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Old 5th September 2011, 06:47 PM   #1
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Default 6n11 + 3886 build questions

Hey all. Let's be frank. I'm a noob. I don't like to be shocked. I don't like things to blow up or even melt. Unless that's the intent, but right now, it isn't.

I purchased and assembled this:

Click the image to open in full size.

Here's the schematic:

6N11+3866 | Flickr - Photo Sharing!

BB won't let me post the pic for some reason.

The power supply that I have has the following secondary outs:

39/0/39: 21.5/0/21.5: 12.5/0/12.5: 3.5/0/3.5

The amp requires 65v and 8v for the tube power and heater. The bridge rectifiers that I installed can handle 800v, 4a. The original specs asked for 200v, 2a.

I'm making a SWAG that the increased voltage to the tube power will result in approx 109v, ish instead of 91v. I'm certainly not an expert on tubes, but it seems that this wouldn't be too much of a problem unless it sends too much output to the input of the chip. Speaking of which, the coupling capacitor that was recommended was a 3.3uf 100v - but the best match that I could find was a 3.3 400v...

The heater voltage I'm not so concerned about because the 12.5v (instead of 8v) goes through a voltage regulator, an LM7806, whose max input is 35v.

The chip voltage will see 21.5v instead of the 25v recommended.

So my questions are:

Will the increased voltage to power the tube entirely screw something up?

How will underpowering the chip affect stuff?

When I wire the transformer to the amp, there are two spaces for the 65v ac input. I am assuming (should I choose to accept the risk) that I can run 39v to each ac input space as there is no positive or negative ac voltage. If I do this, then I would cap off the ct for the trio, right?

If I run the 12.5 to the heater, then I would run one 12.5v and the ct to the two openings and cap off the extra 12.5, right?

Call me a total noob, I'm just trying not to second guess too much and risk melting things....
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Old 5th September 2011, 08:53 PM   #2
ericj is online now ericj  United States
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I wish i had much to add to this, other than to say that i have purchased one of these boards - but not received it yet - and the seller lamely told me that he doesn't have a schematic or BOM.

A few points:

The chip will be happy with the 21.5v, this will actually be safer if you may use 4 ohm speakers.

You could ignore the 12-0-12 secondary and the center tap of the 3.5-0-3.5 and have 7vac for the heater regulator. likely Close Enough assuming there is enough current on the secondary.

If you stay with the 12.5v to the heater regulator you may need a bigger heatsink there.

I don't know the 6n11 but i hear that it is in the 6DJ8 family. The voltage shouldn't be a problem but someone who is better at tube math than i should take a look at the components around the tube to see if they work well for the tube's operating points at that voltage.


Don't suppose you have a higher rez version of the schematic you could send me, maybe a BOM?
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Old 5th September 2011, 10:54 PM   #3
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Ericj, I'd be happy to send a BOM and schematic. I don't see an attachment button for sending them to you via pm. If you'd like, I'd send it to an email. Just PM it to me and I'll get it out to you. I had to convert the PDF to jpg and the prog I used did a remarkably poor job.

I didn't know which forum in which to post this as it is a tube buffered chip amp.

I checked the data sheet for the 6dj8, specifically, and it seems that it can handle more juice. I'm concerned how this will affect the input to the chip.

Right now I think that I'm going to run this transformer just to see if it works and get better transformers for it relatively soon.
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Old 6th September 2011, 12:17 AM   #4
ericj is online now ericj  United States
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well, chinese tubes, like russian tubes, can be a bit of a crapshoot wrt what they are "like". I have no intention of buying an actual 6n11 - I may use a 6dj8 or a 7dj8 or a 6n23p or a 6n1p i already have here. Though I'm guessing that the 6n1p won't perform well at such a low b+.

A lot of people use 6dj8 types in the neighborhood of 100v b+. It can go higher but it's not necessary.

There's conflicting info on what the 6n11 is similar to - some say ecc88, some say ecc85. Very confusing. My guess is that some resistor values might need to be adjusted to make it optimal but it will probably "just work". Assuming same pin assignments. This would be a good reason to post in the tube amp forum - maybe someone there is more familiar with the 6n11.

Some asian audio diy forums have english sections specifically so that they can collaborate with westerners, btw, and singapore forums are typically all english. May be more places out there to look for info.

The higher b+ shouldn't affect input to the chip amp if there is a coupling cap between them.

