Hi!
I have finished my KT88 PP where the input tube is a 6SN7.
I have tried 2 diffrent tubes. Electro Harmonix sounded like crap and GE sounded nice but it's not mine
I have looked at Tung-Sol or TAD. Anyone have any comments on them?
Thanks // Daniel
Try it in SRPP (or use 6N1P-EV if space is an issue), I found the SRPP used as a driver stage works extremely well (and possibly is more tolerant of differing tube sonic signatures).
Hello Green77,
I can guess that you want to roll a few tubes before you tear your amplifier apart and start over. NOS tubes will cost a lot. I would start with a current production Tung-Sol. Tung-Sol 6SN7’s have a reputation for being top of the heap for current production.
DT
All just for fun!
I can guess that you want to roll a few tubes before you tear your amplifier apart and start over. NOS tubes will cost a lot. I would start with a current production Tung-Sol. Tung-Sol 6SN7’s have a reputation for being top of the heap for current production.
DT
All just for fun!
Well they are all the same really, just divide the HT voltage by 2 and lookup the curves at the voltage. Select the current you want and it it for both cathode resistors: job done
The nice thing about the SRPP is that you don't even need anode resistors - just the cathode ones to select the grid voltage and tube current at the 1/2 HT operating point.
Yup , basically determine value of one resistor ! However selecting an operating point that works best ( audibly and electrically ) might not be so simple ?
Hi!
I have finished my KT88 PP where the input tube is a 6SN7.
I have tried 2 diffrent tubes. Electro Harmonix sounded like crap and GE sounded nice but it's not mine
I have looked at Tung-Sol or TAD. Anyone have any comments on them?
Thanks // Daniel
By rolling tubes what you are ding is leaving the bias fixed and then hunting for a tube that works at that bias. Why not adjust the cathode resistor for best sound with the tube you have. It is far cheaper to "roll resistor values" than to roll tubes.
OK, yes roll tubes too. but each tube will have a best operating point so when you compare tubes listen to each tube at it's best setting. Simply doing a tube swap will just find the tube that is the best match for your cathode resistor. This is a good use for a resistor substitution box.
I think this is where small amounts of local feedback help. This can give you linear gain even if there are tolerances in the parts. But that would be a re-design. likely not what you want to do.
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!
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.
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.
My operating conditions is like this :
An externally hosted image should be here but it was not working when we last tested it.
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
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!
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..
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?
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
It's sound really good with a good 6sn7 input tube
An externally hosted image should be here but it was not working when we last tested it.
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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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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