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Old 17th January 2013, 12:03 AM   #1
rrrs is offline rrrs  United States
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Default 211 IT amp, battery vs fixed bias

Hi, been working on this for a while now and got to the point where I am really happy with result and want to “close the box”.
After trying various drivers settled to 2 options, able to switch between; 5687 and 2c51 in parallel; 5687 sounds better to me at higher listening level while I prefer 2c51 at low volume.
5687 gives me a sensitivity of 1.2V on input to get around 6W in class A1 while 2c51 needs about 0.6V on input, so there is 6 db difference.

At this point I just want to finalize the biasing method for the driver stage. Since I am using input transformer any way, there is a possibility to use fixed bias and connect the cathode straight to ground. I worked out two options, one using resistor divider from my negative supply while second one uses “hybrid” method with NiCd 2.4V batteries providing bias voltage while being charged by negative supply.

I would really appreciate opinions on following:
1) Is there real advantage to use battery version (lover AC resistance to ground)?
2) Classic resistor option gives me better flexibility to dial any bias I want as well as better long term reliability, but is there downside?
3) Is there need for first 0.22uF cap that by-passes from bias supply to ground; I found that 1uF on my output stage made a huge difference, improving both, frequency response and distortion, but not sure if that is same with the first stage?
4) One point that concerns me on battery version is that batteries would discharge while amp is not in use and then charge back up once bias supply is turned on. I do switch my heaters and bias supply before the B+, but am not sure how stable voltage would battery in this configuration provide and how long would they last.

I attached schematics for both versions in PDF as they are easier to read that way.
Attached Files
File Type: pdf 211 IT w fixed bias.pdf (93.7 KB, 135 views)
File Type: pdf 211 IT w battery bias.pdf (92.5 KB, 112 views)
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Old 17th January 2013, 05:02 AM   #2
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Why don't you just use cathode bias? I've seen NiCd batteries used in place of the cathode resistor but no where else in the circuit. To be honest I've never seen fixed biased small signal tubes. It's just not practical. Maybe I'm missing something. Why do you want the cathodes connected to ground? What you're doing is unnecessary.
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Old 17th January 2013, 05:12 AM   #3
cotdt is offline cotdt  United States
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Fixed bias sounds better.
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Old 17th January 2013, 05:36 AM   #4
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Have you considered LEDs in the cathode?
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Old 17th January 2013, 06:42 AM   #5
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Quote:
Fixed bias sounds better.

Does it really?
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Old 17th January 2013, 06:49 AM   #6
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Hi!

There are many more factors which contribute to the sound than just the bias scheme. You cannot generally say that one sound better than the other since the whole impelmentation plays a big role. Here my answers/opinions to your questions:

1) Is there real advantage to use battery version (lover AC resistance to ground)?

I see no advantage. It's different. In one case you have a cap in the signal path, in the other case a battery. They are different. One is not necessarily better than the other. If I had the choice I'd prefer a cap.

2) Classic resistor option gives me better flexibility to dial any bias I want as well as better long term reliability, but is there downside?

As has been mentioned above: No real need to use fixed bias. Cathode bias works extremely well and lets the tubes settle at the op point which is best for their emission which is left and the plate voltage. The downside is the bypass cap which is in the signal path which most often is an electrolytic. At your driver I see an electrolytic in the B+ anyways.

I use the ultrapath approach to eliminate the bypass cap from the signal path

3) Is there need for first 0.22uF cap that by-passes from bias supply to ground; I found that 1uF on my output stage made a huge difference, improving both, frequency response and distortion, but not sure if that is same with the first stage?

If your bypass cap makes a big difference, that prooves that the bias supply is in the signal path. Ultrapath with cathode resistor reduces the caps in the signal path down to one (at least for part of the frequency band) and is better controllable IMHO

4) One point that concerns me on battery version is that batteries would discharge while amp is not in use and then charge back up once bias supply is turned on. I do switch my heaters and bias supply before the B+, but am not sure how stable voltage would battery in this configuration provide and how long would they last.

Another argument for cathode bias. Works reliably and adapts automatically as the tubes age. I fussed around with fixed bias too in the path, beeing paranoid about eliminating caps and resistors from teh signal path. Nowadays I am happy with cathode bias

Best regards

Thomas
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Old 17th January 2013, 07:32 AM   #7
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You also have to worry about the high temperatures inside the chassis which may or may not react well with the battery. NiCd batteries are designed to run hot, but what about ambient temps inside? Morgan Jones has a few examples of coin battery biasing in his book, but why bother.
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Old 17th January 2013, 09:40 AM   #8
palmas is offline palmas  Portugal
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950V might be a bit short for 211, also 160V at the driver will not be enough for the swing you need and some headroom.
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Old 17th January 2013, 01:16 PM   #9
euro21 is offline euro21  Hungary
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175V can also be enough.
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Old 17th January 2013, 02:01 PM   #10
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Quote:
Originally Posted by euro21 View Post
175V can also be enough.
LED bias is a bad idea, *unless* the stage is CCS loaded:
- Vf and dynamic impedance change with the signal!
- three green leds in series have rather highish impedance
- does not prevent the tube from running away (an increase in idle current is not throttled by the cathode resistor or anode CCS)
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