Things you should know about tubes — Robert Tomer

R_F · Feb 24, 2018

Something may have changed since 1960. If so, please correct anything that has with direct sourced evidence.

I made screen captures from parts of the book to try to focus mainly on the most relevant bits to audio. I edited out extraneous stuff like references to figures not shown. I did this mainly because it seems everyone today in forums is suggesting that fixed bias is a better idea than cathode bias, something this author specifically opposes. By making this screen captures it's also easier for people to share this information in relevant pieces instead of just linking to the book's PDF.

arts · Feb 24, 2018

Very little (if anything!) has changed regarding tubes vis-a-vis their manufacture and (proper) usage since the time that book was written. Tube technology had reached it's acme during the 1950's/1960's,and until the laws of physics change,there is nothing more to be added.

This is one of the finest books ever written on this subject,by a true Master of the discipline. I actually own this book,and have never found fault with any it's contents.

What has changed is that not enough people read books such as this one.Many can not be bothered to,as they feel that,because of their very forgiving and tolerant nature,tubes don't require a lot of thought,knowledge or consideration of their technical aspects in order to utilize them.In other words,they just ''plug them in'' and can't be bothered to do the math.

R_F · Feb 24, 2018

I've read about tube failures in the inexpensive Chinese-origin tube stereo amps. Some of these amps use fixed bias. I makes me wonder if the main reason for the failures is the use of fixed bias. I wonder how many stereo tube amps on the market are shipping with fixed bias instead of cathode. I have been in the market for a new amplifier and am investigating these design issues so I can make an informed purchase. I'd rather not have to replace tubes often due to a design flaw.

I found another page, also, where he talked about how fixed bias is a problem (page 62):

The book is public domain and can be found in text form here. I could have saved some time by just using the text. I didn't expect it to be archived.

Page 62 discussing fixed vs. cathode bias also — including an illustration:

https://s14.postimg.org/5x2e8es4h/figure4_4.gif
https://s14.postimg.org/s9071ugyp/tester_fixed_bias.gif

spinroch · Feb 24, 2018

I am suitably IMPRESSED!

trobbins · Feb 24, 2018

R_F, fixed bias is not the problem. If there are design flaws then it is not the use of fixed bias, but rather how the amp designer implemented fixed bias. It would be sad if you used 'fixed bias' as a generic amp shopping negative.

Just be mindful that Tomer is presenting summary descriptions in a book meant for the masses, and also that much has happened since 1960 related to other peripheral ways of doing things and in materials.

arts · Feb 24, 2018

You should most definitely NOT use the type of bias method utilized as a way to judge an amplifier. Cathode bias can certainly have some advantages,but these are strictly application dependant.When it comes to higher power applications,cathode bias becomes unusable,for a multitude of reasons.

Many of the failures in inexpensive Chinese manufactured amplifiers are caused by inexpensive Chinese manufactured tubes.Also,some of the economies of design practiced by these companies do not respect the operating parameters of the tubes used,causing tubes of already marginal quality to fail even more rapidly.

gadget73 · Feb 24, 2018

Fixed bias that is not set to a reasonable value is a problem, but so is cathode bias with the wrong value resistor. You get to the same place, just by a different method.

arts · Feb 24, 2018

That and incorrect value grid return resistors.

trobbins · Feb 24, 2018

gadget73 said:
Fixed bias that is not set to a reasonable value is a problem, but so is cathode bias with the wrong value resistor. You get to the same place, just by a different method.

Imho, sort of like having a car and not noticing the tyres have gone flat, or the suspension is now shot, or there is almost no oil in the sump - most problems are the owners fault, or the service mechanic, or the car manufacturer if they detect failing parts and don't issue a service report, or just bad luck if a part fails early. Nowadays, if the amp has survived 50 years, then there is also a path for the owner or service mechanic to appreciate how to make the amp more bullet proof from others experience or modern parts.

GordonW · Feb 24, 2018

Bias type is frequently a trade-off, in terms of tube life.

Fixed bias can, in some cases, give a mechanism for catastrophic tube failure- but, most times, the damage from this can be mitigated, by the use of sacrificial "sense" resistors in the cathode circuit of the tube. Something like a 10 ohm 1/4 or 1/2w resistor (depending on the current draw), can act as a fuse- so, if there is a failure that leads to a short- the resistor will blow and protect everything else.

