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Preamp Design and Bypass Capacitors
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Structo



Joined: 17 Oct 2007
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Location: Oregon

PostPosted: Thu Nov 05, 2009 6:11 pm    Post subject: Preamp Design and Bypass Capacitors Reply with quote

I read somewhere that when bypassing a preamp tube such as a 12AX7 that the voltage rating of the cap should be small because the larger the voltage rating the higher the ESR of the cap.

I was doing some reading at the Valve Wizard site and Merlin says that polarized electrolytic caps can exhibit low frequency distortion.
Too much low freq content can lead to blocking distortion.

He mentions using 6.3 volt electrolytics or even better use non-polarized caps.

He also says that using a non-electrolytic cap such as a poly film cap would sound the best but finding one with a high enough capacitance is difficult if not impossible.

My question to the more knowledgable builders here is:
How important do you feel the voltage rating is on a bypass cap in the preamp with ESR as a consideration?

The last time I changed all my bypass caps (D'Lite) I used NTE 4.7uf 350v caps because the place I bought them didn't have lower voltage caps.

Do you recommend using the smallest possible voltage rating on a bypass cap instead of a large voltage cap, such as 25v or above?
Such as 10v or less?

Have you ever used non-polarized bypass caps?
Or Poly caps for bypass caps?

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tubeswell



Joined: 27 Mar 2008
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PostPosted: Thu Nov 05, 2009 6:25 pm    Post subject: Reply with quote

I have use poly caps for bypass caps for marshallesque type values calling for .47uF, and even .1uF. There is nothing wrong with using poly caps, but using more-expensive (and we are talking in scale of a few cents here) high-voltage poly caps is a waste in a cathode bypass situation, where a 10V or a 20V cap will do the job.

As for polarized caps, 25V work fine in a pre-amp, or 63V or 100V in an output tube. 2CW
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Structo



Joined: 17 Oct 2007
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PostPosted: Thu Nov 05, 2009 6:30 pm    Post subject: Reply with quote

Thanks.

What are your feelings about the voltage rating as far as the ESR of the cap and how that effects the tone?

Somebody at another forum mentioned if you use too high of voltage caps in the bypass that it can add shrill overtones.

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JD0x0



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PostPosted: Fri Nov 06, 2009 1:33 am    Post subject: Re: Preamp Design and Bypass Capacitors Reply with quote

Structo wrote:


He also says that using a non-electrolytic cap such as a poly film cap would sound the best but finding one with a high enough capacitance is difficult if not impossible.


Couldnt you just put a second cap in series if the value isnt high enough?

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greiswig



Joined: 20 Jun 2007
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PostPosted: Fri Nov 06, 2009 1:36 am    Post subject: Re: Preamp Design and Bypass Capacitors Reply with quote

JD0x0 wrote:
Structo wrote:


He also says that using a non-electrolytic cap such as a poly film cap would sound the best but finding one with a high enough capacitance is difficult if not impossible.


Couldnt you just put a second cap in series if the value isnt high enough?


Parallel is the way to add capacitance. Series adds voltage rating.

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JD0x0



Joined: 09 Oct 2007
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PostPosted: Fri Nov 06, 2009 1:48 am    Post subject: Re: Preamp Design and Bypass Capacitors Reply with quote

greiswig wrote:

Parallel is the way to add capacitance. Series adds voltage rating.


Ahh thank you for the correction

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jjman



Joined: 06 Oct 2007
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PostPosted: Fri Nov 06, 2009 2:35 am    Post subject: Reply with quote

HiFi guys are using LED biasing on preamp tubes. A red one provides about 1.6volts and no cap is needed. They run a few in series when more bias voltage is needed, or a different color. Those guys are adverse to caps in general so they know how to avoid them. If you don't want the drawbacks of a cathode cap, I'd check into it.
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tubeswell



Joined: 27 Mar 2008
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PostPosted: Fri Nov 06, 2009 6:45 am    Post subject: Reply with quote

Structo wrote:
Thanks.

What are your feelings about the voltage rating as far as the ESR of the cap and how that effects the tone?

