John Broskie's Guide to Tube Circuit Analysis & Design

05 October 2006

Genius Grants

Radically Different
Voltage Regulator


Stepped Attenuator Resistor Values

In giving my review of Morgan Jones’s new book, Building Valve Amplifiers, I have somehow given the impression to several readers that I sell his book(s) at my Yahoo! Store. I do not; in fact, I do not sell any books. Why not? The sad fact is that I am not even smart enough provide one of those paying links to Amazon.com, which provides a small kickback every time someone follows the link to Amazon's site and buys a book. Oh well. Speaking of not being intelligent enough, a good friend of my mine, Glenn, called the other day to offer solace for our not receiving a MacArthur Foundation (Genius) grant—once again.

"The MacArthur Fellows Program awards unrestricted fellowships to talented individuals who have shown extraordinary originality and dedication in their creative pursuits and a marked capacity for self-direction. There are three criteria for selection of Fellows: exceptional creativity, promise for important future advances based on a track record of significant accomplishment, and potential for the fellowship to facilitate subsequent creative work."

This has been a running gag between Glenn and me for almost two decades, as we both have an immense, but unprofitable, yet important project within us. His is less practical than mine and is more noble because of it. Whereas Glenn seeks to reconcile and unify seemingly disparate and opposed theological/metaphysical systems, I just wish to map out as many of the unexplored territories of tube circuit topology as I can. Neither project is a money maker; both projects require a huge investment of time and effort.

The age of gentlemen scholars is, alas, long dead. So, instead, we both trickle out our efforts, as a full frontal assault is simply impossible in our present situations. But what made being sidestepped this year by the folks at the MacAthur Foundation actually a tad bit poignant was that my wife actually knows one of this year’s grant recipients, which suddenly made having been chosen seem almost possible, although still not at all likely; remember, tube audio is only valued by 0.0000001% of the world’s population. (Besides, the iPod is deemed, by most, to deliver perfect sound, so why bother with tubes?)

Still, I can dream, can’t I? If you are wondering what I would do with the grant money, the answer is easy: I would hire a fulltime nanny, build a soundproof workroom, buy an Audio Precision distortion analyzer, and hire some college kids to assemble about thirty of my circuit designs that I would otherwise never get to build. Once assembled, I would thoroughly test the circuits and publish the results here, as an open-source an endeavor as can be imagined. What a beautiful dream, particularly, the part about a soundproof workroom (indeed, you know it’s bad, when you find yourself envying those in solitary confinement).

Well, enough sniveling. The following design concept is an example of one of those unprofitable, yet important projects. The premise is that I love tube-based voltage regulators, with just cause. Much in the same way as tube-based amplifiers sound better than their solid-state cousins do, so too do tube-based voltage regulators sound better than their solid-state equivalents. Most readers are not about to argue with me on this contention, so what’s new?

 

Tube-based voltage regulators
Given the choice between series and shunt regulators, I like shunt regulators best, but they are not perfect. One topology I would like to experiment with is an SRPP-based regulator that I have mentioned before. Such a regulator could pull the output back in line in both directions, whereas the series can only pull up and the shunt can only pull down. Below is shown an example from Electronics magazine back in 1952.

But even this high-voltage regulator has problems; for example, too little voltage differential across the top triode and too much across the bottom triode. In addition, such a regulator would be difficult to build for truly high voltages, say above 400V, as the triodes will require separate heater power supplies and few triodes can withstand a steady 1,200 volts, for example, which an 845-based single-ended amplifier would require. Moreover, such a regulator could not be used with low voltages or with solid-state equipment. Solid-state equipment?

Why would anyone want to use a tube-based regulator with solid-state gear? Other than better sound, I cannot think of any good reasons. Why can’t we use a tube-based voltage regulator with solid-state circuitry, such as a solid-state power amplifier or DAC? Well, the easy answer is that solid-state circuitry loves low voltages and high current, whereas tube circuitry loves high voltages and low current. All true enough, but our task is not to swing 40-volt and 5-ampere peaks into 8-ohm loads, but simply to erase the one-volt of noise from a power supply rail.

Before showing you how I would tackle the problem, imagine how you would do the same. Many are thinking that heavy-current triodes, like the 6AS7 and 6C33, are the answer. In other words, build effectively half an OTL amplifier configured as a voltage regulator. This solution begs the question: why not just build an OTL amplifier and be done with it?

