Measuring for voltage drop (probe placement)

[Exojam]

I am looking for help in finding out where to place the second probe of my meter (fluke 115) to measure the voltage drop when attempting to bias my amp (one probe on pin 3 of the power tubes).

I am trying to bias either using the Marstan shunt or Aiken method.

This is a 50 watt build from Valvestrom (Metroamp).

Components:
Marstan PT - 1202-118
Marstan OT - 784-139
Marstan Choke - C199

Power Transformer wiring (just in case someone wanted to see it):
AC Line in -
L (Hot) black wire to tip of mains fuse holder, sleeve of fuseholder to one side of power switch, other side of switch to brown (120) PT wire AND to one side of 120vac lamp

N (Neutral) white wire to H on PT, H also goes to other side of lamp via a white wire.

G to new, dedicated ground lug on end of chassis

The other PT AC wires (1,3,4,5) should have individually heatshrinked tip then be bundled out of the way.

PT Secondaries – To rectifier as indicated above.

I use D & F initially, these will yield voltages that are more friendly to modern power tubes, C & G are higher voltage, can easily more the wires if desired.

Heaters – to v5 as indicated above. J to ground lug

I have been able to get the resistance and voltage reading from pin 3 on V4/V5:
V4 = 37.3 ohms - 449VDC
V5 = 38.8 ohms - 448.2VDC

So using my reading and according to steps 4 and 5 of the Marstan shunt method steps below:
4) Now we need to find the current draw needed to get 70% plate dissipation at 450VDC. Take 17.5W divided by 450VDC and you will get 0.0388A(38.8mA). This is the current draw per tube needed to get 70% plate dissipation with 450VDC on the plates.
5) Now we need to find the voltage drop across one half of the primary needed to give the 0.0388A of current. Take 0.0388A times 41 ohms equals 1.59VDC. This is the voltage you will be setting the bias pot to read while measuring across half of the primary of the output transformer. Check both halves of the transformer to make sure that both tubes are reading close to each other. Some transformers have a larger resistance difference between the two halves and will have to have the bias setup as a compromise(balance)between the two tubes. One tube will be set at a higher plate dissipation and the other will be set at a lower plate dissipation than the original calculated plate dissipation.

I would come up with this:
V4 = 37.3 ohms - 449VDC - 38.9ma - 1.45VDC
V5 = 38.8 ohms - 448.2VDC - 39.0ma - 1.51VDC

Again, my problem is I just cannot figure out my second probe placement.

I have attached:
A few pictures of my build.
A screenshot from one of the build manual pictures showing the ONLY place the OT center wire goes. Problem is, is it tied to the B+ line.
The build manual
The Marstan steps

Even though I can get a resistance reading from either the center tab/B+ turret or the HT fuse. I get no reading from those points when the amp is one.

I do have a build thread on the Marshall forum where a few folks are trying to help but I am just not catching it so I thought some different eyes and discussions may help me.

Thanks very much in advanced.

[Exojam]

I apologize but the build manual was to large to attach but can be located from the link below.

http://www.valvestorm.com/50watt%20Kit

[pompeiisneaks]

From that kit, it looks to me like they have a 1 ohm resistor from the pins 1 and 8 tied together to ground on the socket. In your pictures I don't see that OR the connection to ground. It almost looks like pins 1 and 8 are disconnected.

This means you'll get no output. The tubes can't conduct.

see here:

Cathodes.PNG

Once you install those, you'll touch the positive of the DMM to the pin 1 or 8 (or the lead between them) and then the other end can go anywhere that's a chassis ground, including the spot where the other side of the resistor connects to that in the top left of the picture. you don't need to move that ground connection, though, because once you have a good ground, any ground, is ground. Then measure the voltage drop across that 1 ohm resistor and get the mV and that's basically your mA of current (to be exact you should really measure the exact resistance of the resistor and calculate from ohms law, but it's usually a 1% resistor in that spot so you can just live with the very slight variance)

Hope that helps.

~Phil

[Exojam]

Phil,

Thank you very much for the reply and I apologize I did not document that I removed those one ohm resistors, since those in the system I cannot do the biasing in the way I outlined above.

My apologies again, James

[pompeiisneaks]

Not sure I understand, the 1 ohm bias method is one of the easiest to use. If you want to do the transformer shunt method then you do still need to jumper pins 1-8 as well as have a ground for them, even if you don't use the 1 ohm resistor.