Somebody at another forum mentioned if you use too high of voltage caps in the bypass that it can add shrill overtones.


Its more the electrolytic caps that are prone to ESR affecting tone AFAICT - its something to do with the dilectric compound they use to get higher voltage ratings.
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Jana



Joined: 07 Sep 2008
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PostPosted: Fri Nov 06, 2009 4:13 pm    Post subject: Reply with quote

You could also experiment with adding a poly cap in parallel with the electro. Put a .047uf to .68 uf cap in parallel with your 4.7uf cap and see if you hear any difference. This would be similar to the idea of putting poly caps in parallel with the B+ caps to lower the ESR in the power supply.

For what it's worth, I personally just make sure I have a high enough voltage rating to meet the needs of the circuit. If I am using electro caps for bypass I usually go with 25 volt ratings since they are common and my parts box has a lot of those. I don't think twice about using a 50 volt cap, however, if that is all I have in my parts box--I don't consider it a "this will have to do" part but rather "cool beans, I have a part!"

Having said all that, I don't use any electro caps to bypass the Ax7's. Not because of any aversion to electro caps but because my design for my favorite pre-amp circuit uses only one .68uf cap for bypass. As such, I just use a .68uf poly.

Unless it is all you have, using a 350 volt part would be a waste of money and physical space.
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collinsamps



Joined: 06 Mar 2009
Posts: 223
Location: North Carolina

PostPosted: Sun Nov 08, 2009 5:22 am    Post subject: Re: Preamp Design and Bypass Capacitors Reply with quote

Structo wrote:
I read somewhere that when bypassing a preamp tube such as a 12AX7 that the voltage rating of the cap should be small because the larger the voltage rating the higher the ESR of the cap.

I was doing some reading at the Valve Wizard site and Merlin says that polarized electrolytic caps can exhibit low frequency distortion.
Too much low freq content can lead to blocking distortion.

He mentions using 6.3 volt electrolytics or even better use non-polarized caps.

He also says that using a non-electrolytic cap such as a poly film cap would sound the best but finding one with a high enough capacitance is difficult if not impossible.

My question to the more knowledgable builders here is:
How important do you feel the voltage rating is on a bypass cap in the preamp with ESR as a consideration?

The last time I changed all my bypass caps (D'Lite) I used NTE 4.7uf 350v caps because the place I bought them didn't have lower voltage caps.

Do you recommend using the smallest possible voltage rating on a bypass cap instead of a large voltage cap, such as 25v or above?
Such as 10v or less?

Have you ever used non-polarized bypass caps?
Or Poly caps for bypass caps?




To say that larger voltage rating electrolytics have a higher ESR is technically incorrect, because the ESR lowers proportionally to the plate area of the cap getting larger.

It's also true that the thicker the dielectric, the higher the ESR.

So having two electrolytic caps of similar capacitance and voltage ratings where one is short with large plates, and one is long with small plates, the latter will exibit a larger ESR, so when going big, go with short & fat if you want to keep it low.

While ESR adds a tad more series resistance it certainly doesn't make or break the bias point of a preamp circuit or power amp circuit in a tube amp. ESL has more to do with function and tone.

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Andy Le Blanc



Joined: 22 Dec 2007
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Location: central Maine

PostPosted: Sun Nov 08, 2009 5:12 pm    Post subject: Reply with quote

The bypass cap controls degenerative feedback through the cathode resistor.
The incoming signal causes the plate current to fluctuate. The bias voltage,
the voltage drop across the cathode resistor, will also fluctuate.
This condition is not desirable because it reduces the amplification available.

You only need a component voltage rating that at least twice the working voltage,
and if you look at the RCA RC amp tables you will see that the values of
bypass caps recommended to ensure a relatively flat frequency response are
small compared to what you typically see in an instrument amp.

I like diode bias. It sounds good. I like to dress half of a 12ax7 to resemble
a diode, and bias the other side. You can also make one hell of a dirt tone
with the right LED, they make gobs of non-linear harmonic distortion.

The truth is you need to use the parts you can get, if all you can scrounge
is what you got, then your gonna use it until you find better.
No worries, listen to what you make, you can run test software, or put the
DVD player thru it, and swap out parts until you like what you hear.

