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Cathode bias questions

10/1/1999 8:49 AM
Blues Lyne	Cathode bias questions I have a SF Bandmaster Reverb that has been BF'd and has a post PI master volume. I have been using it in a two amp rig with a BF Super Reverb for clean and this one dirty. I put Phillips 6L6wgb's in it to get it to break up earlier. I have been thinking of cathode biasing it to lower the power, and get more break up. I have an article by Kevin O'Connor that suggest a 500 ohm resitor but I noticed that most of the old fenders with 6L6's that were cathode biased used a 250 ohm resistor. Which is a better place to start? Is the difference because the older amps had lower B+ or is there another reason for the difference? The old Fenders also use a 25uF cap, does anybody have a favorite cap value and why? I've also been considering putting in a 5R4 to drop the B+ some. Any suggestions would be greatly appreciated. Thanks, Blues

10/1/1999 8:49 AM

Blues Lyne

Cathode bias questions

I have a SF Bandmaster Reverb that has been BF'd and has a post PI master volume. I have been using it in a two amp rig with a BF Super Reverb for clean and this one dirty. I put Phillips 6L6wgb's in it to get it to break up earlier. I have been thinking of cathode biasing it to lower the power, and get more break up. I have an article by Kevin O'Connor that suggest a 500 ohm resitor but I noticed that most of the old fenders with 6L6's that were cathode biased used a 250 ohm resistor. Which is a better place to start? Is the difference because the older amps had lower B+ or is there another reason for the difference? The old Fenders also use a 25uF cap, does anybody have a favorite cap value and why? I've also been considering putting in a 5R4 to drop the B+ some. Any suggestions would be greatly appreciated.

Thanks,

Blues

10/1/1999 5:18 PM
SpeedRacer	The older Fenders are running lower B+, roughly 360-390V and 250 works great in there IMHO. THey also used output trannies with a higher primary imp, generally 6.6k for the cathode biased amps with 6L6's. The B+ in SF Bandmasters tends to be in the low 400's in my exp, so you could probably pop a zener in there to lose the excess voltage, get yourself into the mid-high 300's and it would set up just fine. To be perfect you might try to find a 6.6k output tranny. Hammond probably makes one that will work nicely. The old CB sound is one of my favorites. YOu should be down to around 18-20W output, which is still plenty loud for leads.. and a nice break up. Try reducing the amount of -fb as well..

10/1/1999 5:18 PM

SpeedRacer

The older Fenders are running lower B+, roughly 360-390V and 250 works great in there IMHO. THey also used output trannies with a higher primary imp, generally 6.6k for the cathode biased amps with 6L6's. The B+ in SF Bandmasters tends to be in the low 400's in my exp, so you could probably pop a zener in there to lose the excess voltage, get yourself into the mid-high 300's and it would set up just fine. To be perfect you might try to find a 6.6k output tranny. Hammond probably makes one that will work nicely. The old CB sound is one of my favorites. YOu should be down to around 18-20W output, which is still plenty loud for leads.. and a nice break up. Try reducing the amount of -fb as well..

10/1/1999 5:26 PM
Blues Lyne	Thanks SpeedRacer, I put a pot on the back to adjust the negative feedback. I'll search for some of the posts on the Zeners. I really miss the archives. Blues

10/1/1999 5:28 PM
Scott	Assuming the amp is currently biased properly now, you can easily determine what the resistor should be as follows: 1. measure the negative bias the amp is currently set at 2. measure the total current flow for both tubes. The safest way to do this is with 1 ohm resistors between the power tube cathode and ground. Or use the transformer shunt technique IF you know what you're doing. 3. Use ohm's law to calculate resistor size : V/I = R. This is slightly conservative since the effective plate voltage will be reduced by the bias voltage. An easy but slightly expensive alternative is to purchase a 750 or 1K 15 watt rheostat (about $20 from Mouser) and then its adjustable.