Others are thinking that some hybrid circuit is the only solution, letting the tube half provide the voltage swing and feedback control, while solid-state pass devices do the heavy lifting. This solution is as obvious as it is compromised, as the goal was to build a purely tube voltage regulator which could power solid-state circuitry.

Ok, so how would I tackle this problem? I would use an electrical lever, in other words, a transformer. Imagine a single-ended amplifier whose output was in series with a low-voltage, high-current power supply rail (or a dangerously high voltage rail). Thus, instead of grounding one end of the output transformer’s secondary to ground, “ground” this end to the raw B+ terminal, leaving the other end of the secondary as the regulated output.

In the above schematic, we see what I have in mind. The output transformer’s winding ratio reveals the transformer’s current step ratio. For example, an output transformer with a 5k primary and an 8-ohm secondary holds an impedance ratio of 625:1 and a winding ratio of 25:1. Thus, an idle current of 100mA can be magnified into a 2.5A current swing. Amazing, yes; magic, no. The inverse of the winding ratio is the voltage step-down ratio. For example, if the triode’s plate can swing 100 volts in both directions, then the secondary’s output can only swing +/-4 volts, as 100V/25 = 4V. In theory, such a circuit, depending on the current drawn by the solid-state amplifier, could erase up to 4-volts of power supply noise. Of course reality will, no doubt, disappoint.

"Wait a minute," many are thinking; "a transformer cannot pass DC currents, so the tube-based voltage regulator could never work to achieve DC voltage regulator." True, it cannot; but its aspirations are not that audacious, as it only works to eliminate AC perturbations from the power-supply rail. Fortunately, must power supply noise is twice the AC line frequency, 120Hz in the USA and 100Hz in Europe. Nearly every output transformer is low-frequency limited, so raising the lowest frequency can only help. (Ideally, a tube power amplifier’s output transformer should be much larger than its power transformer, as 15Hz is 16 times more difficult for a transformer to transfer than 60Hz.)

Now, let’s flesh out a tube-based, AC-only, voltage regulator for solid-state use. Although the triode’s effective transconductance is amplified by the output transformer’s winding ratio, which in this example is 25:1, the result is still not much compared to a good power transistor. For example, a WE 300B holds a transconductance of 5,500µS (or 5.5mA/V), which a step-up ratio of 25 increases to 0.1375A/V; in other words, not much at all, as $1 power MOSFETs can be had with a transconductance of several amperes per volt. If we give our 300B a driver stage, we can effectively increase its transconductance by the driver stage’s gain, which would then be further amplified by the output transformer’s winding ratio. For example, a driver stage gain of 100 would bring the final effective transconductance up to a respectable 13.5A/V, whose inverse equals 0.073 ohms of output impedance. Not bad, as the secondary’s DCR will probably be much, much higher.

In the above schematic, we see the driver stage added. It is a cascode amplifier that offers high gain and phase inversion. Any perturbation at the B+ connection is relayed to the grid and amplifier by the cascode amplifier, which, in turn, drives the output tube, which steers the B+ back inline. If we reverse the phase of the primary winding, the following circuit could be used. The grounded-grid cascode does not invert input signal's phase at its output, so the primary's polarity will have to be changed or a second phase-inverting gain stage added.

Why bother with this last schematic? The answer lies in the output transformer used. Since the both the primary and secondary see a sustained DC current flow, the core will become magnetized, so an air-gapped transformer will be required. Now if the current flows in opposite direction through the primary and secondary, the core will be relieved of some magnetic stress; but if the current flows in the same direction through the primary and secondary, the core will be moved even closer to saturation (not a good thing). So a little flexibility is appreciated.

Well I have been sitting on this regulator design for a long time, but I haven't had the chance to give a good test drive. By the way, if you think the idea is crazy, then you should follow this link from the smart lads at MIT:


Design and Evaluation of an Active Ripple Filter Using Voltage Injection

By Albert C. Chow and David J. Perreault

Massachusetts Institute of Technology
Laboratory for Electromagnetic and Electronic Systems

After I read it, I told myself to get back to work, as the idea looks sound after all. The remaining theoretical problem is that the regulator's own B+ voltage should itself be regulated! The workaround might be to use a pair of push-pull output tubes instead of the solo single-ended tube. Or we might try to use a cathode-follower, single-ended output stage in the regulator.

