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Question on the B- power for fixed bias

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Posted on April 24, 2015 at 02:25:52
howardyu
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Joined: August 9, 2006



I am thinking of DIY a an EL12/EL156 PP amp but have a question on providing B- for the fixed bias.

The following schematic is refferred. Is there a need to build one more set of B- for the other side of a steroe amp? or sharing a single set of B- is fine?

 

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RE: Question on the B- power for fixed bias, posted on April 24, 2015 at 04:54:52
Chip647
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 The bias voltage has very little current being drawn against it. You can share the supply, just add another pot.

 

RE: Question on the B- power for fixed bias, posted on April 24, 2015 at 08:57:44
drlowmu
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 Not addressing your original Q, but make the rectification a full wave bridge, and NOT ever a single diode !! The fixed bias supply voltage is on the control grid of the finals, and needs to be squeeky clean in ripple !!

Have fun.

Jeff Medwin

 

RE: Question on the B- power for fixed bias, posted on April 24, 2015 at 10:27:47
hifipaul
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Use a full wave bridge as Jeff suggests (if possible), and use soft recovery or schotky diodes. A 3 or 4 stage filter, C-R-C-R-C-POT, is called for; ie, well filtered. Use 4 pots, 1 for each tube, so that you can bias the tubes individually (this will require a change from the above schematic). Add a 1 ohm resistor to the cathode of each tube so that you can easily measure tube bias.

 

RE: Question on the B- power for fixed bias, posted on April 24, 2015 at 10:50:47
Triode_Kingdom
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 The same supply can feed both channels. Don't bother with a Schottky or redesigning as a full-wave unless you have time/parts to kill. The half-wave works flawlessly.

 

RE: Question on the B- power for fixed bias, posted on April 24, 2015 at 13:57:39
truep
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 As I recall, that's the "C" supply.

 

RE: Question on the B- power for fixed bias, posted on April 24, 2015 at 14:39:50
Michael Samra
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 Paul
I agree 100% with the full wave supply with Schotkys and higher quality capacitors.So many people engineers included used to discard doing anything with the C minus supply because they felt it didn't draw enough current to warrant anything other than a single diode and a couple caps separated by a resistor. All one has to do is put their scope on the C minus supply source with that simple scheme and you will see audio riding on the rail and being that audio is an AC voltage,certain audio frequencies can get filtered out in a poorly designed C minus supply.I've even contemplated putting a small choke in the C minus line.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Correct..., posted on April 24, 2015 at 14:50:50
Tre'
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 The "A" battery was for the filaments.

The "B" battery was for the high voltage (plate supply). We still use the term, B+.

The "C" battery was for the negative bias supply.

Tre'

Have Fun and Enjoy the Music
"Still Working the Problem"

 

Audio on the bias, posted on April 24, 2015 at 20:13:56
Triode_Kingdom
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 "All one has to do is put their scope on the C minus supply source with that simple scheme and you will see audio riding on the rail "

Where would that be coming from? Even without the bias supply output filter conducting the AC to ground - something it's *very* good at - the differential audio signals from the phase splitter should sum to near-zero on the bias rail. If the tiny amount of remaining signal is bothersome (not to be confused with "audible"), a CCS in series with the bias supply or a better phase splitter might be in order. Installing a FW rectifier and Schottkys won't do a thing.

 

I have a different question about the circuit, posted on April 24, 2015 at 20:47:45
Triode_Kingdom
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 I think the OP's question has been answered by the posts below. So, at the risk of being accused of hijacking the thread...

I've probably looked at circuits like this a thousand times, but it never occurred to me to wonder why the designer installed two resistors across the coupling caps. I think I just figured it out. I wonder if anyone else knows. :)




 

RE: I have a different question about the circuit, posted on April 24, 2015 at 22:31:09
elblanco
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 Loop

 

RE: Question on the B- power for fixed bias, posted on April 25, 2015 at 05:02:56
hifipaul
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An tech that I know explained that any ripple, or noise, in the bias supply manifests itself as intermodulation distortion. This may be why some people like self bias so much.

 

RE: I have a different question about the circuit, posted on April 25, 2015 at 07:00:11
Tre'
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 According to an article I read once by Kairos and Haffler, about a 6550 Williamson circuit adaptation of theirs, the resistors help in coupling the bass frequency.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Question on the B- power for fixed bias, posted on April 25, 2015 at 07:13:35
howardyu
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 Thank you for your correction. That is "C" supply.

 

RE: Question on the B- power for fixed bias, posted on April 25, 2015 at 07:23:55
howardyu
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Posts: 61
Joined: August 9, 2006
.......Use 4 pots, 1 for each tube, so that you can bias the tubes individually (this will require a change from the above schematic).....

………………………………………………………………………

Thanks to replies.

I have a further question on the reply in above. What is the reason for using 4 pots, 1 for each tube? Are there something else affectting the bias?

 

RE: I have a different question about the circuit, posted on April 25, 2015 at 08:23:24
Triode_Kingdom
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 "the resistors help in coupling the bass frequency."