For transformer shunt, you connect the ground to the center tap of the transformer, which should connect into your choke and the first power supply filter caps. Then connect the positive lead to the anode of one tube (Pin 3) and measure the resistance and write that down, repeat for the other pin 3 (they should be slightly different) then connect them up, power on, with volumes at 0 on everything, and measure the voltage drop. That divided by the resistance gives you plate current. On one half. You should then shut off, swap the postive lead of the DMM to the other Pin 3 and turn it back on and repeat the measurement of voltage for the second one.

~Phil

[martin manning]

You can keep the 1Ω resistors in place and they will not affect your results using the OT primary resistance technique. Note you are not doing a (current) shunt measurement, you are using the OT primary resistance in the same way as the 1Ω cathode resistors are used.

The voltage measurement you want is from the CT (B+) to the plate (pin 3) on each side, the same two points as you used to measure the resistance (since you are looking to calculate current from a known resistance and a voltage drop!).

[Exojam]

Martin,

Thank you for the reply and information.

That is my problem, when I have the amp powered on, I get absolutely no voltage reading from the OT centertab/B+ turret to pin three of either power tube, while look8ng for a voltage drop.

I can set my fluke to auto, manual, whatever and nothing for a voltage drop.

When I was using the Marstan (method link right below in the attachments), I get these values:
V4 = 37.3 ohms - 449VDC - 38.9ma - 1.45VDC
V5 = 38.8 ohms - 448.2VDC - 39.0ma - 1.51VDC

Than the next step states (the numbers in the below paragraph are from the manual).
5) Now we need to find the voltage drop across one half of the primary needed to give the 0.0388A of current. Take 0.0388A times 41 ohms equals 1.59VDC. This is the voltage you will be setting the bias pot to read while measuring across half of the primary of the output transformer. Check both halves of the transformer to make sure that both tubes are reading close to each other. Some transformers have a larger resistance difference between the two halves and will have to have the bias setup as a compromise(balance)between the two tubes. One tube will be set at a higher plate dissipation and the other will be set at a lower plate dissipation than the original calculated plate dissipation.

And shows the below diagram (see attachment).

38DD5619-A947-4759-A0D8-2658B0C58D5B.png

There is my problem, my results states I should have a 1.45 and 1.51VDC for a voltage drop.

BUT I have not idea where the second probe should goes as one has to be on pin.

Hope that clears things up.

James

[martin manning]

You are measuring correctly, one probe on the CT, the other on pin 3. Do you have the cathodes of the power tubes grounded? No voltage drop across the OT primary means there is no current flowing.

[pompeiisneaks]

You are measuring correctly, one probe on the CT, the other on pin 3. Do you have the cathodes of the power tubes grounded? No voltage drop across the OT primary means there is no current flowing.

This is exactly what I was trying to explain earlier, it's ahrd to tell from the pictures, but if you just 'removed' the 1 ohm resistors, without replacing them with a wire, you have no ground for your power tubes and they can't work, you've opened the circuit like a switch so nothing can flow. Either put the 1 ohm resistors back in jumpered between pins 1-8 then to that grounding lug, OR just use a straight wire between 1-8 and then to the ground lug on the socket nut.

~Phil

[sluckey]

Exojam, you're missing a big point. You will never see a voltage drop on the OT because there is no current flowing through the tube and the OT. And there can never be any current flowing through the tube and the OT until you connect pins 1 and 8 to chassis ground, either directly using a wire or indirectly using a 1Ω resistor as per the kit instructions.

[pompeiisneaks]

Sometimes a picture is worth a thousand words:

Grounds.PNG

This is your amp from your photos. I've drawn red lines that show where there MUST be either a wire, or a 1 ohm resistor to from the two pins and that grounding lug.

As an aside, I see what looks like a burn mark on the orange wire in the middle of those two, and can't be sure but it may show the bare wire underneath, if so cover that with something, or you're prone to other problems.

~Phil

[Exojam]

Thank you guys SOOOOO much.

Yes, that is what I what missing. 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.

Thanks again since I thought I was loosing my mind!

Now I will give it a go tomorrow or the next day.

[sluckey]

I'm very curious... Why don't you want to bias that amp using the very good and simple instructions on page 28 of the instruction pdf?

[martin manning]

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!

[Exojam]

I'm very curious... Why don't you want to bias that amp using the very good and simple instructions on page 28 of the instruction pdf?

Doing some reading of bias methods, I had read there were two more accurate ways to accomplish it, so I wanted to give them a try.