 

Alternate stepped attenuator resistor values
The full Aikido stepped attenuator kit is stil on sale and it ships with the 32 resistors required to make an input impedance of 100k. This value works well with circuits featuring robust contemporary tube gear, but other values are sometimes needed; 25k for solid-state amplifiers and 250k for vintage tube amplifiers, for example. Thus, the table below lists the needed resistor values for 600, 5k, 10k, 20k, 25k, 50k, 75k, 100k, 200k, 250k stepped attenuators.

 

Total Input Resistance

 

600

5k

10k

20k

25k

50k

75k

100k

200k

250k

R1

123

1,028

2,056

4,112

5,140

10,280

15,420

20,560

41,120

51,400

R2

221

1,845

3,690

7,380

9,225

18,450

27,675

36,900

73,800

92,250

R3

299

2,494

4,988

9,976

12,470

24,940

37,410

49,880

99,760

124,700

R4

361

3,009

6,018

12,036

15,045

30,090

45,135

60,180

120,360

150,450

R5

410

3,419

6,838

13,676

17,095

34,190

51,285

68,380

136,760

170,950

R6

2,317

19,310

38,620

77,240

96,550

193,100

289,650

386,200

772,400

965,500

R7

1,026

8,549

17,098

34,196

42,745

85,490

128,235

170,980

341,960

427,450

R8

603

5,024

10,048

20,096

25,120

50,240

75,360

100,480

200,960

251,200

R9

397

3,307

6,614

13,228

16,535

33,070

49,605

66,140

132,280

165,350

R10

277

2,312

4,624

9,248

11,560

23,120

34,680

46,240

92,480

115,600

R11

449

3,744

7,488

14,976

18,720

37,440

56,160

74,880

149,760

187,200

R12

113

941

1,881

3,762

4,703

9,405

14,108

18,810

37,620

47,025

R13

28.3

236

472

945

1,181

2,362

3,543

4,724

9,448

11,810

R14

7.12

59.3

118.6

237

297

593

890

1,186

2,372

2,965

R15

1.79

14.9

29.8

59.6

75

149

224

298

596

745

R16

0.60

5

10

20

25

50

75

100

200

250

The resistor values are absolute, not standard; pick the closest 2% or 1% values. As you can see, the spread of resistor values listed above is too great for me to stock, so I am only able to provide the resistor pack for the 100k attenuator.

 

Moving valve amplifiers
I began this blog entry by mentioning Morgan Jones's book. Well, here is a snapshot of Morgan moving a power amplifier he built. (He agreed to let me publish it here. I thought I would have to get a release from Brad Pitt as well, but Morgan is adamant that he didn't use a leg double for this photo.)

//JRB

 

     


