Tech help needed. Different readings between direct plate, cathode & bias probes.

[jasonguitar]

I'm confused by the differing results of the direct plate current measurement vs cathode resistor measurement vs bias probes measurement with this Supro Golden Holiday. The amp is dual 6v6, 5y3 rectified, cathode biased. I'm missing the boat somewhere cause I've never experienced these drastically different results before! Hopefully one of the kind resident experts can enlighten me. I'd sure appreciate it.

Here's the data:

SCREEN v:260.3
SCREEN Current: 8.7ma

LT 6v6 PLATE to CT:248 ohms
RT 6v6 PLATE to CT:462 ohms
LT PLATE v: 277v - CathodeV= 268.8v
LT PLATE v drop: 42v
RT PLATE v: 262v - CathodeV= 253.8v
RT PLATE v drop: 57.2v
LT PLATE current: 42v/248ohms=169ma??
RT PLATE current: 57.2v/462ohms=123ma??

CATHODE Voltage: 8.2v
CATHODE Resistor: 200 ohms
TOTAL CATHODE Current:41ma(/2 = 20.5ma per tube)

BIAS PROBE reading LT 6V6: 76ma???
BIAS PROBE reading RT 6V6: 77ma???

What gives?

[MWJB]

OK, you are experiencing difference in readings because you are using different methods to measure/determine your plate current. Each has its shortcomings...

Firstly, I am a little concerned at the vast disparity between DCR of the OT primaries. The difference in voltage is not necessarily an issue, but I really wouldn't expect on side of the primary to measure twice the other. Can you, unplug from the wall, pull the tubes apply a AC voltage source (half a volt from a variac - measured- to 6.3 if you have suitable source) to the secondary, then measure AC voltage developed from CT to each end of the primary, should be reasonably equal. It's possible that the OT is shot. The DCRs will be a little off because it has a cheap OT and there will be an imbalance in the amount of wire from one side to the other.

Have you measured the "200ohm" cathode resistor? Is it exactly 200ohms? Even so, I frequently find that this method, whilst good for "rule of thumb" throws up differing results to shunting the OT, or known good probes. It's not a big issue, rarely is cathode bias so sensitive that an mA or two here in there is "life & death".

I've never heard of measuring plate current by dividing voltage drop accross the primary winding's DCR...this plainly does not appear to work.

8.2v/200ohms= 20.5mA per tube @ 260vdc...seems ball-park.

Your bias probes are clearly wrong, some are not suitable for measuring cathode biased current, others just require one probe to be used, one at a time in PP cathode biased applications, see mfrs instructions on this.

[jasonguitar]

OK, you are experiencing difference in readings because you are using different methods to measure/determine your plate current. Each has its shortcomings...

Firstly, I am a little concerned at the vast disparity between DCR of the OT primaries. The difference in voltage is not necessarily an issue, but I really wouldn't expect on side of the primary to measure twice the other. Can you, unplug from the wall, pull the tubes apply a AC voltage source (half a volt from a variac - measured- to 6.3 if you have suitable source) to the secondary, then measure AC voltage developed from CT to each end of the primary, should be reasonably equal. It's possible that the OT is shot. The DCRs will be a little off because it has a cheap OT and there will be an imbalance in the amount of wire from one side to the other.

Have you measured the "200ohm" cathode resistor? Is it exactly 200ohms? Even so, I frequently find that this method, whilst good for "rule of thumb" throws up differing results to shunting the OT, or known good probes. It's not a big issue, rarely is cathode bias so sensitive that an mA or two here in there is "life & death".

I've never heard of measuring plate current by dividing voltage drop accross the primary winding's DCR...this plainly does not appear to work.

8.2v/200ohms= 20.5mA per tube @ 260vdc...seems ball-park.

Your bias probes are clearly wrong, some are not suitable for measuring cathode biased current, others just require one probe to be used, one at a time in PP cathode biased applications, see mfrs instructions on this.

Thanks for the reply.
I was thrown off because thus far, all cathode biased amps I've worked on have given me the same readings between the probes and the cathode resistor. I guess I've finally discovered that isn't always gonna be the case.

I was concerned about the drastic difference in the OT primary resistance as well. I will test as per your suggestion which was next on my to do list.

I was told by another tech and have also read somewhere about the voltage drop across the OT primary measurement for determining plate current. I have never tried this method until now and did so as a check against the two different measurements I was getting using the other methods. I guess we'll scratch that one!

If the OT turns out to be OK and with the approx 20.5ma per tube with plate voltage of 268.8v I'm getting only 5.5 watts static dissipation per tube. Looks like I need to lower the cathode resistor value to get the bias current up.

[MWJB]

"If the OT turns out to be OK and with the approx 20.5ma per tube with plate voltage of 268.8v I'm getting only 5.5 watts static dissipation per tube. Looks like I need to lower the cathode resistor value to get the bias current up."...maybe, but check for anything that might be pulling down your B+ first, like shot filter caps, 277v at the plate is low even for a 6V6 cathode biased amp.

