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Small IDHT tube cathode bypassing

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Posted on July 6, 2015 at 00:44:48
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Hi again all,

I have another capacitor-related question. A comment by a commercial amp builder recently that I read recently went along the lines that the cathode bypass capacitor for small IDH tubes (e.g. input tubes) essentially inaudible - that differences between capacitor types are inaudible. Does this reflect others' experiences? What would be a rationale - apart from listening experiences - that could explain this?

Just wondering. Thanks in advance for your thoughts.

Regards,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

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RE: Small IDHT tube cathode bypassing, posted on February 6, 2017 at 12:59:56
used-hifi
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Location: Surprise AZ
Joined: March 18, 2003
 This was a good Thread about "RK"/ bypassing

some of you guys claim that there is NOT much AC voltage across the cap, so it will have little to no change in sound...... but many including myself hear drastic changes! Why? you mights ask.... well its NOT the voltage that makes caps sound different In RK bypasses ITS THE AC (music)CURRENT so if you have crappy hi esr or low current pulse type capacitors THIS WILL CHANGE HOW IT SOUNDS IMO.

I hope this gives some incite to others to try to properly bypass there RK's


Lawrence

 

RE: Small IDHT tube cathode bypassing, posted on July 10, 2015 at 13:55:02
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Firstly, thank you to all those who have taken the time to contribute. I found it informative and enjoyed reading the suggestions, even if I don't understand some of it.

There has been a little back-and-forth below, but thankfully it was for the most part healthy debate and not personally targeted. Against my better judgement, I was involved in a brief negative exchange, but I think the outcome will be positive. It did encourage me to think about something else though - it is off topic, but I thought it might be worth mentioning.

In my experience, the people (and teams) who have made the most valuable contributions to field have been able to draw on a wide range of disciplines, approaches and resources in their pursuits. They start with a purpose; the desired outcomes crystallise and appropriate methods are developed as they bring a coordinated effort to task. They approach from many directions - they have perspective, yet their purpose is always in mind. They bring harmony to superficially incompatible concepts. They can synthesise, interpolate and extrapolate. They are theoretical and pragmatic. They try, fail and learn. They can speak the nomenclature and lay-terms. They make the complex understandable for the rest of us. They ask "dumb" questions, yet cut to important details in a flash. They are honest yet tactful. Their ego is secondary to the cause. They are inclusive. Ultimately, they bring an understanding to their endeavours. By contrast, extreme approaches, for example either narrowly quantitative or qualitative, are by comparison ineffectual.

This is especially pertinent when the something may appear objective or purely quantitative, but its end-use or interpretation is personal. Relevance? The reproduction of music can - as a high level concept - be characterised as an objective endeavour. Conversely, the listening and enjoyment of live music - let alone reproduced music in one's own home - is personal.

Regards,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Small IDHT tube cathode bypassing, posted on July 7, 2015 at 05:45:08
cpotl
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Posts: 1002
Location: Texas
Joined: December 6, 2009
 "A comment by a commercial amp builder recently that I read recently went along the lines that the cathode bypass capacitor for small IDH tubes (e.g. input tubes) essentially inaudible - that differences between capacitor types are inaudible."

One can get some estimate of how audible the effect might be by putting an oscilloscope probe on the cathode while a signal is playing. This will show how much audio signal is appearing across the bypass capacitor. For an ideal cathode bypass (with infinite ideal capacitance) there would be zero signal. There will in actuality be some non-zero signal, but with a competent design it should be so minimal that the difference between one type of capacitor and another should be inaudible.

Of course, what people think they hear is a very different story, more likely related to the price they have paid for the capacitor...

Chris

 

Absolutely Correct!, posted on July 8, 2015 at 19:20:57
Triode_Kingdom
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 "There will in actuality be some non-zero signal, but with a competent design it should be so minimal that the difference between one type of capacitor and another should be inaudible."

