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Old 10th May 2009, 01:00 PM   #1
tmhajw is offline tmhajw  United Kingdom
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Default increasing coupling capacitor value in we 91

Hi There
Just built a WE 91 (Sound Practices..) only my 2nd power amp build. I've used Russian teflon 0.22uf coupling caps between the 6sj7's and 300bs, and, always curious, am wondering if I can get a richer mid/bass if I bypass these with 1uf Russian POI's...
The schematic stipulates cap values as between 0.1 and 0.47uf, so what are the downsides of having a cap value of 1.22uf or so?!
Thanks
Thomas
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Old 10th May 2009, 04:57 PM   #2
pointy is offline pointy  United Kingdom
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I think you mean de-coupling caps these are to block inter stage feed back and some DC. if so you will not get much from making them a higher value and you should use (0.1uf or 0.22uf or a 0.47uf)

there are better places to do what you are looking for such as pre-amp and the in-put of the power-amp.

I think you may find your answer at the in-put to the power-amp maybe
the resistor to ground could be a little bit lower in value.
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Old 10th May 2009, 05:56 PM   #3
tmhajw is offline tmhajw  United Kingdom
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Oh, what does altering the resistor at the input to ground do? I assumed (?!) this only altered the gain...
It's a 220k at the moment, what sort of value do you suggest?!
Thanks for your reply- I'm a total 'newbie' at this...!
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Old 10th May 2009, 06:16 PM   #4
IDSKoT is offline IDSKoT  United States
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Excuse me, but I don't know another way of subscribing to a thread other than posting. So, subscribed. Interested in this.
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Old 10th May 2009, 06:42 PM   #5
pointy is offline pointy  United Kingdom
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still use the same resistor value but use a multimeter to get one at the lower end of the tolerance band. this should give you some tonal change in the direction you wished.
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Old 10th May 2009, 07:10 PM   #6
tmhajw is offline tmhajw  United Kingdom
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Now I'm intrigued...!
OK I just so happen to have a handful of 220k carbon films, most of which I've just measured 218/219k on my DVM. A couple measure 216k.
I've just lugged (!) down the amp and measured the ones in there, across the input jacks, one is 219k the other 218k.
So swapping these with the 216k will alter the tone of the amp?! How does that work, please?!
Thanks Thomas
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Old 10th May 2009, 09:15 PM   #7
pointy is offline pointy  United Kingdom
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you should be able to get a good tone within the 220kohm tolerance band ,which is the stated value on the designers schemo .
what sometimes happens is the designers will want you to use a resistor at one end of the tolerance band but they will only put the shop supply value on the drawing.
so with some components you may need to try a few to find the best set up.
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Old 10th May 2009, 09:25 PM   #8
kevinkr is offline kevinkr  United States
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Increasing the value of the interstage coupling capacitor (C7) will only improve the low end response provided that there are no other design issues, and the output transformer and/or speaker load are not the culprit to start with.

I assume the question pertaining to the 220K resistor relates to R5 in the SP schematic. (I have most issues of Sound Practices.) Changing the value of this resistor will have no effect on the sound at all with most sources unless their source impedance is extremely high and that resistor then represents an appreciable load on that source. It is shunt connected across the source and is usually dominated by the low driving impedance (ac!) of the source which may be several orders of magnitude less.

IMO the place to start is with C4 (cathode bypass capacitor) - which at 16uF is not nearly large enough for good bass response. A 100uF motor run type is a reasonable choice, and likewise substantially increasing the value of C2 (47uF or so) will help in conjunction with the other change.

The design as implemented will not have particularly good performance below 100Hz because of the high supply source impedance at these frequencies, and because of a very early roll-off with substantial peaking in the low-mid bass region due to C4.

I strongly recommend you pick up a copy of Morgan Jones "Valve Amplifiers" (3rd Edition) and optionally a copy of his "Building Valve Amplifiers" as well, this will make things a whole lot clearer.
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Old 10th May 2009, 09:26 PM   #9
pointy is offline pointy  United Kingdom
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within a 10% band it maybe as low as a 200k ohm resistor
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Old 10th May 2009, 11:10 PM   #10
tmhajw is offline tmhajw  United Kingdom
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Yes actually I must admit I did encorporate a few changes from the schematic when I built it...
For C4, the cathode bypass for the 300b I used a 47uf polyprop;
C1 & C2 the p/s caps I increased to 100uf & bypassed;
C5 the pentode cathode cap I used a 100uf & bypassed.

So you suggest increasing C4 preferably with a motor run 100uf; I must admit there's not a lot of room in there now! Guess I can drop in a 100uf electrolytic across the polyprop....would this be reasonable?

Out of interest, to go back to my original question, what are the downsides to increasing the size of the coupling cap C7 to around 1.22uf?!
Thanks
Thomas
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