OTOH- cathode bias, due to the fact that the tube usually has to be biased to a higher current value at idle to minimize distortion, compared to fixed bias, means that the tube is "cooking" at idle significantly harder than the same tube usually would be in fixed bias. This wears out the tube faster, as a result.

This dichotomy of failure mechanism was demonstrated, when Rogue Audio went from cathode bias (in the M120) to fixed bias (in the M150). Basic same amp design otherwise (some other minor changes, but nothing big). The tubes in the M120 usually would wear out within a couple years of steady use, under cathode bias- but, in the M150, they would not only make more power, but usually last longer. Now, there would sometimes be some catastrophic failures- but they wisely put 1/4 amp fuses in the cathode circuit of the output tubes- and with those in place, I personally have seen, the worst failure of one of these amps, be only a blown screen resistor, on top of having to replace the tube and fuse. Not a bad thing to fix (the amp was working in less than an hour once it was on the bench)...

Regards,
Gordon.

R_F · Feb 25, 2018

Can anyone provide direct evidence to support the use of fixed bias? I appreciate the feedback but since Tomer is so adamant in his book, which appears to me to be quite strong as a source, I'd like to see data or something more convincing. : )

BillWojo · Feb 25, 2018

Just bought a copy of the book off of Ebay, $4.99 shipped. They must be in reprint as the seller has a bunch of them.

BillWojo

trobbins · Feb 25, 2018

R_F, so far you have pointed to page 62 in Tomer's book. You appear to then make a giant leap of connection between Tomer's discussion on that page, and use of fixed bias in an 'amplifier' as a significant cause of 'tube failures'.

Do you appreciate what Tomer is discussing on page 62 and how that may or may not relate to how an amplifier works? Do you have any statistically significant evidence where a properly working amp is causing 'tube failures' - presumably relating to the use of fixed bias in the output stage (given that fixed bias could be used by all stages in an amp if needs be).

R_F · Feb 25, 2018

The book is in the public domain but I have also seen it for sale on Amazon for something like $15. It's also apparently being sold as an ebook even though it's in the public domain in full form via the Internet Archive.

Trobbins, please refer to these pages from Tomer and let me know if there are any leaps I'm making based on what he is saying on those pages:

page 20:

That seems to sum it up, but there is also this bit:

It seems clear enough, without considering page 62's contents at all, where Tomer stands on this issue. Page 62 was an addition, you'll notice, actually. This is what the opening post said:

Fixed vs. cathode bias — which is better?

Under that was the link for page 20.

BinaryMike · Feb 25, 2018

R_F said:
Can anyone provide direct evidence to support the use of fixed bias? I appreciate the feedback but since Tomer is so adamant in his book, which appears to me to be quite strong as a source, I'd like to see data or something more convincing. : )

Here's an authoritative citation that you might have missed: http://audiokarma.org/forums/index....dvantage-to-cathode-bias.434801/#post-5570812

trobbins · Feb 25, 2018

R_F, ta that's more explicit.

My view is that Tomer's 1960 book needs to be taken in to perspective - especially failures relating to fixed bias. Fixed bias in the form of a separate diode rectified negative supply used for an output stage was pretty much new to the 1950's, and I'd suggest only gaining a reasonable level of use in the latter half of that decade. Selenium or copper oxide diodes were still being used, and silicon diodes had only been available and priced to suit that application in the latter half of the decade. Fixed bias was being leapt on as a means to avoid cathode bias in the every increasing battle for higher wattage power amps - amps with fixed bias would I suggest be typically used in 40W plus output stages, so the high power available from the power supply and the need for larger dissipation valves were the scene. The need for diodes and pots and a setup requiring a voltmeter, and an instruction manual that may only have referred to setting a bias voltage (not a bias current along with anode voltage and some maths), and a simple tube-swap/replace mentality would certainly have led to a general view of failures coming through to the attention of reliability practitioners that were from 'fixed bias' amps.

