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Old 8th Mar 2020, 10:44 am   #1
tri-comp
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Default Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Split from this thread:
https://www.vintage-radio.net/forum/....php?p=1328438


Hi,

I was given that book as a birthday present several years ago.
The circuit diagrams within are quite full of errors that are perhaps not easily spotted by novices.
On pages 154 and 156 the EZ81 rectifier has one side of the filament connected to the cathode (HT output voltage) and the other side to ground through a 100-Ohm pot. That should certainly lite that valve up ...for a sec.
On pages 147 and 150 the powersupply diode-bridge is connected to the transformer wrongly.
On page 162 in the powersupply for the negative grid voltage the filtering electrolytic cap's C10 & C12 are shown with wrong polarity.
There are bound to be more, I'm sure.

Best regards,

/Torben
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Old 8th Mar 2020, 12:21 pm   #2
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Thumbs up Re: Build your own Audio valve amplifiers.

I think you'll find that in many books to name a few Audio Measurements and understanding hifi circuits by Crowhurst, valve amplifiers 3rd edition jones, valves revisited smoyzi, power supply handbook Fielding and vacuum tube amplifier basics Jurich. I'm sure theres plenty more which I have but not read.
But it's a good point and worth the effort of pointing out.
But no circuit design is without its flaws and errors whether practically or theorectically.
But despite the errors they are all informative and that's the beauty of valve equipment theres no right or wrong way of designing and building them.
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Old 23rd Dec 2020, 8:45 am   #3
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Default Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Some more mistakes in addition to post #5.

Picture 1:
The value of C8 (100 uF) is way higher than the maximum in the datasheets for the EZ81 (50 uF).

Picture 2:
The two cathode resistors of 150 Ohm each for the KT88 are connected parallel (also on the design of the circuit board!) giving 75 Ohm, while they should be series connected to give 300 Ohm. Not good for the KT88 at all (and OPT? and power transformer? there's only a fuse on the primary side). It will start to pass way too much current...

Picture 3:
The cathode of the upper half of the ECC81 sits at 232V, while the filament supply for the valves is earthed, so sits at 0V (not shown in the picture but it does). The maximum cathode to heater voltage of the ECC81 is 90V. Not good for the cathode/filament isolation; very high risk on it failing...

Like Torben I got the book as a present. I did write to the publisher (in 2010) about the mistakes I found, and they aknowledged them. But as far as I know, they still sell this book without warning buyers for the mistakes it contains.
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Old 23rd Dec 2020, 2:01 pm   #4
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Quote:
Originally Posted by Robert Gribnau View Post
Picture 1:
The value of C8 (100 uF) is way higher than the maximum in the datasheets for the EZ81 (50 uF).
C7 actually...

This is probably the most minor of the errors - the EZ81 is operating way below its capabilities with just a pair of ECL82's to load it. So, it probably isn't going to mind too much with a few extra μF's

Curious that the transformer is shown with just a single heater secondary, how does the EZ81 get warmed? EDIT - just noticed that it is the same heater winding as all the remainder - which is thus simultaneously referenced to HT+ and to chassis!!

The others: KT88 cathode bias is horrendous! Unless the resistors were meant to be 600Ω each, which would make things right, in parallel. But I guess there's a parts list on another page, which would confirm or refute this.

Of course, 600Ω is not a standard value, and 75Ω is but only in E24. Whereas 150Ω is, in E6 and E12, so why not just use a single resistor - as a 7W wirewound if necessary?

And, 232V heater-cathode is asking for trouble!

Good spot. Some books unfortunately are written by people who failed as engineers, this might be one of them.

Last edited by kalee20; 23rd Dec 2020 at 2:02 pm. Reason: Within text!
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Old 23rd Dec 2020, 2:42 pm   #5
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Yes, I meant C7... talking about mistakes...

The parts list for the KT88 amplifier also contains the mistake ("R10, R11 = 150 Ohm, 3W"). My guess is that the author just mixed up parallel and series connected.

For me it's not so much that the book contains errors, but more that the author and publisher must know about part of these errors by now and still sell the book without pointing them out (erratum).
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Old 23rd Dec 2020, 3:03 pm   #6
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

I read this book in a library years ago, and I wish I had a copy now but it's disappeared from shelves. Shame, it deserves to be digitised.
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Old 23rd Dec 2020, 5:26 pm   #7
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by Robert Gribnau View Post
Picture 1:
The value of C8 (100 uF) is way higher than the maximum in the datasheets for the EZ81 (50 uF).
C7 actually...

This is probably the most minor of the errors - the EZ81 is operating way below its capabilities with just a pair of ECL82's to load it. So, it probably isn't going to mind too much with a few extra μF's
It may just be possible that C7 should not be there at all. Marked on the diagram next to EZ81 is is a current of 180mA with an input of 200 to 250V. Also L1 is curiously specified as having a value of 8...10H which suggests that it might be intended for choke input filter operation.

Referring to the Philips EZ81 data sheet under choke input operation, for 250V input and 180mA load current a choke value of 10H is specified. This arrangement would comfortably supply both amplifiers.