I simulated this to see if that's true. Not only does the 470K not improve bass, there's a very slight degradation. My theory on why the resistors are there actually has nothing to do with frequency response.








 

RE: I have a different question about the circuit, posted on April 25, 2015 at 09:01:25
Steve O
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 Typically this circuit provides a response step function at subsonic frequencies for improved subsonic stability under closed loop conditions (controlled phase shift).
It may also reduce blocking under overdrive conditions but there are better means of achieving this.

I would think a truly revelatory simulation needs to include both real-world OPT characteristics AND the closed loop.

 

RE: Question on the B- power for fixed bias, posted on April 25, 2015 at 09:13:07
hifipaul
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Even matched tubes will vary a bit in bias voltage + they drift over time. I found it best to be able to bias every tube individually.

 

RE: I have a different question about the circuit, posted on April 25, 2015 at 09:34:39
Tre'
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 The article I read is linked below,

"The output stage is coupled to the driver stage through a resistive capacity network which provides conventional RC coupling at signal frequencies and attenuated direct coupling at subsonic frequencies.

This again introduces an improved low frequency phase characteristic which adds to the stability margin of the amplifier."

Question, does your theory have anything to do with the charging and discharging of the coupling cap and the so called "blocking" that that can cause?

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

+1 (nt), posted on April 25, 2015 at 09:40:03
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
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 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Audio on the bias, posted on April 25, 2015 at 09:49:12
Michael Samra
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 Jack
What I was referring to was when you put the scope on the bias supply source while music is playing,you can sometimes see audio riding on the C- rail even tho it may be little..Since the C- rail is joined to the output side of the output coupler thru a 270k grid leak resistor,that's where I was able to see audio.The caps were old obviously and I realize it's not a lot of audio but it was there and even tho it's separated with a grid leak resistor to the last filter cap in the C- supply,it makes you wonder how much of the audio signal may getting filtered out by that cap when it's good.
The only reason it made me take more interest in the C- rail is when I had a Citation 2 I was working on that had an open bias winding and Jim just had me put small toroid in its place with a bridge and when I did that,it was made a sonic improvement over my Citation 2s that were built the same way. After that I just put that little toroid in all four of my Citation 2s even tho the bias winding was good..I figure a well executed C- supply with a lower impedance may have a positive effect? Then again maybe I'm over analyzing this.It won't be the first time.LOL
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Yes, I think you're both correct, posted on April 25, 2015 at 11:18:14
Triode_Kingdom
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 I simmed it again, this time down to 0.1 Hz. Phase is much better controlled below 10 Hz with the 470k resistors, and especially below 1 Hz. I also see that this would only be important in a system with significant NFB, where the amplitude of subsonic frequencies is greater. Otherwise, with no loop and no parallel resistor, signals below 1 Hz are so severely attenuated, it doesn't much matter what the phase is. In addition, without NFB, no instability can result from this.

OK, that tears my theory to shreds. I was thinking about what happens if the bias supply fails. The 470K resistors would bias the tubes on hard, instantly blowing the cathode fuses and protecting the outputs. I should have known the designer(s) weren't focused on that. The fuses would probably blow the instant the supply fails anyway. :)






 

RE: Audio on the bias, posted on April 25, 2015 at 11:39:53
Triode_Kingdom
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Location: Central Texas
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 "it makes you wonder how much of the audio signal may getting filtered out by that cap when it's good."

In terms of grid signal, I don't see the difference between grounding a tiny portion through the C- rail filter, or connecting the grid resistor directly to ground. In any event, it's the C- filter that the grid resistor sees, not the rectifier ahead of it. Maybe you're hearing a reduction in IM due to more effective filtering at 120 Hz. Even with a half-wave rectifier though, ripple should be really tiny. Difficult to imagine how it could produce audible levels of IM unless the filter is defective...

 

RE: Audio on the bias, posted on April 25, 2015 at 19:23:21
Michael Samra
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 In terms of grid signal, I don't see the difference between grounding a tiny portion through the C- rail filter, or connecting the grid resistor directly to ground.

I agree with you because the grid leak resistor sees AC ground when it hooks to the C- supply but to be honest,I only saw the audio on a couple dyna MK3s and one other fixed bias amp. As far as effective filtering goes that would make sense but being there is so little current involved how effective does it have to be?..Mcintosh used a single 10uf cap in their C- rail but for some reason the the full wave bridge with the little toroid seemed so effective.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Audio on the bias, posted on April 25, 2015 at 20:28:08
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 "it makes you wonder how much of the audio signal may [be] getting filtered out by that cap when it's good [and with a large enough value to have a low reactance down to the lowest frequency of interest WRT the AC signal]."

None. The better the cap the more it acts like a piece of wire and the grid resistor will act like it IS grounded.

When the cap is bad, or it's reactance is too high, then the signal to the grid of the tube will be altered by the cap. Not the other way around.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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