# Checking 5e3 Bias

Discussion in 'Shock Brother's DIY Amps' started by Tele Slacker, Dec 9, 2019.

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1. ### Tele SlackerTDPRI MemberSilver Supporter

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Built my 1st kit amp last July... a mojotone variety at their Burgaw facility. Absolutely love the amp, and am absolutely hooked. Have since built a 5f1 and now building a friend a 5e3 in an old repurposed chassis and cab.

trying to learn a little more each day. In that spirit, I decided to learn about bias by measuring some of the vitals of my deluxe. I read the RobRob output xformer resistance method, and watched the Uncle Doug bias videos... both really REALLY helpful.

After doing the step by step, I had the following measurements:

1st column is resistance from pin 3 of referenced 6v6 to pin 8 of rectifier tube, measure resistance across one side of OT (taken with amp off). ΔV is voltage drop across the same, obviously with amp powered on for a while. Plate Voltage is taken from pin 3 of respective 6v6, to chassis ground.

Was surprised by the big difference (my perception) in voltage drops across the different legs of the output xformer. Is this normal? I plan to try a different set of 6v6 to compare, but I expected things to be closer to equal, and a greater overall wattage/dissipation (assuming I have my head on reasonably straight as to what I’m looking at).

wondering what some more experienced eyes thought of these results.

Thanks for any feedback.

2. ### King FanFriend of Leo's

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Before tackling that, have you measured bias using Rob's much simpler 'cathode resistor voltage drop' method?

Last edited: Dec 9, 2019
3. ### petebFriend of Leo's

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The resistance in column one drives the situation. Expect a 5-10% difference here. Yours are exactly 10%.

Since the OT resistances are 10% off expect the current draw and plate dissipation to be 10% off if the tubes are matched. It looks like yours are not. Not a big problem.

You like the sound?

The plate voltage is low. That’s why the plate dissipation is low. If the plate dissipation is low and it sounds good, don’t worry about it. Did you subtract the cathode voltage from the plate voltage?

4. ### Tele SlackerTDPRI MemberSilver Supporter

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I have not. I will give that a try tonight.

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5. ### Tele SlackerTDPRI MemberSilver Supporter

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Absolutely! But, I’m also aware that a singing beautiful tone can sometimes be tubes in distress. Just want to make sure there was nothing glaringly wrong, based upon more experienced eyes.

I followed the referenced uncle Doug video for these numbers... I didn’t see any reference to subtracting cathode voltage using that method (I may have missed it?).

6. ### King FanFriend of Leo's

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Cool. Useful for cathode bias amps and tons simpler. Rob gives a step-by-step:

To use the Tube Dissipation Using Cathode Resistor Voltage Drop calculator enter your Tube Type and Plate-to-Cathode Voltage at the top of the page, then enter the Number of Tubes that share a cathode resistor, the measured Voltage Drop across the cathode resistor and the Cathode Resistor's Ohm Value and click Calculate. If your amp has 1 ohm bias setting resistors then enter '1' into the Cathode Resistor's Ohm Value field. You can get an accurate resistance measurement of cathode resistors by turning the amp off and simply measuring resistance across the cathode resistor. It's more accurate to actually measure the resistor's ohm value rather than relying on its marked rating. The calculator subtracts 5.5% of the cathode current as screen current.

To measure voltage drop across the cathode resistor set your multimeter for DC Volt measurement, clip the black probe to chassis ground (so you can use just one hand to do the measurement) and put the red probe on the tube socket cathode pin . You can also put one probe on each leg of the cathode resistor. The voltage shown on the meter is the 'voltage drop' (disregard any minus sign). Note that voltage measured at the cathode pin is the same as the voltage drop across the cathode resistor so you don't have to actually put your meter pins on the legs of the cathode resistor to measure its voltage drop.

You can get a more accurate bias measurement if you actually measure the resistance of your cathode resistor. It may be marked as a 250 ohm resistor but actually measure 238. To measure the cathode resistor turn the amp off and measure the resistance from the tube cathode pin to ground.

Last edited: Dec 10, 2019
7. ### jtcnjTele-Holic

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Its been a while; this may not be completely accurate but here goes.

Agree on the cathode resistor method if cathode bias.

For the transformer method as an exercise:

I dont think you can measure anode to cathode as the resistance across the tube. I could be wrong though.

I recall measuring R across Plates pin 3 to CT of OT - amp off, then voltage across same points - amp on.

Ohm's law gives current through each half of OT, almost same as total current through tube (add screen current of 2-4ma rule of thumb? = total current through tube.
P=I * V(plate pin 3 to ground) per tube.

On the few I have done, the voltage across the OT were not identical.

8. ### jtcnjTele-Holic

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I think I have that a little off.
If you use the voltage across the cathode resistor and resistor value to calculate current, it is the total tube current. You subtract some current to account for screen current, then calculate the plate dissipation.

Using the transformer method is normally without a bypass cap. So if doing this with a cathode bias, you would need to measure plate voltage from pin3 to pin8 and use that with the current calculated through each half of the OT.

And, I believe accounting for screen current is used as part of determining the total current draw a design would call for and matching a suitable PT to it.

9. ### Tele SlackerTDPRI MemberSilver Supporter

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My notes weren’t very clear to anyone looking... sorry about that. I was measuring resistance (then Vdrop) from pin3 of output tube (one side of transformer), to pin 8 of rectifier tube/center tap of OTransformer... at least that was my intent.

I did the tube dissipation using cathode resister method... much much easier, getting the attached results. Wattage still seems a little lower than I expected... but good with tone, so leaving alone.

I guess the disadvantage to this method is that you miss any tube “imbalance” by looking only at the current thought resistor, as it’s an average of the 2 tubes?

