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Old 28th April 2012, 10:11 PM   #1
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Default Broskie auto bias

I am in the process of building an amplifier with toroidal output transformers and had heard that they were more sensitive to DC imbalance. I could adjust bias all the time but I thought it might be better if a circuit just did it for me.

I was researching the subject and came across this blog entry at TubeCad. Folks have mentioned that it has been built and works well.

Anyway, I laid out a PCB for this. Anyone see any problems with how I have done this?

One tube is the master and the other just tries to maintain the same current as the other. I added a relay to put the circuit in a defined state before the HT- supply comes up. I want bias voltage to start out at the negative rail.

Thanks in advance for your comments/critiques.
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Old 30th April 2012, 02:14 PM   #2
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Maybe the weekday crowd has some comments?
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Old 30th April 2012, 04:03 PM   #3
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I missed that excellent article. I always enjoy reading Broskie's work.

I like Blumlein's original circuit. It seems very clever. Broskie's premise is that the original circuit (that seems to perform very well) can be improved with solid state components, but did he really close on that point? If he did, I missed it.

Broskie made this statement: "Blumlein didn’t have access to transistors and OpAmps; we do. (I know that some are already getting nervous. But why not take advantage of a technology that’s cheap, effective, doesn’t wear out, and, most importantly, isn’t in the audio signal path? Anxiety or arrogance? Fashion or foolishness?)". I find the original circuit to be elegant in it's simplicity and with just a pair of cathode resistors rather than a single and a length of wire, it too is cheap, effective and doesn't wear out.

It would be fun to bread board some of those and compare the results.

Last edited by Captn Dave; 30th April 2012 at 04:08 PM.
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Old 30th April 2012, 05:09 PM   #4
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I think the original Blumlein Garter circuit would be an excellent choice for this application. IMNSHO no need for fancy and potentially unreliable solid state circuitry in this application.

I'd use really good quality film caps for the cathode bypass caps.
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Old 30th April 2012, 05:28 PM   #5
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Blumlein garter works really well and sounds excellent. i used it in a headphone amp with toroidal outputs.
The problem with Broskies bias circuits are that they slave one triode to the others bias. It maybe that the one is running very hot and this then forces the other really hot. In the end this maybe fairly unstable and hard on valves.

Broskie discusses this in a subsequent article where he goes back and looks at the garter again - and concludes that it is intrinsically superior because it avaerages the imbalance between the two halves. He then goes onto design a version where one of the resistors is replaced with a transistor which preserves all of the benefits of the circuit without the hit of doubling the bias voltage.

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Old 30th April 2012, 05:48 PM   #6
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Here's the follow-up article - Blumlein’s Garter Circuit Revisited.

One thing that has always bothered me is the value of the two cathode resistors; why not use two in series to make up the necessary value (thereby not doubling the bia voltage)?
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Old 30th April 2012, 06:01 PM   #7
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Quote:
Originally Posted by Captn Dave View Post
I like Blumlein's original circuit. It seems very clever. Broskie's premise is that the original circuit (that seems to perform very well) can be improved with solid state components, but did he really close on that point? If he did, I missed it.
Blumlein's circuit is very elegant in its simplicity and offers pretty good performance, but not as good as the circuits Broskie fleshes out. In the end, gain has to be used to get the imbalance down to near zero. If you missed it, it is all of the stuff after "A Modern Approach to Bias Balance." All of those approaches will give essentially zero imbalance as opposed to the 8.5% imbalance of the Blumlein circuit in the example.

I have an aversion to cathode bias since it allows bias voltage to vary with signal, particularly in a class AB output stage(which is what this is going in). Also, it must be noted the Blumlein's circuit doubles the cathode resistor power dissipation. That's the price paid for the performance increase.
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Old 30th April 2012, 06:13 PM   #8
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Quote:
Originally Posted by kevinkr View Post
I think the original Blumlein Garter circuit would be an excellent choice for this application. IMNSHO no need for fancy and potentially unreliable solid state circuitry in this application.
This is going in an amp with a McIntosh-style Unity Coupled output stage with a source follower driver. I think it would be an awkward choice for this application, fixed bias seems much more natural to me, especially since the negative supply is mandatory for the source followers already.

As far as reliability goes, I'm not asking much of the semiconductors. My guess is they would outlast a hot resistor...
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Old 30th April 2012, 06:14 PM   #9
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The example given is an extreme example - which illustrates how effective it really is. In normal operation, with the same triode on each half, expect the imbalance to be less than 1%. As I said I use it in exactly the scenario you intend to and it works excellently.

I would look at the second implementation of the transistor garter as a compromise which offers the best of both worlds without undue complexity.

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Old 30th April 2012, 06:27 PM   #10
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Quote:
Originally Posted by Shoog View Post
Blumlein garter works really well and sounds excellent. i used it in a headphone amp with toroidal outputs.
The problem with Broskies bias circuits are that they slave one triode to the others bias. It maybe that the one is running very hot and this then forces the other really hot. In the end this maybe fairly unstable and hard on valves.

Broskie discusses this in a subsequent article where he goes back and looks at the garter again - and concludes that it is intrinsically superior because it avaerages the imbalance between the two halves. He then goes onto design a version where one of the resistors is replaced with a transistor which preserves all of the benefits of the circuit without the hit of doubling the bias voltage.

Shoog
That's not really a problem with the design I posted. The bias will be set on the master to a particular value. Insert a KT88, 6L6, EL34, doesn't matter. There should be no 'running hot' as any correctly functioning tube will settle at the same current in the master slot. The slave will match that current.

As far as safety goes(a point Broskie brings up in the garter bias revisited article), I have a separate comparator that will shut the amp down should any output tube draw too much current.
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