DCAudioDIY.com

DC Area Audio DIYer's Community
It is currently July 22nd, 2019, 9:04 pm

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 27 posts ]  Go to page Previous  1, 2, 3  Next
Author Message
PostPosted: February 17th, 2019, 10:43 am 
Offline

Joined: July 17th, 2016, 6:24 am
Posts: 600
David McGown wrote:
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


I am no expert but my reading suggests that fixed bias has high 2nd harmonic distortion. Cathode bias reduces the percentage of harmonic distortion at the expense of gain. Bypassing the bias resistor In erases the gain and the harmonic distortion.

Isn’t lower gain with lower distortion better?

_________________
Shashi


Top
 Profile  
 
PostPosted: February 18th, 2019, 1:15 pm 
Offline

Joined: February 24th, 2018, 6:16 pm
Posts: 16
Doesn't self bias require some sort of small cathode resistor to allow actually measuring plate current?

Mark


Top
 Profile  
 
PostPosted: February 18th, 2019, 2:08 pm 
Offline
Site Admin
User avatar

Joined: February 28th, 2013, 10:38 am
Posts: 1108
natandmark2 wrote:
Doesn't self bias require some sort of small cathode resistor to allow actually measuring plate current?

Mark


That's the most common way, but it doesn't have to be done that way. You can measure across any known resistance through which plate current flows, including the OPT primaries.

Roscoe


Top
 Profile  
 
PostPosted: February 18th, 2019, 4:13 pm 
Offline

Joined: February 24th, 2018, 6:16 pm
Posts: 16
That's a good idea. I don't have the equipment for measuring low resistance values with high accuracy, so I could use a small value precision cathode resistor to establish an accurate plate current, measure the corresponding opt primary voltage drop and then remove the resistor.

Mark


Top
 Profile  
 
PostPosted: February 18th, 2019, 5:51 pm 
Offline
Site Admin
User avatar

Joined: February 28th, 2013, 10:38 am
Posts: 1108
natandmark2 wrote:
That's a good idea. I don't have the equipment for measuring low resistance values with high accuracy, so I could use a small value precision cathode resistor to establish an accurate plate current, measure the corresponding opt primary voltage drop and then remove the resistor.

Mark


DCR on an OPT is at least in the 10s of ohms, you ought to be able to measure the resistance accurately. Keep in mind that the voltages are quite high though, so be careful...


Top
 Profile  
 
PostPosted: February 18th, 2019, 6:15 pm 
Offline

Joined: February 24th, 2018, 6:16 pm
Posts: 16
I guess it depends how accurately you want to set plate current bias. My trusty fluke voltmeter will indeed measure 10 ohms or less, but probably at best 5 to 10 % accuracy because of various series resistances that come into play. But if I use a 20 ohm 1% tolerance temporary cathode resister and set my bias at 50 mA I'm measuring 1.0 volts which I can easily measure at about 1% accuracy!

I'm not certain how sensitive output distortion is to the accuracy of the bias setting -- that would be interesting to test. That is, what would happen to output distortion if for example you deliberately set a 5% difference in bias current between the two output tubes (I'm assuming push pull of course) or if you set the bias 5% less than specified.

The high voltages involved are certainly a very significant safety issue -- I'm guessing there aren't many commercial amps that would use such a method for setting bias!

Mark


Top
 Profile  
 
PostPosted: February 18th, 2019, 7:25 pm 
Offline
User avatar

Joined: December 14th, 2013, 2:19 pm
Posts: 710
Attachment:
figure-8-6as7-current-balanced-pp-amp-sch.jpg
figure-8-6as7-current-balanced-pp-amp-sch.jpg [ 498.26 KiB | Viewed 788 times ]


Here is another option if you are going Class A. Adjustable solid state current sinks in the tails of the output cathodes. This allows precise setting of bias current. Note that there are 500 ohm resistors in the tails of the CCS. Measurement of voltage there should be excellent. In addition, even if your meter is off by 50% (!), it wouldn't matter to DC balance, as long as they are the same. Dave Berning has found that even very small imbalances in output primary current cause a great increase in distortion. This ensures that source of distortion is not an issue.

You can switch out any power tube for another and the current will remain exactly the same. What will vary is plate voltage, but that is of little concern so long as the tube isn't defective.

You can, if you like, use a toroidal output transformer, since there is virtually no DC current imbalance to saturate the core.

The downsides:
1. Limited power of a Class A amplifier and the additional heat associated with Class A operation of any type of output device.

2. The capacitor joining the cathodes is critical. That's how the AC signal gets amplified. Otherwise the CCS in the tails would absorb the differences and nothing would be amplified. The best sorts of caps are indicated here. But, the needed caps are much smaller than cathode resistor shunt caps, allowing film types instead of electrolytics.

3. More parts than a simple resistor and shunt capacitor. Complexity.

Having said all that, the bottom line is the sound. We heard this amp, built by Richard Sears, at a club meet a few years back at David M.'s home. To me, it sounded amazing, powerful, liquid, full, pick your terms, it drew me into the room from the kitchen where we were listening to Tubedriver tell us about road racing. So you know it was something. I knew nothing about the design before hearing the amp, but wow!

Stuart


Top
 Profile  
 
PostPosted: February 18th, 2019, 11:07 pm 
Offline

Joined: March 2nd, 2013, 2:43 pm
Posts: 153
Location: Potomac, MD
Traditional self bias requires the amp to draw more or less constant power regardless of the signal level. This implies class A operation. If you try to violate this by driving the tubes harder, the bias voltage will be unstable. Fixed bias allows Class A-B or B, and much greater power output for the same operating conditions for the tubes.

David


Top
 Profile  
 
PostPosted: February 19th, 2019, 3:22 pm 
Offline

Joined: February 24th, 2018, 6:16 pm
Posts: 16
Hmm, well, there are at least a few commercial class a-b amps that at least claim to use self bias -- I think including the McGary Audio amp we heard a while back at a DCHFG meeting.

Mark


Top
 Profile  
 
PostPosted: February 19th, 2019, 8:25 pm 
Offline

Joined: July 17th, 2016, 6:24 am
Posts: 600
dberning wrote:
Traditional self bias requires the amp to draw more or less constant power regardless of the signal level. This implies class A operation. If you try to violate this by driving the tubes harder, the bias voltage will be unstable. Fixed bias allows Class A-B or B, and much greater power output for the same operating conditions for the tubes.

David


David, Can you explain the operational details of Fixed and Cathode bias? My question is specifically centered around the bias topology and how it effects the operational characteristics of the tubes.

For example, the grid voltage can be set to -5V and Cathode 0V which gives -5V grid bias
or
the grid voltage can be held at 0V and the cathode at +5V which again gives -5V grid bias voltage.

What exactly happens in the tube which makes the first one operate in AB mode and later in A mode?

Thx.

_________________
Shashi


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

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: Bing [Bot] and 5 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