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Old 28th August 2010, 05:17 PM   #11
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Originally Posted by ashok View Post
Yup , basically determine value of one resistor ! However selecting an operating point that works best ( audibly and electrically ) might not be so simple ?
Selecting a good operating point is simple - read this link, you already have the HT so you just have a point on a vertical line to select, which gives you the cathode voltage and expected anode current, which gives you the resistor. For instance a 2mA current at Vg = -2V gives you a 1k resistor.

It must be easy, because even I can do it


For the OP: if you bypass the cathode resistor you can try a stereo pot on the cathode and dial in a good sound for any tube - then just sub in fixed resistors when you have got it. Perhaps put a series resistor (chosen from looking at the load lines) in with the pot to prevent cooking the tube though!
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Old 28th August 2010, 08:12 PM   #12
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I can't see any mention of your operating conditions for the 6SN7. This tube runs best with plate current ~8mA. I'm surprised you were disappointed with EH6SN7. Mine sound OK and they have a good rep. generally.

What might work well for you is a mu follower (or beta follower if you're a bit short of B+ voltage). This would provide relatively high gain compared with conventional resistor plate load, lower distortion and lower OP impedance to help drive your OP tube.
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Old 29th August 2010, 09:14 AM   #13
Green77 is offline Green77  Sweden
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My operating conditions is like this :

Click the image to open in full size.

Dont' know if it's the best for a 6SN7?

And one more question. Is the resistor in the blue box nesessary? Becuse i don't have it in my amp..must have missed it And is 100K ok?

Thanks
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Old 29th August 2010, 09:41 AM   #14
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Originally Posted by Green77 View Post
My operating conditions is like this :

Click the image to open in full size.

Dont' know if it's the best for a 6SN7?

And one more question. Is the resistor in the blue box nesessary? Becuse i don't have it in my amp..must have missed it And is 100K ok?

Thanks
For 8mA you need 4.5V at the cathode with your rail, so use a 4.5/8 = 560 ohm cathode resistor (Rk). Also you will need a 165/8 = 20 - 22k resistor on the anode (Ra).

At 8mA the tube will be quite hot I think. Personally I would run it at around 5mA, which gives a 5.5V cathode, Rk = 5.5/5 = 1.1k, Ra = 165/5 = 33k

You need the 100k resistor but I would change it to 470k or even 1M. It saves the tube if the grid gets disconnected.

In fact I had a similar thing last night, a dry joint on my bias PCB caused the -25V bias to a GU50 to fail, I noticed this when the GU50 anode was glowing a nice bright red/orange !! The glow was very even and most impressive - and did not damage the tube - but you may not be so lucky with a lesser tube (and will eventually damage transformers etc!).

P.S. These new anode resistors will need to dissipate c. 1Watt so should be rated above that!
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Old 29th August 2010, 09:52 AM   #15
Green77 is offline Green77  Sweden
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Thanks, i will put a 470K resistor in the "blue box"

So i run my tube cold with Ra=47K and Rk=500 Ohm?

What happen's if i change the Ra to 33K? Will there be more volt to the next stage and the volum will be higher? That i don't need

Thanks //Daniel
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Old 29th August 2010, 12:02 PM   #16
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Originally Posted by Green77 View Post
Thanks, i will put a 470K resistor in the "blue box"

So i run my tube cold with Ra=47K and Rk=500 Ohm?

What happen's if i change the Ra to 33K? Will there be more volt to the next stage and the volum will be higher? That i don't need

Thanks //Daniel
With your old circuit you have 2.6V running through 500 ohms = 5.2mA.
IF that 2.6V is correct - but I doubt that.

5.2mA through 47 ohms = 47 * 5.2 = 244V, leaving < 90V of plate voltage. In my opinion this is a non-optimal operating point and my numbers will work much better

The volume should be similar but the DC level has changed - if you are capacitor coupled it should be fine, if not - it won't. Only you know the full schematics for this amp..
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Old 29th August 2010, 01:04 PM   #17
Green77 is offline Green77  Sweden
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Thanks for explaining

I messured 2,6V at that point so it's correct. And there is no cap to next stage

I think i leave it as it is

Regards //Daniel
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Old 29th August 2010, 01:19 PM   #18
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Originally Posted by Green77 View Post
Thanks for explaining

I messured 2,6V at that point so it's correct. And there is no cap to next stage

I think i leave it as it is

Regards //Daniel
OK then the slight weirdness is there to eliminate a capacitor - a laudable aim.
As you are stuck around 88V on the anode (plate) I would choose a more suitable tube - maybe an ECC88?

Have you the rest of the schematic or is that secret?
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Old 30th August 2010, 07:56 PM   #19
Green77 is offline Green77  Sweden
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Hehe, no it's not secret schematic. I've built TriodeDick's Bill. With some changes..

It's sound really good with a good 6sn7 input tube


Click the image to open in full size.
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Old 30th August 2010, 09:03 PM   #20
rongon is offline rongon  United States
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That's another variation on the ol' Mullard 5-20, no?

How does it sound with the KT88's in UL mode but with no global feedback at all? I'd think the output stage would have enough 3rd order harmonic distortion to sound a little "off." But heck, what do I know?

ALso, the voltages don't seem to pan out. That 6SN7 voltage amp should be drawing 5.2mA (2.6V/500 ohms = 5.2mA). 5.2mA across that 47k plate resistor should drop 144.4V. That leaves only 100V at the 6SN7 anode, not 125V as indicated on the schematic.

If there really is 130V appearing at the top of R12, then there's -30V bias on those ECC99's. That can't be right...

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