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Old 31st August 2009, 12:03 PM   #1
costa23 is offline costa23  Indonesia
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Default je lab 300b

in je lab 300b design, the 6sn7 input has no bypass capacitor,
what's the pros n cons if i add bypass capacitor in the 6sn7 input part.
thanks
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Old 31st August 2009, 07:13 PM   #2
smbrown is offline smbrown  United States
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In my opinion, the result will be even more gain than what you already have which is too much. The double 6SN7 (or 76 to 6SN7) provides an awful lot of gain. And yes, I've built it. Same as other similar designs like the Richart Flesh & Blood amp. Also, leaving that cap out of the circuit is good in that it leaves a cap out of the singnal path. For even more fun, try a pentode driver for your 300B, or you can use a single stage and a choke or CCS load on the anode and be done in one stage instead of two. Lower part count, simpler amp.
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Old 31st August 2009, 09:57 PM   #3
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that point isn't about opinion, it is about engineering fact.

a bypassed cathode resistor will provide more stage gain as he says by virtue of not allowing feedback to be developed across the resistor.

it is local feedback, note local feedback and feedback around single stages, this is an opinion as you cannot prove or disprove it factually, is not an evil in the way multi stage feedback is, in that it does take away a lot of musicality, subtle nuances, and evens out the whole sound.

however, I prefer capacitors in.

also note that I discerned an insignificant difference in cathode caps, only a black gate was mildly better than most and to such a degree I wouldn't lose 5 seconds worrying about which one to put in there, it is totally uncritical so any decent make will do very well indeed

There is nothing particularly bad about capacitors, even if they are in the signal path, I have no idea where this accepted evil of caps. comes from.

it like many things in audio is absolute rubbish, spouted to make people look good or shed out loads on useles expensive capacitors, note all things are about money in one way or another, and this is just another one. its shocking how far people will decieve and things become accepted just to get money.

Chokes are more of an evil than caps. trust me, they add significant resistance, hence causing power loss, resonate electrically with associated capacitances, change their inductances due to voltage, current, temperature, vibration

simply said, the purpose of minaturisation and progress in electronics, a massive aim is to eliminate chokes, in graphic eq's, power supplies, output transformers, and speaker crossovers, many of those is quite demonstratable in improving sound

a pentode driver usually gives very poor hi end extension, do try, I ahve done most things, and its very weak on current.

your 300b needs it for 2 reasons, 1, its a large input capacitance, that draws current, 2, the prior stage has to be a low output impedance, in other words, hi current

consequences are a dire slew rate and reduced hf extension.

soudwise, this driver stage is rather thick and heavy to me in the bass/lower end, however it is quite 'bubbly' , dynamic and enjoyable in the midrange.

why don't you just pull it out and put the cap in and see and decide for yourself, and go to the literature and learn what it does? its all out there somewhere.

it is true that this 2 stage driver has a lot of gain, however, one thing that confirms to me that they spout junk is that a 2 stage amplifier is nonsense, as it will inevitably be driven by an active preamp, effectivly making 2 active driver stages, making a mockery that a single stage driver and output tube amplifier is pure and simple.

there is little difference between an integrated 3 stage and a 2 stage with an active preamp stage. and I for one cannot believe the ignorance of designers who ignore this simple evident fact.

remember this when someone tries to hard sell a 2 stager.

Last edited by lt cdr data; 31st August 2009 at 10:08 PM.
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Old 1st September 2009, 12:41 AM   #4
costa23 is offline costa23  Indonesia
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thanks a lot
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Old 1st September 2009, 11:45 AM   #5
smbrown is offline smbrown  United States
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Yes, not all pentode drivers are created equal. And yes, hf rolloff can be an issue. I tried the one on the Angela Instrument site (still there?) and though it was very dark sounding. I then tried the one developed by Thorsten using an EL84 and thought it sounded great.

As for two stage amps, I was thinking last night about this post and thought I should mention that yes, you'll likely need an active preamp with maybe a 4 or 5 volt output, so thanks for mentioning that Lt Cmd. Or, another option with the JE labs amp is to simply call it an integrated amp and put a volume control an an extra input or two on it.
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Old 2nd September 2009, 10:01 AM   #6
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Hi lt cdr data,

Quote:
Originally Posted by lt cdr data View Post
a pentode driver usually gives very poor hi end extension, do try, I ahve done most things, and its very weak on current.
You might try something like this:

Click the image to open in full size.

Regards,

Tom Schlangen
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If in doubt, just measure.
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Old 2nd September 2009, 10:11 AM   #7
Boeing is offline Boeing  Japan
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Thanks for Mr Tubes4e4 for sharing EF80 cure. I used 6SJ7 with pentode connection, to drive 300B, this is first time I see EF80 do it. As the cure you post, I think EF80 properly be the good driver for 300B. I have some EF 80, I will give it a chance.
Thanks for your sharing.
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