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Post subject: 5F11 Fixed Bias
Posted: Wed May 14, 2014 12:40 pm
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Aspiring Musician
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Found this on the web. Just wondering if it would work fine for fixed bias on a 5F11 Vibrolux, or is there a better way? I've read that an additional 100K 3 watt resistor in series on the band side of the IN4007 adds some additional protection also?

Thanks for any input.

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Post subject: Re: 5F11 Fixed Bias
Posted: Wed May 14, 2014 3:23 pm
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Wrong wiring at the pot and missing resistor to ground .


1- Rectifier diode go to one side of the pot . You must have a filter caps here in the amp
2- Other side of the pot ,put a resistor 15K to ground .
3- Center tap of the pot ( wiper ) go to the 10 K .


Test; 1- remove all output power tubes
2- read if you have bias negative voltage at each output power tube socket
3- Turn pot to read max negative voltage , at least -30 volts DC


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Post subject: Re: 5F11 Fixed Bias
Posted: Wed May 14, 2014 3:38 pm
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IMO best is on your amp , you have a 56 K resistor at the output of your bias rectifier .

Remove on leg of this resistor and put your 10 K pot in series with this 56K

Disconnect the wire going to output tubes and put it in center tap ( wiper of the pot ).

Test like I tell you in my first answer .

Too much negative bias voltage , put lower resistor in place of 56 K

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Post subject: Re: 5F11 Fixed Bias
Posted: Wed May 14, 2014 5:45 pm
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So more like this. Sorry for the crude drawing. I should be able to then replace the 56k with different values if needed to get it in a good range. I haven't built this amp yet so I'm just planning.

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Post subject: Re: 5F11 Fixed Bias
Posted: Thu May 15, 2014 12:16 am
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Yes, that is the correct wiring for the bias pot. If you find adjusting the leg-to-ground resistor a bit of a hassle (the grounded leg should be soldered onto the pot's body). Get this resistor's value into the nominal negative voltage ballpark with the bias pot turned to midpoint. Then, change the resistor next to the diode on the separate tagboard, in the bias supply. This resistor is inline with the circuit. So, increasing the resistor value decreases the negative voltage in the circuit. And vice-versa.

The pot-to-ground resistor has the opposite effect on the voltage. Increasing this resistor's value, increases the negative voltage in the bias supply circuit. And vice-versa.

Good luck! Tell us how it works out.


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Post subject: Re: 5F11 Fixed Bias
Posted: Thu May 15, 2014 1:06 am
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upnorth wrote:
So more like this. Sorry for the crude drawing. I should be able to then replace the 56k with different values if needed to get it in a good range. I haven't built this amp yet so I'm just planning.



+1


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Post subject: Re: 5F11 Fixed Bias
Posted: Thu May 15, 2014 7:18 am
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Thanks for the help. So I could start with say, a 47K for R2, pot to ground. Changing R1 from that point would be for fine tuning the negative voltage to -31v with the pot at midpoint?

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Post subject: Re: 5F11 Fixed Bias
Posted: Thu May 15, 2014 8:05 am
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Minus 31 V is a good point to start but do not mean you are safe . Test your bias with reading with tube and plate voltage .

When all you tube are well biased and job finish, pot in the mid is good but is not a must


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Post subject: Re: 5F11 Fixed Bias
Posted: Thu May 15, 2014 9:25 am
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Yes, thanks. I'll be sure to do the math after this amp is built. I've got a good tester for Ik/Vp (Amp Head).

Thanks again for all the help. I should study up on adding the resistors to measure plate current also.


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Post subject: Re: 5F11 Fixed Bias
Posted: Thu May 15, 2014 2:16 pm
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upnorth wrote:
Thanks for the help. So I could start with say, a 47K for R2, pot to ground. Changing R1 from that point would be for fine tuning the negative voltage to -31v with the pot at midpoint?

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Yeah, 47k-ohm would be nice starting point. What I do for these circuits is soldered a fixed lead to the body of the bias pot. Say, a 1/2-inch long solid piece of wire. Make a hook at the open end. Then, solder the supply-to-ground resistor (R2 in your diagram), from this lead's hook to the pot's terminal. You can then change the resistors without over-heating the pot. After you find the final resistor value, you can remove this lead and solder the final resistor directly to the pot's body.

The final resistor(s) setup MUST BE TESTED with the 6V6GT's in the amp. You are shooting for a desired end-point idle bias current with the bias pot near midpoint, using new tubes. Don't rely only, on the (-) 31VDC reading. This (-) 31VDC is only a nice starting point.

You may still need to tweak resistor(s) values, after measuring idle bias.

That's why altering that 10k-ohm resistor (R1), before the diode is easier. It's on a separate board. Lot less potential heat damage. If you do this, put a small alligator clip onto the diode. To prevent heat damage to it.

Good luck! Keep us informed.


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Post subject: Re: 5F11 Fixed Bias
Posted: Fri May 16, 2014 1:28 am
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BMW2002Ti wrote:



The final resistor(s) setup MUST BE TESTED with the 6V6GT's in the amp. You are shooting for a desired end-point idle bias current with the bias pot near midpoint, using new tubes. Don't rely only, on the (-) 31VDC reading. This (-) 31VDC is only a nice starting point.

.



I do this mod lots of time and never tested with output tubes in for safety . I alway had same negative voltage with or without tubes .


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Post subject: Re: 5F11 Fixed Bias
Posted: Fri May 16, 2014 3:46 pm
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Stratele, I test with a fresh set of output tubes. So, that the bias pot will be near the midpoint with new tubes. Each amp and tube set will have slightly different midpoint, as a function of the true negative voltage on the signal grid pins and the current passed by the specific tubes.


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Post subject: Re: 5F11 Fixed Bias
Posted: Fri May 16, 2014 6:21 pm
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I've got a pretty new matched set of EH 6V6's which from past experience seem to be in the middle of the pack for bias setting. JJ's needed to be turned up, RCA's down. Of course it depends on many other factors as well.


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Post subject: Re: 5F11 Fixed Bias
Posted: Sat May 17, 2014 12:48 am
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BMW2002Ti wrote:
Stratele, I test with a fresh set of output tubes. So, that the bias pot will be near the midpoint with new tubes. Each amp and tube set will have slightly different midpoint, as a function of the true negative voltage on the signal grid pins and the current passed by the specific tubes.



Each amp is different .

Do you mean you adjust negative voltage to have, say -30 VDC at output tube socket , with no tubes in .
And when you put tube in socket you don't have this -30 VDC ?


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Post subject: Re: 5F11 Fixed Bias
Posted: Sat May 17, 2014 12:50 am
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upnorth wrote:
I've got a pretty new matched set of EH 6V6's which from past experience seem to be in the middle of the pack for bias setting. JJ's needed to be turned up, RCA's down. Of course it depends on many other factors as well.


If I understand what you said my answer is ; that is why we need to check and adjust bias each time we use another tube (set ) in a amp .


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