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Old 2nd May 2010, 03:27 PM   #21
TheGimp is offline TheGimp  United States
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Quote:
Originally Posted by rotaspec View Post
I read somewhere that an unbypassed cathode resistor means a dramatic change in the internal resistance of the tube compared to bypassed (but I can't remember which way it goes).

I have played with the bias on 6N1P until I am blue in the face, and I haven't found a plate current or load that I am completely happy with. Even without any real test equipment, I can tell by listening that there is some distortion there. A 6N3P seems to sound better in the same circuit (but it is a complete socket rewire job, so comparisons take some time). At least the mu is similar (36 vs 33), so NFB changes are probably not required. I would love to see spectra of the two of them under similar operating conditions.
I ordered 10 6N3P=EVs off ebay to try, although from the one data sheet I've seen they don't look as linear as the 6N1P-EV to me.

What bias conditions are you proposing? What type of bias?

Cathode Resistor, bypassed or un-bypassed?

LED cathode bias?

What plate load?
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Old 2nd May 2010, 09:31 PM   #22
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The Russian pdf data sheet for the 6N3P (not -EV) I have is 148kb in size for comparison to yours. I do have another sheet, but it's in Djvu format and 1.1Mb. It is however much more detailed in its curves.
If you look at unbypassed cathode resistor bias of around -2v for a B+ of 250v and any load from 25k to 68k it looks pretty linear, but I have run a 220k load and it still sounded good (defying the characteristic curves). That's if I'm reading the curve correctly. To my mind that makes it a good candidate for a front end.
It looks very good with a CCS of about 4 to 6mA as well, but I haven't tried that yet.
I have tried IR LED bias of -1v and again it sounded much better than the curve would suggest.
To my ears (remember I don't have any other distortion measuring equipment) bypassing the cathode in either case either made no difference (Rk) or actually sounded worse (LED). However, the capacitor I used was just a junkbox one.

The 6N1P only really seems to improve when the available B+ is cranked toward the 400v end so I can get enough current through the tube at a sensible load. Ian found the same thing when we were doing the Mengyue Mini experiments.
However, it may be a good candidate for a CCS plate load, and that is on my list to try.
I am running a 6N1P with a plate load of 22k B+=250v and about -1.3v (bypassed Rk) bias in my 6P1P SE computer amp at the moment and it actually sounds quite good. Again, the curves say it shouldn't be very linear at all.
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Old 2nd May 2010, 11:14 PM   #23
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Default 6N3P curve

Here's a screen grab from the djvu file for the 6N3P:
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File Type: jpg 6N3P_curve.jpg (340.8 KB, 382 views)
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Old 3rd May 2010, 03:06 AM   #24
TheGimp is offline TheGimp  United States
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I've been using this one:

http://klausmobile.narod.ru/td/data/_6n3p.GIF

My latest testing has been with LED bias combined with CCS plate load, and may to some degree account for my liking the 6N1P(EV).

Just got back from Nasheville TN (Flooding) last night with our son, and I haven't had a chance to do anything in the shop today.
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Old 11th May 2010, 04:02 AM   #25
TheGimp is offline TheGimp  United States
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The 6N3P-EBs came in today, but it's going to be a couple of weeks before I can get back to the workbench. What operating point are you proposing to run at?

Right now the work bench is covered with engine parts from my 1975 Harley Davidson that I took apart to fix a low compression problem back in Feb. 2009 and never put back together.

I ended up tearing it all the way down and split the cases to install a new connecting rod set and rebuild it from the bottom up after I found a SS cam shim got chewed up by the cams and pumped through the engine.
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Old 11th May 2010, 05:31 AM   #26
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I just looked at a schematic I drew when I used the 6N3P-EV to drive EL84 in class A SE.
I had a 47k plate resistor and a IR LED for bias of 1.05v, and B+ was around 280-300v from memory (I forgot to write it on the drawing, sorry). I think I ended up with about 2.5mA plate current.
Bear in mind, this was the input tube for an amp to suit my Mac Mini computer which has a low AF output level (<500mV).
It would be interesting to see how it responds to a CCS plate load.
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Old 12th May 2010, 01:29 AM   #27
TheGimp is offline TheGimp  United States
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I'll try it with a CCS plate load and LED bias. If I make the CCS variable I can try different current settings to see how it behaves.

With luck the son and I will have the engine off the workbench this weekend. I'm messing with RK (Road King) air shocks right now.
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Old 12th May 2010, 02:39 AM   #28
TheGimp is offline TheGimp  United States
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I think I found some old HP IR LEDs I can try as well. They measure from 1.04 to 1.14V.

My B+ is a bit lower than yours, so I'll can use the HP supply on it and crank it up.
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Old 13th May 2010, 11:04 AM   #29
Ian444 is offline Ian444  Australia
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Gimp,

Very interesting experiments, really enjoying reading the thread. A few notes/observations if I may, hope its OK.

