idle with 98db speakers at listening position.

The maker of the amp suggested that this buzz is coming from AC heaters, to me it sounds like PS ripple. I will post the PS schematics tonight.
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Old 9th October 2012, 11:36 PM   #8
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Here's the power supply drawing.
Couple of notes on it: PS uses two power transformers connected in parallel. All secondary CT are wired together and connected to mains ground and chassis. A wire connects mains ground to signal ground bus where all components are grounded in a star ground fashion, including PS filtering caps (see attached photos).

At the moment both filtering caps have their negative terminals connected directly to the start ground. Would there be a way to optimize power supply a bit? I am thinking of having filter cap minus terminals connecting to the CT first and connecting to the star ground after the PS is filtered.
Another improvement would probably be changing series of electrolytics with high capacity film caps (like Clarity TC or new Wima MKP4)
Attached Images
File Type: jpg LDM300bPS.jpg (28.0 KB, 156 views)
File Type: jpg IMG311.jpg (600.8 KB, 153 views)
File Type: jpg IMG315.jpg (808.1 KB, 130 views)
File Type: jpg IMG313.jpg (692.4 KB, 115 views)

Last edited by ClefChef; 9th October 2012 at 11:47 PM.
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Old 10th October 2012, 12:59 AM   #9
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Quote:
Originally Posted by Vinylsavor View Post
Hi!



It will depend a lot on personal taste if the sound of such a circuit is preferable or not. It will also greatly depend on the system. Best for such things is to try and listen. If this is your first DIY amp, build it the old school way first. Then experiment with such circuits.

I prefer cathode bias with individual resistors and ultrapath caps.

Best regards

Thomas
I prefer individual resistors as well...
Here's what I would like to do: rewire the amp so it uses existing 6DM4 damper diodes in power supply rectification instead of hexfreds, add ccs 300b heater supplies (tent labs or Rod Coleman), change input tube from 5687 to 6n6p, rewire 300b for individual bias resistors/capacitors, add UltraPath.
Of course this would no longer be LDM UltraFi the way it was designed, although the changes can always be easily reversed if so desired...
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Old 17th October 2012, 01:48 AM   #10
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Changing electrolytic PS caps to large films produced excellent results. I also separated 300b cathodes - I prefer the sound of the amp this way.
At this point the amp sounds incredible and I would like to keep it that way. Having a little DHT AC heater hum seems like a good compromise.
If funds permit I would try large film caps on 300b cathode bypass. I would also like to try out a pair of grid chokes.
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