# New 5F6-A: what's up with these measurements

Discussion in 'Shock Brother's DIY Amps' started by TelZilla, May 13, 2019.

1. ### shortcircuitTele-Meister

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What's the bias voltage measured at pin 5 of the power tubes? Have you measured the resistance of the 1 ohm resistors connecting the cathodes of the power tubes to ground? All of this can be explained if those 1 ohm resistors are actually orders of magnitude higher.

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2. ### robrobPoster ExtraordinaireAd Free Member

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"If there is a bias pot, why is there a -20 cathode voltage?"

-20mv, probably meter error.

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3. ### muswell_hillbillyTele-Meister

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Let's say the meter leads were swapped. Also assuming the cathode resistors are actually 1 ohm, would this measurement (20 mV / 1 ohm = 20 mA) then be consistent with the voltage drop reported across the primary windings?

Last edited: May 16, 2019
4. ### corliss1Friend of Leo'sPlatinum Supporter

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Nope - while related, current (in mA) isn't a measurement of voltage drop (DC volts)

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Withdrawn

6. ### corliss1Friend of Leo'sPlatinum Supporter

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The more things get posted here the more questions I have on this amp for sure

7. ### petebFriend of Leo's

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Does this amp have the one ohm cathode resistors?

Then a positive 20 mV on the cathode would mean 20 mA of current thru the cathode, which would make sense with the calculated plate current of 19 mA. (leaves 5% screen current)

The numbers look good, 19 mA of current for 55-56% of plate dissipation for a 14 w 6v6.

Last edited: May 16, 2019
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8. ### TelZillaFriend of Leo's

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It is. Typo. Will correct.

9. ### TelZillaFriend of Leo's

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I checked them. They are 1 ohm. When you say bias voltage, you mean pin 5 to ground in V DC, correct? I will check when I get home

Sorry, that was a typo. It's a 5F6A, running two JJ 6L6GCs.

You and me both, brother. I'm stuck on two things at this point:
• That tiny voltage drop across the Primary winding of the OT. What could explain that other than a bad transformer?
• The low B+, even with a GZ34, which should (with 2019 US wall voltage) give higher readings than Leo's. Especially coupled with the fact that I got higher readings last week.
I'm quite worried I cooked the OT or some other component(s) in the bias circuit. But I never smelled smoke, nothing looks crispy, I never blew a fuse, and I'm using my lightbulb current limiter. What else should I check before ordering a new OT / all the bias parts (\$\$\$\$)?

Last edited: May 16, 2019
10. ### shortcircuitTele-Meister

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Yep. It should be negative with respect to ground. Normal bias voltage for a 6L6 should be around -40V. If there's something wrong with your bias circuit and the voltage is too negative, then there will be a very little current flowing through the tube, and therefore very little voltage drop across the OT.

11. ### petebFriend of Leo's

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Now that I see that the tubes are 6L6, the bias is 8W/27.5W is 30% plate dissipation.

This is the low end of the acceptable spectrum and why your amp sounds fine.

I think your OT is fine but maybe not your fixed bias voltage.

what is the fixed bias on pin 5 of the power tubes?

like short circuit says -40V here would be acceptable for 6L6 tubes.

your amp is biased cold. I would expect your bias is -50 volts or more than 50.

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Back....if you set the amp to draw some current, the B+ will come down. Ime and imho, 30% of max plate dissipation is not where I would want to her that amp.
And....any certain bias voltage will cause different tubes to yield different current draw, plate voltage, and therefor different plate dissipation numbers. I don’t adhere to any certain, arbitrary bias voltage number. Start cold and move to higher current draw to be safe.

13. ### TelZillaFriend of Leo's

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Just another question, cause I'm sure you're not sick of me yet...

If you refer to the schematic in post #24 or the layout in post #30, you'll see the 1.5K resistors between the grid (pin 5) of the 6L6's and the 220K grid leak resistors. These aren't on the original Fender schematic, or the Ted Weber schematic. What's the function of these resistors?

14. ### TelZillaFriend of Leo's

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B+ is low, not high. And the bias pot is all the way clockwise. I can't bias it hotter by using the bias pot- that's the issue. As you say, 30% of plate dissipation is not where I want this biased.

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15. ### corliss1Friend of Leo'sPlatinum Supporter

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So if your pot is maxed out it is essentially a straight wire. What would one do in that situation to increase the voltage in that area?

16. ### muswell_hillbillyTele-Meister

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I believe these are grid-stopper resistors:

http://www.valvewizard.co.uk/gridstopper.html

17. ### TelZillaFriend of Leo's

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So CW is lowest resistance? I had thought it was the other way around. Although when I think about the way a volume pot works, CW= no resistance makes sense.

V= I*R, so increase resistance. But if I roll it back, my voltage drop shrinks and I end up with even less plate dissipation. I actually did all the measurements with the pot all the way CCW and got something like 2 watts per tube. Maybe swap the 15k resistor for a higher value? But I'm not sure why that's any different than rolling the pot back

18. ### corliss1Friend of Leo'sPlatinum Supporter

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I'm not sure CW or CCW with your build, but one way will provide the most negative resistance possible and one way will provide the least.

If turning the pot to the "most" side doesn't give you enough, you need to get more voltage to that circuit. I'm not sure if you already mentioned it somewhere that I missed, but did you provide detail on how you did the adjustable bias?

19. ### muswell_hillbillyTele-Meister

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According to the schematic, and assuming a 50 VAC bias supply:

will provide the following range of bias voltages:

EDIT: Apologies - In the original post, I left out the 1K series resistor which lowers the available bias voltage.

Last edited: May 16, 2019

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