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Old 5th March 2013, 05:57 PM   #1
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Default 100 W-class Circlotron

I have lately been testing with a circlotron monoblock with two JJ EL509. My goal has been to design about 100 W class amplifier with these tubes. The present circuit is based on my earlier prototype I built couple years ago. Then it was a test platform for different output tubes and transformers. At that time I made some test with 6L6 and EL34 but now wanted to go to higer power level. I added an adjustable screen grid voltage to accommodate my circuit for EL509 that requires relatively low G2 voltage.

The whole project has been delayed because I did not have suitable power supplies but finally – after I had burned a dozen of power NFET’s – I managed to get ready my adjustable 600 V / 300 mA power supply. The other supply is two series connected Heathkit IP-2717 which are a bit light, but will do.

The required 100 W could be achieved with some 450 V supply voltage and 500 ohms load impedance, but then the linearity of the output tubes is not optimal. Better alternative is to use higher load impedance (1k) and higher supply voltage (550 V).
It follows that the driving voltage level at the output tubes is very high (> 500 Vpp) and this in turn places high demands on the linearity and capacity of the driver stage, but this challenge can be solved.

The output transformer I used is quite unusual. It is Polish Indel TGL40/001 that is originally 4k to 8 ohms type. This transformer contains four separate parallel connected secondaries and by connecting two in series and two in parallel the required 1k to 8 ohms output impedance was created.
One good feature of this transformer is it’s low ( 1,4 mH) leakage inductance and this can be clearly seen. The frequency response exceeds above 40 kHz without any GNFB.
Usually the high end frequency response is problematic with circlotrons.

Below is the present circuit:

Click the image to open in full size.

http://kuva.termiitti.com/image/29593.GIF


I made some tests before and after applying 10 dB global negative feedback.

Without GNFB I got as follows:

+Ub = 550 V, Ik = 55 mA/tube, Ug2 = 120 V, Ug1 = -37 V, Rload = 8 ohms

Pout(max.) = 120 W @ THD = 2,0 %

THD at: 1 W = 0,10 %, at 30 W = 0.27 %, at 60 W = 0,33 % and at 100 W = 1,2 %

Frequency response (-1 dB) <20 Hz….40 kHz

Zout = 1,8 ohms


With 10 dB GNFB :

THD at 1 W = 0,05 %, at 50 W = 0,11 %, at 110 W = 0,39 % and at 120 W = 0,53 %

Frequency response (-1 dB) <20 Hz….70 kHz

Zout = 0,16 ohms

At 30 Hz and 110 W, THD = 0,70 %

Last edited by artosalo; 5th March 2013 at 07:16 PM.
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Old 5th March 2013, 07:01 PM   #2
Ketje is offline Ketje  Belgium
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Looks very nice.Some remarks however.
I hope each EL509 has it own heatersupply ,Vkf=150V max !,connected to its cathode.
The poor 6N1 has a hard time with Va=300V max (Svetlana says only 250V)
What is the use of R65 ?
How come you get 350V after the two 33k ? If one 500V goes up the other goes down the same amount and v.versa.Netto from 500V with two resistances in parallel to 350V ,how about 2x 100k.Or do I misunderstand ?
Mona
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Old 5th March 2013, 07:20 PM   #3
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There was a mistake in the drawing (maybe not the only one...).
The 350 V supply voltage for 6J38P is stabilized with two zeners that was not drawn but I added those now.

R65 is just a "grid stopper" for the constant current supply.
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Old 5th March 2013, 07:40 PM   #4
Ketje is offline Ketje  Belgium
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Now you have 3x D2
The 200V zener will become rather hot (>1W)
Mona
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Old 5th March 2013, 07:44 PM   #5
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Quote:
Originally Posted by Ketje View Post
...The 200V zener will become rather hot (>1W)
Mona
Both zeners are 5 W and stay cool.
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Old 7th March 2013, 05:52 AM   #6
tinitus is offline tinitus  Europe
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Old 1st May 2013, 05:46 PM   #7
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Default First unit completed

I finally got one of the monoblocs done and one more is under process.

I built the power supply and amplifier on separate boards. Then possible repair and modifications are easier to accomplish.
There are a current meter for both EL509 to monitor the standing current if the bias tends to fluctuate. With full 100 W output power to 8 ohms load the anode current of one EL509 is 150 mA.
At idle it is some 55 mA.

The test results are a little better now when I adjusted the GNFB to 12 dB. For example there is a THD sweet spot at 70 W; only 0,08 %. At 100 W THD is essentially below 0.5 %.
Un-weighted hum level is 90 dB and A-weigted below 100 dB.

I will do a complete test for both amplifiers as soon as I get the 2nd unit ready.

There will be no volume control, but a switchable 10 dB / 0 dB input attenuator.




Click the image to open in full size.



Click the image to open in full size.
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Old 1st May 2013, 10:49 PM   #8
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Nice looking amp, does it cost a lot to make PCBs for just 2 mono-blocks? Also could you please post the power supply schematic?

TIA,
Jaz
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Old 2nd May 2013, 05:00 AM   #9
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I got 10 amplifier and 5 power supply pcbs for 110 USD including shipping from iTead studio.
When the pcb size is 100 mm x 100 mm the unit price is very low. Therefore I try to fit my designs to this size if possible. I did not make the pcb for just two amplifiers.

I will update the schematics soon.
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Old 2nd May 2013, 04:55 PM   #10
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Here are the updated schematics:

Click the image to open in full size.

Click the image to open in full size.

Last edited by artosalo; 2nd May 2013 at 05:00 PM.
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