Measuring voltage drop on a Twin Reverb Build

[72Blazer]

I'm still learning here.  I measured resistance on the OT windings and came up with 21.4R on AB pair of 6L6s and 21.1R on pair CD of the 6L6s.  I then measured the voltage drop and came up with -1.15v on AB and -1.12 on CD.

I watched Uncle Doug's video where he was Biasing a Twin Reverb and the voltage drop he measured were -2.32.  Mine seem low? or is this normal/acceptable?

Plate current is 53.7mA on AB pair and 53.0mA on CD pair

Plate Voltage 435v on AB pair and 430v on CD pair. 

Multiplied .02685 x 435v and got 11.67 watts on AB pair.

Multiplied .0265 x 430v and got 11.39 watts on the CD pair.

Am I in the ballpark (although running a little on the cold side)?

FWIW the amp sounds just fine, quiet, and reverb and tremelo working fine.  I used Sluckey's tremo-nator mod.

Vr
J

[sluckey]

I'd say you are really cold. I'd crank the bias up so each tube is idling at about 20 watts.

[72Blazer]

Bias pot is cranked all the way clockwise and still can't get voltage drop any higher than -1.15v.  Using a 27k resistor on the bias pot.  Should I lower that value?

[72Blazer]

I tried a 10k resistor on the bias pot and get a voltage drop of -2v

Now I get 20 watts at idle

[AlNewman]

I'm not familiar with the twin reverb, but does 21 Ohms seem low across the transformer?

[72Blazer]

FWIW, in Uncle Doug's video, he measured 35 and 38 Ohms on that particular twin reverb OT

[AlNewman]

Ok then.  Maybe you'd like to try placing a 1 ohm resistor between each pair of tube's cathodes and ground.  It can make measuring very simple.  You need to take into account screen current, but it's pretty failsafe, fwiw.

[72Blazer]

I've got a 1ohm on the pairs of tubes-but not each of the 4 tubes. What is the advantage of a resistor on each tube? 

[sluckey]

I've got a 1ohm on the pairs of tubes-but not each of the 4 tubes. What is the advantage of a resistor on each tube?

Allows you to see exactly how much current is flowing through each individual tube. If you only have one resistor shared with two tubes you may have a reading of 60mA. But maybe one tube is pulling 40mA and the other tube is pulling 20mA. Not well matched.

[72Blazer]

Thanks-should have figured that out myself.  I think I will go ahead and do that while I got the chassis out.

[72Blazer]

Installed them.  I do have one pulling ~5 mA more than its sidekick.

[AlNewman]

So what are your voltage drops and B+?

[tubeswell]

Installed them.  I do have one pulling ~5 mA more than its sidekick.

Try swapping the tubes in each pair around - there will be a combination of the four tubes that is a 'least mismatch' due to the effect of paralleling particular pairs through each side of the OT primary

[72Blazer]

First off thanks to all who took the time to help me out here.

I swapped out two of the 6L6s with some other used ones I had
Here are the results now:

I again measured resistance on the OT windings and came up with 21.2R on AB pair of 6L6s and 21.3R on pair CD of the 6L6s. 

Voltage drop -1.8v on AB pair and -1.95 on CD pair

Plate current is 42.4mA on AB pair and 46.0mA on CD pair

Plate Voltage 423v on AB pair and 428v on CD pair.

Got 18 watts on AB pair and 19.5 watts on CD pair

I’m thinking this is a better result.

Open to more feedback here.

Vr
J

[sluckey]

Got 18 watts on AB pair and 19.5 watts on CD pair

So, that's about 9 to 10 watts per tube. The 6L6 are rated for 30 watts. You are still biased very cold. It's common to bias each tube to idle about 70% (20 watts).

[72Blazer]

I think I realize now that I was reporting 1/2 of the actual here.

iow:  the actual PCA for AB pair was 85ma on the AB and 92ma on the CD side.

I had divided by 2....

Duh

[sluckey]

Try this... Measure plate voltage for each tube. Measure cathode voltage (millivolts) for each tube. The millivolts on the cathodes converts directly to milliamps. Multiply the plate voltage times the cathode milliamps for each tube. What are the four numbers you get?

[72Blazer]

Honestly I am unsure how to do the math but here are the numbers:

                Plate      Cathode

A           450v      45mA
B           450v      53mA
C           422v      52mA
D           420v      50mA

[72Blazer]

A = 20.7w
B = 23.3
C = 21.9
D = 21.1

[72Blazer]

Would it be beneficial to switch B and D tubes with each other?

