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Old 9th October 2006, 11:05 PM   #1
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Default Aikido PSU help

After a bit of musing, and an email from John Broskie, I have decided to go for the Aikido octal-tube preamp. I'll order a single stereo board and get going with the project.

John suggested that I try either 12sn7 tubes (4) or mix them with a tube such as 6DJ8. I cannot find 12sn7s available, but have been able to find 6sn7 tubes. Anyway, I do not need a huge amount of gain, so about 20db will be fine.

OK, so I need to design a power supply.

My thoughts are:

320-0320 (Hammond 276X) into
5U4GB into
a choke input filter using TWO 12H chokes (Hammond 193B)
two 50uF caps.

According to PSUDII, I given a 20mA draw (4 X 6sn7 @ 5mA each). I should see around 260V.

Anyway, does this PSU appear suitable.

Broskie's information gives resistor values for 250V. Would it be terrible to use the same values with B+ of 260V.

Otherwise, could anyone recommend a suitable tube rectified PSU that gives 250V at B+?

Thank-you,
Charlie
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Old 9th October 2006, 11:41 PM   #2
SY is offline SY  United States
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The extra 10V will not make any significant difference. Go for it.

I don't remember the maximum input cap rating for the 5U4, so make sure that 50uF is ok. Data sheet will tell all.
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Old 9th October 2006, 11:54 PM   #3
Sherman is offline Sherman  United States
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Quote:
Originally posted by SY
...
I don't remember the maximum input cap rating for the 5U4, so make sure that 50uF is ok. Data sheet will tell all.
Max cap input for a 5U4 is 40uF. Make your first cap 40uF or less.

I have made several PS with 5U4s and have used 40uF motor run caps and 32uF electrolytics in the first position in a cap input filter without problems. (Actually I did have some arcing with one Chinese tube but all the others (Sovtek, NOS) I tried were happy with 40uF.
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Old 10th October 2006, 11:46 AM   #4
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5U4 is a little over kill for this application - you can also try the 5V4 or 5Y3 family... I really like the old 5Z4G or 5Z4GT (GZ30) British rectifiers for preamp use.

The 12SN7 should be readily available - the best part is you can still pick up some choice '40s examples for decent prices versus the 6SN7. If I had known how much vintage 6SN7s had gone up in value, I would have invested in them instead of stocks
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Old 10th October 2006, 04:07 PM   #5
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Coupla things:

1) The total current draw of the 6SN7s is dependant on the resistors that you use. If you use 1000R for the input tubes and 300R for the output tubes (as Broskie suggests), then you'll draw 5mA for the input tubes and 10mA for the output tubes, for a total of 30mA. (http://tubecad.com/2006/05/blog0064.htm)

2) 6SN7s really appreciate a higher B+ than 260V. 300V is a better target.

3) I am using a 3.3uF cap before the choke to up the voltage. A preamp doesn't see the large voltage swings and current demands that a power amp does, and thus I personally don't find choke-input to be needed for a preamp.
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Old 10th October 2006, 08:33 PM   #6
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Default How do you calculate these?

Jayme,

I have a general question - when you have a schematic that does not show the operating point, how do you calculate it? For example, in this case we have a plate voltage of 260V on a 6SN7. The cathode resistor Rk is 1000R. How do you determine that the tube will draw 5mA (given that you have the plate curves, etc. available)?
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Old 10th October 2006, 08:58 PM   #7
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How about just a simple power supply with a Hammond 269JX > A Full Wave Rec, pair of Paralleled 47uF caps, and a 1.2k-1.5k/47uF RC filter. Draw your B+ from the RC filter which is pretty clean and with the noise rejection of the Aikido should be plenty decent for a Simple power supply... (figure a 30mA draw as above post indicates)
A simple bridge rec + 47000uF cap to get DC heaters...

Very low tech but... I don't see how it'd be a bad supply as this is not a Power amp so current demands are minimal.
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Old 10th October 2006, 09:29 PM   #8
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Renegade,

OK, so I look at Vl1 to get the voltage of B+ with a 30mA load ???? In this case, the mean V is about 281V.

So, I get the PSU built. Now how do I check it without connecting it to the actual preamp ??? I mean, how can I use my multimeter to check the B+ voltage without a 30mA draw from the multimeter ?? Maybe, this is rather a dumb question, in which case....duh!

Regards,
Charlie
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Old 10th October 2006, 09:29 PM   #9
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Quote:
I have a general question - when you have a schematic that does not show the operating point, how do you calculate it? For example, in this case we have a plate voltage of 260V on a 6SN7. The cathode resistor Rk is 1000R. How do you determine that the tube will draw 5mA (given that you have the plate curves, etc. available)?
This article by Broskie shed some light in the calculations of operating points. http://www.tubecad.com/articles_2003...ier/index.html

Good luck

Erik
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Old 10th October 2006, 10:06 PM   #10
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Default Calculating operating point on existing schematic

Erik,

The question is not how to establish an operating point when designing a new topology, but rather how to infer/determine the operating point of an existing schematic (that you did not author). In this example, we have the Aikido topology with 6SN7. The 6SN7 has a plate voltage of 260V, and a cathod resistor of 1000R. How does one determine the bias point here?

Do you merely pick an example bias voltage - e.g., -3V, then run the numbers (V=IR on the cathode resistor), obtain a current, look at the plate curves to see if it works (i.e., the operating point falls on the correct curve in the plate characteristics chart)? If not, pick another bias voltage and recalculate.
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