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Old 16th April 2012, 03:10 AM   #1
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Default 300B amp cathode bypass

Hello, everyone!

I built Andrea Cuiffoli's "new reference" 300B amplifier (my schematic is attached) as well as a BK-12m speaker kit (with Fostex 126en). The overall sonic performance is unsatisfactory. The voices are recessed which makes you want to turn up the volume, which only makes it sound loud and doesn't add to the presentation, so I just turn it back down.

I suspect the 2200uF cathode bypass (Andrea recommended) is too high. I want to try the 220uF Nichicon KZ. I used the formula for calculating the bypass cap value, and 2200uF is not even in the ballpark for this application...

Any thoughts? Thanks!
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Old 16th April 2012, 03:18 AM   #2
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I'd try the 220uF. The quality of the bypass cap on the D3A is also critical. I would look around for a few different ones to try and would recommend shunting these with film caps as well.

I'm not a big fan of FR speaker systems, but before concluding the problem is with the amplifier design can you test the amplifier with some other speakers?

What is the story with the GZ34 diagram? I'm assuming you have a filament supply and that this is a graetz style bridge circuit.
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Old 16th April 2012, 03:21 AM   #3
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Sure wouldnt hurt to give it a try, a KZ is inexpensive compared to other options. If youre worried about the cap being too big you could try bypassing it with a small poly film, maybe 0.22-1uf? At first glance the 0.47uf coupling cap looks a little excessive too...that would be my first two considertions...
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Old 16th April 2012, 03:29 AM   #4
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Looks like your resistor at the coupling cap after the 0.47uf is high by 100k compared to the design....he shows a 200k and you show what appears to be a 300k....i am not sure what effect exactly this has on the filtering effect at the 0.47uf/300k junction but it may cause some rolloff...you will need someone here a little more savy than myself to calculate that...
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Old 16th April 2012, 03:35 AM   #5
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Have you checked your B+? also, in reviewing the original schematic he is showing x2 120R at the power supply right after rectification...i am not sure of that means one 120R per channel or if he intended it to be a net 60R per channel....either way you have a 120+68R per channel which is definitely more than stock but, I also see you have about 14v more on your power trannys secondary...either way, simple to check your voltages and see if you are not too low....that could answer someof your issues as well as all the operating points would be off some depending on how far off your B+ is....just a couple of observations...
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Old 16th April 2012, 03:41 AM   #6
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Lastly, then I will shut up, i noticed the cathode bypass on the D3a is not given on the originsl schematic, i doubt it would change much but 470uf seems a bit high? You could also try the LED on the D3a cathode and possibly a CCS on the plate set around 20mA....
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Old 16th April 2012, 03:41 AM   #7
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The grid resistor is 200K (sorry for the chicken scratch). The extra 68Ohm resistor brings B+ down to the desired 400V since I'm indeed using a different power transformer. This schematic is a bit out of date since I got new tubes that now show ~62V at the cathode, so it's more in line with spec on that point also.

It seems that electrolytics are the only choice for the 220uF/100V type caps, and Nichicon KZ has been mentioned as among the best in various places. Are there choices I'm not aware of?

By the way, I just tried bypassing the cap with 1uF (the only value I have handy) and I think things improved somewhat, but still not great. Now I think (from just listening) that the frequency response is uneven.
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Old 16th April 2012, 03:44 AM   #8
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Thanks for the replies and suggestions! The D3a bypass of 470uF is, again recommended by Andrea (I also e-mailed him about this, but the language barrier is preventing effective communication).
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Old 16th April 2012, 07:25 AM   #9
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Default 300b

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Originally Posted by mariner4 View Post

It seems that electrolytics are the only choice for the 220uF/100V type caps, and Nichicon KZ has been mentioned as among the best in various places. Are there choices I'm not aware of?
Oh, I suppose you could bypass with a film type, some have done it here but it requires a bit of real estate...parallel a couple 100uf 200v Solens or similar...

Another thought would be to lower your coupling cap to a much smaller value to effect a high pass with a rolloff around 30hz...not much need to have much, if any, of the 6w dedicated to frequencies you are not using...again, i would have to do a little research on how to calculate it, cant do itin my mind off hand. I am sure someone here watching can help here....like i mentioned earlier, at first glance 0.47uf seems a little big...
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Old 16th April 2012, 07:33 AM   #10
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As another has mentioned, your choice of speakers has probably more to do with the sound and frequency response than anything you could do with amp mods...i havent looked at the 126e frequency response in a while but I would bet it is anything but flat in the range you are concerned with...i dabbled a little with several fullrange designs an personally found all of the ones I tried terribly lacking...just one persons humble opinion....i never could afford the ultra expensive Lowter and similar so maybe I quit too soon...if you have access to some reasonably effecient two pr three ways with a flat response i would give it a listen with them. Did you add any BSC to your 126e?
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