6DJ8/ECC88 mu-follower - diyAudio
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Old 13th April 2006, 08:01 AM   #1
SSassen is offline SSassen  Netherlands
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Default 6DJ8/ECC88 mu-follower

Here's hoping you gents can give me a few pointers. I'm currently looking for a line stage with quite a bit of gain. I am restricted to using the 6DJ8/ECC88 tube and have a 180-volt HT available to me. I currently run the 6DJ8/ECC88 in SRPP with the lower kathode resistor bypassed with a capacitor which yields about ~28x of gain. However the mu-follower looks to have better gain and better distortion figures on paper, but information on how to calculate it is scarce (or I'm looking in all the wrong places).

Any tips, suggestions or examples are most welcome!

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
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Old 13th April 2006, 08:15 AM   #2
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Hi Sander,
it took a while to ger that old post (where fdegrove posted such a schematic in reply to a question of mine) but here it is: DIY tube output in DAC - needs advice...

Cheers

Andrea
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Old 13th April 2006, 10:48 AM   #3
SY is offline SY  United States
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Mu follower calculations are treated in good detail in Morgan Jones's "Valve Amplifiers."

If you're already getting a gain of 28, it will be difficult to get it much higher- the mu is the ceiling, unless you're willing to cascode. With 180V, it's possible as long as you don't need gobs of swing.
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Old 13th April 2006, 11:02 AM   #4
SSassen is offline SSassen  Netherlands
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Thanks SY,

It is an output stage for a dac, input is ~0.06Vrms and output should be as close to 2Vrms as possible. Load impedance is 10K ~ 100K hence it is not required to drive a very low load, nor deliver high voltage swing. THD and gain however are important, THD should be as low as possible and gain should equal Mu.

Currently the schematic looks as follows, also showing the passive I/V conversion and 3rd order Bessel filter for the dac.

Click the image to open in full size.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
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Old 13th April 2006, 11:04 AM   #5
SSassen is offline SSassen  Netherlands
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Also, I do own Morgan Jones' book, but he doesn't go into great detail about the Mu-follower, or at least not to an extent that allowed me to work it into a working Mu-follower with the 6DJ8/ECC88.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
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Old 13th April 2006, 11:19 AM   #6
SY is offline SY  United States
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Do you have the Third Edition? It has about 5 or 6 pages detailing the mu follower, with a worked example using an E88CC (which is an exact equivalent of the ECC88).

Why are you restricted to ECC88? That gives you a severe gain limitation. Cascoding isn't an option unless you can add a buffer to the output.
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Old 13th April 2006, 11:31 AM   #7
SSassen is offline SSassen  Netherlands
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Hi SY,

What pages exactly? I just skimmed the book again and the only examples I could find using the E88CC are on page 115 - 118, but I can't seem to rework those into anything that's usuable with my HT, they use a 285-volt HT, which is >100-volts more then I have available.

I'm not restricted to the ECC88 though, it is just the highest Mu tube I have available. And according to Morgan Jones' book it is a excellent choice for a low-distortion valve with good linearity.

I'm actually upgrading the output section of a commercial dac, so I have to work within the confines of the dac, it has two noval sockets on the PCB and, as mentioned, a 180-volt HT. Here's a closer look at the PCB with all tube components removed.

http://hardwareanalysis.com/images/a...arge/11817.jpg

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
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Old 13th April 2006, 12:02 PM   #8
SY is offline SY  United States
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It might be worthwhile to follow those calculations, but use the voltage restrictions you have. For a 2V max signal, you could go down as low as 40V on the lower tube. Or for better linearity, the beta follower might be more suitable and waste less precious B+. If it were me, I'd lose the fancy topology and just run the tube as a conventional grounded cathode, LED bias, CCS load, and direct couple to a CCS-loaded cathode follower. That will get you a gain of 30 or so with excellent linearity.

For higher gain, consider an ECC81 run the way I described; that will be good for a gain of 45-50. With 120V on the plate, 2mA, the distortion at 2V will be miniscule.
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Old 13th April 2006, 01:31 PM   #9
ttan98 is offline ttan98  Australia
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Sander,

No need for the pi inductor input filter as audio tube preamplifiers are not so susceptible to rf. Most AUDIO tubes act as a low pass filter in itself.

If you don't believe me look as most famous preamplifiers, none of them have a inductor low pass input filter.

You need this filter with solid state preamplifier esp. when your power amplifier(close by) is a Class D type...
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Old 13th April 2006, 09:03 PM   #10
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watch out for the heater to cathode spec.......the ECC88 it's pretty low...

richj
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