I have an old Edison-12 poweramp, with EL84 push-pull output stage, that was originally cathode biased.
I replaced the cathode bias resistor for a 1R - used a small mains transformer and biased the EL84's manually (fixed ?) at 35mA (actually 30mA anode + 5mA screen).
it transformed the amp, from a polite low-power amp, to a lively punchy one.
If you use separate trimmers for each tube, you can balance them accurately.
I replaced the cathode bias resistor for a 1R - used a small mains transformer and biased the EL84's manually (fixed ?) at 35mA (actually 30mA anode + 5mA screen).
it transformed the amp, from a polite low-power amp, to a lively punchy one.
If you use separate trimmers for each tube, you can balance them accurately.
Dave Gillespie's version of "enhanced fixed bias". ttp://www.audiokarma.org/forums/showth ... p?t=466229
Dave's original EFB article is located here:
http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf
and a simple implementation of it, with listening impressions, is located here:
Article: Installation of the EFB(tm) Bias Circuit in a Dynaco ST-35,
and a more involved implementation:
Power Supply Mods for a Dynaco ST-35
http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf
and a simple implementation of it, with listening impressions, is located here:
Article: Installation of the EFB(tm) Bias Circuit in a Dynaco ST-35,
and a more involved implementation:
Power Supply Mods for a Dynaco ST-35
EFB is better known as tracking bias. It's an old if little-used idea. A circuit is arranged so that if the B+ rises, the bias voltage increases (more negative) proportionately, and vice versa, to maintain a more consistent operating point. It's the same concept as power scaling well known to guitar amp builders. I also coined the term ayalodyne to denote the proportional tracking of electrode voltages.
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Well, I modified an old cary SLM-100 mono blocks auto-bias to manual bias and hat a better over all sound...
after this modification, a more direct clear sound was audible (listen to jazz & classic using magnepan 3.6 speakers)...
Just imagine the large electrolyte cap over the cathode
While the cap gives a large absorption energies, just my point...
Plan careful for trim pot failure, while this will cost you some power tubes or may have a fuse on the anode voltage as almost all cary's have)
Hp
Depends on your design philosophy. Auto bias has the advantage of requiring no user intervention. Fixed bias isn't so user friendly since it requires periodic monitoring and adjustment. The various forms of servo bias can help reduce the need for user management at the cost of added complexity and inclusion of the dreaded Sand Devil.
Your choice. I opted for fixed bias in my designs since it gives better sonic performance.
YMMV
User Friendly ?
Last time I checked this was "Do It Yourself Audio"
We want user intervention, user friendly is a No No!
+1 on fixed bias being better sounding (for HiFi).
How about reliability? Drifting op point because of temp variation. Injection of noise via heaters?
Not me! The DIY part is in the design and construction, not the maintenance. I want it to work forever without me having to open it up and find out where I went wrong!
1. With non-regulated B+ and non-regulated fixed - bias which are generated from the same HV winding:
When the Mains voltage increases, both the B+ increases, and the fixed bias increases.
When the Mains voltage decreases, both the B+ decreases, and the fixed bias decreases.
That means that the tube current remains relatively constant, even though the mains voltage changes.
2. If you regulate Either the B+ Or regulate the fixed bias, the tube current changes when the mains voltage changes.
In that case, you should regulate Both B+ and fixed bias supplies.
Someone else needs to answer about EFB, I have never used that.
3. in any situation, when the mains voltage is at its maximum, make sure that the tube current, tube voltage, plate dissipation, and screen dissipation are all within the tubes maximum specifications.
(Mains voltage varies from day to day, and hour to hour).
4. I have used adjustable fixed bias, battery bias, and self bias.
Self bias is my favorite.
For any parallel tubes, use individual bias (individual pots for fixed bias; and individual parallel RC for self bias).
The same recommendation of individual bias goes for non-parallel tubes in push pull (a single 'push' tube and a single 'pull' tube).
Push pull transformers "love" balanced push and pull current.
5. Self bias tends to have some auto regulation of tube current, even if the B+ is not regulated. More B+ causes more self bias, which makes the
tube current relatively constant.
6. Tube rolling?
Individual self bias makes it really easy.
The above are all just my opinion.
EL84 tubes:
Happy building and good sounds for listening.
When the Mains voltage increases, both the B+ increases, and the fixed bias increases.
When the Mains voltage decreases, both the B+ decreases, and the fixed bias decreases.
That means that the tube current remains relatively constant, even though the mains voltage changes.
2. If you regulate Either the B+ Or regulate the fixed bias, the tube current changes when the mains voltage changes.
In that case, you should regulate Both B+ and fixed bias supplies.
Someone else needs to answer about EFB, I have never used that.
3. in any situation, when the mains voltage is at its maximum, make sure that the tube current, tube voltage, plate dissipation, and screen dissipation are all within the tubes maximum specifications.
(Mains voltage varies from day to day, and hour to hour).
4. I have used adjustable fixed bias, battery bias, and self bias.
Self bias is my favorite.
For any parallel tubes, use individual bias (individual pots for fixed bias; and individual parallel RC for self bias).
The same recommendation of individual bias goes for non-parallel tubes in push pull (a single 'push' tube and a single 'pull' tube).
Push pull transformers "love" balanced push and pull current.
5. Self bias tends to have some auto regulation of tube current, even if the B+ is not regulated. More B+ causes more self bias, which makes the
tube current relatively constant.
6. Tube rolling?
Individual self bias makes it really easy.
The above are all just my opinion.
EL84 tubes:
Happy building and good sounds for listening.
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No, both I and V increase, and about in the same proportion, which IS what we want. We want V/I related to load impedance.
Yes, with both V and I rising with line voltage, the dissipation rises as square of voltage. Just like with ANY rational load. (Incandescent lamps, and most LED lamps, are irrational loads.)
Elsewhere a smart designer spent many words describing a bias which reduced current as voltage rose. OK, if you want a fixed-power heater. But meanwhile the audio power available reduced either-side (low OR high) of design line voltage, and faster than voltage changed. A good cure for a non-problem.
But yes, you have clearly outlined why a Zener in the bias (only) is probably a sign of a too-young (low-bid) designer.
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