Boutique aficionados and elitists never want to talk objectively about this concept. If the AC voltage across the bypass were actually zero, the capacitor could contribute *nothing* to the sonics of the amplifier. It would be presenting the same electrical characteristics as a perfect piece of wire for all signals. This is also true of filter caps in the power supply. The absence of AC signal voltage across them would indicate operation as perfect decoupling caps, capable of contributing nothing, regardless of internal construction or materials. The fact is, I've heard dozens of enthusiasts say over the years that they could hear little or no difference from changing coupling caps or adding filtering (when the original components were in known-good working condition). My aural experience has been similar, and has been repeatedly confirmed by observation of physical phenomena in the circuits I've worked with.


 

RE: Small IDHT tube cathode bypassing, posted on July 7, 2015 at 19:05:00
drlowmu
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Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 " but with a competent design it should be so minimal that the difference between one type of capacitor and another should be inaudible."

Sorry, WRONG, wildly wrong, as often you are. Oh well, at least you are consistent.

1994, Sound practices, Marzio and Jelasi authored :

" When constructing equipment capable of attaining high peaks of musicality, the quality of the passive components assumes critical importance. This is particularly true for capacitors. Note that in the list of components, electrolytic capacitors are not recommended, not even as power supply filters. We bypassed the high value capacitors (with exception of Cl which is placed in a non-critical position) with others of small capacity in silvered mica and/or paper and oil. This practice is common among extremist experimenters who know how much the quality of these components can influence the final result ".

Marzio and Jelasi used THREE carefully-selected capacitors in 1994 on their driver and finals stages' Rks, as shown in their schematic and parts list.

Jeff Medwin

 

RE: Small IDHT tube cathode bypassing, posted on July 7, 2015 at 20:08:44
cpotl
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Posts: 1002
Location: Texas
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 If different bypass capacitors lead to different sounds, then this will necessarily correlate with different levels of audio signal appearing across them. I take it this is not being disputed?

My definition of a "competent design" was one where the bypass capacitor was chosen so that the audio signal developed across it is negligible. There will then, more or less by definition of "negligible," be no audible difference between one such choice and another. If a given choice of capacitor fails to achieve the goal, then I would not call that a competent choice, since it will lead to some colouration of the sound.

Chris

 

RE: Small IDHT tube cathode bypassing, posted on July 9, 2015 at 06:03:45
drlowmu
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Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 What you write is a pile of baloney. You obviously are a theorist and you must NEVER build good revealing amps and actually LISTEN to the parts. Every film cap you buy sounds different than another brand, of the same exact specified value.

There are people who will spend up to and over $2,500 for a single film cap. Think its "perfect sounding" versus a 60 cent WIMA MKP series caps? Likely not. Think it sounds different? Of course it does !!

What you theorize and post has no relationship to reality. Reality is what you HEAR, the end-result when you play a good amp through a good audio system.

Audio performance, "how it sounds to you", is ALL that really matters. I am done discussing this with you. Post on, as much as you feel compelled to do so.

Jeff Medwin

 

RE: Small IDHT tube cathode bypassing, posted on July 9, 2015 at 06:25:53
cpotl
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Location: Texas
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 "There are people who will spend up to and over $2,500 for a single film cap."

They are fools.

If they want to understand what is going on, they could put an oscilloscope across the capacitor, and look at the audio signal. If it is not negligible, they could either bypass the capacitor with another one that corrects the deficiencies, or they could chuck out the original one and get another. Any way it is done, the solution costs a few dollars, not thousands.

Chris

 

RE: Small IDHT tube cathode bypassing, posted on July 9, 2015 at 06:24:43
cpotl
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Posts: 1002
Location: Texas
Joined: December 6, 2009
 Based on you response, I think you do not understand the basic principles involved. You surely cannot dispute that the different sounds, if they are real, are the result of different audio signals appearing across the bypass capacitor? If the cathode resistor is bypassed properly, there will be a negligible audio signal across the capacitor. If it is not done properly, because of a bad choice of capacitor, or insufficient capcacitance, there will be an audio signal, almost certainly frequency dependent, and that will imply a colouration of the sound from the amplifier. Now, you may like that sound, and there is nothing wrong with that, but it does then mean that you like to achieve a "sound effect" via a non-ideal bypassing of the cathode resistor.

It is as simple as that.

