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Old 18th December 2013, 08:41 PM   #21
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Quote:
Originally Posted by DF96 View Post
Difficult to comment on a moving target. Show us your current circuit.

As I said, the SRPP has too much gain. The unbalanced SRPP (with better PSRR) has even more gain. You can't improve the wrong circuit; you have to change the circuit.
Yes, shortly i will. Just alot of other work

Is it possible to use this tubes together with another circuit? I have not seen any other circuit than a srpp with 6n3 (or more tubes) I will try one if you have one. If it's lower gain then ofcourse just better. I like it simple



I just have little or no experience in tubes/circuits as you can read.

BUT i can solder and and understand pins and so on so if i have a schematic written i can probably make it work. (so why did i buy this crap hehe

Last edited by Max Martin; 18th December 2013 at 08:45 PM.
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Old 18th December 2013, 08:42 PM   #22
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Originally Posted by Max Martin View Post
Also Russian tubes arrived today. 2x 6n3p-EB and one 6X4-R. From first note there was a big difference. All kinds of instruments sounds much more "true" and everything is so clear. Really impressed just after couple of hours compared to the chinese.
That's seems to be the general consensus for users rolling in NOS Russian tubes.

jeff
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Old 19th December 2013, 12:04 PM   #23
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Originally Posted by cjkpkg View Post
Is the heater voltage referenced to B+/4?
Max,

You should not get hum (from the heater at least) in your circuit. Heaters have been operated with 6,3V a.c. before with inputs of 2 mV, but correct balancing/centre-tapping and routing of leads are necessary.

What Cjpkg was referring to above is the matter of heater-cathode voltage. If the heater supply is earthed, the top tube cathode sits at about +80 - +100V with regard to the heater. While this is in order spec-wise, it allows for the cathode to get leakage electrons from the heater (inside the cathode tube, operating like a diode). If the heater supply is ac, such leakage will definitely cause hum.

It is therefore better and also common practice to operate the heater at a d.c. voltage higher than that of any cathode, so as to cut-off any diode-operation. In your circuit that would mean at least some 120V positive, as the top cathode sits at some 100V above earth. This is dangerous as the maximum neg. heater-cathode rating (100V) is then exceeded in the bottom triode.

The best you can do is to try connecting the heater supply to a resistor tapping of about +80V above earth (bypassed to earth). This is especially necessary when operating heaters from 6V.a.c. as explained above. Occasionally one gets a particularly 'leaking' tube between heater and cathode, which must then be discarded or used elsewhere.
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Old 19th December 2013, 01:49 PM   #24
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Originally Posted by Johan Potgieter View Post
Max,

You should not get hum (from the heater at least) in your circuit. Heaters have been operated with 6,3V a.c. before with inputs of 2 mV, but correct balancing/centre-tapping and routing of leads are necessary.

What Cjpkg was referring to above is the matter of heater-cathode voltage. If the heater supply is earthed, the top tube cathode sits at about +80 - +100V with regard to the heater. While this is in order spec-wise, it allows for the cathode to get leakage electrons from the heater (inside the cathode tube, operating like a diode). If the heater supply is ac, such leakage will definitely cause hum.

It is therefore better and also common practice to operate the heater at a d.c. voltage higher than that of any cathode, so as to cut-off any diode-operation. In your circuit that would mean at least some 120V positive, as the top cathode sits at some 100V above earth. This is dangerous as the maximum neg. heater-cathode rating (100V) is then exceeded in the bottom triode.

The best you can do is to try connecting the heater supply to a resistor tapping of about +80V above earth (bypassed to earth). This is especially necessary when operating heaters from 6V.a.c. as explained above. Occasionally one gets a particularly 'leaking' tube between heater and cathode, which must then be discarded or used elsewhere.
Hallo Johan, nice name. It's my real name too =)

Now i learned something. Very useful info for me. I was just thinking about the grounding cuz when i'm connecting "via the board" the heater automatic gets earthed together with the rest (caps and so on) but now with ac i don't have any ground at all. Just direct from transformer to heaters.
If i would like to bypass to earth, what would your suggesting be in terms of resistance? what is normal? 10ohm or 10K? I really don't know how to calculate that.

If i would like to have DC i clearly need to buy a new transformer that delivers maybe 8-9V so i can get proper regulation and voltage out from this L7806.

Here is little updated schematic how it looks today:
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Old 19th December 2013, 04:11 PM   #25
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Regarding the sound i feel that low end is not so strong. To big /small decoupling cap? Can i go up to 3,3 or 4,7?
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Old 19th December 2013, 07:58 PM   #26
DF96 is offline DF96  England
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Some datasheets recommend no more than 20k resistance between cathode and heater, with exceptions for cathode followers and LTPs.

Nothing inside a valve envelope should be left to 'float', as it could pick up stray electrons and go negative or emit electrons (thermally or via secondary emission) and go positive.
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Old 19th December 2013, 09:10 PM   #27
cjkpkg is offline cjkpkg  United States
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just run 2 300 ohm resistors from your heater AC terminals at the transformer and then tie the centers either to ground or to a voltage divider from your B+ with something like a 300K on top and a 100K on the bottom to ground.

Click the image to open in full size.
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Old 19th December 2013, 11:02 PM   #28
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Seems legit.
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Old 20th December 2013, 11:06 AM   #29
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Ok thanks a bunch. I will try that first one with two 330ohms. Nearest value i have and see and hear. Later i try also the second you wrote.

Nice, thx
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Old 20th December 2013, 01:20 PM   #30
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Quote:
Originally Posted by cjkpkg View Post
just run 2 300 ohm resistors from your heater AC terminals at the transformer and then tie the centers either to ground or to a voltage divider from your B+ with something like a 300K on top and a 100K on the bottom to ground.

Click the image to open in full size.
One thought. How about the heater wiring to the tube rectifier? What i can see it's not "grounded" either before intering 6u4.. Look at the pic. Maybe it's possible to do the same on that 6.3v from transformer? (transformer have two independant 6,3V)
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