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Auto-bias - easy to convert to or not?
Auto-bias - easy to convert to or not?
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Old 11th July 2008, 06:31 AM   #1
GlidingDutchman is offline GlidingDutchman  South Africa
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Default Auto-bias - easy to convert to or not?

Hi there

Is auto-bias just another fancy name for fixed bias?

How easily can a "manual bias" system be converted to auto bias?

I am busy remodding a tube amp and to keep with a clean look thought why not convert it to auto-bias.

It is an SE design using a single 6C33C tube.

D
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Old 11th July 2008, 06:44 AM   #2
analog_sa is offline analog_sa  Europe
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Default Re: Auto-bias - easy to convert to or not?

Quote:
Originally posted by GlidingDutchman

Is auto-bias just another fancy name for fixed bias?


No. Highly recommended that you do read some book on tube circuits.

Auto bias simply means cathode bias. Conversion may be easy but depends upon power supply as you need the extra volts that the cathode resistor will drop. Some other parameters change too.

Why do you want to convert?
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Old 11th July 2008, 07:17 AM   #3
GlidingDutchman is offline GlidingDutchman  South Africa
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Why? For sake of ease of use and sleek looks.

I am an old QUAD mkII user and never had to "bias" - it is a new thing for me as I got the SE am as a gift from a friend.

Probably then have to install a small ammeter on the chassis with a selector knob to make it more convinient.

Any suggestions?

D
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Old 11th July 2008, 08:24 AM   #4
analog_sa is offline analog_sa  Europe
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Suggestions? Yes, show a circuit. Is this a diy amp?

The conversion will certainly change the sonics among other things. Are you really looking forward to a cathode cap?

What's wrong with the ammeter? There is also the option to use a window comparator with led indication or even go for a full blown, micro-adjusted automatic bias.
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Old 11th July 2008, 09:02 AM   #5
GlidingDutchman is offline GlidingDutchman  South Africa
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Here is the schematic.

Driver tube: 6H30
Power tube: 6C33C

D
Attached Images
File Type: jpg hobkirk se amp - p12363.jpg (30.5 KB, 483 views)
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Old 11th July 2008, 09:09 AM   #6
analog_sa is offline analog_sa  Europe
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Seen this? Not a particularly slim design but a very nice well thought out circuit.
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Old 11th July 2008, 09:11 AM   #7
analog_sa is offline analog_sa  Europe
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The circuit above is incomplete and won't work. The question is whether the PS can be set for an extra hundred or so volts.
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Old 11th July 2008, 09:27 AM   #8
GlidingDutchman is offline GlidingDutchman  South Africa
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The circuit is not incomplete and does work.

It is a prototype amplifier developed by Alan Hobkirk from ISIS Audio in Capetown.

D

See attached image of amp section - the part of the circuit diagram I attached.

I have also mapped the PSU sections. Four in total.

Heaters
High Tension for output
Bias
and driver section
Attached Images
File Type: jpg 2008_07_10 001s.jpg (32.4 KB, 386 views)
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Old 11th July 2008, 09:33 AM   #9
analog_sa is offline analog_sa  Europe
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Quote:
Originally posted by GlidingDutchman
The circuit is not incomplete and does work.


Whatever you say chief
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Old 11th July 2008, 09:41 AM   #10
GlidingDutchman is offline GlidingDutchman  South Africa
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Quote:
Originally posted by analog_sa



Whatever you say chief
Am I missing something? Yesterday when the amp arrived I turned it on and had a listen - it works and have good gain.

...

D
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