Chris

 

RE: Small IDHT tube cathode bypassing, posted on July 15, 2015 at 02:54:36
amnesiac
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Posts: 717
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 If I was rich I would pay my minions to stack up silver foil oil caps with bipasses on my cathodes.

And I would only use one eye minions because well you know why!

 

RE: Small IDHT tube cathode bypassing, posted on July 6, 2015 at 14:34:15
SETguy
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Posts: 35
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 Here's another option for you to consider which should work quite well with the 6C6: Try grounding the cathode and use a 10 meg grid-leak to create the bias (this necessitates AC coupling to the source)! I use this method almost exclusively when using high-mu triodes in the front end (and contrary to popular belief, this does not result in hum issues); some pentodes, like the 6C6, also can be used that way. GE published some recommended values for the plate/screen load as a function of supply voltage which you can look up (if you can't find it, look up the 6J7- same tube electrically). The advantage of this method is that there are no frequency response issues, elimination of 2 components from the signal path and, as the tube ages, it will automatically adjust its bias based on the available space charge.

 

RE: Small IDHT tube cathode bypassing, posted on July 6, 2015 at 15:15:45
drlowmu
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Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Hi,

But isn't there a huge disadvantage, that you have to add a series coupling cap in the signal path. Not only that, its placed at the lowest signal level of the entire amplifier.

I build two stage DC amps because I want to eliminate all caps that are in series with the signal path. Sure sounds better to me than ANY cap couple SET amplifier !!

If you have to bypass an Rk with film caps, that is not any disadvantage at all. Its an advantage.

You can select the best caps, plural, and get the amp to play exactly the way you want it to, in EVERY range, at the speaker voice coil, and to your ear, where it counts the most.

Of course, you have to experiment and listen, and TEACH YOURSELF how to bypass properly, what values and brands work best.

Quality - wise, Dennis tells me in 7-2015, Mundorf Silver and Oil has an entire line up of values, that will do the job. Usually, I experiment, even snuck in some Russian surplus caps here and there. Dealer's choice !!

No, my fellow hobbiests, having an Rk to bypass can be a BIG advantage, if you teach yourself how to do it well ! Yes, it costs, but it costs more to fumble on an amp, and never get it to perform at a certain level. I'm getting too old to be listening to mediocre amps, its gotta please me, its the WEAK link in the High Efficiency audio chain, at least 98% of the SETs out there are mediocre in my opinion.....

Jeff Medwin

 

RE: Small IDHT tube cathode bypassing, posted on July 6, 2015 at 22:34:49
Al Noakes
 'I build two stage DC amps because I want to eliminate all caps that are in series with the signal path'


Perhaps you should remove all cathode bypass capacitors ? Any signal through the grid also flows through these

Al

 

RE: Small IDHT tube cathode bypassing, posted on July 7, 2015 at 00:09:10
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Yes sir, great idea. I'll just put in an inexpensive solid state POS, a LED, etc etc, and have no frequency compensation, sound shaping at all.

Jeff Medwin

 

"frequency compensation" and "sound shaping", posted on July 8, 2015 at 07:58:13
Lew
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Posts: 10911
Location: Bethesda, Maryland
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 These terms are very revealing of your goals. That's perfectly OK, but many do not share those goals. And those goals are not attainable without introduction of "colorations" that most of us would like to avoid. (Coloration seems to be one of your goals, on the other hand.)

Anyway, you like to deride those who criticize your approach without having tried it. In this case, try using a Schottky or red LED and NO bypass capacitor, on the cathode to ground connection to bias a small signal triode. You might change your tune, or maybe changing your tune would constitute "sound shaping".

 

RE: "frequency compensation" and "sound shaping", posted on July 9, 2015 at 19:35:53
91derlust
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 These terms are very revealing of your goals. That's perfectly OK, but many do not share those goals. And those goals are not attainable without introduction of "colorations" that most of us would like to avoid. (Coloration seems to be one of your goals, on the other hand.)

That is the conclusion I have arrived at as well. Although ultimate neutrality is not my primary goal, I'd still like to avoid gross colouration and maintain acceptable transparency, while achieving other goals.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: "frequency compensation" and "sound shaping", posted on July 9, 2015 at 22:35:26
drlowmu
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Location: East of Kansas City
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 Both of you speak from NO listening experience with this technique, especially properly executed. Your lack of experience allows me to write off your "guesses and comments" as useless, which IMHO, they are.

