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Old 19th May 2009, 09:32 AM   #1
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Default EL36/6CM5 SE design critique

OK firstly thanks to those who have assisted in my various threads before. You should recognise the family lines of this proposal if you look back on my earlier posts...

Take one EL36 in triode mode. CCS load the anode at 40ma, bias at -52V, giving a Vq of 315V. Cap couple it to a torroid with a 5k2:8 impedance ratio. See Tom's excellent triode curves for the EL36 for the details.

Drive this with a CCS loaded EC900 biased at -1.5V. Set the CCS at 6ma. Anticipate a swing of 108V from a 2V RMS input. Curves for this were compliments of Yves.

Linearity to burn in both of these tubes at these points and the best part of three fifths of five eigths of damned-all distortion.

Power supply needs to do 600V @ 40ma and 280V @ 6ma plus the admittedly sickening heater requirements of the EL36 (hey, series them to keep current load bearable) and the less demanding heater requirements of the EC900. Don't forget the -52V bias.

Right, the torroids are an experiment, but at about $15 USD equivalent locally, its worth a play. In addition, they can be configured to output to 8 or 16 ohm loads! The rest owes me about $25 USD equivalent so whatever...

Likes - unbelievably linear curves of the two tubes, simplicity in the design, reasonable Po (around 7 watts), easy access to the parts, nothing exotic. Similar but different to the 807SE circuit I discussed earlier, so a variation that builds a family.

Dislikes - 600V is a little daunting and we are just a gnats outside the max Va for the EL36...

Feel free to criticise - I love to learn!
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Old 19th May 2009, 10:13 AM   #2
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Hi,
Small issue: The transformer will be more like 4600:8 as you should use open-circuit voltage(10V) to calculate.

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Old 19th May 2009, 10:18 AM   #3
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Enjoy! That is one gorgeous sounding tube!

We did a LOT of work with this one (but not CCS loaded) at my forums.

Here's a bunch of info:

http://geek.scorpiorising.ca/GeeK_Zo...p?topic=2816.0
http://geek.scorpiorising.ca/GeeK_Zo...p?topic=1322.0
http://geek.scorpiorising.ca/GeeK_Zo...p?topic=1551.0
http://geek.scorpiorising.ca/GeeK_Zo...p?topic=3215.0

Search will bring up 1/2 dozen more.

Cheers!
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Old 19th May 2009, 11:11 AM   #4
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The maximum rating for EL/PL36 screen voltage is 250V. Sure, this is for TV duty, so you can probably exceed the ratings a little, but I think that more than 300V will burn up the screens almost immediately. For triode mode, with screen and anode connected, 300V supply (for fixed bia + CCS anode load) would be the highest I would dare use.

Also, I believe Yves pusblished some good curves on this Tubes forum, showing best performance from the PC/EC900 CCS loaded, when operating at 10..14mA maybe ~130..150V. The curves get much more congested at high voltage, low current. A 150V supply would be worth considering here.

I hope he won't mind - I attached them again here!
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Old 19th May 2009, 11:34 AM   #5
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Default Re: EL36/6CM5 SE design critique

Hi aardvarkash10,

instead of CCS-loading the trioded EL36, I would do a "power SRPP" with it, driving a DC-decoupled toroid (cheap, plain PT...) Cheap toroids used as PTs usually have a highish shunt capacitances in excess of 1nF, and SRPP is just ideal to drive that. Expect a mind boggling bandwidth and stunning square wave response. Just go for a transformer Q of about 0,7

Serge DaSilva / Switzerland built quite some big amps using this topology and making it popular. I made fewer amps w/ smaller tubes this way. An example circuit is attached. Mind, the driver stage need not to be SRPP topology, too. Actually, a mu-follower (or pentode based CCS) probably would have been a better choice.

Other than Serge, I mostly use an additional gNFB of about 6..9dB from OPT secondary to driver cathode. With that in mind, lots of empirical experiments showed that a reflected impedance Ra ~ 1,7*rp(trioded) gave optimum Po at very low distortion. From that, you easily can compute the voltage ratio of the toroid to be used.

Hint: compute/develop for a secondary load impedance of 5 Ohms, so in the end, you can use both 4 ohms and 8 ohms speakers Just dont worry, Zout is very low and hence DF very high.

