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Old 26th March 2009, 03:57 AM   #1
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Unhappy Changing grid leak bias preamp to cathode bias

Hi..it's me again. I'm thinking my questions are below your standards since I am not getting responses, but how about giving a rookie a little help? I expext you all knew as little as I do at some point, right? Anyway, I'd like to change the grid leak bias front end of a couple amps to cathode bias. Doesn't sound too impossible, but I'd like a little guidance before I torch something up unnecessarily. Since I deal with mostly guitar amps, I like to use stompboxes to boost the front end, and have heard this is dangerous with grid leak bias. I like the sound of octal preamp tubes...especially 6SJ7 and 6SC7...more than 9 pins like 12AX7, but a lot of older amps with octals are grid leak bias. Just a little info is all I'm asking...I catch on quick. Thanks in advance...
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Old 26th March 2009, 04:24 AM   #2
kevinkr is offline kevinkr  United States
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I think grid leak bias was used a lot in the days when good high value electrolytic caps were hard to get - keeping the impedance low in the cathode circuit at low frequencies helped keep hum coupling to a minimum between the filament and cathode. (This was quite effective against electro-static coupling, but not electro-magnetic coupling.)

I would measure the voltage drop across the plate resistor of each stage you want to convert, and then change the input grid resistor to 1M ohm and temporarily install a 5K - 10K 1W pot in the cathode circuit. Adjust the pot so that the plate voltage drop closely matches the value you measured when the stage had grid leak bias, turn off the amp and measure the resistance. Replace the pot with the closest fixed value resistor you can find, and in addition add an electrolytic bypass cap of 22uF - 100uF in parallel with the cathode resistor. (You might need to tune this value to sound right.)

Before you do anything though do read the safety and high voltage thread if you are not thoroughly familiar with safe practice around high voltages.

Regarding your comment about being ignored, lots of people here, like you, are comparative newbies - they may look at thread and not know the answer, others who do may be comparatively busy at a particular moment and can't answer in a timely fashion. I have no access to the forum during the normal working day and others may be in the same boat.. Hopefully my answer was helpful, and do post or email me if you have further questions.
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Old 26th March 2009, 08:38 AM   #3
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Yes, do as Kevin suggests, but one other thing I would mention.

Grid leak bias typically uses very high values of grid leak resistor, like 10 Meg. When you replace this with 1 Meg (or less) to set up your cathode bias conditions, the effective input impedance is greatly reduced, by a factor of at least 10 to 1. This will normally require the preceding coupling capacitor to be increased in value by the same factor, to maintain the same low frequency response. So, you may find you lose a lot of bass unless you increase the value of the preceding coupling cap tenfold or more.
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Old 26th March 2009, 11:18 PM   #4
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You also need to be aware of some limitations.
If this is for older style "octal tubes" and especially if it is for a Hi mu triode like 6SL7 (for example) then residual grid current can still upset the bias point. Grid current gets worse as anode current increases.

RDH gives some useful "rules of thumb". If using fixed bias then Rg1 should be no higher than 2 times the anode load resistor value. If using cathode bias then Rg1 can be up to three times the anode load resistor.

Grid Leak Bias is inherently noisy and should be avoided in low signal level parts of the circuit - no good for phono preamps for example.

Cheers,
Ian
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Old 26th March 2009, 11:59 PM   #5
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Thanks guys...very cool advice, I can handle this. I have heard of using a pot in circut to find values for cathode bias resistors in the output, but didn't think to apply it here. So when I put the pot in the cathode circut, I attach to the beginning of the pot and the wiper? For the 1 meg resistor on the input grid, is 1/2 watt ok? Or should I be safe and go to 1 watt? And it makes sense that I would have to bump the cap up accordingly...I think I would have realized that from the bass response.
OK, now if someone could check out my post in re my Bell 2122, I would appreciate it...those guys make smokin' guitar amps with just a bit of work.
Thanks again everybody...I'll be in touch !!!
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Old 27th March 2009, 07:52 AM   #6
m6tt is offline m6tt  United States
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The grid leak resistor can be as low as 1/4 watt (or lower), but it is preferable (but not necessary) to use a carbon comp type. These aren't spiral cut with a laser like some film resistors...the spiral adds a small inductance which can introduce RF into your circuit.

Yes, either side of the pot & the wiper. If you're working with guitar amps or even hifi (honestly I've come to enjoy doing the calculations, but it's nice to check a little high and low), get a 1-2K wirewound 5w pot...you can use it to do most preamp cathode resistors & small power stage cathode resistors.

If it's the input stage, you can lose the cap that goes from the jack to the grid...that was there to keep the grid DC biased, now the bias is applied to the cathode, it's unnecessary.

Go to tubecad.com, and read the "common cathode amplifier" PDF...a couple of pages in it gives a calculation for cathode resistor with a plate resistor present. Use a plate resistor that was 1.5-2.5 times the plate resistance of the tube I was using. Use the calculation for the cathode resistor given that plate resistor, especially for input stages. For later stages, I often find I have to increase the cathode resistor a couple of hundred ohms to prevent grid clipping (guitar amps). It's a good read though, and I recommend keeping it around.

I just started learning this stuff a few years ago (just graduated from university), had a few questions ignored or even ridiculed! Still, it's a hell of an opportunity to read, and it amazes me that the internet even lets us all share knowledge like this...there aren't really many people in the world that know the experience of wiring together that first thing that works, if you think about it.
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Old 27th March 2009, 12:09 PM   #7
SY is offline SY  United States
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Bill, grab a copy of Morgan Jones's "Valve Amplifiers" and read about diode and LED bias. I've found these methods to be a superb way of biasing tubes without the severe disadvantages of cathode bypass caps.
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Old 27th March 2009, 11:19 PM   #8
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Reply to m6tt: It sure is a good place to pick some knowledge up. All mine has been pretty much seat-of-the-pants. I had a fleet of 52 guitar amps to keep running and no decent tech for miles! I remember when I first bypassed an unbypassed preamp tube and put a switch on it...I almost wet myself!! I'll check that link out...thanks !
Reply to all: Thanks for all the advice...tomorrow is my day to try it out. I'll let you know how it works out...
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Old 28th March 2009, 05:26 PM   #9
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Quote:
Originally posted by SY
Bill, grab a copy of Morgan Jones's "Valve Amplifiers" and read about diode and LED bias. I've found these methods to be a superb way of biasing tubes without the severe disadvantages of cathode bypass caps.

Copy that;
I use the LED trick where possible and also a CCS in the anode. This extra bit of solid state offers fantastic performance especially with an active Baxandal tone type stages.
Do the sums first; alot of red led's from various vendors have Vop between 1.6-2.5V and when using frame video tubes (EF184 as triodes are excellent for this) in preamp stages when Ia is between 5-10mA, the LED and CCS are excellent linearising tricks.
The reward; Replacing tubes becomes non critical and performance near identical esp distortion.

Morgan Jones highlighted this, but historically, I wonder why (apart from costs) it has taken so long for this perfomance adv. to be taken notice.


richy
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