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Old 7th July 2004, 06:09 AM   #1
garbage is offline garbage  Singapore
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Default how to calculate lower operating point?

hi

i'm a tube newbie and would like to build some thing like the below but at a lower operating point:
Click the image to open in full size.

any idea what is a straight-forward method to determine the resistor values that i need to change in order to make it work with a B+ of say 480V?

thanks.
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Old 7th July 2004, 07:19 AM   #2
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As I recall, this is the "choked monkey" design. Utterly horrible as you waste half the power in that 3k 25W resistor below the 300B. Just remove that and cap couple the 6AQ8 to the 300B.

Let's see, you need more than that. First, get a new power transformer that does 400VDC or so (something around 350-0-350VAC). If it's more convienient, you can use a 5U4 instead of 6AU4's. Change the first filter caps from 200uF 450V to 40uF 450 or 500V. Replace the 47 ohm 5W resistor between the 6AU4 cathodes and 1st filter cap with a short. Same for the 3k 25W, remove it. Move this end of the 65H choke to B+ instead. You'll need slightly more resistance in the 6AQ8 cathode, probably 470 ohms. Insert a .22uF capacitor in series with the 200 ohm grid stopper on the 300B, and at the junction between them, add a 100k grid leak resistor to ground.

Or you can look for another 300B SET amp circuit, there's enough out there that you WILL find exactly what I described, only in schematic form.

Tim
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Old 7th July 2004, 07:41 AM   #3
garbage is offline garbage  Singapore
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hi tim

thanks for the detailed post.

may i ask what is the function for the 0.22uF cap before the grid stopper?

and is the 100k grid leak to be placed after the 65H choke?
(ie 6AQ8 plate - 65H choke - 100k grid leak - 0.22uF - 200R - 300B grid)

with the above 2 additions, will the design still be a "choked monkey"?

actually i am interested in building a "choked monkey" 300B set.
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Old 7th July 2004, 11:36 AM   #4
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The .22 seperates the high (+400V) 6AQ8 plate voltage (which is pretty high for something like that... some resistors and capacitors should be used to bring that down so it doesn't throw sparks...) from the 0V potential of the 300B grid. In the circuit shown, the 300B floats at +200V or so by way of the big resistors below it. Its operating voltage then comes from the extra high supply voltage. Capacitors block DC but they're not perfect so you need a grid leak to ensure the 300B grid stays at 0V. Its cathode will naturally float itself up to around 60-80V due to current flow through the resistor (um 800 ohms or so is it?).

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Old 7th July 2004, 11:59 AM   #5
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Default Re: how to calculate lower operating point?

Quote:
Originally posted by garbage
any idea what is a straight-forward method to determine the resistor values that i need to change in order to make it work with a B+ of say 480V?
It wont work very well at all with only 480V. The extra B+ is the price you pay for direct coupled designs.

If you want to do a DRD / Monkey style amp with only 480V, use a 2A3 or 45 as the output valve - not a 300B.
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Old 9th July 2004, 08:06 AM   #6
garbage is offline garbage  Singapore
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Lightbulb 6EM7/300B DRD?

Hi All

Thanks for the response so far.

How does the circuit below look alright with the indicated resistor values?

I want to use a B+ of 480V as this would make it easier to source for caps in the power supply section.

This is my first time trying to calculate the values, can someone help verify if my calculations are more or less ok?

Method of calculation taken from Jeremy Epstein's site.

Suppose I have a 480V B+. And the following information:

300b operated as follows
Vplate : 300v, 65mA
Vgrid : 120v (bias -60v)
Vcathode : 180v

Assuming the choke is 65H and 250R, this should give me resistors of 2750R and 4000R at the cathode of 300B.

6EM7 operated as follows
Vplate : 100v, 20mA
Vgrid : 3v (bias -17v)
Vcathode : 20v

This should give me 1000R for bias at the cathode of 6EM7.
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File Type: jpg 6em7_300b v0_1.jpg (15.4 KB, 320 views)
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Old 9th July 2004, 02:35 PM   #7
garbage is offline garbage  Singapore
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let's see if i can post the circuit as an image straight here without the need to download it.

Click the image to open in full size.

oh... it does not work.
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Old 9th July 2004, 02:51 PM   #8
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I feel the person best suited to explain this is Steve Bench:-

http://members.aol.com/sbench101/

And check out 'Loadlines'

We're not worthy, etc...
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Old 10th July 2004, 03:21 AM   #9
garbage is offline garbage  Singapore
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hi powertriode

i did email steve a few days back regarding drawing of loadlines.
he's really nice and was quick to reply my query.

i'm not sure if it would be nice of me to send him a circuit diagram for verification.

if i don't get any replies from this thread, i'll probably ask steve if he would take a look at my circuit.

thanks.
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Old 13th July 2004, 06:33 AM   #10
garbage is offline garbage  Singapore
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Realised that i am using the wrong unit for 6EM7.
It should be unit 1 instead of unit 2.

Thus with the following conditions:
486V B+, assuming 102R output transformer resistance

300B operated as follows
Vplate : 300v, 65mA
Vgrid : 120v (bias -60v)
Vcathode : 180v

6EM7 operated as follows
Vplate : 120v, 1.8mA, bias -1v

Assuming the choke is 65H and 250R, this should give me resistors of 2848R and 55R at the cathode of 300B.

And bias of 1V at 6EM7 with 1.8mA would require resistor of 555R.

nb. I have not calculated the loadlines for driver and output tube yet.
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File Type: jpg 6em7_300b v0_2.jpg (15.3 KB, 155 views)
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