[jasonguitar]

"If the OT turns out to be OK and with the approx 20.5ma per tube with plate voltage of 268.8v I'm getting only 5.5 watts static dissipation per tube. Looks like I need to lower the cathode resistor value to get the bias current up."...maybe, but check for anything that might be pulling down your B+ first, like shot filter caps, 277v at the plate is low even for a 6V6 cathode biased amp.

So as far as the OT goes, with 6.3vac on the secondaries I get 235vac on the left side from CT to Primary and 470vac on the right side.

As far as plate voltage is concerned, the filter caps are all new and tested good. With only the rectifier tube installed I get 425v from CT to ground and 420v LT and 421v RT from OT primaries to ground. When the tubes are installed it drops to 277LT & 262v RT.

[MWJB]

Your OT is shot. Replace it.

[Enzo]

OT? Or PT? 6.3v secondaries and voltages around a CT sounds more like a PT to me. Or am I misreading?

[g1]

Enzo, I think you are misreading. The 6.3 was injected into the OT secondary to see if the two sides of the primary were close to being balanced.
These readings are very suspicious, both the ohms readings and the voltages with the 6.3 applied to the secondary are roughly 2:1.
This suggests to me that the primary is miswired. Sounds like one of your ohms readings (462) is across both halves and the other (248) is across only one half.
Can you disconnect the primary and verify that the CT is really the CT?
Also, when I use the voltage drop measurement method I measure from CT to either plate, then divide by the resistance. This way you measure only the actual voltage drop across the primary half. Then there is no fuss with cathode voltage, screen current, or any other problems.

[jasonguitar]

Enzo, I think you are misreading. The 6.3 was injected into the OT secondary to see if the two sides of the primary were close to being balanced.
These readings are very suspicious, both the ohms readings and the voltages with the 6.3 applied to the secondary are roughly 2:1.
This suggests to me that the primary is miswired. Sounds like one of your ohms readings (462) is across both halves and the other (248) is across only one half.
Can you disconnect the primary and verify that the CT is really the CT?
Also, when I use the voltage drop measurement method I measure from CT to either plate, then divide by the resistance. This way you measure only the actual voltage drop across the primary half. Then there is no fuss with cathode voltage, screen current, or any other problems.

Yes we are discussing OT troubleshooting. Very interesting. Could it be this thing has been miswired? The supposed primaries are both the same color wire and the CT is a different color. Guess I need to disconnect em and ohm them out before I decide for sure that this needs to be replaced. Thanks.

[Enzo]

Oh yes, you did way that actually, I lost my focus. Getting senile here.

You're right, that two to one ratio of voltages, and the roughly two to one resistance does suggest the center tap and one end may have been swapped. That would be a pleasant surprise and a bullet dodged.

[jasonguitar]

Oh yes, you did way that actually, I lost my focus. Getting senile here.

You're right, that two to one ratio of voltages, and the roughly two to one resistance does suggest the center tap and one end may have been swapped. That would be a pleasant surprise and a bullet dodged.

A pleasant surprise indeed and a "bullet dodged"! Can't believe it was wired wrong! That's why it ended up on my bench; to fix someone else's screw up and learn something in the process. Thanks gentlemen for the insight and sound advice. Always greatly appreciated! Now lets see where the bias is really at.

[jasonguitar]

So all is looking and sounding pretty good here. The plate voltage does seem a bit low still though considering most Supros of this era with similar power supply and output section seem to be running around 325. B+ at CT is about 305v. Cathode to plate is 286v on the left and 284.5v on the right. Lowered the cathode resistor to 100 ohms and I'm getting approx 33ma per tube now for a static dissipation wattage of 9.42watts per tube. I've got 241.8v on the screens with a shared 1k screen resistor. 17ma total screen current.

[g1]

That's great the OT wasn't shot. What method did you use to get the new bias figure and how do the different methods now compare?
Also, in your original post you took the voltage dropped across the tube and divided by the resistance of the side of OT primary, this is incorrect. You need to measure the voltage at the same place you measure the resistance (CT to plate). Then you will find the reading should be in agreement with the other methods.

[jasonguitar]

That's great the OT wasn't shot. What method did you use to get the new bias figure and how do the different methods now compare?
Also, in your original post you took the voltage dropped across the tube and divided by the resistance of the side of OT primary, this is incorrect. You need to measure the voltage at the same place you measure the resistance (CT to plate). Then you will find the reading should be in agreement with the other methods.

Thanks for straightening me out on the CT to plate voltage measurement. So I just checked the amp again. Ditched the probes but I'm still getting differing measurements if I divide the cathode resistor measurement for 2 tubes compared to the CT to plate measurement. Here's the data:

Left Plate to CT resistance: 206 ohms Voltage from CT to Lt plate: 14.6v/206ohms= 70ma x 290v(plate to K)= 20 watts
Right Plate to CT resistance: 236 ohms Voltage from CT to Rt plate: 17.2v/236ohms= 72ma x 287.2v(plate to K)=20.67 watts

K voltage 6.8v/100ohms K resistor = 68ma x 290v(plate to K) = 19.72/2 tubes = 9.86 watts per tube?

What gives?

[MWJB]

Measuring voltage drop accross the primary & dividing by DCR plainly does not seem to work. I have never heard this method described for reading plate current before.