Sincerely,

Jeff Medwin

 

Jeff, we are done., posted on July 10, 2015 at 16:06:05
91derlust
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 Jeff, I deleted my previous response. I do not want to get caught in negative exchanges with you - they are miserable experiences that achieve nothing that justifies the effort.

As you know, I sought the type of experience you apparently demand. The reality is that you repeatedly mandated that experience be limited only to your Rx. Any expression of creative or independent thought - especially experimentation with other methods - has been vehemently discouraged. This behaviour has been repeated continually in our personal correspondence. Because of this, I requested that personal communication between us cease. I will endeavour to extended this agreement to public exchanges as well.

All the best to you Jeff,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: "frequency compensation" and "sound shaping", posted on July 10, 2015 at 12:09:45
elblanco
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Posts: 3486
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 Jeff,

you like the sound of a dynamic volume expander and distortion. That is okay, and there is absolutely nothing wrong with creating a unique sound generating machine.

 

RE: "frequency compensation" and "sound shaping", posted on July 10, 2015 at 14:42:22
rage
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Posts: 792
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 dynamic volume expander and high distortion I disagree with.

bandwidth limited yes.

 

RE: 19KHz ..., posted on July 13, 2015 at 06:47:00
deathtube 667
 There was a post by tube wrangler that claimed -1/2dB at 19KHz or something to that effect.

Perhaps the EML tubes and all his other special techniques allow for such performance from a SE 7B4 DC 2A3 circuit.

 

RE: 19KHz ..., posted on July 13, 2015 at 06:53:26
rage
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 my point was that claims of high distortion are exxagerated.

 

RE: excessive distortion ..., posted on July 13, 2015 at 07:05:27
deathtube 667
 Based on your ARTA graphs, the distortion of the SE 12AX7 DC 2A3 was NOT unusually high from what I remembered seeing.

IMO, it looked OK.

Not sure how much your SE 2A3 circuit deviated from what DF or JM have done.

 

RE: excessive distortion ..., posted on July 13, 2015 at 07:39:11
rage
Audiophile

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 since jeff's circuit is a moving target I'm probably off by a mile. :)

 

That's perfectly OK, too., posted on July 10, 2015 at 07:34:34
Lew
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 I'll take the high road on this, because there's no such thing as "winning" this argument. But for the record, I meant no criticism of your preferred approach; I merely recommended that you might try another that is diametrically different, just to hear the result for comparison.

 

RE: "frequency compensation" and "sound shaping", posted on July 8, 2015 at 12:09:40
Al Noakes
 With a Schottky biasing a wimpy triode such as a 12AX7 , it may work but the impedence will be far higher due to the low current through the diode . Y ou would also need far more diodes stacked to get the required bias . I seem to remember Naz proposed a solution using a star power LED for around 1mA or so (archives?) .

Al

 

Good point, posted on July 9, 2015 at 07:16:46
Lew
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 But it does not negate the basic idea. I've never done it with a 12AX7 or with currents as low as 1mA. I got the idea originally from Naz.

 

RE: Good point, posted on July 9, 2015 at 18:18:58
jetrexpro
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Posts: 57
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 91,
May I ask what output tube you plan to directly couple the 6C6 driver tube to? I to very much like the sound of the 6c6 pentode.
Thanks jet

 

RE: Good point, posted on July 9, 2015 at 22:18:20
91derlust
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 My pleasure! IIRC, you had once posted some of your experiences with resistors on a similar amp on A'gon... Are you still running without a cathode bypass on the 6C6?

I will be directly coupling the 6C6 to a 2A3 run at 42mA and 250V. Now, before people think that I am blindly following the Jeff and Dennis Rx:

> The operating point has been reported as sounding pretty decent by a few people.
> I think Emission Labs ~mu 4 tubes might sound really nice with a ~1:6 mA:V ratio. This is something that Thorsten recommends with 300Bs, but it might apply to 2A3s and maybe even EML 45s.
> I am loading the 2A3 with 4K2, not 2k5. I have two beautiful 5.9Kg custom nanocrystalline beasts sitting on my shelf...
> The cathode bypass will be sized for a ~8Hz -3dB point, not 20 - 25Hz (which could work well in the right amp, but I am not experimenting with that for this build).