Rich
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Old 23rd Dec 2020, 5:36 pm   #8
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Default Re: Build your own Audio valve amplifiers.

180mA might be just the transformer rating - which would be rather extravagantly expensive for a pair of ECL82's!

Removing C7 would indeed give a choke-input filter - but voltage would fall quite a lot. Unlikely to be used in a simple amplifier like this. (It's much kinder on the rectifier valve, though...)

Now that Graham has merged the threads, I see that the HT short-circuit had already been picked up in the earlier thread.
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Old 23rd Dec 2020, 6:47 pm   #9
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Default Re: Build your own Audio valve amplifiers.

Heater pin numbers.

Lawrence.
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Old 26th Dec 2020, 5:40 pm   #10
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Default Re: Build your own Audio valve amplifiers.

It would seem that a thread listing the errata for this book would be a useful sticky in the audio or homebrew sections.
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Old 26th Dec 2020, 6:05 pm   #11
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Default Re: Build your own Audio valve amplifiers.

It would indeed.

But it would be even more useful, pasted inside the book itself!
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Old 26th Dec 2020, 6:59 pm   #12
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Default Re: Build your own Audio valve amplifiers.

I'm assuming this is based on data sheets and simulation as opposed to an actual build? Don't forget all valve data sheets are published using simulations and are riddled with mistakes as well..
Personally I would build it and then see what you get. Plus if you have the test gear to do so test it and run real simulations with the test gear.
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Old 27th Dec 2020, 10:55 am   #13
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Default Re: Build your own Audio valve amplifiers.

Well it all started because I chose to buy this book. Which for me is a must have well written and easy to understand publication. Plus incredibly hard to get hold of. But I agree as David said this is for sale section not really a discussion thread. It would be very interesting indeed if someone actually built this amp I may get round to it at some stage. Then see if the errors mentioned in the real working unit circuitry if it is all wrong and basically blows up.
What did interest me was and I quote "why use resistors in series instead of just one resistor". I would suggest have a read round why you would use this method. No amp is the same unless it's a mass produced product. This book is not intended for a company to mass produce the designs within this book. Its experimentation. That's the fun part.
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Old 10th Jan 2021, 9:38 am   #14
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

It looks to me that you doubt that what I presented as mistakes are realy mistakes.

The first schematic (the amplifier with ECL82's) is a stereo amplifier with the one EZ81 feeding both channels. So the current demand is not particulary small for an EZ81. The maximum value for the reservoir capacitor has to do with the risk of flash-overs. The higher the value of the reservoir capacitor, the higher the 'current bursts' in the intervals when the voltage on a plate is higher than the voltage on the reservoir capacitor. So that's why there is this maximum value in the datasheets for the EZ81 (and other rectifiers). It limits the amount of current during these 'bursts'.

The second one (the parallel connected resistors of 150 Ohm each) is clearly a big mistake. Just take a look at a datasheet for the KT88 and you will see. Or a different approach: The schematic indicates 30 V at the cathode of the KT88. With a total cathode resistance of 75 Ohm the cathode current would have to be: I = V / R = 30 / 75 = 0,4 so 400 mA (!). The dissipation (of anode + screengrid) would than be: P = I x V = 0,4 x 285 = 114 Watt (!). Both values are way over the absolute maximum values in the datasheets for the KT88 (being 230 mA and 46 Watt).

The third one (one cathode of an ECC81 sitting at 232 V while its filament sits at 0 volt) is also clearly a mistake. At the start of my rediscovery of this hobby some 15 years ago I did not know about this limit yet. I was intrigued by srpp configurations so I tried almost every valve type I had at the time in srpp configurations. After I destroyed a third EF85 (I did not know about vari-mu either) that was in the upper half of the srpp configuration, I wanted to know what had caused these failures. Because of that I 'discovered' this maximum value for cathode to heater voltage. Since than I take this maximum value very serious. And in this particular case with an ECC81 that maximum value of 90 Volt is violated by 142 Volt (!).

The nature of the mistakes make me doubt wether the author has built (all) his amplifiers before publishing them. Maybe the first mistake would not manifest itself right away (it would shorten the lifespan of the EZ81 though). But remember the other mistake in the first schematic (at the cathode of the EZ81 B+ is connected to the filament supply of all valves + the filament supply is connected to earth through a 100 Ohm adjustable anti-hum pot). That other mistake would manifest itself right away. For sure the second mistake would manifest itself right away (with luck the fuse blows before any serious damage is done). The third mistake I think will manifest itself in a matter of minutes at most.

Like I already wrote: The author and publisher aknowledged these mistakes in 2010.
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Old 13th Jan 2021, 2:14 am   #15
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

This publisher used to have a monthly column "het lek" (the leak) with errata in their magazines. It was said that if you wanted to build one of their designs, always wait at least a month and look for the leak.

The cathode of the EZ81 being tied to the heater, might have been a mistake made in the final drawing only. The author might have drawn it correctly while the editor used a standard library symbol for a directly heated rectifier or an indirectly heated one wired that way.
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Old 12th Jun 2021, 11:10 am   #16
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

One more example with multiple mistakes (btw: the values of the parts in the schematic match with the parts list in the book).

