Auto-bias - easy to convert to or not?

GlidingDutchman · 11th July 2008, 06:31 AM

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

analog_sa · 11th July 2008, 06:44 AM

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?

GlidingDutchman · 11th July 2008, 07:17 AM

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

analog_sa · 11th July 2008, 08:24 AM

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.

GlidingDutchman · 11th July 2008, 09:02 AM

Here is the schematic.

Driver tube: 6H30
Power tube: 6C33C

D

analog_sa · 11th July 2008, 09:09 AM

Seen this? Not a particularly slim design but a very nice well thought out circuit.

analog_sa · 11th July 2008, 09:11 AM

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.

GlidingDutchman · 11th July 2008, 09:27 AM

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

analog_sa · 11th July 2008, 09:33 AM

Quote:

Originally posted by GlidingDutchman
The circuit is not incomplete and does work.

Whatever you say chief

GlidingDutchman · 11th July 2008, 09:41 AM

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

11th July 2008, 06:31 AM	#1
GlidingDutchman diyAudio Member Join Date: Nov 2007	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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Fixed bias vs. auto bias	HFGuy	Tubes / Valves	30	14th October 2011 09:45 AM
How easy is it to convert from 110V AC to 240V AC?	spikeyfaz	Analog Line Level	19	26th December 2007 07:14 PM
Auto Bias?	lpd	Tubes / Valves	0	18th February 2007 03:33 AM
benifits of auto bias	hacknet	Tubes / Valves	11	26th October 2004 03:04 AM
auto bias on a Quicksilver KT-88 amp	Original Burnedfingers	Tubes / Valves	4	15th February 2003 02:40 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

11th July 2008, 07:17 AM	#3
GlidingDutchman diyAudio Member Join Date: Nov 2007	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

11th July 2008, 08:24 AM	#4
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	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.

11th July 2008, 09:09 AM	#6
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	Seen this? Not a particularly slim design but a very nice well thought out circuit.

11th July 2008, 09:11 AM	#7
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)