I will likely post the final build here, but would be happy to keep you updates as to my failures and successes as the build progresses.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Good point, posted on July 13, 2015 at 10:03:53
Al Noakes
 I've got a fixed bias 2A3 on the bench at present . I have three pairs of 2A3 : RCA/Shuguang2A3C/Sovtek2A3 . All need more than 42mA to start singing , I use 48mA at present with 260V A-K with a 5k OPTX . May be worth bearing in mind once you've got it up and running to play with the op points a little

Al

 

Thanks Al. May do just that. nt., posted on July 13, 2015 at 13:06:24
91derlust
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 .
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Good point, posted on July 10, 2015 at 01:30:21
jetrexpro
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Posts: 57
Joined: April 19, 2010
 91, I am still running my 6C6 driver without the cathode bypass cap and I like the way it sounds but it provides a little less gain to the output tube. Please do keep me posted on your amplifier.

 

Cool! Will do. nt., posted on July 10, 2015 at 02:09:02
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 .
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Small IDHT tube cathode bypassing, posted on July 7, 2015 at 01:16:10
Al Noakes
 I think you should look at how the signal flows in your amp as you don't seem to understand about signal paths and impedences . What is wrong with SS in the cathode ? You don't need a bypass cap on a diode/LED as the impedence is low , look at the impedence of a bypass cap at 20Hz and compare this with the couple of ohms a schottky diode offers . Obviously you have never tried or even considered this...

Al

 

Really enjoying the discussion gents..., posted on July 6, 2015 at 19:04:44
91derlust
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Joined: December 25, 2014
 Thanks to you both!

I have thought about using Mundorf SiO... should be nice in an amp that has a gentle roll-off in the highs (as does the 6C6-2A3... unless I use positive feedback to equalise it). Our local distributor usually has pretty competitive pricing on them.

Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Really enjoying the discussion gents..., posted on July 7, 2015 at 00:18:15
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 How about using a REAL triode for the 2A3 driver tube, only THREE parts inside it ???

2A3 is easy to drive in a DC configuration.

The WE91 is narrow band and nothing special at all. Dennis used to throw them in the dumpsters when re-doing theaters, and replace them with his modded Dyna ST-70s, 'till he figured out he could resell them abroad.

Jeff Medwin

 

RE: Really enjoying the discussion gents..., posted on July 7, 2015 at 14:58:42
91derlust
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Posts: 1101
Joined: December 25, 2014
 The subject title of the original post was misleading... I should have said IDH tube not IDHT (old habits die hard!). The 6C6 pentode meets my practical requirements and I also like it sonically.

When it comes time to try something different I may go Willie style and try an all DHT amp.

BTW, did you make a conscious decision to troll, or does it simply come naturally:

"The WE91 is narrow band and nothing special at all. Dennis used to throw them in the dumpsters when re-doing theaters, and replace them with his modded Dyna ST-70s, 'till he figured out he could resell them abroad."

You know full well that what I am doing is rather different to the WE91. Please, share your experience of the circuit I will be building. Based on your own demands for experience, you must have considerable experience, else you would be, by definition, a hypocrite.

Regards,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Small IDHT tube cathode bypassing, posted on July 6, 2015 at 14:14:19
twystd
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Posts: 2723
Location: Austin,Texas
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 I agree with others that the bypass cap on any tube, small, large, IDH or DH is CLEARLY audible. I prefer eliminating them altogether when possible, I've gone from battery bias, to diode bias lately, and really like it, LED bias is nice as well. I find filament bias to be very interesting on DHTs, but haven't gotten around to trying it yet.

twystd

 

I agree - avoid bypasses like the plague, posted on July 7, 2015 at 15:26:32
andy evans
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 I spent a whole day at one point with a huge stack of capacitors, sitting on the carpet linking them up in various combinations. All possible combinations sounded worse than none, even though there was clearly an impedance mismatch which was far from ideal. At that point I gave up.

