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 Post subject: 6N7 metal tube preamp
PostPosted: 18 Nov 2012, 07:26 
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A was a given a box of tubes and two happened to be 6N7. These can be purchased on the 'net so if you build the preamp which will be posted here more tubes are available. Take a look at the schematic Matt (once again) has designed. I'm happy to go first for the build. Anyone want to join me??
Attachment:
6N7.jpg

I hope he doesn't mind me posting the hand drawn schematic.

:!: The schematic has been updated. See: 6N7 metal tube preamp schematic. (4 posts down) :!:


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PostPosted: 18 Nov 2012, 10:34 
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Hi Mark, I always liked the metal tubes, but I suspect you will not get the gain you expect. Based on RCA's specs...the absolute max is 30dbv, a probable value would be about half that. I didn't run the numbers, but I suspect this is a case where the sim is not all that close to real world. I find this to be on occasion a bit annoying, but then if this was an exact science then all our projects would be perfect from the git go. Still the tubes could be fun to play with and who really needs 26 dbv anyway. If you use then as a line stage it may be over kill and for a really high gain preamp, under kill. Nice line driver perhaps. Possible good use as a phase inverter for a PP amp. A guitar amp pp drive ...even better unless they are micro phonic.

Good listening
Bruce

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PostPosted: 18 Nov 2012, 12:04 
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@Bruce: Thanks for the comments. Currently I'm using my latest chip amp (SynergyII\Cu) as a power amp so I need very little gain to drive it. I think the chip amp has too much gain but not quite the mellowness I'm use to. The 4S Universal I made to go with it also has too much gain, I can't win. That is why I was interested in the 6SN7 cathode follower as a perfect partner for the cool sounding high gain chip amp. I'll build that later for now I'll give this a go.

Not just that I think the tubes are quirky looking - what better reason to build something with them. Thanks to Matt, I can and the compact PS' I have been building later should drive them no problems.

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PostPosted: 18 Nov 2012, 17:46 
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So... How do I say this without sounding like a complete buffoon? Well, here it goes. I screwed up! Do NOT use the circuit provided above, it is HORRIBLY wrong! :eek:

The load resistor should be 10kΩ NOT 100kΩ. Of course, this changes the entire analysis. I'll post the correct circuit and complete analysis shortly.

Sorry everybody. :blush:

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PostPosted: 18 Nov 2012, 18:34 
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So lets try this again. In my first attempt I was getting an analytical solution that didn't even come close to the graphical one. At first I attributed this to the fact that I graphically determined rp and µ from the plate characteristics. However, this shouldn't have accounted for the magnitude of mismatch I was seeing. Then I finally noticed that I had drawn a 10kΩ load line but used a 100kΩ load in all my calculations. Oops!

Here is the updated preamp schematic with the proper values.
Attachment:
6N7 Schematic.jpg

Here is the associated load line diagram. The blue lines are for the load line analysis and the green are used for the graphical determination of rp and µ. As always, the brown lines are cathode resistor lines.
Attachment:
6N7 Load Line.jpg

One more thing. Remember that this preamp requires one tube per channel. The heaters are 0.8A per tube so make sure you have 1.6A available for the heaters.

Now we can build our preamps.


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PostPosted: 18 Nov 2012, 19:08 
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One more thing. Please note that with a total plate current of 7.6mA, the plate resistor will be dissipating 0.58w. This means that you'll need a 0.6W resistor minimum; 1 1w resistor would be better.

Also note that this circuit is also suitable for the large 7 Pin S shaped type 53 and 6A6. The 6A6 has identical heater requirements. The type 53 has a 2.5v 2A heater. These are old discontinued tubes but there are a bunch of both types available on eBay.

This could be a fun project.

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PostPosted: 18 Nov 2012, 20:40 
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At the risk of incurring Mark or Gio's wrath, I'd like to make a small slightly off topic jaunt. I had an idea... :idea:

If the big heater currents of these tubes are causing you concern, there is another answer. You could use the little 6C5 triode for each channel. They have a heater requirement of 6.3V, 300mA per tube. Here is a simple schematic and the load line design.
Attachment:
6C5 Schematic.jpg
Attachment:
6C5 Load Line.jpg

Lots of these are also available on eBay for not too much money.

Now back to your regularly schedule thread, already in progress.


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PostPosted: 19 Nov 2012, 01:15 
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@Matt: Thanks for the update I was just about to start construction of the preamp section then onto the PS. I calculated Rk wattage requirements but never even thought about Rload wattage requirements. I purchased some more parts at work today to start construction but now will have to mill through boxes of parts to see if I have a 1W 10K R. I have tomorrow off and could have got a good start. I may need a fast trip down to work - lucky only 14mins away.

Also my compact PS will not cope with the filamnet current requirements. This was to be my next investigation but (thanks to Matt) I now have the answer. This preamp will require a fully blown PS. Now do I have the power tranni?

And Matt if you want to go off thread in the name of DIY - knock yourself out!! Thanks again Matt for all your effort. Now to search out some components.

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PostPosted: 19 Nov 2012, 06:53 
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Carbon or metal film??

I have a 10K 1W carbon resistor for the load resistor OR I have 20K 1/2W metal film. I can use one carbon resistor or two metal film resistors. What would you use. Personally I don't mind a little carbon in my amps but are your thoughts.

I have all I need to build the 6N7 preamp.

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PostPosted: 19 Nov 2012, 11:06 
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mwhouston wrote:
Carbon or metal film??
In my amps is the one place where I do everything possible to reduce my carbon footprint. :D I'm a firm believer in metal film and wire wound resistors. They are 30dB quieter than carbon and much more reliable. I'd go with the metal films and never look back.

Carbon is for grilling steaks, not building amps.

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