DCAudioDIY.com

DC Area Audio DIYer's Community
It is currently July 22nd, 2019, 8:32 pm

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 27 posts ]  Go to page 1, 2, 3  Next
Author Message
 Post subject: Fixed bias vs self bias
PostPosted: February 15th, 2019, 9:15 am 
Offline

Joined: January 15th, 2015, 7:19 am
Posts: 866
Location: Baltimore MD
Just curious what are the advantages/ disadvantages to these two methods.
I have a 6550 family Williamson type amp I am rebuilding and it has a complicated method of fixed bias that I would like to simplify.


Top
 Profile  
 
PostPosted: February 15th, 2019, 10:12 am 
Offline
Site Admin
User avatar

Joined: February 28th, 2013, 10:38 am
Posts: 1108
With fixed bias you can get the same power with a lower B+. Also, if the amp allows individual adjustment for each tube, you can balance the currents in each tube. The advantages of self bias are it's a lot simpler, and you don't have to screw with it...

Roscoe


Top
 Profile  
 
PostPosted: February 15th, 2019, 10:27 am 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 266
To add to Roscoe's comments:

Fixed Bias -
Advantages:
1. Allows adjustment of bias level to a specific plate current. Usually, this can be performed on individual tubes, or a group of tubes with DC balance pot between pairs of tubes.
2. Does not require matched pairs of tubes.
3. Voltage across tube is constant regardless of bias setting. Only adjusting plate current.
4. Lower input impedance into output transformer since there is no cathode resistor in series with plate resistor.
5. More power for a given B+ voltage since voltage is not being wasted across cathode resistor.
6. Allows one to re-bias tubes as they age.
7. Can allow for individual replacement of output tubes upon failure of an tube (just replace and re-bias).
8. Generally, a tighter sound (better bass) due to lower impedance.
Disadvantages:
1. More complicated - as you noted.
2. Failure of bias supply can result in damage to tubes and output transformer due to excessive current (redplate). Bias supply need to be reliable in its design and component selection.
3. Noise in negative bias supply is injected into grid of output tube. Usually not much of an issue, most bias supplies are very low current and consist of a half-wave rectifier with a cap-input filter. However do not want to use a regulated supply since that will upset the bias with any change in B+, and increases complexity (more failure-prone).

Self (Cathode bias)
Advantages:
1. Very simple (cathode resistor)
2. Very reliable - failsafe
Disadvantages:
1. Requires careful matching of tubes. Cannot replace tubes individually upon failure without impacting balance.
2. Cannot adjust bias levels (one can install a rheostat or adjustable wirewound resistor for limited bias adjustment, but impacts both voltage across tube as well as current.
2. Lower power output due to voltage drop across cathode resistor
3. Higher impedance (due to cathode resistor)
4. Cannot adjust tubes as they age (though "Blumlein garters" can help in maintaining balance).

You cannot use self bias and expect to have the same sound if it was originally a fixed bias amp. It a matter of debate on which is preferred.

Just to make thing complicated, a combination of self and fixed bias can be a good compromise arrangement, which provides the failsafe advantage of the self bias with the adjustability of fixed bias.

David


Top
 Profile  
 
PostPosted: February 15th, 2019, 10:50 am 
Offline
User avatar

Joined: July 24th, 2015, 4:17 pm
Posts: 907
Location: Parkville, Maryland
This topic has always generated controversy. Sid Smith (Radiocraftsman and Marantz) preferred cathode bias because it provided better tracking of the music dynamics. General Electric (and later Jadis) liked a combination. Cathode bias to set a starting point together with adjustable fixed bias. You would have to do some research to find out the reason as it escapes my memory. And then a lot depends on the tube type. My Carys have 300B drivers that are set with fixed bias and the 845 output tubes that utilize cathode bias. Maybe ask Dennis Had (Cary) why those choices were made.

With the audio mania that we all suffer from -- fixed bias allows tube rolling. With a design based on 6550s you can play with brands and types. KT-88s, 6CA7s and EL-34s (with the EL-34 suppressor grid tied to the cathode) can work in a 6550 circuit with fixed bias. You have only to adjust for the cathode current that best benefits the tube of choice. With lower plate and screen voltages you can even roll in 6L6s, or KT-66s, or KT-77s, or whatever.

When I had a push-pull amp. it had B+ that enabled me to try them all. KT-88s gave the most bang for the buck, KT-66s (run as triode) the best musical presentation.

_________________
Walt


Top
 Profile  
 
PostPosted: February 15th, 2019, 10:53 am 
Offline
User avatar

Joined: July 24th, 2015, 4:17 pm
Posts: 907
Location: Parkville, Maryland
I almost forgot. With fixed bias you can run with a cathode tied directly to ground for best dynamics. Yes -- there is risk with that method.

_________________
Walt


Top
 Profile  
 
PostPosted: February 15th, 2019, 10:59 am 
Offline
Site Admin
User avatar

Joined: February 28th, 2013, 10:38 am
Posts: 1108
David McGown wrote:
To add to Roscoe's comments:

Fixed Bias -
Advantages:

4. Lower input impedance into output transformer since there is no cathode resistor in series with plate resistor.
8. Generally, a tighter sound (better bass) due to lower impedance.

Self (Cathode bias)

Disadvantages:
3. Higher impedance (due to cathode resistor)

David


All of these are only accurate if the cathode resistor is un-bypassed.

Quote:
Self (Cathode bias)
Disadvantages:
2. Lower power output due to voltage drop across cathode resistor


Only if the B+ remains the same. If you increase the B+ by the bias voltage, you can get the same output.

Roscoe


Top
 Profile  
 
PostPosted: February 15th, 2019, 11:52 am 
Offline
User avatar

Joined: July 24th, 2015, 4:17 pm
Posts: 907
Location: Parkville, Maryland
Quote:
All of these are only accurate if the cathode resistor is un-bypassed.



Cathode bias sets gain as a byproduct of setting idle current. Cathode bypass enables full gain without interfering with the static bias setting.

And as I have discussed here many times, the quality of the bypass capacitor(s) should be considered as a signal-path component with the best possible products used in that application.

_________________
Walt


Top
 Profile  
 
PostPosted: February 16th, 2019, 11:51 am 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 146
Pelliott321 wrote:
Just curious what are the advantages/ disadvantages to these two methods.
I have a 6550 family Williamson type amp I am rebuilding and it has a complicated method of fixed bias that I would like to simplify.


I'm curious as to what is compicated about the bias method. Can you share a schematic?

The Eico HF-60 had a two-step bias arrangement. There was a pot to adjust the bias voltage, and then another pot to balance the two output tubes. It took a lot of fiddling to get the bias just right. ;-)


Top
 Profile  
 
PostPosted: February 16th, 2019, 4:17 pm 
Offline

Joined: January 15th, 2015, 7:19 am
Posts: 866
Location: Baltimore MD
I will provide the schematic on Monday, I'm tied up today and tomorrow. I apologise, I should have provided the schematic in the first post, my bad


Top
 Profile  
 
PostPosted: February 17th, 2019, 9:43 am 
Offline

Joined: July 17th, 2016, 6:24 am
Posts: 600
Good thread.

Here is an online cathode bypass calculator. It shows the FR of the amps for each type tube by changing the bypass capacitor values.
https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

I presume the bypass cap effects the phase just like caps in in-line filter circuits.

_________________
Shashi


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 27 posts ]  Go to page 1, 2, 3  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group