Regards,

Tom Schlangen
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Old 19th May 2009, 01:19 PM   #6
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Default Supply voltage for EL36 triode

At high voltage and 40mA surely the triode curves are deeply into the curved region. And, since the most remarkable thing about the EL36 is the enormous peak current - why run high voltage anyway? at 230V the Mullard PL36 spec shows over 800mA peaks are available. I would think that 180V/70mA would make a better amplifier than 315V/40mA. The screens would then never be in danger of burnout.

If you still like the idea of High Voltage, you could approach it in stages.

Get a mains Trafo with 230V secondary and build a standard bridge circuit for 300V. If you want to see how 600V sounds, just change the bridge to a doubler - instant 600V dc!

If you try 600V supplies, use a stopper resistor in the G2 terminal to help preseve screens. (This may be a good idea anyway, to suppress oscillation)
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Old 19th May 2009, 10:04 PM   #7
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Default thanks guys

Lars - good point on the OPT - thanks for that

Tom - yeah, I just re-read the SRPP Deconstructed article again and got a bit excited about that topology for an output. Might try it on a smaller setup using a triode/pentode bottle - I have some little ocsillator/mixer tubes that look like goers for a headphone amp. Meantime, I'd like to learn a bit more about CCS topologies (and the math is easier)...

Rod - I have a datasheet that shows 550V max at G2 as long as there is no current. Anyone experienced this? Yeah, the deflection tubes will do big pulse current, but CCS loaded, I figured I'd go conservative. I could always lift the current and drop the voltage in the proto-stages and see what occurs...

I dropped the current on the EC900 to get it into a suitable range for the output swing without over-driving the EL36. Its still amazingly linear down that low, but if it gives problems, I can also raise the CCS current and modulate the line input instead to reduce the swing.

The bridge/doubler thing is already built! And the trannie (250VAC @ 150ma) has multiple tappings on the secondary too so I can mess with it a bit.

Hiya my Canadian friends! - yeah, geekzone has been a cruising zone (sounds a bit ummm, risque...) and I have been through some of the EL36 stuff there too. Gotta say that building your own OPTs and the like is a little beyond me at the moment, although I have an 807 project that requires it. And I just found a 78 yo ex-winder with ALL the stuff to do it and the time to teach me, so colour me happy!
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Old 21st May 2009, 07:57 AM   #8
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Default Re: EL36/6CM5 SE design critique

Quote:
Originally posted by aardvarkash10
Take one EL36 in triode mode. CCS load the anode at 40ma, bias at -52V, giving a Vq of 315V. Cap couple it to a torroid with a 5k2:8 impedance ratio. See Tom's excellent triode curves for the EL36 for the details.

Drive this with a CCS loaded EC900 biased at -1.5V. Set the CCS at 6ma. Anticipate a swing of 108V from a 2V RMS input. Curves for this were compliments of Yves.

Linearity to burn in both of these tubes at these points and the best part of three fifths of five eigths of damned-all distortion.
Interesting. I've been re reading Patrick Turner's JBS amp page and am contemplating a larger version of it, but your op point is closer to what I was looking at. Eight per channel in push pull sounds like fun.

I have all the parts, including a 2kVA 240V iso trans for HT.

I'll watch for a while as I have other stuff to complete first (like my glacial speaker projects). I'm looking forward to seeing how they measure at this op point.
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Old 21st May 2009, 09:20 AM   #9
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Default hiya brett

Patrick's stuff leaves my head spinning... His obsessive approach doesn't align with my "build on a shoe-string slap it together" tendancies, but I admire his logic and depth of understanding.

That by way of a warning - any learning from me wil be slow and often inconclusive. It regularly involves smoke, and I often get sidetracked. If you want to build something before you draw a pension (or your final breath), don't wait for me...

Case in point - Tom's discussion of SRPP approaches took me off to looking at the smaller valves in my boxes and has led to a couple of nights designing a SRPP headphone amp using the little 100V line trannie that JAYCAR does as the output load. Multiple taps on the OPT allows for differing headphone impedences, and the little bugger is designed to do 0.5W into 8 ohm on a 20k primary impedence. And again - I have all the bits!

So many tubes, so little time.
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Old 21st May 2009, 09:37 AM   #10
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No worries. I have a bunch of high power SS to use here, but as I also have the other bits, I thought it might be fun. I'm more interested in the performance of the EL36 at that op point than anything else, but I reckon it will be OK 'near' there too.

My design idea is strongly reminiscent of PT's big poweramps.

They will be driving 98dB/W speakers (<300Hz).
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