OT resistance

[dude]

I have several Hammond AO-43 OT's. Looking to put one  in a Blues Junior.  Measuring the resistance from the secondary to primary I get a large difference, 207 ohms and 181 ohms. the original BJ OT and most all I measured are fairly close, does this mean the Hammond OT might not work properly or give a diffident bias to each tube when biasing by the shunt method..?

[kagliostro]

Measuring the resistance from the secondary to primary I get a large difference, 207 ohms and 181 ohms.

 

How did you measured difference from secondary to primary, they are separated winding

also secondary will have only few ohm of resistence

excuse me but what you say is a no sense

Franco

[pdf64]

Perhaps dude meant the resistance from either end of the primary to its CT?
If so, the discrepancy doesn’t seem that large.
Bear in mind that if any primary turns are shorted, the transformer’s primary inductance and hence impedance will be crippled.
Resulting in the amp’s power output being a tiny fraction of what it should be.

[sluckey]

Dude! Think about what you just said.

[dude]

Perhaps dude meant the resistance from either end of the primary to its CT?
 

Yes, sorry

[dude]

Perhaps dude meant the resistance from either end of the primary to its CT?
 

Yes, sorry

Yeah, it's a crazy question, ha! I meant the CT of the OT to each plate lead.

[HotBluePlates]

... I get a large difference, 207 ohms and 181 ohms. ...

207Ω / 181Ω = 1.14 ---> ~14% tolerance.  Some places we would call that a "small difference."

... does this mean the Hammond OT might ... give a diffident bias to each tube when biasing by the shunt method..?

The OT winding resistance doesn't matter to the tube.  The screen voltage and bias voltage matter a whole lot more to what current the tube will pass.

It does mean that you will need to write down resistance figures for each transformer (and maybe half-primary?) so that you keep track for the math.

I have several Hammond AO-43 OT's. ... does this mean the Hammond OT might not work properly ...

The Hammond AO-43 is "push-pull EL84 to 8Ω" and the Blues Junior is "push-pull EL84 to 8Ω."  Seems like it oughta work just fine.

[kagliostro]

OK, I was thinking you where talking about the two semi windings of the primary measured with the CT as reference but wanted you to say that


The differences in resistance between the two semi windings is simply due to the way the windings are wound


Think to the support of the windings empty, put on the number of windings necessary to obtain the correct turn ratio to the secondary (say 200 turn for the first semi winding) then wound It (say) 10 wound of the secondary, after that wound the other 200 turns of the other primary semi winding (the other 200 turns) ...... now think to what you have, the first time the wire was wound directly on the support but the second time the semi winding (also now 200 turns) was wound on the first semi winding plus the secondary so 200 turns are longer than the previous 200 turns and obviously length Is more and resistance too


There is an old way to wound primary for PP Amps that result in same resistance, but not often applied, you must wound on the support the 200 wounds using paired wires, so the length of the first and second semi windings are the same and the same is the resistance


To be sure if the first and second windings are on spec you simply must avoid to measure resistance, put a small AC voltage on the SECONDARY winding and measure the voltage on the two semi windings of the primary (using CT as reference) to be OK the two voltage must be equal or very very close each other


Franco




Obviously windings are wound in a more complicated way (number of layers) but this Is a semplified description

[JPK]

I have several Hammond AO-43 OT's. Looking to put one  in a Blues Junior.  Measuring the resistance from the secondary to primary I get a large difference, 207 ohms and 181 ohms. the original BJ OT and most all I measured are fairly close, does this mean the Hammond OT might not work properly or give a diffident bias to each tube when biasing by the shunt method..?

Indeed it will but it won't really matter. I just finished a 5E3 with a Hammond 1760E OT. Out of curiosity I measured the secondary resistances right out of the box and noticed a bit of difference (one leg to CT was 8.7% higher than the other, cold resistance). It ended up biasing at 80% and 86%. Not the same at all but concluded it was totally acceptable (I created a thread, linked below). Amp sounds great. Tubes have a nice orange glow.


https://el34world.com/Forum/index.php?topic=29644.msg326264#msg326264

[PRR]

> large difference, 207 ohms and 181 ohms

The DC resistance is typically far smaller than the audio impedance, so as to be "negligible".

You have a 26 Ohm difference in the two sides of an 8,000 Ohm winding, or 0.3%. That's small.

[dude]

Thanks for the reply's, much appreciated. 
I'm a "Jack of all Trades but Master of None", most of you guys don't realize your education in electronics is a gift, takes years to learn and develop. I read info over and over for it to stick and sometimes it never does. I'm lucky at this point in my amp building life I know what I know and can build an amp. I've come to the point where a layout is much harder to understand than reading a schematic, in a schematic it's all there, thanks mostly to you guys. Reading, studying articles, books, etc is great but with no teacher to "ask a question" over sticky points, it brings a lot of frustration.  You guys spent years learning, it's a gift, others don't have it and I appreciate everyone sharing their knowledge with me and anyone who asks.
Have a great Holiday and Merry Xmas to all.
Al, aka, The dude.