Will drop you a PM with my email address. Thanks!
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Old 6th September 2011, 01:12 AM   #5
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I used a 6N1P. I didn't trust the 6N11 that I was looking at. I'll see how it works out tomorrow...I'm setting up a test box (literally a box) with the appropriate innards and outtards to hook up a cd player and run it to some speakers. Guess I gotta quit looking at it and see if it will function
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Old 6th September 2011, 01:53 AM   #6
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What i have heard about the 6n1p is that it does not really sing until it's up over 200v b+. Some run it near 300v which requires some additional complexity since you cannot ground the cathode with that voltage on the plate.

I have 6n1p-ev and 6n6p-i in a futterman-style headphone amp and they sound dreamy and buttery smooth, but that's at 250v b+.

But the 6n1p may work fine for you anyway. And at any rate, having a 6dj8 style tube means that there is a vast variety of similar tubes out there.

Anyway, your 6n1p has a 600ma heater. Your 12v secondary will produce about 16vdc after rectification and filtering, so your regulator will have to drop 10 volts and pass a little more than half an amp. It will run HOT. If the 3.5-0-3.5 secondary has enough current, I suggest starting with that.

Don't lose any sleep over having caps rated for a higher voltage. That is never a problem - as long as it physically fits and is the same (or near) uF, the extra voltage tolerance just means it's physically larger and may have a longer lifespan.
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Old 6th September 2011, 10:46 AM   #7
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I see your point.

Ignore the 12.5/0/12.5, use the 3.5 x2 so as not to overtax the regulator.

I have some JBL lx300 speakers - not the most sensitive by any means, but they are the best speakers that I have right now unless I go with a 4 ohm MB Quart 6x8 pair that I haven't installed in the car...I think I'll take a chance with the JBLs and see.

When I run a simulation of the power circuit, it looks like 21.5v ac will produce a supply voltage of around 13.9v and 14.9v. Adding the absolute values of the average yields ~ 29v. The datasheet for LM3886 says that this will produce an output of about 41 watts into an 8 ohm load and much more than necessary for a 4 ohm load to be heard...

As far as the tubes - I'm not familiar with Chinese tubes. I've rolled the tubes in several pieces of musical equipment, generally BK Butler stuff and in a few amps, and I've found Russian tubes are good to my ear without spending a ton on some NOS mullard that I'll never use enough to appreciate. At least in the musical gear. So, I went with the Russian version as a start and I'll change it to see if it makes a difference as things go on.
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Old 6th September 2011, 06:16 PM   #8
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I would be (and am) tempted to ignore the heater rectification and regulation and just run 6vac to wires tacked to the bottom of the tube socket, since i've never had a problem with properly-run AC heaters, but whatever.

Double-check the current ratings for the lower voltage secondaries on your trafo. You may have to use a 7806 and a big heatsink. and maybe a resistor to drop some volts before it gets there.

In any case, keep an eye on it. As long as you are dropping at least 1.2v at the regulator, you are fine. Less than that and the regulator may not be performing well. In which case, it could potentially just be replaced with a power resistor. Heaters aren't tremendously picky.

The thing about 4ohm vs. 8ohm in an lm3886 is that you have to run the 3886 at fewer volts to safely drive 4 ohms. 18-22vac transformers are routinely recommended for 4-ohm lm3886 applications.

As for myself i have a 300VA 25-0-25-0 Antek toroid I plan to use, which means i may not have enough volts on the tube section. I am considering building a voltage doubler into the upper side of the AC section to supply more volts to the tube B+ than to the chip. Since the tube won't be pulling much current this should be fine.
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Old 6th September 2011, 06:40 PM   #9
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Also you're forgetting that AC voltage is measured as RMS, but after rectification and smoothing you will see peak voltage, less diode drops. Multiply by 1.414 and (probably) shave off a volt for diode losses, and you are in the neighborhood of +/-30vdc across the lm3886.

And we are still making assumptions about transformer regulation, the actual voltage at your ac mains, etc. It could easily be a few volts more than 30.

Last edited by ericj; 6th September 2011 at 06:44 PM.
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Old 9th September 2011, 05:19 AM   #10
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So my board finally showed up.

Curious discrepancies between BOM, schematic, board, and datasheet around R1 and L1.

These are the output bits on the chip amp. L1 is there to prevent RF from feeding back into the amp. The resistor in parallel from it is to assure that the output is never unterminated (e.g. when the amp is producing near-RF of it's own for some reason).

Old BOM and new BOM both agree on 0.22uH, but the datasheet recommends 0.7uH. I guess someone felt that 0.7uH cut out too much of the 'air'? probably harmless. Not arguing with it.

For R1 the old BOM says 10Kohm, the new BOM says 0.15ohm, datasheet says 10ohm.

Like i said, this network is about preventing bad things from happening at high frequencies.

I wonder what values are used by gainclones and the like?
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