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Old 13th December 2004, 02:21 PM   #1
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Question bias arrangements for P-P amps.

Can you tell me the differences between those three bias arrangements?
The bias point is the same but with different resistor arrangement.
I didnít put the bypass cap or caps. Iíd like to know the differences with the circuits bypassed and un-bypassed.

Which from the three is the best and why? At first glance, I thought that these three are the same because the bias is the same but theyíre NOT.

At the second circuit some guys put 10R resistor and some other 33R or other values. Is this value critical or not?
(Of course I know that if you change this value you must change the other resistor to have the same bias point.)
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Old 14th December 2004, 02:31 AM   #2
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Hi,

Quote:
I didnít put the bypass cap or caps.
Well, you should have left them in as that's where the differences lay for some.

a) separate cathode resistors for each tube, better than c) IMO which sports a common resistor for both tubes.

b) I assume the bottom resistors to be capacitively bypassed, the 10R (or higher) separate cathode resistors provide some current feedback. A topology not uncommon in transistorized circuits.

c) The most common and most economical way. Soundwise I don't like it but YMMV.

Another way, there's more than one way to skin the proverbial cat, is to use method b) or a combination of b) and c) where the smaller resistors are adjustable to counteract tube inbalances.

My prefered way is to use fixed bias.
Although it's the more expensive one it does away with the need for bypassing the cathode resistors (recommended) with the never perfect caps and is slightly more efficient and to my ears sounds better too.

Cheers,
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Old 14th December 2004, 02:42 AM   #3
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Circuit (a) provides individual cathode bias for each tube. If one tube stops conducting for any reason, the other tube won't be harmed. Without bypass capacitors, there will be current negative feedback, which will reduce the gain significantly and increase the effective plate impedance by an amount = Rk * (mu+1). With or without bypass capacitors, circuit (a) helps to balance the tubes. Mild class AB1 is possible with circuit (a) provided you use large bypass capacitors. This is probably the best arrangement for cathode biasing OP tubes.

Circuit (b) doesn't make a lot of sense to me. The only reason I can think of for having the 10 ohm resistors in there is to provide a means of measuring the cathode current of each tube independently, for checking the balance. Otherwise, in effect it's the same as circuit (c).

Circuit (c) provides a shared cathode bias for the tubes. If one tube should stop conducting, damage could result to the other tube because it will be under-biased - this is mainly true of output tubes, which are commonly biased close to their maximum dissipation. Circuit (c) doesn't help with tube balancing. Although, in theory, the shared cathode resistor doesn't need bypassing, in practice it may be necessary to improve the sound in cases where the tubes themselves are not exactly balanced.
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Old 14th December 2004, 07:35 PM   #4
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Thank you guys for the theory. I knew it but the reason I ask this question was to hear the comments about tubes balancing.
So the best way for tubes balancing is (a).
I thought it was (b).

Here I have another question.
What do you say about matched pairs that a lot of companies out there sell?
Is it necessary to try to find only matched pairs for better performance?
Suppose we buy a matched pair. Will this matched pair be matched in one or two months? Are tubes getting older the same? Or over here we have another myth?

@fdegrove
I think circuits with self bias have more stability than those with fixed bias.
And are simpler to build, too.
I havenít tried to build with fixed bias, though. So, what do you say? Give it a try in the future?
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Old 15th December 2004, 12:07 AM   #5
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Hi,

Quote:
I think circuits with self bias have more stability than those with fixed bias.
Depends on how you do it but basically I see no reason for less stability with a fixed bias set-up.

Quote:
And are simpler to build, too.
True but I think it gained popularity with most manufacturers because of its reduction in cost....Pennypinchers, ya know.

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So, what do you say? Give it a try in the future?
Why not? Easy enough to try it out for yourself.

Cheers,
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Old 15th December 2004, 02:44 AM   #6
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Matched pairs do not usually stay that way for long but at least they'll tend to stay closer than two tubes which started out differently. How well the matching works depends on who did the matching and what procedure they used. If I'm using tubes which aren't hideously expensive, I'll buy about twice what I need and match them myself at the desired operating point.

Do use cathode bypass caps!

