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Old 17th July 2012, 10:01 PM   #1
marcins is offline marcins  Poland
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Question SE on 300B

I planning build something similar to this amplifier 300B Mk2 Triode Amplifier, but with some small changes.
I have capacitors for this amp. For one chanel:
- for power supply 1x20uF Mundorf TubeCap, 4x47uF Mundrof TubeCap, 1x30uF Mundorf TubeCap
- for amplifier 1x47uF Mundorf MCap, 1x100uf Mundorf MCap, 1x Mundorf Supereme Silver OIL 0.22uf.

Power transformers, chocke and output transforemer, I have form Polish manufacturer - EDIS (Ogonowski).
As volume potentiometer I use Khozmo Attenuator - High Quality Audio & Industrial Attenuators :-)

Now I looking for resistor, especially for cathode resistor. I found something like this TDO japan made resistors, but I can't found any opinion abut this resistors. Maybe some of you had to do with them?
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Old 17th July 2012, 11:20 PM   #2
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Why spend 10€ on a cathode resistor ? EDIS, the transformer manufacturer, has very good C-Cores. This would be a better alternative to spend the money.
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Old 18th July 2012, 08:05 AM   #3
marcins is offline marcins  Poland
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I already have the output transformer - as in the description and the fotakch above;-) I have OT on EI96 core (0.3 mm).
I think it's priced at 10€ for resistor in not so much at the amount that I gave for capacitors.
Other options for cathode resistor is 2 x Mills MRA-12 1,8k in parallel. But this give me 900ohm, not 880 ;-)
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Old 18th July 2012, 09:30 AM   #4
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Originally Posted by the_manta View Post
Why spend 10€ on a cathode resistor ? EDIS, the transformer manufacturer, has very good C-Cores. This would be a better alternative to spend the money.
+++ I Agree...
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Old 18th July 2012, 09:48 AM   #5
marcins is offline marcins  Poland
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But I already have the output transformers;-) At the core of EI96, 0.3mm blade.
Other option for cathode resistor is 2 x Mills MRA-12 1,8k in parallel.
And I have one more question. I have two power transformer. Each have 380-0-380V 200mA, as in the original Angela project - 300B Mk1 Tube Amplifier. But this 200mA is not too much for one chanel? :-)
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Old 18th July 2012, 03:03 PM   #6
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It takes much more that expensive parts to make a great amplifier. I would start with a good schematic. The most expensive capacitor in the universe will still sound veiled compared to NO capacitor; you need to direct couple the driver to the output tube.

You can't just take someone's schematic and expect to construct the exact same thing. There is always testing and experimenting to achieve the best sound. For instance you are worried about the power transformer. Yes it might provide more voltage than you need. That might require a cathode resistor of 900 ohms. You won't know until you build the unit and then test it and refine it.

If I were building this amp, instead of buying one $15 resistor I would buy a bunch of modestly priced resistors say 750, 800, 880, 900, etc. to "dial in" the operating point of the output tube. This is an iterative process. Even a recipe for chocolate cake will require some experimentation to get it to your own "taste".
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Old 18th July 2012, 03:29 PM   #7
marcins is offline marcins  Poland
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Yes, you're right. I remember how I built small SE amplifier on the EL84. I chose value of the cathode resistor soldered it to the wires :-)
It can actually do the same here, first I solder cheap resistors on the wires and, as already establishes the value of this can I change it to something better.
But I am still worried about the winding 380-0-380 - it seems to me that a lot of 200mA is too much for one channel of the 6SN7 + 300B.

And one more thing:-) I still like my EL84 SE, but I want to have my own 300B;-) I buy some expensvie parts, because somehow seem more suited to audiophile - 300B;-)
I add some old photos of my EL84, and my OTl on 6as7g ;-)
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Last edited by marcins; 18th July 2012 at 03:39 PM.
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Old 18th July 2012, 04:05 PM   #8
disco is offline disco  Netherlands
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Originally Posted by marcins View Post
But I am still worried about the winding 380-0-380 - it seems to me that a lot of 200mA is too much for one channel of the 6SN7 + 300B.
Trannies are wound to a voltage at a certain current.
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Old 18th July 2012, 04:22 PM   #9
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Default the numbers don't add up

Yes, the power transformer will probably provide more voltage than the schematic calls for, but it will run cool and unstressed. So in the end that is a good thing. This means that you will need to be prepared to re-engineer the voltages on the voltage amp, driver and output tube. Since the front end is direct coupled you will need to juggle different values of resistors to get the thing working properly.

I should mention in relation to this that the voltages and currents listed on the schematic don't add up anyway, so quite a bit of testing and experimenting will be required to make it work properly. A quick for instance: we know that the first tube is drawing 3mA (3V/1k ohms) exactly; if we multiply 3mA by the plate resistor we get 204 volts. If we add that to the plate voltage we get 296V. The schematic lists B+ for this stage at 320V so we have a big error right there. The exact same problem arises with the second stage: the numbers don't add up.

So instead of worrying about expensive parts you should be building the circuit and experimenting with component values to determine exactly which parts you will need. This amplifier can't work as shown on the schematic, but the circuit topology will definitely work once you get all the proper voltages and component values worked out.
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Old 18th July 2012, 05:10 PM   #10
kevinkr is offline kevinkr  United States
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The text with the schematic states pretty clearly that the voltages provided are only approximate, but I'm wondering what sort of multi-meter was used to measure them as they do not jibe..

They aren't extremely critical although I will say that available driver headroom is not much. One worthwhile improvement might be to consider a choke load (plate choke) on the second stage in lieu of the resistor - noting that some adjustment to plate voltage to that stage will be required, this will give you headroom to spare and a significant improvement in linearity as well.
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