Still On Sale


Past blog posts

27 Sep 2006
Aikido Headphone Amplifier Recipe
Reiffin's SE Cathode-Follower Power Amplifier

19 Sep 2006
They're Back and They're New!

Cathode-follower
push-pull power amplifier design

3 Sep 2006
New Lower Price!
Actually Building Projects
Cathode-Follower Output Stages Once Again

05 Sep 200678
Book Review: Building Valve Amplifiers

28 Aug 200677
Mystery Solved
More Balanced-Output DACs and Tubes

21 Aug 200676
Death, Taxes, and Spam
Balanced-Ouput DACs & Diff Amps

12 Aug 200675
Balanced-Ouput DACs and Tubes


05 Aug 2006
Transformer-Coupled Aikido

28 July 2006
Einstein's amplifier
Aikido-inspired amplifier for Einstein

07 July 2006 
NOS

01 July 2006
Low Output Impedance
Aikido Cathode Follower

27 Jun 2006
DACs & Tubes / Bodies & Souls
   
15 Jun 2006
Ultra-Linear Aikido
  
07 Jun 2006
Transformer-Coupled Power Buffers
Complex Tube Power Buffers

04 Jun 2006
Buffers and More Buffers

BUF634
Power Buffers for Loudspeakers
Crazy but Good Idea

27 May 2006
Hybrid Promise

Soft-Clipping Circuit

22 May 2006
Spoiled

The 6082
Free DC Voltage for Heaters

13 May 2006
Aikido Tubes: Results and Values
Aikido and 300B Amplifiers

07 May 2006
New PCBs
Aikido Hybrid Headphone Amplifiers

27 Apr 2006
Aikido Low-Impedance Headphone Amplifier

21 Apr 2006
Aikido Headphone Amplifiers

17 Apr 2006
Getting R15 & R16 Straight

11 Apr 2006
24-Volt Aikido Amplifier

07 Apr 2006
PCB update & More Circuits

01 Apr 2006
Hybrid Aikido Amplifiers


Three-Switch Stepped Attenuators
26 Mar 2006

Printed Circuit Boards for the Aikido Amplifier
18 Mar 2006

European Triode Festival &
Aikido amplifier PCBs

10 Dec 2005

Jeb’s Amplifier
09 Sept 2005

Radiotron Designer’s Handbook
14 Aug 2005

Paying up front
04 Aug 2005

Aikido Amplifier Once Again
PDF
15 July 2005

A Wrong Turn
PDF
07 July 2005

Cathode-Follower Power Amplifier Design
PDF
29 June 2005

Cathode-Follower Power Amplifiers
PDF
22 June 2005

Auto-Bias and the Circlotron OTL Amplifier
09 June 2005

More Auto Bias Circuits
21 May 2005

Broskie Auto-Bias Circuit
09 May 2005

Class-AB Auto-Bias Circuits
29 Apr 2005

Amplifier Auto-Bias Circuits
16 Apr 2005

Common-Cathode Amplifier Design Ideas
09 Apr 2005

Big Amplifiers
19 Mar 2005

"Which Tube Should I Use?" Typos
PDF
10 Mar 2005

Which Tube Should I Use?
& Unbypassed Cathode Resistors

02 Mar 2005

All-Tube SRPP on Steroids
16 Feb 2005

SRPP translated again
12 Feb 2005

Mixed-Technology Hybrid
09 Feb 2005

Broskie-Macaulay Amplifier
04 Feb 2005

Taylor Source Follower on Steroids
28 Jan 2005

Aikido enhancement
PDF
25 Jan 2005

More Aikido Testing

22 Jan 2005 PDF

T-Rex & Shunt Regulators
PDF
18 Jan 2005

Aikido Amplifier Revisited
PDF
16 Jan 2005

Settling dust
13 Jan 2005

Making Sense of the CES
PDF
10 Jan 2005

E-mail from Australia & Curry amplifier
PDF
05 Jan 2005

Reflections on perfect reflections
PDF
04 Jan 2005

Perfect amplifier & perfect reflections
PDF
01 Jan 2005

The Einstein amplifier
30 Dec 2004

A second helping of crumbs
29 Dec 2004

A Feast of Crumbs
28 Dec 2004

Murray Amplifier
23 Dec 2004

More Translating Glass to Silicon
20 Dec 2004

OTL E-Mail
18 Dec 2004

Error in Power-Supply Schematic
More OTL
10 Dec 2004

Gomes & SE+ & Error Take Off
04 Dec 2004

OTL Regrouping
03 Dec 2004

OTL Amplifier Design
28 Nov 2004

32-ohm Speaker Design
24 Nov 2004

Tube-Based Computer Amplifier
23 Nov 2004

Translating Glass to Silicon
19 Nov 2004

Special Use for Gomes Amplifier
17 Nov 2004

More Aikido
15 Nov 2004

Gomes Vs XPP
12 Nov 2004

DC Coupled
11 Nov 2004

Aikido Variations
10 Nov 2004

Solid-State E-Mail
09 Nov 2004

Aikido Amplifier
08 Nov 2004

Where Have I Been
07 Nov 2004

The Experiment
01 Dec 2003

Unusual Circuit
21 Oct 2003

Differential SRPP from Poland
17 Jun 2003

Tubes and Headphones Once Again
03 Apr 2003

Simple PP Amplifiers (Updated 6 Nov 2004)
21 Mar 2003 (Updated 6 Nov 2004)

Simple Single-Ended Amplifiers
22 Feb 2003

LPs Covers Again
& a Letter to the Editor of Electronic Products

19 Jan 2003

Zen Amplifiers
20 Dec 2002

Letters from Acme Tube Design
E-mail from Erno Borbely
29 Nov 2002

 

 
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