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Old 21st July 2012, 12:06 PM   #11
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Hi!

The simplest way to bypass the signal around the cathode cap is the ultrapath connection from B+ to cathode. In same cases (depends on transformer and tube) the cathode bypass cap can even be completely omitted. The only drawback is the need for a very ripple free B+

Best regards

Thomas
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Old 21st July 2012, 01:15 PM   #12
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Quote:
Originally Posted by Davec113 View Post
I was wondering, since the driver tube's anode is ccs loaded, if it would be possible to get rid of the cathode cap in the driver stage as the ccs would stabilize the current?
You can take the cathode electrolytics away without any effect to frequency response or DC-bias, and since your anode has a CCS loading the gain will be reduced very little too.
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Old 21st July 2012, 02:11 PM   #13
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Hi Dave,

You asked for a picture of the 4P1L in filament bias, and here's a roughly drawn one for you!

I haven't tried the 126B. That's rated 30mA, but only 44H inductance. 126C is rated 15mA and 105H. For a driver stage the 4P1L could be run at around 20mA or even up to 25mA.

andy
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Last edited by andyjevans; 21st July 2012 at 02:14 PM.
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Old 21st July 2012, 03:08 PM   #14
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Ammended version - the 4P1L is used in triode mode with grids 2+3 connected to plate (pins 2+3+4). Filaments are wired in parallel (1+7 and 8)

Andy
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Old 21st July 2012, 03:36 PM   #15
Sheldon is online now Sheldon  United States
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Quote:
Originally Posted by artosalo View Post
You can take the cathode electrolytics away without any effect to frequency response or DC-bias, and since your anode has a CCS loading the gain will be reduced very little too.
Yes, easiest thing to do - unsolder one connection to the bypass cap. Put in a temporary switch if you like to do A/B.

More advanced option, for fun (ala Wavebourn), is to convert the ccs to a gyrator (fixed anode voltage, but high impedance at audio frequencies). This allows the current to adjust as the tube ages, just as it would with cathode bias.

Sheldon
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Old 22nd July 2012, 02:01 AM   #16
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Quote:
Originally Posted by andyjevans View Post
Ammended version - the 4P1L is used in triode mode with grids 2+3 connected to plate (pins 2+3+4). Filaments are wired in parallel (1+7 and 8)

Andy
Thanks! This looks interesting, I will look into it... maybe for my next amp and not the SSE though. I think I will start with removing the cathode cap in the driver stage of my SSE, that's a simple enough mod.
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Old 26th February 2013, 12:04 PM   #17
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I tried it on my KT-88 based SE which has an D3a driver with a CCS as anode load. Simply removed the 1500uF electrolytic cap there and the sound has cleared up. I havent done a A to B compare..
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Old 26th February 2013, 03:50 PM   #18
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remove c10, c20. and add a 2.2uf poly cap across c11 and c21 also.
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Old 2nd March 2013, 12:00 AM   #19
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Its the funniest thing...

Everyone talks about caps as if they are the culprits in "the sound" (problem). After much esquisitely precise testing ... I have found that the "cathode cap" is not in itself the "problem", but rather tightly controlled fixed bias (which is what a resitor + capacitor makes). In essence, the resistor and capacitor form a tiny, compact power supply in series with the cathode, raising its voltage relative to the grounded grid. To all the well-informed here, this is nothing at all new, or even "newly revealed". It just is.

For instance ... so long as one's using a separate winding of the transformer for filament power, it is pretty straight forward to make a low current negative power supply ... a few rectifiers, capacitors, a very low current choke, and so on ... With nothing more sophisticated than a quality pot, one can get 0 to -15 volts of bias for the grid. Grids just aren't connected by a high-value resistor to round, but rather, to the wiper of the pot sitting across ground and the C- supply.

The effect of this kind of supply is exactly the same as using a cathode-resistor-and-capacitor (of sufficiently high quality capacitor and low-value low-impedance double-bypass caps, of course). Using a DPDT switch and careful calibration of the ampere flow through the cathode ... one can do A/B comparison trivially. Amplification remains a constant. Whatever "problem" the fixed bias has given the system, remains in both cases.

Not so... when one uses the bias-in-cathode resistor WITHOUT the capacitor. Now suddenly the tube will behave much less like a Gm (transconductance) amplifier, and more like a cathode follower (with multiplicative gain on the anode load). Almost all nonlinearities that the transconductance-as-a-function-of-current curve produces with fixed bias are removed. Not all, must most. In simple circuits without fancy constant-current or cascode loads, the net effect is reduced gain and increased linearity. It is hard to do the "a/b" switch ... requires a 3PDT switch to do it right and an OUTPUT pot to reduce gain from the cap-bypassed higher-gain configuration. Doable, but not easy. The sonic difference though is quite remarkable. "Floating bias" (or loosely, cathode-follower) response is more linear - but the effect seems to also be a slight muffling of the sound. Overall - when the whole system gain is pepped up just a bit to accomodate that ... the sound becomes lovely and more rich due to increased symmetric linearity.

So... there you are. Stuff wrought from hundreds of hours of fiddling.

GoatGuy
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Old 2nd March 2013, 07:41 AM   #20
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Thanks. I agree that the sound without the bypass caps is a bit thin. I will try putting in some bypass caps, keeping it around 500uF low ESR. I previously had 1200uF, low ESR.. Lets see.
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