From the 1960's, the market for higher wattage amps has dictated pretty much the generic use of fixed biased output stages. Bias supply reliability has improved using silicon diodes, failsafe or more reliable pots, cathode current sensing for bias setup, a better appreciation of screen resistors for protection, and a general trend to using a lower idle dissipation level than 'design centre max'. More recently, the use of secondary side fusing in the power supply, and either the use of a cathode fuse or 'poor-man fuse' current sense resistors, has further reduced collateral damage from failed output valves that may have muddied the 1950's scene of where the fault lay.

If there is now a resurgence of amp failures - seemingly directed at low price valve amps from a plethora of new manufacturer entrants able to sell internationally into an eager market - then yes one could try shopping for amps that if poorly designed or made have less chance of blowing themselves up.

Retrovert · Feb 26, 2018

Emphasis added:

trobbins said:
My view is that Tomer's 1960 book needs to be taken in to perspective - especially failures relating to fixed bias. Fixed bias in the form of a separate diode rectified negative supply used for an output stage was pretty much new to the 1950's, and I'd suggest only gaining a reasonable level of use in the latter half of that decade. Selenium or copper oxide diodes were still being used, and silicon diodes had only been available and priced to suit that application in the latter half of the decade.

That is the key point which must not be overlooked in placing Tomer's work in context vis-à-vis bias failure: the (then) new technology of selenium rectification was failing in the field. More so for selenium than copper oxide, which was less common in consumer items.

Published limits for selenium rectifier were routinely exceeded as a cost-savings measure, a practice existing throughout the lifecycle of selenium rectifiers. The datasheets even encouraged nefarious excess with, roughly paraphrasing, warnings to never exceed the current limits unless, of course, one is making a consumer device intended to only lasted a few years, in which case have at it with gusto.

Poor design practice is why selenium rectifiers failed with alarming regularity in audio and television equipment, yet lasted decades in welders and industrial equipment. Design practice optimizing for manufacturing cost, not lifespan. Same thing happened with tubes, another subject Tomer addresses in detail.

So Tomer's warning about failures in fixed bias chernobyling output tubes was a largely consequence of poor design for the screen supply which caused rectifier failure and thus bias failure.

Retrovert · Feb 26, 2018

BTW: it is generally more useful to the discussion to post text as actual text, i.e. cut and paste from the PDF, than as a bucket of dead pixels from which one cannot readily quote.

GordonW · Feb 26, 2018

Retrovert said:
BTW: it is generally more useful to the discussion to post text as actual text, i.e. cut and paste from the PDF, than as a bucket of dead pixels from which one cannot readily quote.

I would normally agree- but I think this is one of those PDF files that was scanned in as images, and never OCR-processed. So, there's no text to cut and paste.

I've run into this repeatedly with old book scans. Unless someone is willing to put in the time to OCR process them (which has to be HEAVILY proofread afterward), it's the best that's going to happen for a lot of books...

Now, one COULD take the image of the text into a picture editing program, and crop it to only have the lines that they want to quote- and post that as an image- then respond to that. A bit clunky, but it would get the point across.

Regards,
Gordon.

GordonW · Feb 26, 2018

BTW: I just posted this link in the other thread (from Dave Gillespie) referred to above- it's also relevant here. Kevin Hayes' (of VAC) treatise on bias- and why he came up with the iQ Intelligent Bias sysem:

https://www.google.com/url?sa=t&rct...iQsystem.pdf&usg=AOvVaw2t-v0BlzANJ6K1C_-h44RI

Kevin doesn't have the graphs he sometimes uses, in that PDF- I am going to try and find them, and scan them in- but, in those graphs, he shows the transient bias shift of both cathode bias and traditional fixed bias, during normal amp operation. It's a bit remarkable how they are almost INVERSE to each other. This is why it can be useful, to put small value cathode resistors in a fixed-bias amp (such as the "bias sense" resistors I oftentimes use and recommend)- it can add a bit of the cathode bias tendency to the fixed bias tendency. Some of the error is likely cancelled out, in operation.

No, it's not nearly as effective as Kevin's iQ bias system- but it's better than traditional fixed bias or cathode bias alone...

Regards,
Gordon.

Things you should know about tubes — Robert Tomer

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