10/1/1999 5:28 PM

Scott

Assuming the amp is currently biased properly now, you can easily determine what the resistor should be as follows:

1. measure the negative bias the amp is currently set at
2. measure the total current flow for both tubes. The safest way to do this is with 1 ohm resistors between the power tube cathode and ground. Or use the transformer shunt technique IF you know what you're doing.
3. Use ohm's law to calculate resistor size : V/I = R.

This is slightly conservative since the effective plate voltage will be reduced by the bias voltage.

An easy but slightly expensive alternative is to purchase a 750 or 1K 15 watt rheostat (about $20 from Mouser) and then its adjustable.

10/1/1999 6:52 PM
Ted Matsumura	Scott, Could you give a math example of this? I've used some of these data points to get the power (dissipation) data, but not to select cathode resistor, which is fixed at 250 ohms in a 5E3 clone. Here is the data I have been collecting, though I have not until now been noting the bias or the ma between pin 8 on rectifier and pin 3 on power socket. With given set of tubes and AC V, I might have 355 VDC on plates, 19 VDC across 250 ohm (5W) cathode resistor (and 25 mfd cap) giving (19/250)355 or 27W, or subtracting the cathode voltage, then (19/250)(355-19) for about 25W dissipation. If I use ohms law with the #'s that come to mind of the recent bais #'s, the formula comes nowhere near to putting a 250ohm cathode resistor in there. What am I missing? Thanks.

10/1/1999 6:52 PM

Ted Matsumura

Scott,
Could you give a math example of this? I've used some of these data points to get the power (dissipation) data, but not to select cathode resistor, which is fixed at 250 ohms in a 5E3 clone. Here is the data I have been collecting, though I have not until now been noting the bias or the ma between pin 8 on rectifier and pin 3 on power socket.

With given set of tubes and AC V, I might have 355 VDC on plates, 19 VDC across 250 ohm (5W) cathode resistor (and 25 mfd cap) giving (19/250)*355 or 27W, or subtracting the cathode voltage, then (19/250)*(355-19) for about 25W dissipation.

If I use ohms law with the #'s that come to mind of the recent bais #'s, the formula comes nowhere near to putting a 250ohm cathode resistor in there. What am I missing? Thanks.

10/1/1999 8:08 PM

Scott

Ted,

Your data as stated:

quote:
"With given set of tubes and AC V, I might have 355 VDC on plates, 19 VDC across 250 ohm (5W) cathode resistor (and 25 mfd cap) giving (19/250)*355 or 27W, or subtracting the cathode voltage, then (19/250)*(355-19) for about 25W dissipation."

is correct for a 5E3 Deluxe using 2-6V6s being run near the maximum plate dissipation rating of 13 watts. The thing you might have overlooked is the cathode resistor is sinking the current from BOTH power tubes and therefore the 25W you calculate is the total dissipation. Each tube is running at 12.76 watts (assuming perfect balance). If you were to unsolder the output trans CT and insert an ammeter to measure the current you will measure 76 ma, or if you use the transformer shunt method you will get 38 ma. DO NOT MEASURE CURRENT THIS WAY UNLESS YOU KNOW WHAT YOU'RE DOING AND THE DMM IS RATED FOR THIS SERVICE!!!. Cheap DMMs may blow up during the intial surge (I know from experience).

Here's a good trick to make current measurement safer. Instead of grounding the cathode resistor and cap, connect them to one terminal of a chassis mount fuse holder (like Bussman HKP), and ground the other terminal of the fuse holder. Install a 3/16 A fuse. Now, you have a convenient place to open the circuit for current measurement and a fuse to protect the OPT if a tube shorts. Use alligator clip meter leads for no hands current measurement.

10/2/1999 12:47 AM
Troy	So with 6L6's setup for cathode bias; what's the lowest wattage rating Rk for common pair? I see Mouser sells 15W and 25W cements. Would 15W do the trick since most tweed Fender's used 10W ratings? Troy

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