[dude]

Sorry to beat an old dog but trying to bias a Blues Junior that l put a Hammond AO43 OT in, l’m having an issue.
Biasing by OT resistance method, the ohms on each plate of warm OT, amp off, no voltage, l have a difference of 25 ohms. I get a wide difference in voltage drop across each plate to OT CT, one plate is .985v other is 2.75. Switching tubes the voltage drop follows the same plate, not the tube. “I assume” doing the math for mA bias setting with matched tubes, l’m going to get a difference of mA setting and dissipation for each tube even though they are matched?
I was told above the resistance in the OT plates is only 25 ohms, in specs.
Again, l assume finishing the math the bias is going to be like around 9 or 10 mA difference?
So, is my Hammond OT bad? What am l doing wrong? The resistance of the plates of the Hammond is 212 ohms vs 187 when warm.
The amps sounds ok but l do hear a frizziness  on the tail end of a hard hit chord, like maybe a bias issue.
The OT CT is 328.9v
The plates are 322.8 and 326.2v


Maybe l should finish the math

[PRR]

> the voltage drop follows the same plate, not the tube.

The "plate" is inside the tube. Not the output transformer. What are you really saying?

Why not check current with a cathode resistor?

[AlNewman]

I think my blues jr measures around 100 ohms/side of the output transformer.
Would that create some issues?

[dude]

> the voltage drop follows the same plate, not the tube.

The "plate" is inside the tube. Not the output transformer. What are you really saying?

Why not check current with a cathode resistor?

When l say plate, sorry l mean socket. This is a PCB board and it’s almost impossible to get a 1 ohm resister across the cathode to ground, traces are unreachable.
If l take OT CT voltage, 328v and substrate the plate 322v, l get 6v difference. Take the 6v divided by the OT resistance on that side (187ohms), l get .032 amps or 32 mA too hot for el84.
The other socket, 328v minus 326v = 2v. 2v/212ohm (resistance of the OT on that side is 212). I get .009 amps or 9mA, way too cold for el84. Are these tubes way mismatched, seems it’s the big difference in OT resistance..?
If l measure voltage drop from the OT CT to plate with meter, l get 2.52v one side and .9 the other, the voltage difference from plate to OT CT. Two ways gives me difference readings.
What am l doing wrong? Should l take the average of the two OT windings, big difference between 187 and 212..? Give big difference in mA bias

[sluckey]

May I suggest... Disconnect the OT plate leads from the board and connect the dangling plate leads to a couple of 1Ω resistors. Then connect the other end of the resistors to the board at the point the OT plate leads had been connected. IOW, put the 1Ω resistors in series with the OT plate leads. Now you can accurately measure the mV drop across the 1Ω resistors to get the plate current. This is the same idea as putting the resistors on the tube cathode. Just be careful with the probes.

[AlNewman]

Another thing I forgot, was the ribbon cable on the phase inverter interfered quite a bit with my multimeter, to the point where I could hardly get a voltage reading from plate to center tap on that side...  It helped if I angled my probe away from it, but basically I couldn't really trust the reading on that socket.  Hope that helps.

[PRR]

> does this mean the Hammond OT

I'm sure the Hammond is better than the Blues Jr.

[dude]

May I suggest... Disconnect the OT plate leads from the board and connect the dangling plate leads to a couple of 1Ω resistors. Then connect the other end of the resistors to the board at the point the OT plate leads had been connected. IOW, put the 1Ω resistors in series with the OT plate leads. Now you can accurately measure the mV drop across the 1Ω resistors to get the plate current. This is the same idea as putting the resistors on the tube cathode. Just be careful with the probes.

Thanks, l’ll try that, could l leave the 1 ohm R’s attached permanently in series in the primary leads? Somewhere safely mounted between the bd and tube plate. I’m aware this method is dangerous but if so, what wattage 1 ohm would l use? B+ is 328

[sluckey]

Thanks, l’ll try that, could l leave the 1 ohm R’s attached permanently in series in the primary leads? Somewhere safely mounted between the bd and tube plate. I’m aware this method is dangerous but if so, what wattage 1 ohm would l use? B+ is 328

Sure. Just find a way to safely secure the resistors. If your OT leads have quick disconnect terminals you may consider crimping some quick disconnects on the resistor leads. Like these...

     https://www.lowes.com/pd/IDEAL-16-Count-Disconnects-Wire-Connectors/3363062

1Ω 1Watt is what I use.

[dude]

Thanks you Sluckey, I have about 5 Hammond OT's from an AO-43, all of them have about 25 ohms difference in their windings. Big difference in tone from that small BJ OT, night and day.  Yes, the Blues Junior has clip on leads but the only issue is I soldered the primary leads to the clip-on terminals on the Bd and getting the solder off without taking the bd out again might be an issue, don't want to loosen the solder on the opposite side.
So across that 1 ohm, the reading is in "mV"  the exact same as if across the cathode to ground of the el84?

PRR, yes indeed the Hammond is well made and sounds great vs the small cheap original Fender that comes stock. Thank you for your help, as always.