Thanks for the feedback. Going to play around with the different test methods to learn more, while trying not to die.

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10. ### petebFriend of Leo's

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True

Note: when the amp is processing signal, it’s not the DC resistance that matters, it’s the impedance to AC that matters, which depends on the number of winds. The numbe of winds may be balanced but the DC resistance is not due to one winding being within the other, making it shorter in length, decreasing resistance.

You could try swapping the two power tubes positions. This has a 50% chance of decreasing your over all imbalance.

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11. ### King FanFriend of Leo's

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Good job; your voltage drop numbers look right from here. Just out of curiosity, what value did you measure (enter) for plate-to-cathode voltage in the first part of Rob's calculator? What's your B+?

FWIW, here are some 'typical' 5e3 voltages our friend Telenut62 drew up years ago. Yours may not match exactly, but the relative values are good to compare.

12. ### Tele SlackerTDPRI MemberSilver Supporter

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I realized after posting those numbers I had an erroneous value in plate-to-cathode voltage. Retested and re-enter the (correct) numbers, and got this... still seems a little anemic on paper (overall love the sound though... but this is my first deluxe, so comparisons are limited).

Thought I would try that swap and see what changes. Will do that tonight and rerun the test using cathode resistor method and OT method.

13. ### King FanFriend of Leo's

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So at a glance, Telenut's plate-to-cathode voltage is about 342V and yours is 257? Cool, literally. I'm glad it sounds good.

But now you got us wondering -- should it be that cool?

What's your DC voltage at the positive end of the first filter cap?

What PT are you running?

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14. ### dan40Tele-Afflicted

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What PT are you running in this amp? If using a standard 5e3 PT, that low of a plate voltage would indicate a very high current draw pulling the plate voltage down that far. This does not seem to be the case so you must be running a different PT in this build. What b+ voltage are you reading at the first filter cap?

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15. ### Tele SlackerTDPRI MemberSilver Supporter

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Think I can solve the mystery.... I'm a dumb@ss. Given the alarm at the cool plate voltages, I went through process more methodically, listening and reading slooooowly, and came up with this:

...which appears straight up spot on. Believe it's time to give Rob a donation... been mooching too much not to pony up.

Think I had too many cooks-in-the-mental-kitchen, trying to understanding more than one method at a time. I also had amp running through a lightbulb limiter, which was probably not the best idea while trying to discern actual operating voltages.

Thank you for your patience. I appreciate you taking the time to help me learn. If you have any more advice (other than 'get a new hobby')... let me know. Appreciate y'all!

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16. ### Tele SlackerTDPRI MemberSilver Supporter

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Just for my continued amusement and education.... I was poking around with VOM, comparing voltages to Telenut's send voltages... on V1 I am about 30 volts lower on Pins 1 and 6 (plates of the 12a()7. He's using a 12ax7, while I have a 12ay7 in V1.

Does that account for voltage differences there? The B+3 voltage is in line with those readings (~250V).

17. ### King FanFriend of Leo's

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Good for you. I love a happy ending...

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18. ### dan40Tele-Afflicted

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Yes...a 12ay7 will bias up in the circuit differently than a 12ax7. Different bias points will equate to different plate voltages. Even two different 12ax7's will give slightly different readings on each plate of the triode. This is because every tube is slightly different and will settle in at a slightly different operating point.

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19. ### Tele SlackerTDPRI MemberSilver Supporter

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thanks for the feedback.... just thought 30 volts less than ‘standard’ may be excessive. Again... love the sound.... zero complaints. But I’m just using this as a learning opportunity.

Along those line... just to confirm suspicions... I ran the amp from light bulb limited power again last night (using 100 watt bulb), and checked some voltages. Sure enough (and probably not surprising to you guys), big differences in plate voltages. But if I understood Uncle Doug accurately, using the light bulb limiter during bias checks was strongly encouraged. May need to use a higher wattage bulb?

If you'll continue to humor my basic "Whos's bigger" comparison questions:
I am seeing a ~12volt difference from Plate to Cathode of each 6v6 tube.. consistent result, measured two ways...
• directly, pin3 to pin8 of inner tube: 12.88V
• directly, pin3 to pin8 of outer tuber: 12.64V
• indirectly, measuring inner output tube pin3 voltage (to ground), subtracting Output Transformer center tap (to ground): difference = 12V (no better resolution with my VoM)
• indirectly, measuring outer output tube pin3 voltage (to ground), subtracting Output Transformer center tap (to ground): difference = 12V
In Rob's how-to, he's seeing a voltage drop of ~1.33V (albeit over a lower resistance/impedance... ~ 72-80 ohms). Again... seem like a dramatic difference (nearly 10x what I'm seeing), with resultant plate currents that are more than double (if my math is right). Is the PTranny resistance/imp create this different because mine is 384-0-384 (compared to Rob's example 355-0-355)? Or am I just looking at this all wrong?

20. ### dan40Tele-Afflicted

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Doing your bias calcs with the light bulb limiter in circuit will result in false readings. The light bulb will naturally limit the current draw which will then cause your b+ voltages to fall quite drastically. This will end up resulting in false bias readings when the amp is off the limiter and running at the normal plate voltages. Always do your measurements with the amp plugged straight into the wall, volume control at zero.

The difference in cathode voltage readings between the two 6v6's are caused by the tubes drawing uneven current from each side of the OT. If you purchase another pair of tubes that are matched a bit closer, the voltage readings will be closer to each other. There is also a slight difference between each half of the OT's windings which will cause a small difference in readings. If the amp sounds good and the tubes show no signs of redplating, I would leave it be and enjoy it. If it's really bugging you, a fresh pair of tubes with a tighter match will bring those readings a bit closer.

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