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Old 18th June 2012, 07:15 PM   #11
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Quote:
main difference being that the heaters of two halves are in series (not center tapped by pin 9 like 12AU7) and consume more current?
Yes the heater current is higher. And only 6,3v possible.

Pin 9 of 6N1P should be grounded. (Or rather may be grounded) You don't have to connect it to anything. But theoretically is should shield the 2 halves better if you have it connected to ground.

Last edited by Bas Horneman; 18th June 2012 at 07:40 PM.
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Old 18th June 2012, 11:31 PM   #12
tuhkam is offline tuhkam  Estonia
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Hi all!

I managed to to raise the voltages a bit by moving the choke between the two electrolytics and changed the R2 to 6.3k and moved it between the last electrolutic and the tiny film cap.
Now the voltages from the PSU are 232V and 226V. It seems difficult to obtain much higher voltages if I leave this high resistance choke in there. (tried also without the choke, way too much mains hum)

I have also installed the 6N1P, I got the impression it sounds better than 12AU7, the frequency range seems a bit extended both ways. Now it appears, my preamp heater winding is a bit weak for 6N1P, gives only 5.8-6.1V depending on the 6N1P specimen. This winding has no specified current, the other one (used for 6P14P) has 3A written on it.

Could I consider heating all three tubes from the 3A winding?


One of my output tubes got bad, it started to distort on bass notes, had to replace it. These tubes likely have decades of TV duty behind them, I can't expect too much.

I should try to find another choke with less resistance, so I could get both voltages in the 250-300V range.

I remember an odd thing from a week ago when I changed the 12AU7 anode resistor values (20k, 33k, 47k, 100k), it had also a big influence to the 6P14P voltage going to OPTs. Of course the bias point of 12AU7 changed accordingly, currently it's around 166V with 6N1P.

Is this normal, or have I missed something?

As time passes I'm getting more and more joy out of this device
Of course safety is vital at these voltages, I armed myself with a capacitor discharge tool a couple of weeks ago.
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Old 19th June 2012, 06:54 AM   #13
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Nice and clean design.
As people said about the PS filter, the voltage of your power transformer calls for C-L-C filtering.From first stage supply to second, an RC filter will do the decoupling, as you already have on the schematic ((R2/C4).

100nF first to second stage capacitor will probably be as good as it gets.

Recommended Grid stopper resistor for EL84 is 10Kohm, your mileage may vary of course.

Pin 9 of 6N1P is just a shield between the 2 sections, you should ground it. You are correct about the heater connectors.

Wire it up and light it up!
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Old 19th June 2012, 09:40 AM   #14
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Quote:
Could I consider heating all three tubes from the 3A winding?
This might be A helpful read........


The Valve Wizard
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Old 19th June 2012, 07:01 PM   #15
tuhkam is offline tuhkam  Estonia
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Soonerorlater, thanks!

That was some good reading. I'm thinking on artificial center tap, or a pot with the wiper as adjustabe center tap instead of the plain AC heater I'm using now.

One thing that seems strange.
The page suggests that heaters specified 6.3V can be run as low as 5V (80%) without signifficant negative consequences.
"Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher."

This picture, also posted somewhere on this forum, encourages rather some overvoltage.
Click the image to open in full size.
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Old 19th June 2012, 07:28 PM   #16
tuhkam is offline tuhkam  Estonia
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Just for the fun if it, here are some photos of this ugly thing.

The right edge of the particleboard was cut to measure by the stongest man in the village using brute force.

The machine is built of electronic waste, except for main power switch, some connectors and a few resistors.
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File Type: jpg IMG_4566.jpg (293.3 KB, 380 views)
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Old 19th June 2012, 10:09 PM   #17
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Originally Posted by tuhkam View Post
Hi all!

I managed to to raise the voltages a bit by moving the choke between the two electrolytics and changed the R2 to 6.3k and moved it between the last electrolutic and the tiny film cap.
Now the voltages from the PSU are 232V and 226V. It seems difficult to obtain much higher voltages if I leave this high resistance choke in there. (tried also without the choke, way too much mains hum)
Hi

Some observations: If I understand right you have 232V before, and 226V after R2. That means that your 6N1P's draw less than 1 mA together. Looking at the data sheet it seems that they shuold be run a 5+ mA to be reasonably linear.

Secondly, if your 500ohms value for the choke is correct and you have 232V after the choke then you are dropping close to 100V (depending on PT regulation) over 500 ohms. That indicates that you are drawing some 100mA per output tube!!!
Think you should check that out.

/Olof
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Old 19th June 2012, 11:31 PM   #18
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tuhkam,
I think (from your post above) that you have miss-interpreted that graph.

It is saying that at 100% of heater voltage you get between 3,00 and 10,000 hours of tube life.

At 85% of heater voltage you get 3 hours tube life.

That is why all of the old text books and application notes specify plus or minus 5% maximum from nominal heater voltage.

For 6.3V rated heaters that means between 6.0 and 6.6 volts.

Cheers,
Ian
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Old 20th June 2012, 08:12 PM   #19
tuhkam is offline tuhkam  Estonia
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Quote:
Originally Posted by hemgjord View Post
Hi

Some observations: If I understand right you have 232V before, and 226V after R2. That means that your 6N1P's draw less than 1 mA together. Looking at the data sheet it seems that they shuold be run a 5+ mA to be reasonably linear.

Secondly, if your 500ohms value for the choke is correct and you have 232V after the choke then you are dropping close to 100V (depending on PT regulation) over 500 ohms. That indicates that you are drawing some 100mA per output tube!!!
Think you should check that out.

/Olof
hemgjord, thanks for taking the time

the choke measures 500 ohm, when measured with a multimeter, I have no idea about the inductance. The resistance seems rather on the high side for a tubeamp choke?
I have another lonely output transformer that might qualify as a choke, that one measures around 350 ohm. Would this be more suitable?

In order to raise current draw of 6N1P should I decrease cathode resistor to 150 to 250 ohm? Or a change to anode resistor value to get to the 5mA?

On the output tubes, would doubling the cathode resistor value bring the current draw down to around 50mA?
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Old 20th June 2012, 08:42 PM   #20
TheGimp is offline TheGimp  United States
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The inductance of the choke is presenting an impedance which swamps it's dc resistance. This is what is causing the large voltage drop across the choke, along with the selected first cap. Increasing the value of the first cap will present less ripple to the inductor and a higher dc voltage resulting in a greater DC out.

Try placing a 2uF cap before the inductor.

Decrease the cathode resistor for the 6N1P and you will increase the current. You may need to change the anode resistor as well as the anode voltage will decrease due to greater voltage drop across the anode resistor at the higher current.

That said, I find that the 6N1P with a cheap red LED in the cathode (1.6-1.8V) works well and has low distortion even at low current. I use a .5-1mA adjustable current source and set the anode voltage around 115V.
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