Measuring for voltage drop (probe placement)

[Exojam]

And why not leave the resistors in place so you have the option? As I said above, they won’t affect the results you’ll get using the OT primary resistance method.

Nowhere in any document I read or got from anyone else, that it is OK to drop the resistor just place a piece of wire in it’s place.

I’ll bet you’ve never seen a recommendation to remove the one ohm resistors and leave the cathodes ungrounded either!

You are correct, none of the other two methods I read about even mentioned removing the resistor or wire. I guess they understood people following those steps would be advanced enough to know. Which you could see I was not.

But I feel it is a learning process and you guys taught me that something has to be there. You taught and I learned, sounds like a win win to me.

[pompeiisneaks]

More accurate, in that you can bias it as closely to 70% w/o going over, then yes, but if you bias it with the 1 ohm resistor method you'll get screen current included which is usually about 5mA max. This means you'er just a little cooler. If you want to still try the other method, though, as has been stated, you don't need to remove the 1 ohm resistors to ground. They do nothing to the transformer shunt method of measuring the actual plate current.

I also would state that measuring current on 1 ohm ground resistors at lower voltage potential, is safer as well. Instead of measuring the full B+ across the transformer windings.

~Phil

[Lynxtrap]

Sorry, but this discussion worries me. Install the 1 ohm resistors and use those to measure and calculate the bias! I say that with the best intentions.

[Exojam]

Lynxtrap,

My I ask why it worries you?

This is my first amp build and I tried to read everything I could on this subject but felt it necessary to ask for help. And if you say that with the best intentions, that is how I will take.

I have no issues what so ever in dropping thos resistors back in. About to head over to the amp and start to solder.

James

[Phil_S]

I wouldn't speak for someone else, however, I also briefly was concerned. It appeared you were using the shunt method to check bias. The shunt method is a direct measure of current flowing, employing by inserting the meter in series between the OT and the power tube. I soon realized you were measuring voltage and calculating current using the DCR of the OT. Measuring current directly is potentially a serious danger and should only be done by someone who is very clear about what's involved and how to do it safely. FWIW, I've built a dozen or so amps, and I'm not comfortable with the thought of using the shunt method. Measuring voltage and indirectly determining current using Ohm's law is considerably more benign, with ordinary concerns for safety. On that point, I felt you were on the right track and didn't comment.

I've done what you suggest, calculating current by finding voltage and DCR. I've done it with the 1 ohm resistors. The difference is screen current, which, for guitar amps, can be estimated and it's good enough. If you really want to know, many times there is a 1K screen stopper and from placing your meter across that resistor, you will derive the actual screen current. Use the 1 Ohm resistors. It's really the simplest, easiest way to do it and it is accurate enough. Also, it's cheap to do, about as close to free as it gets. This is the right price for you own safety.

[Exojam]

Well the 1 ohm 1% resistors are back in place. One of them read 1.1 ohm and one read 1.2 ohm.

I will start some measurements either tomorrow or Friday since I have a med appointment tomorrow.

I greatly appreciate all of your concerns. This may be my first amp build but not my first run around with current that will stop the heart. I was an Avation Electrians Mate in the Navy working on A-6E Intruders. But I do realize planes are not amps but both will whack you with one slip of the hand. I hope no takes that as “just because of my previous job means I know everything about amplifiers”, just trying to give a little background.

More importantly, again I appreciate your thoughts of my safety, that is very nice to know you guys think that way for someone who only had one post on this forum!!!!!

One thing that did happen when the resistors were going in was I had to trim a little of the ground wire that goes over to the speaker jacks which in my opinion left it to taught and I feel I want to replace it to provide a little more slack in it.

Thanks so much again and I will be sure to post my updates in a day or two.

47E3A188-4920-49B1-9B64-0F39E468DF36.jpeg

[Phil_S]

It can be difficult to get an accurate reading on a 1Ω resistor. If they are spec'd at 1%, which is about as good as it's going to get, then they are probably just that. If you cross your meter leads, what does the meter read? I believe this tells you about the low end reading it can manage with accuracy. I suspect only the really high end meters can resolve an accurate reading on a 1Ω. If it was me, I'd treat it like it is 1.0Ω. Fix that ground wire and cover that burnt spot on the orange wire. It seems like you are out of the woods on this. Careful now, amp building is addictive.

[Lynxtrap]

Lynxtrap,

My I ask why it worries you?

I'm not so worried anymore
At that point I knew nothing about your experience with electronics and working around potentially lethal voltages. Those other methods of measuring bias can be dangerous, and it appeared that you lacked some understanding of how the circuit works, what you were supposed to measure and how you should do it. That could be a recipe for disaster.

The 1 ohm resistor method is pretty safe and usually gives a close enough reading for our purposes IMO. I realize that "close enough" might not do in military aviation

[Exojam]

It can be difficult to get an accurate reading on a 1Ω resistor. If they are spec'd at 1%, which is about as good as it's going to get, then they are probably just that. If you cross your meter leads, what does the meter read? I believe this tells you about the low end reading it can manage with accuracy. I suspect only the really high end meters can resolve an accurate reading on a 1Ω. If it was me, I'd treat it like it is 1.0Ω. Fix that ground wire and cover that burnt spot on the orange wire. It seems like you are out of the woods on this. Careful now, amp building is addictive.

Yeah that embarrassing burn mark, I was very ticked when I did that.

[Exojam]

Lynxtrap,

My I ask why it worries you?

I'm not so worried anymore
At that point I knew nothing about your experience with electronics and working around potentially lethal voltages. Those other methods of measuring bias can be dangerous, and it appeared that you lacked some understanding of how the circuit works, what you were supposed to measure and how you should do it. That could be a recipe for disaster.

The 1 ohm resistor method is pretty safe and usually gives a close enough reading for our purposes IMO. I realize that "close enough" might not do in military aviation

Well you are correct in that I did not understand how that part of the circuit worked so instead of just proceeding I called out for help and you folks came a running!

[Exojam]

So I just hooked up my meters (Fluke 115 and a cheap one) to check the bias and I am coming up with a 6ma difference between this set of matching Mullard XF2 tubes. V5 is giving me the higher reading and it does not matter what meter is on which tube, same reading. I am just not sure if this points to a component or if it is just “the way it is”.

I have attached a picture of the reading and a screenshot of the bias calculator which shows my range with the 430VDC I have for a plate voltage.

DF33D01D-D1CC-41E8-A585-E6EEBD391897.png

986B5C21-79D5-4DD4-BC9C-404AB3285A96.jpeg

[Lynxtrap]

So I just hooked up my meters (Fluke 115 and a cheap one) to check the bias and I am coming up with a 6ma difference between this set of matching Mullard XF2 tubes. V5 is giving me the higher reading and it does not matter what meter is on which tube, same reading. I am just not sure if this points to a component or if it is just “the way it is”.

Swap the tubes between the sockets. If the difference "follows" the tube so to speak, the difference is in the tubes.
Matched tubes will seldom measure exactly the same.

[Exojam]

Will do.

I know I have swapped them before but I took them out while working on so I cannot remember who was where. Stinks getting old!

[xtian]

That's not a bad mismatch. Wouldn't bother me.

[Exojam]

Swapped and now down to about a 4ma difference.