[sluckey]

I have one of those AO-43 (AO-24158) OTs in my spare parts bins. Primary readings are just like yours. BLU-RED is 213Ω. BRN-RED is 186Ω. BLU-BRN is 399Ω. (Measurements with Klein MM700). This is not a defect. It's just the way the primary is wound to have the same number of turns on each half of the winding. One half is wound to a certain number of turns. Then the other half is wound on top of the first half for the same number of turns. Naturally the wire will be longer for the second half, therefore the longer wire will have more resistance. This is not a problem for proper operation of the OT, but I see that it can be problematic for the way you are trying to calculate bias and PD.

[dude]

I have one of those AO-43 (AO-24158) OTs in my spare parts bins. Primary readings are just like yours. BLU-RED is 213Ω. BRN-RED is 186Ω. BLU-BRN is 399Ω. (Measurements with Klein MM700). This is not a defect. It's just the way the primary is wound to have the same number of turns on each half of the winding. One half is wound to a certain number of turns. Then the other half is wound on top of the first half for the same number of turns. Naturally the wire will be longer for the second half, therefore the longer wire will have more resistance. This is not a problem for proper operation of the OT, but I see that it can be problematic for the way you are trying to calculate bias and PD.

YES, I figured that out after trying the resistance method for biasing.  I've been reading and "understanding" about transformers and biasing methods. This Blues Junior board is one of the lousiest PCB that I ever work on, the traces are very small and the slightest melting of solder, they'll come off, luckily the traces can be carefully scraped and fixed if they do.
I took a pic below of the two male plate push on connectors, I attached 1 ohm 1 watt R's as you suggested. Haven't turned on the amp at this point.  Just that knowing there's high voltage on those R's I'm thinking will these R's blow and take out the tubes or OT? just my lack of knowledge. The R's aren't on stand-offs but they ain't moving, they are tight and away from components.
So, I just set my Klein MM700 meter (same as yours) to DC volts and take a reading across each resister, I'll get a mV reading, just as if I'm using the tube's plate and cathode?  I installed a bias pot, little blue box pot, I can then just turn the pot to the correct mV for the tubes to bias?
Sorry for the redundancy, but I want to be positive I got this right, can you confirm from the pic that all looks good and I'm correct.         

[sluckey]

You have it all correct. Just be careful. Even though there will only be a few millivolts ***BETWEEN*** the probes, there will be B+ voltages on both probes.

Your mounting of the resistors actually turned out better than I had expected. I'd still feel better if the flying lead was secured with a chassis mounted turret, terminal strip, or even better, a tip jack such as these...

[PRR]

> there will be B+ voltages on both probes.

Which may be why we usually do the resistors in the *cathode* circuit? Non-lethal metering. I haven't tried plate metering since the early days of Disco. Before "Staying Alive", which is relevant since "Ah, ha, ha, ha,.." is now the official beat for CPR, which I almost needed after taking plate voltage through the chest.

[HotBluePlates]

Electronics, Nostalgia, Pop Music, and medical skills all in 2 sentences!!

[dude]

> there will be B+ voltages on both probes.

Which may be why we usually do the resistors in the *cathode* circuit? Non-lethal metering. I haven't tried plate metering since the early days of Disco. Before "Staying Alive", which is relevant since "Ah, ha, ha, ha,.." is now the official beat for CPR, which I almost needed after taking plate voltage through the chest.

Ever since l got 353vdc up my arm, l’m extremely cautious. I was working on an old Bassman amp, unplugged. Drain the caps but caps have memory as l learned. Only had one hand in there, back in the day l was told never to hold the chassis with a loose hand.
The shock went up my arm to my shoulder, never forget that feeling. I still had a heart beat but felt ill and weak..?  I checked the cap l touch, 353vdc. Same finger that touched the cap contacted the chassis.  Good thing l didn’t have the other hand tight on the chassis. I now use bleeders or a clip on bleeder on every amp l work on. Lucky to be here, 353vdc across my chest would have probably been my last moment. This BJ PCB has no safe B+ spot on the bd for a 220k.
I’ll use clips on my meter leads before l turn on amp when biasing. Thanks all

[sluckey]

I’ll use clips on my meter leads before l turn on amp when biasing. Thanks all

Those big ole gator clips that come with the MM700 will put a lot of strain on that flying resistor. I suggest using light weight gator clip test leads to minimize stress.

[dude]

Those big ole gator clips that come with the MM700 will put a lot of strain on that flying resistor. I suggest using light weight gator clip test leads to minimize stress.

I hear ya, l was even concerned with the thickness of the 1 watt R leads when soldering, doesn’t take much strain to break or weak’n the leads. Might be best to just not use the clips, probably best to run wires to those test jacks you posted mounted somewhere safe. I might re-think how l can wire this set-up safer. It’s not my amp and the owner, my son in law, might not be as careful as me. Thanks again

EDIT: The more l think about it, l don’t like HV going through those tip jacks, probably best to use cheap gator clips.