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dobbhill



Joined: 22 Jan 2005
Posts: 425
Location: Louisiana

PostPosted: Sun Nov 08, 2009 6:22 pm    Post subject: Reply with quote

You can also use fixed bias........
Never done it on a guitar amp, but have used it in Hi-Fi with excellent results.
D

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Structo



Joined: 17 Oct 2007
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PostPosted: Sun Nov 08, 2009 6:28 pm    Post subject: Reply with quote

Thanks all, a very good discussion. Very Happy

@ Mr. Collins,
You have stated pretty much what I have discovered in my research.
I wonder how the ESR affects the frequency since we know a resistor in series can filter them on grids.
ESL is another factor that I ran into that is also part of the scenario.

Hmmm, I wonder what an LED would sound like?
I assume you would put the cathode to ground?

Yesterday I was browsing Mouser and I really had difficulty in finding low voltage electrolytic caps.
I ended up ordering some 4.7uf 50v caps.
It was interesting as I chose the properties of the caps that as soon as I clicked on 6.3v or 15v or 25v the availability went to 0.

So there must not be a very big demand for small axial E caps with low voltage ratings anymore.

I didn't scour the net looking for them either.

I ended up ordering some Sprague/ Vishay 4.7uf 50v ones.

I actually found some of those at the local electronics place the other day, I'm not sure what brand they are but they are tiny!
I have not idea how old they are.

I was looking at a few photos of D clones here and it looked like the top builders use 50v caps.


I was referred to this link at another forum. Pretty interesting how this guy pretty much dispels all the BS about caps and other audiofool myths.
Although he addresses mostly solid state gear he does have a section on tube amps.
He also states that the lead length definitely affects the capacitance.
He says in order to only have the capacitance of the cap itself, you have to make sure the leads are as short as possible.
He says that even a few millimeters can affect the value.
I hadn't heard that before, although I am aware that two conductors separated by a dielectric is a capacitor in theory, I just never read the part about the cap leads affecting the value.

http://sound.westhost.com/articles/capacitors.htm

If you don't feel like reading it that is fine. In his conclusion he states,

" I have never seen the specifications for snake oil as a dielectric, but I expect it to have rather poor performance overall. With 'magic' components, in the end everyone loses. DIY audio is supposed to be fun, not an endless search for the mystery component that will make everything sound wonderful. Sad news ... that component does not exist."
Laughing

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Structo



Joined: 17 Oct 2007
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PostPosted: Sun Nov 08, 2009 6:31 pm    Post subject: Reply with quote

dobbhill wrote:
You can also use fixed bias........
Never done it on a guitar amp, but have used it in Hi-Fi with excellent results.
D

When you say fixed bias are you talking about a non bypassed stage?

I know most Hi Fi guys hate bypassed preamp stages, at least that is what I have read.
They don't seem to like any capacitor because it can color the sound.
But I believe you still have to use decoupling caps between stages.

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paulster



Joined: 10 Oct 2006
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PostPosted: Sun Nov 08, 2009 8:40 pm    Post subject: Reply with quote

collinsamps wrote:
To say that larger voltage rating electrolytics have a higher ESR is technically incorrect, because the ESR lowers proportionally to the plate area of the cap getting larger.

It's also true that the thicker the dielectric, the higher the ESR.

So having two electrolytic caps of similar capacitance and voltage ratings where one is short with large plates, and one is long with small plates, the latter will exibit a larger ESR, so when going big, go with short & fat if you want to keep it low.

A very good observation.

If you're fussy about these things then the trick is to compare datasheets when choosing caps as you may find that going up a voltage notch can have a positive (or negative) benefit.

Structo wrote:
I wonder what an LED would sound like?

Tom

Did you ever hear any of (our former friend here) Gary Moore's amps? Those fabled BITE lamps were the cathode bias LEDs.

Although, to be fair to the poor LEDs, I don't think they'd sound that bad if properly implemented.

Game over for your amp during a show if you get a preamp tube fail short though as that LED will be long gone.
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