The only 6P1P-EV's I've seen on ebay or bought myself are NOS Svetlana. The Chinese made 6P1, not sure if they are still in production or not, some NOS are from the Beijing factory pre 1986 I think, now closed down/moved, and later ones are from the Shuguang plant. They have different markings.

It is good to hear you are still trying LED's as cathode load on the output tube. They are very difficult to work with, as the LED array needs to be designed for a specific current at a specific voltage and they do not bend. If the B+ happens to rise on the mains, the LED's will sink more cathode current and hold the bias voltage fairly constant. They are not forgiving nor adjustable, but fitting them to the Mengyue and a Simple PP gave similar results, deeper bass, more clarity, quite remarkable. I see you were interested in the RLD thread where some solid state circuit replacements for LED arrays were suggested by you and Wrenchone. Have you tried a listening test between LED's and your suggested cct? Such a cct, if it matches LED performance, would be brilliant. I don't know how LED's compare to fixed bias however, I wonder if the benefits of LED arrays are exactly the same as running fixed bias? Why is everyone (as in the tube forum) so quiet about this? I mean, if fixed bias was the ducks nuts, then LED arrays would not be in existence. I bought some parts to try fixed bias in the EL84 amp, and am really keen to try a transistorized cathode load also, (using just my ears to judge). The benefits of a transistorized cathode load is the ability to adjust the bias voltage and therefore cathode current, instead of buying 100 or 200 LED's to get an array happening after matching every individual LED to make equal strings of equal current and voltage etc etc etc, 10 hours later....the only way to adjust bias with LED arrays is with screen current regulation or simply combination bias where you use the LED array and fixed bias with pots to adjust the grid voltage. If you go that far mosfet source followers are not much trouble to add (another good one, if you are in the mood for extra clarity). If fixed bias was good, you would just do fixed bias, and forget all the other stuff.

I have not found any considerable difference in sound quality between the 6P1P-EV and JJ EL84 tubes. EL84 seems to make more power more easily, which might explain why its popular.

That Klausmobile site is wonderful.

Tube List (English)

Tube Tester Files - 6N1P - 6H1?

Tube Tester Files - 6N3Pi-E

Glad to hear that HP supply is coming in handy

Edit: the "3mA / 120V closeup" on the 6N1P answered many questions for me.

Last edited by Ian444; 13th May 2010 at 11:10 AM.
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Old 13th May 2010, 03:29 PM   #30
TheGimp is offline TheGimp  United States
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Thanks Ian, constructive criticism is always welcome.

I'm just trying to learn so I do a lot of experiments. Simulations are interesting and can lead to good insight, as well as studying formulae and theory, however they are all apporximations of what the tubes actually do.

I really would like to understand tube operation to the point where I could modify the tube models for some of the russian tubes to get them to more closely match the real world, but getting the time is another matter.

The "3mA / 120V closeup" is indeed interesting. I usually go to the Klausmobile site first for data on Russian tubes (probably should call them Soviet rather than Russian as there were many tube mfgs and all were not in Russia propper).

I think the issue of Fixed Bias vs cathode bias is a matter of circuit complexity and how many power supplies do you need.

However, cathode bias does tend to provide some small amount of NFB as most cathode devices will increase voltage drop with increased current flow (except ideal LEDs, Vshunts, etc). This small amount of NFB helps stabalize the circuit. This is not true with fixed bias, and hence makes it more sensitive to voltage variation and less stable.

I would really like to try the same amp with different bias mechanisms at the same operating point and measure changes in distortion, then do listening comparisons. It will be difficult to switch between them on the fly though, and it would require switching bias to two tubes at the same time if a PP amp were used. SE might be easier in this regard.

My experiments with screen control of bias indicated that it is feasable, but possibly only over a very narrow range and may not compensate for large variation in tube parameters without causing increased 2nd harmonic distortion due to compression of the curves. This is unfortunate as it looked like a good cantidate for bias adjustment with cathode bias.

I'm beginning to think this is like pushing on a balloon. Push in on one side and the rest just shifts to take up the change. We try to achieve static bias to null the load current through the transformer, but in the process we shift the plate curves and change the gain of the tube. So we are statically matched, but not dynamically matched which results in increased distortion.

If instead we adjust the dynamic matching and leave the bias through the transformer to shift, we effect the B-H curve positionand have a particular effect on low frequency distortion.

It almost requires two mechanisms to (1) adjust static bias current, and (2) adjust the plate curves to match for matched gain and minimum distortion. In class A this is not as critical, but I think that in AB1 it will really cause problems as one tube shifts into B operation while the other is in A operation. Missmatched tube characteristics will result in miss matched gain of opposite phases of the waveform. Thus higher 2nd order harmonic distortion.
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