Pre-stage/phase splitter:

What are we looking at? At first glance it looks like a long-tail pair but the ‘tail’ is very short and the values of the anode resistors (100K and 15K) differ enormously. The resistor of 15K is at the anode of the triode with the grounded grid, while normaly you see the higher valued anode resistor at this side.

I do not see how this design can produce output voltages of the same amplitude but with opposite phase. I think that the side with 15K will produce way less amplitude than the side with 100K.

The feedback looks strange to me since it is applied to the common ‘tail’.

Between B = 283 V and the top of R3 the voltage difference is 55 V. Even with the slider of P1 turned fully downwards (which by the way would be impossible because there has to be only 260 V at R4) you would still need a current of: I = V / R = 55 / 25000 = 2.2 mA.

But there is no 2.2 mA to start with when looking at the cathode voltage and cathode resistors. I = V / R = 2.2 / 1247 = 1.76 mA.

So however this stage is supposed to work, it can surely not work with the given values/voltages.

Output stage:

In the schematic (and in the text) it says that you have to adjust the cathode resistance so that the anode current per EL84 is 44 mA. The cathode voltage would than be about 8 V.

Lets suppose that the resistance for dc per half of the primary is about 200 Ohm (wich is rather high for an output transformer of quality). Than the voltage loss in one half of the primary would be about 10 V. So the voltage between cathode and anode of the EL84’s is 300 – (8 + 10) = 282 V. The voltage between the cathode and screen grid will be slightly higher, lets say 285 V.

Looking in the datasheets for the EL84 you will see that it is impossible that the anode current is only 44 mA at Vg2 = 285 V. I think something between 60 and 65 mA is right. But than the anode dissipation per EL84 would be between 17 and 18.4 Watt, which ofcourse is way over the limit of 12 Watt.

Power supply:

It is impossible to get 300 Vdc from 2 x 200…220 Vac and an EZ81 at about 100 mA like this amplifier is supposed to draw. I do not think you will even reach 250 Vdc.

The value of C8 right after the EZ81 is 100 uF. The datasheets give 50 uF as maximum. Higher values at least shorten lifespan.

The cathode of the EZ81 is connected to its filament, which in turn is connected to the filaments of the other valves. The filaments are grounded through anti-hum pot P5, so B+ is practically shorted to ground.
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Old 12th Jun 2021, 11:28 am   #17
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Beyond the basic errors that can be seen, there is also a certain lack of sophistication. For example, there are no parasitic oscillation stopping measures on the ultra-linear EL84 outputs.

If someone is wanting to start building valve amplifiers, choosing one of these and getting it going successfully is likely to demand as much knowledge as it would take to do your own design from scratch. The Mullard designs would be a much safer start and already do all that is necessary.

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Old 12th Jun 2021, 1:03 pm   #18
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Hi!

On the subject of book "howlers", of which the book that is the subject of this thread contains many, (and I am in agreement with all of them!) I am taking great care in the writing of my T & M Theory and Repair Book to make sure that silly "howlers" like this don't get into my text and diagrams, and I will also be making sure that maker's amendment sheets (where appropriate) are included in diagrams or referenced where needed!

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Old 12th Jun 2021, 1:07 pm   #19
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Quote:
Originally Posted by Robert Gribnau View Post
What are we looking at? At first glance it looks like a long-tail pair but the ‘tail’ is very short and the values of the anode resistors (100K and 15K) differ enormously...

I do not see how this design can produce output voltages of the same amplitude but with opposite phase. I think that the side with 15K will produce way less amplitude than the side with 100K.
Absolutely in agreement.

And, the grounded grid - why not tie directly to ground, why the parallel capacitor and resistor?

Quote:
Originally Posted by Robert Gribnau View Post
The feedback looks strange to me since it is applied to the common ‘tail’.
Yep - so -ve to one side, +ve to the other.

Quote:
Originally Posted by Robert Gribnau View Post
The cathode of the EZ81 is connected to its filament, which in turn is connected to the filaments of the other valves. The filaments are grounded through anti-hum pot P5, so B+ is practically shorted to ground.
Again yes - connecting heater to cathode is best practice (even though the EZ81 is rated for high h-k voltage). But it needs a separate heater winding - sadly missing...

Basically, an appalling diagram.
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Old 12th Jun 2021, 4:03 pm   #20
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Default Re: Mistakes in the book "Build your own Audio Valve Amplifiers" by Rainer Zur Linde

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by Robert Gribnau View Post
The feedback looks strange to me since it is applied to the common ‘tail’.
Dear heavens... I hadn't spotted that one!

Effectively the feedback signal modulates the Gm the forwards path experiences.... it's a frequency mixer! Should help make even order distortion quite nicely. If only the author had known it could have been patented as a "valve sound enhancer"

There are too many issues to be considered typos missed in proof reading. The Ray Bradbury "Fahrenheit 451" approach may even be justified, preventing others falling into the traps therein.

To debug a bad design, you have to start by knowing enough to have designed it properly yourself. Therefore the only people who can use this sort of information don't need it.

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