Soon after I discovered filament bias with the 26 tube I was using, and life just got better and better from that point, which is now quite a few years ago just after Thomas Meyer started telling us about his filament bias experiments. I built a 26 preamp with a LL1660 and filament bias, then the Coleman regs came out in their first version and it got better still, then several updates of the Coleman regs came out and here we are today. I now use the 4P1L in place of the 26.

Going back to cathode bypasses for me would be like living in a mud hut.

 

RE: I agree - avoid bypasses like the plague, posted on July 9, 2015 at 23:14:11
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Andy,

From my experiences, and a fairly good understanding of this subject, you made at least three or four mistakes in this experimenting you reported above to us.

(1) One day is nothing !!!

(2) Do you know, have a good handle, on what values play what part of the music's spectrum? Likely you don't know what values to use.

(3) Only the very best film capacitors can possibly work on Rk Bypassing.

For example, Mundorf Silver and Oil would be a good minimum single brand suggestion.

Q : Were any and all of the film caps you used equal or better than that?

I would guess not.

It takes lots of listening, thought, insight, and money to do this right.

The above fact, (and the three points I outline above ), is where most DIYers will certainly fail.

Dennis Fraker is the only person "I" know of in tube audio, who knows how to do this really well, and since about 1989 BTW !!

Jeff Medwin

 

RE: I agree - avoid bypasses like the plague, posted on July 7, 2015 at 18:04:16
twystd
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 Andy, filament bias certainly works with the 4P1L, that tube is a natural for it. I'm sure the Coleman reg with filament bias is the hot setup for DHTs. However, implementing that on a tube like a 300B seems to raise some serious heat sink issues, that I don't know how to solve, as I'm pretty much a dolt when it comes to solid state.

I've been getting into some Russian tetrodes, when run in triode mode. They are just as, if not more, linear than DHTs, plus they are indirectly heated, so you don't have to mess with a filament supply.

With a driver tube, like the 5e5p or 6e6p-dr, you can diode or LED bias them, and avoid the bypass cap. With a power stage, like in my 8 watt 6p36s 2 stage integrated SET, that I'm working on now, the only way I can eliminate the cathode bypass cap on the power stage, is to go fixed bias. Unfortunately that necessitates a negative bias supply, but that's still a lot easier than a DHT with a Coleman reg and filament bias.

BTW, from what I've read, I'd sure like to hear your 4P1L preamp. Bet it sounds mighty fine!

twystd

 

Yes - filament bias on outputs doesn't go much beyond 4P1Ls, posted on July 11, 2015 at 06:59:56
andy evans
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Posts: 4378
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 I always wanted my output stage in filament bias, and didn't achieve this until I hit on PSE 4P1Ls. The thing is that you need quite a low bias voltage. Maybe 10Y is another possibility. There's not much else I can think of. Bias voltage on 2a3 and 300b is too high - the demands on filament bias would be ridiculous.

But 4P1Ls work like a charm. I have 2 of them in parallel, but 3 in parallel is quite do-able.

 

RE: Small IDHT tube cathode bypassing, posted on July 6, 2015 at 13:07:56
91derlust
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Posts: 1101
Joined: December 25, 2014
 Thanks for taking the time to to share your thoughts all.

Regards,
91
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Small IDHT tube cathode bypassing, posted on July 6, 2015 at 09:28:53
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 The commercial amp builder is dead wrong . LMAO. Amazing,..... but sadly typical in audio!!

Maybe eighty percent of what you hear in a two stage amp, is the driver tube. EVERYTHING about the driver tube is totally audible, and it demands to be built "as optimized" as you can get it. After all, if the driver stage loses ANY information, it is never received and "made up" in the finals stage, is it ? That logic does not take any EE degree, does it.

Rk bypassing on driver tubes should always be entirely done with only the highest-quality films.

Since no single cap plays the entire range of music perfectly, it is important to adjust the amp so it plays, at the speaker's voice coil, as you WANT it to, in as honest and as linear a fashion as possible. Only your ear is the judge here.

