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Old 24th October 2005, 03:17 PM   #1
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Default 6C33 fixed/auto bias ?

In p-p audio power amps ....Do 6C33 power tubes behave better using cathode (auto) bias or fixed bias ? Despite my vendor matching these tubes (understandably not keen on the idea); over time I' finding ther'e gradually drifting out...of characteristics. I've heard this is a common problem (is it ??) even though the tubes is still relatively new.......I've tried using cathode bias to
equalise which helps thd at lower pout........but when compared to 88's and 34's in fixed bias these are way more reliable over time.

This 6C33 tube was designed bomb proof....so whats on the move ?

replies welcome.
richj
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Old 24th October 2005, 04:40 PM   #2
protos is offline protos  Greece
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I am designing a 6c33 amp but I haven´t played with them yet.
From all the info I have picked up on them
1.High gm means diffcult to match tubes and they tend to drift.
2.Experienced users suggest a long break in time first with heaters and then powered up.Then they stabilise.
3.Cathode bias is safer of course.However depending on bias level you must get alot of dissipation on the cathode resistor.eg 200ma x 70v= 14W.So you really need a big aluclad heatsinked resistor there ideally.
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Old 24th October 2005, 04:54 PM   #3
dhaen is offline dhaen  Europe
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I think they probably are bomb-proof in their original application - power regulators...
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Old 24th October 2005, 04:57 PM   #4
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Quote:
Originally posted by protos
200ma x 70v= 14W.So you really need a big aluclad heatsinked resistor there ideally.
Why not make a current source such as LM317 with a large resistor to dump this current into; the LM317 is a floating device so with 300 ohms load this should work OK; the voltage setting resistors can easily be calculated from the formula's.
With 2000 muF on the input and maybe output this works fine.
I have had it running (on EL34's at 50 mA without a sink for years; here of course yes the resistor and regulator must be heatsinked.
I have found it makes a really rich sound - basically because absolute current matching makes the output trannie behave better.
success albert
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Old 25th October 2005, 03:34 PM   #5
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Quote:
over time I' finding ther'e gradually drifting out...of characteristics. I've heard this is a common problem (is it ??) even though the tubes is still relatively new......
In my experience it is only a problem if the anode dissipation is close to max and if the tubes has not been burned in. I have used more than 30 6C33C and never had a problem with drift, some tubes are used for more than 5 years.

Anode dissipation shoul be less than 40W and the burn in procedure is: heater only for at least 1 hour, follow that by burn in with at least the same current as is used in the circuit and the same or higher anode dissipation for at least one hour. Heater must always be switched on 30 seconds before anode voltage.

Regards Hans
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Old 25th October 2005, 04:31 PM   #6
protos is offline protos  Greece
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Quote:
Originally posted by triode_al


Why not make a current source such as LM317 with a large resistor to dump this current into; albert
Are you suggesting a ccs on top of the output tube acting as a load?
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Old 31st October 2005, 02:46 PM   #7
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Quote:
Originally posted by protos


Are you suggesting a ccs on top of the output tube acting as a load?
In fact I use it as a variable cathode resistor. This will enerate the bias; it can be combined if you like with a (large) bit of fixed bias (e.g. 50V), if you like, then the CCS will generate less heat and get less Vtotal (in this case 70V-50V=20V left over to maintain in a tight grip.

I hope this diagram (sorry, only thing I have is Powerpoint) is clear.
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Old 31st October 2005, 02:48 PM   #8
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I see the picture (JPEG) doesn't show up.
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Old 31st October 2005, 03:55 PM   #9
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As for me, I have some thought about using of 6C33C:

1) NEVER use it with fixed bias.
2) NO SENCE to use it with low plate dissipation or one heater - there a a lot of another tubes to get less power.
3) If you want output power as much as possible (15-18W) - use it with 200mA current and 270-300v anode voltage.
But if you want the better sound with a little bit less power (11-13W) - use it with 280-300mA and 200-220v at the plate.

So, we MUST implement autobias in any case. This can be done in a few ways:
1) traditional way - cathode resistor with capacitor.
You need a powerful resistor at the heatsink (P=~20-22W) and el.cap.
2) MOSFET at the cathode - the same power but no El.Cap.
3) Small cathode resistor (current sensor) and servo loop to the grid with OP, mossfet pt another tube(s).

But there is another way, that I'm working now (exaclty - output stage is ready, and I'm working with the driver/input stage).
We call this method "autofix", becouse it has all advantages of the autobias and also a fixed bias, and some additional features.

In a few words - we put autobias resistor after the rectifier bridge, before El.Cap at the plate power supply. Then - through diode we get the negative voltage from the point where this resistor is connected to the bridge.
After - we put RC-filter and get the negative voltage that depend of the lamp current? even highet, so I put the trimmer and obtained the posibility to control the lamp's current.
Cathode of the 6C33c is connected to the "-" of the power supply El.Cap - this is the ground.

Advantages:
1) no resistor, no capacitor athe cathode. Cathode is sitting on the ground.
2) Power supply El.Cap charged from the rectifier through the resistor - less peak current.
3) the value of the bias resistor is 1/2 of the traditional autobias resistor - > half of the heat dissipation.
4) Possibility to change the bias with trimmer.

Preliminary schematics: http://altor.sytes.net/audio/se33/se33.gif

(caution: this file is changed time to time, due to development process, and may have some not critical mistakes in values, etc., but the idea - should be clear).
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Old 31st October 2005, 04:36 PM   #10
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Quote:
NEVER use it with fixed bias.
What do you base this on? It works perfectly with fixed bias!

Quote:
If you want output power as much as possible (15-18W) - use it with 200mA current and 270-300v anode voltage. But if you want the better sound with a little bit less power (11-13W) - use it with 280-300mA and 200-220v at the plate.
Sounds very inefficient, have you measured output power at these settings?

In my SET I used 220V and 200mA and it gave a genuine 13W output power using 600 ohm anode load. It is an amplifier designed by Dakesue of San-ei which is the first who designed an amplifier with 6C33 outside the eastern block. He and many others believes that the 6C33C sounds better at around 200V and should not be used at as much as 300V. If you use a 6C33C at 60W anode dissipation it will drift, with ~40-45W max it will never drift, (assuming it is burned in correctly).

Regards Hans
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