[sluckey]

Honestly I am unsure how to do the math but here are the numbers:

                Plate      Cathode

A           450v      45mA        20.25W
B           450v      53mA        23.85W
C           422v      52mA        21.94W
D           420v      50mA        21.00W

I would adjust the bias until tube B was idling at 21W then recheck all.

A = 20.7w
B = 23.3
C = 21.9
D = 21.1

How do you compute those numbers???

Would it be beneficial to switch B and D tubes with each other?

Possibly. Try and see.

[72Blazer]

I did not like the results of switching the tubes

wound up with 25w on tube B (445v x 57mA)

Switching them back

I'll try and dial Vdrop back to get tube B under 20w and then measure and report back

[tubeswell]

Honestly I am unsure how to do the math but here are the numbers:

                Plate      Cathode

A           450v      45mA
B           450v      53mA
C           422v      52mA
D           420v      50mA

Try putting A and D together on one side and B and C together on the other side

[72Blazer]

Tubeswell-many more adjustments made...

Here is the latest

A           448v      42mA     18.81w
B           448v      45mA     20.16w
C           446v      42mA     18.73w
D           446v      40mA     17.84w

Is swapping tubes as you suggested still warranted?

[AlNewman]

Honestly I am unsure how to do the math but here are the numbers:

                Plate      Cathode

A           450v      45mA
B           450v      53mA
C           422v      52mA
D           420v      50mA

That looks pretty sweet.  I might even turn that down a bit.  Unless you're playing Madison Square Gardens.  Luckily it looks like you have adjustment and don't have to change nuts and bolts.

[tubeswell]

Tubeswell-many more adjustments made...

Here is the latest

A           448v      42mA     18.81w
B           448v      45mA     20.16w
C           446v      42mA     18.73w
D           446v      40mA     17.84w

Is swapping tubes as you suggested still warranted?

each side within 3mA looks pretty good


But if you want to experiment for your own edufication, take a journey of discovery etc YMMV. (Swapping the pairs around vis-a-vis the OT Primary will change it up and maybe even it out even more (or not). each half of the primary will have a different DC resistance which will interact with the bias point to affect tube current etc. you will only find out by trying.).

[sluckey]

Tubeswell-many more adjustments made...

Here is the latest

A           448v      42mA     18.81w
B           448v      45mA     20.16w
C           446v      42mA     18.73w
D           446v      40mA     17.84w

Is swapping tubes as you suggested still warranted?

I'd call that well done. Amp should sound clean and powerful. I bet you really see the advantage of having a separate 1Ω cathode resistor for each tube now.

[72Blazer]

Thanks again all. 

This was a really good bias lesson for me and yes the ability to measure each tube with the 1ohm cathode resistor rules!

The amp is quiet and functional.  Wayyyy too much volume/power for me.  It's in a head cabinet although I bet it never leaves the shop.  My next build is gonna be a 5F-1 Champ.  I mustered all the parts and that we will be a joy to carry over to my buddies for house for jam nights.

You guys are the best and thanks for helping an old retired Air Force "scope dope" with no technical background learn a little fraction of electronics theory.  You know it's funny, two buddies of mine whom I work with are retired Air Force radar maintenance technicians.  They had all the electronics training back in the day but neither one pays guitar nor have the desire to pickup a solder gun!  But I get it, I don't ever want to stare at a CRT and radar blips again either!

I'll take some pics now/post them here and I'll stand by for feedback on lead dress and soldering critique.

Vr
J

[72Blazer]

Pic 1

[sluckey]

I'm an old retired radar technician. Play guitar and electronics is a hobby, especially as relates to guitar. Racked up about 45 years working on AF and FAA radar systems. Never thought of it as "work". Still enjoy soldering and being in company with others that share my passions for guitar/electronics.

[72Blazer]

Thanks for keeping those "trons" flowing all those years.  "turning and burning"

[sluckey]

Hey, we worked for you guys! Even more so when dealing with the FAA controllers.

[shooter]

Never thought of it as "work"

yep, same, some days I would just smile, shake my head, and think "can't believe they're paying me for having fun"

[72Blazer]

I am still supporting the Air Force.  41 years total now.  It has been my life since I enlisted in 1981.  Still part of the team and its great.  Great American's from every part of the US.