This requires multiple high-quality film bypasses ( get out your wallet ), to insure ALL the ranges are played, and all the music's nuances remain intact through the entire audio amplifier.

To date, ( since 1998 as far as I can tell, and in 2006, when I first attended RMAF to obtain direct experience ), I am very sure Dennis Fraker leads the audio world in Rk bypassing.

My present build, a Telefunken 1/2 12AX7, DCed to a Type 45, has a total of 24 film Rk capacitors doing duty on the Rk bypassing for the four triodes. I've been through three iterations of cap types the last few weeks, to get a more acceptable-to-me desired result.

AS a mater of fact, EVERY cap in a SE amp is IN the circuit, and EVERY cap needs to be fully bypassed IF you want the best possible result!! Cowboy logic, works !!

Other important build pointers,

(a) the film caps' orientations ( re: outer foil ) must each be properly determined and each installed properly "polarized",

(b) they must not touch each other's bodies, and

(c) not touch the chassis. Now you know.

Only YOU Raymond, can find the answers needed, ONLY by building and listening for yourself, until it suits you, and YOU are happy with the build.

Have fun, its a very cool trip.

Jeff Medwin

 

RE: Small IDHT tube cathode bypassing, posted on July 6, 2015 at 05:37:12
Al Noakes
 Inaudiable ? He must have tin ears . I don't claim to have golden ears or the ability to discern cable changes over the phone like some here but I do hear cathode bypass caps . Film caps are simply too big , electrolytics leave a nasty footprint on the sound . A combination of electrolytic paralleled with film doesn't sound as good as films . I tend to avoid cathode bypasses altogether favouring LEDs , stacked schottky for drivers and fixed bias for output stages . No matter what people claim about their boutique caps or 'multiple ear-tuned cap bundles' in the cathode is that these are also in series with the grid . I would rather have a smaller , superior quality cap coupling to the grid rather than a coupling cap and a huge cap in the cathode which are both in series with the signal . YYMV and all that

Al

 

Maybe no bypass?, posted on July 6, 2015 at 13:06:51
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Thanks for your thoughts Al.

I suspect my next build - which has a 6C6 pentode input directly coupled to the output tube - will have very high gain. Even when bypassed the plate impedance is so high that the plate R essentially sets the output impedance. So, perhaps I could experiment with running no cathode bypass capacitors... it would lower gain and increase plate impedance, but this may not be an issue. Maybe I should look into that?

Regards,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Maybe no bypass?, posted on July 6, 2015 at 14:48:24
Al Noakes
 High gain ? Use a lower value load and you'll get lower Rout and lower gain from the driver . Why not place the cathode circuit and G2 of the 6C6 within the cathode circuit of your output valve ? You may only need a single bypass cap between output stage cathode and 0V as you'll be running tens of milliamps through the cathode bias stack . I have only tried this with a triode connected D3A but it should in theory work with a pentode . The 'full monkey' used a similar scheme if I remember correctly

Al

 

RE: Maybe no bypass?, posted on July 6, 2015 at 19:06:41
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Ah, thanks Al... I remember you explaining that when we last "chatted" on the forum. Thanks for the option!

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Maybe no bypass?, posted on July 6, 2015 at 22:36:01
Al Noakes
 Knock up a breadboard and give it a try :)

Al

 

RE: Maybe no bypass?, posted on July 6, 2015 at 13:40:39
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 There are a number of techniques available to send the cathode resistor bypass cap. "boogeyman" running. LEDs were mentioned. Another method is the use of a NiMH cell stack between the cathode and ground. Charge the stack up before 1st turn on. The "trickle" of current between cathode and ground keeps the stack charged.


Eli D.

 

Thanx Eli - had forgotten about the battery. nt., posted on July 6, 2015 at 19:21:14
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 .
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Small IDHT tube cathode bypassing, posted on July 6, 2015 at 05:23:31
Triode_Kingdom
Audiophile

Posts: 10037
Location: Central Texas
Joined: September 24, 2006
 Many people believe that this extends to power IDHTs as well. One possible explanation is that the ESR of nearly all modern caps is sufficiently low over the audible spectrum as to render their internal characteristics otherwise not a factor.


 

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