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2A3 without cathode bypass cap???

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2a3 without Cathode bypass Cap ???

Hello

After finishing new ,clone type Sun audio 2a3 amp I noticed that
I run amp without cathode caps, only 750 0hm resistor .
Sound was incredibly clear and detail with 55 ma current and 240 volts between anode en cathode .

Afterwards I added 100 uf caps on cathode but Psu turned unstable ?
Voltage started to drift up and down , making some low frequenzy pumping. Psu and Circuit is exactly same as Sun 2a3

I would be pleased of advise running amp without caps on cathode?
Sound is so good so i would add cap only if its necessary

Thanks

Papparazzi
 
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Something wrong here, does the amplifier use negative feedback from the secondary? I am not that familiar with the Sun amp, but a lot of Japanese SE designs do use global feedback - if so you might have it inverted (positive feedback) - try swapping primary leads (or secondary, but NOT both) if such is the case.

Running without a cathode bypass cap will result in very high plate resistance with the 2A3 - about 3.5K in fact which will result in very poor low frequency performance due to the finite inductance of your output transformer primary, this will result in very limited low frequency output and lots of distortion. Overall output power and linearity will be compromised due to the lack of voltage gain in the output stage, and the severe mismatch between the output tube and transformer. Distortion will be (much) higher, and output power much lower.. Damping factor will be <1 as well. I can't recommend this under any circumstance. (I've tried this too.)

FWIW I much prefer fixed bias with no cathode resistor or bypass cap required in the output stage.
 
Hi

Thanks for your comments !!!!

First , forgot use cathode caps when turning power on first time . When i noticed it i was suprised how good was sound . I have no global feedback .simple cascode Jelabs/Sun driver circuit . No lack of bass at all , sound is very clear maybe too much highs ? Primary load is 2,5 k Tranny has also 3.5 k tap.

Amp has no electolytics at all 47uf/500v pp caps used .Also in driver Cathode



Regards

papparazz1
 
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Joined 2004
Paid Member
papparazzi said:
Hi

Thanks for your comments !!!!

First , forgot use cathode caps when turning power on first time . When i noticed it i was suprised how good was sound . I have no global feedback .simple cascode Jelabs/Sun driver circuit . No lack of bass at all , sound is very clear maybe too much highs ? Primary load is 2,5 k Tranny has also 3.5 k tap.

Amp has no electolytics at all 47uf/500v pp caps used .Also in driver Cathode



Regards

papparazz1

Too much highs are most likely the consequence of a rising response due to insufficient primary inductance relative to the excessively high output tube rp you get when you have no cathode bypass cap. You need to find out why the amplifier motorboats with the caps installed - that could be an indication of unintended feedback through the supplies or some other as yet undiscovered error. Once you sort this out it will sound much better with the cathode bypass cap in place than without.

I avoid electrolytics whenever possible, but I really prefer fixed bias in output stages regardless of cap used.

I had an infatuation with unbypassed cathode resistors in voltage amplifier stages at one time, measurements later confirmed what many were saying about distortion - it is counter intuitive but in a lot of cases unbypassed cathode resistors while providing local feedback actually increase thd over a bypassed configuration, much worse is what happens to noise performance particularly with low signal levels and high mu tubes. Cathode resistor johnson noise, current induced noise and other noise depending on resistor composition get gained up by mu+1 which can turn a relatively quiet stage into something with an output noise voltage of perhaps as much as 300 - 400nVrtHz or worse which is rather noisy actually. (And is totally avoidable unlike some other noise sources)
 
Thanks again of all Info replyers !!!

I added coupling to all stages . BG for 6sn7 and 100uf Elna for 2a3 output. Sounds more coloured but still good , and as cathode caps do, add noise and spoils clarity whatever cap type I use

One moment I thought I was reaching something special . On my Bis record Handel - Emma Kirkby -London Baroque recording
i could hear that there are Two violins on very highest notes playins same tone ;thats with No caps in output stage . Adding caps at output made all sound same as before..
But still great soundind Scratch Build Sun clone

Papparazzi
 
Hi

Here is picture on right ,Left is my SE KT88 amp


Papparazzi
 

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Lot's of different opinions on this, I see...

I run my SE amps without cathode bypass caps.
I think they sound much better without them.
I don't use those amps for bass, though, only from 200Hz and up.

I don't use cathode bypass caps at all unless I absolutely have to ...
 
I don't use cathode bypass caps at all unless I absolutely have to ...


Hi

No caps in in output cathode brings some Clarity magic but sounds littlebit "Weird" on mids and Highs , as some upper harmonics are missing?
I changed Back to Elna Cerafines and wich made sounb again "Normal" wit typical Colorations

Thanks again to all of advice !!!!



Papparazzi
 
I found out that Nichicon Muse BP (bipolar) caps have higher clarity when they are used as bypass caps, at least they did so in some of my circuits.
However, I also figured out that managing not to use any bypass caps IF feasible resulted in the best sonic characteristics. I share the idea of using a fixed bias if it is possible.
 
I found out that Nichicon Muse BP (bipolar) caps have higher clarity when they are used as bypass caps, at least they did so in some of my circuits.
However, I also figured out that managing not to use any bypass caps IF feasible resulted in the best sonic characteristics. I share the idea of using a fixed bias if it is possible.

According to the calculator I posted a link to a few posts ago, I get a treble boost with no bypass caps.
 
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