And I've been toying with a new way to do cathode bias- if all goes well this weekend, I might be able to see if it actually works...
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Old 16th December 2004, 09:44 PM   #7
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Default Biasing

Quote:
Originally posted by SY
Matched pairs do not usually stay that way for long but at least they'll tend to stay closer than two tubes which started out differently. How well the matching works depends on who did the matching and what procedure they used. If I'm using tubes which aren't hideously expensive, I'll buy about twice what I need and match them myself at the desired operating point.

Do use cathode bypass caps!

And I've been toying with a new way to do cathode bias- if all goes well this weekend, I might be able to see if it actually works...
Hello ,
The Williamson biasing scheme is a good starting place for cathode bias but for low ra valves I prefer a compound bias scheme using a CCS tail and variable grid bias . Always find it best to use an adjustable system with a single tail that does not use bypass caps , I'd love to try Blackgates but just can't justify the cost ! Everything I build also uses bias meters , very useful for real-time balancing , also if the output stage is biased into class A the meter dancing from side to side indicates clipping . One point to note , if high slope low mu valves are used such as 6C33 , best to use the compound scheme mentioned otherwise HT is being thrown away heating up the cathode bias resistors .

cheers
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Old 17th December 2004, 11:42 AM   #8
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@SY
Quote:
Do use cathode bypass caps!
Of cource,I'll use bypass caps. I draw the schems just to saw you the resistors arrangement.
Quote:
And I've been toying with a new way to do cathode bias- if all goes well this weekend, I might be able to see if it actually works...
Will you post it to diyaudio??????

@316a
Can you post any schem of your output stage?
Itís very interesting using a CCS tail and a variable grid bias!
What kind of CCS are you using?And what kind of variable grid bias?

Quote:
Everything I build also uses bias meters , very useful for real-time balancing , also if the output stage is biased into class A the meter dancing from side to side indicates clipping .
Very cool!
Quote:
I'd love to try Blackgates but just can't justify the cost !
Yes they're too expensive.I'm planning to use NIPPON caps bypassed with Audyn MKP.
But I like very much your idea with CCS tail.
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Old 17th December 2004, 12:01 PM   #9
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I'll post the idea now, since it's possible that my wife will intercept all my efforts to get a few hours of work in ("But, it's SHOPPING weekend!!!"): an array of LEDs. With red LEDs, for example, you get about 1.7V drop. So five in series will have an impedance of something like 25 ohms and a drop of 8.5V, about right for an EL84. But wait! The LEDs are happy at a 20mA current, so four strings in parallel could handle the max current of a pair of EL84s and at the same time, drop the string impedance to roughly 6 ohms.

Now, this is 20 LEDs per pair of tubes. Fortunately, LEDs are small, so this could be built up on a 3cm square piece of perfboard, which isn't much larger than a power resistor. And at about 10 cents apiece in a bulk pack at my local parts emporium, the cost per side is about $2, which is pretty good compared to a non-inductive wirewound and a high quality bypass. Overload performance ought to be spectacular- instant overload recovery with no time constant.

The downside will no doubt be the dazzling light.
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Old 17th December 2004, 02:31 PM   #10
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Default Funny light-bulbs

Quote:
Originally posted by SY
I'll post the idea now, since it's possible that my wife will intercept all my efforts to get a few hours of work in ("But, it's SHOPPING weekend!!!"): an array of LEDs. With red LEDs, for example, you get about 1.7V drop. So five in series will have an impedance of something like 25 ohms and a drop of 8.5V, about right for an EL84. But wait! The LEDs are happy at a 20mA current, so four strings in parallel could handle the max current of a pair of EL84s and at the same time, drop the string impedance to roughly 6 ohms.

Now, this is 20 LEDs per pair of tubes. Fortunately, LEDs are small, so this could be built up on a 3cm square piece of perfboard, which isn't much larger than a power resistor. And at about 10 cents apiece in a bulk pack at my local parts emporium, the cost per side is about $2, which is pretty good compared to a non-inductive wirewound and a high quality bypass. Overload performance ought to be spectacular- instant overload recovery with no time constant.

The downside will no doubt be the dazzling light.
Hello ,
Interesting ! Never even considered LEDs for an output stage .

...but how do the b*ggers sound and measure ?

Dave Slagle mentioned using coloured lightbulbs for cathode bias a while ago on the Audioasylum .

...suppose Christmas tree lights could also be used

cheers
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