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

[jasonguitar]

Going back through the posts, it seems that your plate voltage rose when you fitted a smaller cathode resistor, this is not right, it should have dropped.

I had changed the cathode resistor before we figured out that the OT was wired wrong.

[jasonguitar]

I'm still wary of the OT too, did we ever get round to determinining a turns ratio? Weren't you getting 470VAC from end to end of the primary, with 6.3VAC (measured) injected at the secondary? 470/6.3= 75, 75*75= 5625...even with a 2ohm load that's a primary Z of 11.25K...seems very unlikely.

What would you suggest I do?

[MWJB]

Right, in your very first post you said that plate voltage to cathode was 262v, this was with the 200ohm cathode resistor, your latest voltages are higher than this with a 100ohm cathode resistor...this makes no sense.

At around 280-290vdc plate to cathode, you should be getting mid 30's plate current with a 200ohm resistor. Fit a 220-250ohm cathode resistor.

If the largest voltage accross your OT primaries is 470VAC with 6.3VAC on the secondaries, it should be nearer 200VAC.
200/6.3 = 32
32*32= 1024
1024*8ohms = 8200ohms (typical area for 2x6V6).

You should see 180VAC to, say, 210VAC accross the primaries, end to end, if your amp runs a 8ohm speaker. I still suspect that your OT is shot. Something is guzzling up current. Try another OT, or redo the 6.3VAC test measuring carefully.

[jasonguitar]

Right, in your very first post you said that plate voltage to cathode was 262v, this was with the 200ohm cathode resistor, your latest voltages are higher than this with a 100ohm cathode resistor...this makes no sense.

At around 280-290vdc plate to cathode, you should be getting mid 30's plate current with a 200ohm resistor. Fit a 220-250ohm cathode resistor.

With a 246.5ohm cathode resistor I get a cathode voltage of 8.4v. Left plate is 292v, Right plate is 290v.

[jasonguitar]

If the largest voltage accross your OT primaries is 470VAC with 6.3VAC on the secondaries, it should be nearer 200VAC.
200/6.3 = 32
32*32= 1024
1024*8ohms = 8200ohms (typical area for 2x6V6).

You should see 180VAC to sat 210VAC accross the primaries, end to end, if your amp runs a 8ohm speaker. I still suspect that your OT is shot. Something is guzzling up current. Try another OT, or redo the 6.3VAC test measuring carefully.

Redid the OT test again to be sure. Measured 6.3vac coming out of the variac applied to the secondaries. Reading 466vac across the primaries and 233vac per side.

[MWJB]

A 6V6 push pull OT usually has a primary impedance of 4.7K to 8.5K, this impedance can be established by the formula above. Your primary impedance for an 8ohm load is 43.8K. Ergo, your OT is shot. Replace it.

This is why your bias reads low.

[jasonguitar]

A 6V6 push pull OT usually has a primary impedance of 4.7K to 8.5K, this impedance can be establishe by the formula above. Your primary impedance for an 8ohm load is 43.8K. Ergo, your OT is shot. Replace it.

This is why your bias reads low.

Bummer. I guess a basic Deluxe replacement OT should be fine. 25 watt 6.6k ohm CT, 8ohm secondary.

[MWJB]

Yes, it'll be pretty bullet proof, a 5E3, or tolex Princeton PT would be OK in a cathode biased design.

[jasonguitar]

Yes, it'll be pretty bullet proof, a 5E3, or tolex Princeton PT would be OK in a cathode biased design.

Just did the OT test with a 25watt Triad I had laying around. Like you said it should, it has 220vac across the primaries on the 8ohm tap. I'd use it for this amp but its too damn big! I'm worried about the size of ANY replacement cause the original is a little thing attached to the speaker frame of oneof the 6x9s.

[jasonguitar]

Just ordered the OT. Should be here by Wed. Looking forward to being done with this thing! I really appreciate all the help. I do have an issue with the speakers being in pretty bad shape. May have to order a pair from Weber as they seem to be the only ones who make 4ohm 6"x9" guitar speakers. Then I'll have to figure out where to mount the new OT. No room on the chassis.

[WholeToneMusic]

In support of what MWJB said about the transformer shunt method. Current always takes the path of least resistance that is why it goes through the meter.

[jasonguitar]

In support of what MWJB said about the transformer shunt method. Current always takes the path of least resistance that is why it goes through the meter.

Thanks. I got it. I'm familiar with the method and understand the logic. Having not used it for a while, I was second guessing when I asked the question about my meter input resistance. I had gotten a bit confused by some comments I'd read recently saying the shunt method was innacurate due to meter impedance variations, which doesn't really make sense. I brain farted and was thinking of things in reverse. As long as my meter has significantly LESS resistance than the OT when measuring current(which it does) it should be pretty darn accurate. Like you said, current takes the path of least resistance. The shunt method gave me approx the same result as the voltgage drop across the primaries and the bias probes and is likely the MOST accurate of the 3.

[g1]

OK, I'm going to try to clear up this shunt business. It is not an inaccurate method as someone else said. What I said was that it was inaccurate with MY meter.
There is a spec. for current readings called "burden voltage". It is the voltage that appears between the probes when measuring current with your meter. From this spec. you can calculate the resistance of the "shunt" of your meter. Some meters spec. at 1 or 2 mV per mA. This translates to 1 or 2 ohm shunt. You can measure the resistance with another meter if you have one. Set the meter to be measured to mA range and make sure the probes are in the mA terminals. Measure the resistance between the probes with the second meter.
Some meters have a burden voltage of 10mV/mA OR MORE. This would be my meter (Meterman 37xt) also Fluke 29 and others. This means your shunt is 10 ohms! I measured mine and read 12.7. It is not a cheapo meter, true RMS, just not very good spec. for current. On the 10A range it has a much better burden voltage but measuring 50ma on the 10A range is not very accurate.
So when I shunt a 150R winding at around 50mA I am reading about 6% low. When I shunt something like Hiwatt or Marshall with around 50ma through a 50R winding my 10ohm shunt gives me a reading around 20% low.
Anyone who uses shunt method should find out their burden voltage or measure their "shunt" with another meter. If you have a burden voltage of 1 or 2 mV/ma or the shunt of your meter measures 1 or 2 ohms (or less is even better) the shunt method should work fine for you. Make sure to follow all safety precautions.
Hope that made sense!

[jasonguitar]

Anyone who uses shunt method should find out their burden voltage or measure their "shunt" with another meter. If you have a burden voltage of 1 or 2 mV/ma or the shunt of your meter measures 1 or 2 ohms (or less is even better) the shunt method should work fine for you. Make sure to follow all safety precautions.
Hope that made sense!

I'm getting 2.7 ohms across the probes on the ma setting.

[g1]

That sounds good. I would call that a shunt. What my meter gives I would call a low value parallel resistance.
A few questions for you:
1) what are the supro speaker impedances and are they series or parallel?
2) does the OT attached to the spkr. look stock?
3) what kind of bias probes do you use and do you use the voltage or current range on your meter with the bias probes?
4) were there sound quality problems with the amp when you started working on it?