Bias

cmoore
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Bias

« on: October 29, 2008, 04:41:49 AM »

What kind of bias, output, cathode resistor voltage etc. etc. are you guys seeing with your VC's? Seems like i am running at 42MA's. Was thinking of changing the cat resistor to a 2 watt 6-700 ohm. What are you guys experiencing?
Thank You


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winston_smith
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Re: Bias

« Reply #1 on: November 02, 2008, 07:19:37 PM »

Hi,

What's the plate voltage?


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cmoore
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Re: Bias

« Reply #2 on: November 06, 2008, 11:45:57 PM »

Hey Winston-
Every thing seemed to be OK, so I put the chassis back in the cab. But I think it was up there pretty high, like 390 volts at the plate. But maybe I don't know how to check the bias on a cathode biased amp. Do I take the voltage at the cat (pin 8?) and divide that by the cat resistor? I am assuming the parallel cap does not affect the impedance because there is no DC going thorough this "by-pass" cap. Afraid I need wiser men to guide me at this point. I can tell you that the reissue TungSol draws more current than the RCA 6V6. I think.


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winston_smith
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Re: Bias

« Reply #3 on: November 07, 2008, 10:21:37 AM »

Hi,

Yes, that's how you do it. Measure the voltage drop across the cathode resistor (with your meter set on VDC), then divide the result with the value of the cathode resistor (according to Ohm's law).

You said you have 42ma and 390V plate voltage. Well, this means over 16 watt. That's a little too high for a 6V6. The conservative rating of a 6V6 is 12 watt, but most of them do quite well at 14 watt, even higher. 16 watt, however, is too much. Things are complicated because of the voltage drop of the rectifier tube. If you replace the cathode resistor (with one of a higher value), the plate voltage will also go up a little bit. Personally, I would try to lower the plate voltage and leave the cathode resistor at the stock value. There are several ways to do that. First, if you have a Sovtek 5Y3 rectifier, try to put a NOS rectifier tube, as the Sovteks are not real 5y3s and their voltage drop is smaller. If this doesn't do it, you can always insert a dropping resistor before the first filter cap (5 watt/a few hundred ohms). The old Fender schematic shows a plate voltage of less than 350, but the real champs had well over 350V. Try to settle at 370.

Please tell us how this is going.

W.S.


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cmoore
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Re: Bias

« Reply #4 on: November 07, 2008, 10:11:22 PM »

Yeah, OK,
Let me pull the chassis out again and double check all the numbers I gave you. I will check to see what rec tube I have in there also.
Thank You


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cmoore
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Re: Bias

« Reply #5 on: November 07, 2008, 11:23:02 PM »

My apologies, the plate voltage was not 390. Embarrassed

The rec tube is a JAN Phillips 5Y3.
Pin 2 - 352
Pin 4 - 319VAC
Pin 6 - 318VAC
Pin 8 - 352
TungSol reissue 6V6
Pin 2 - 229
Pin 3 - 345
Pin 4 - 345
Pin 7 - 22.9
Pin 8 - 22.9
So if I am reading things the right way, I have almost 23 volts on top of a 470 ohm resistor. That is about 47 milli amps. Seems like I almost have a small camp fire sitting on my desk. This is what made me think I might need a higher value cat resistor, to get the amps down. But would that make the plate voltage too high?
I'm at your mercy.


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winston_smith
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Re: Bias

« Reply #6 on: November 08, 2008, 02:53:33 AM »

Hi,

Well, your voltage readings are very close to the fender schematic. It seems that your power tube is still working well beyond its specs, however. Let's do some math:

Current = 22.9/470= 0.048A
Power = (345-22.9)*0.048 = 15.4 watt

I would say it would be for the best to target 14 watt. A JJ 6V6 would stand 15.4 watt, I don't know about a Tung Sol reissue. As the plate voltage is already low, I think it would be best to increase the value of the cathode resistor. Not much. Try something between 500 and 600 ohm and redo the math. Anyway, I think your voltages look quite good. It's not mandatory to do any changes. It's just that the power tube will wear out at a faster rate. Good luck and keep us posted!

W.S.


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cmoore
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Re: Bias

« Reply #7 on: November 08, 2008, 04:01:58 PM »

Thanks W. -
I have calls into a few friends to see if they have any large resistors in the 5-600 ohm range. If not, then it's Mouser time. Will post again ASAP.
Thank you so much for the guidance.


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cmoore
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Re: Bias

« Reply #8 on: November 19, 2008, 11:06:29 PM »

OK-
Took awhile to get that resistor... So now I have a 3 watt 680 ohm cat resistor. That voltage reads 26.7V, the plate is at 360V, and a steady 369V coming off the rec tube. If I am doing the correct math, that makes about 39.2 mA. So the numbers look "better" but the current still seems kind of high. The amp sounds good. So if their are no objections, I think I will just leave everything where it is. Thanks again for all the help.
Ooops, another math question. Does this add up to 14 watts?
Thank You


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winston_smith
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Re: Bias

« Reply #9 on: November 20, 2008, 01:00:08 PM »

Hi,

Good to hear from you again.

Current = 26.7/680 ~ 0.039ma
Power = (360-26.7) * 0.039 ~ 13 watt

You are right where you should be. You can even lower the value of the cathode resistor (at around 600 ohm), but you shouldn't bother with that. If the amp sounds good, than this is it. Congratulations!

W.S.


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cmoore
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Re: Bias

« Reply #10 on: November 20, 2008, 10:11:19 PM »

Hey W.S. -
Thanks again. And thanks for the the power calc formula. The INTERNET has been good to me. Wink


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