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

[jasonguitar]

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?

1)The speakers are two 4 ohm 6"x9"s wired in series.
2)The OT is the stock unit riveted to the speaker frame and has the same serial number as other amps I've seen pics of.
3)1ohm-1%-3watt cathode voltage drop measurement probes. Millivolts.
4)The amp sounded like a fuzz face with a weak battery. Had every loud hum.
Filter cap can was shot and there were a few wax coupling caps in the preamp which were exceedingly leaky. The tremelo wasn't working(shorting footswitch jack wasn't shorting). Most of the other preamp coupling and decoupling caps had been replaced previously. My friend who owns it doesn't know what the history of this thing is. If you've ever seen a gut shot of these, the output tranny is wired to a terminal strip and the wires are very brittle. I could tell it had been re-soldered so the leads probably broke off before and it was re-wired incorrectly. Even though it wasn't working right it sounded kind of cool and my friend liked it. He just wanted me to go through it and wanted to get the trem to work. Even with the low bias/bad OT it sounds good now.

I just got the new OT in. Have to wire it up and figure out if I can mount it the same way or not. Looking forward to hearing this thing healthy.

[g1]

Hope this will get your voltage reading across cathode resistor in line with the probe readings. Please do all the bias measurement methods you tried before and post the results.
If you still have access to those pictures of other amps could you see if maybe the speakers were wired parallel?

[jasonguitar]

Triple checking things before I go installing the new OT, and second guessing as g-one's question about the speaker wiring got me thinking... it turns out I made a huge oversight here. The two 4ohm speakers are actually running in parallel for a 2 ohm load. 2 schematics verifiy this.
I tested the OT for turns ratio/impedance. At about 75vac across the primaries I get 1vac on the secondaries.
75*75 = 5625:1
5625*2 = 11250ohms - Within proper range for 6v6 push pull at 2ohms?

[g1]

That's what struck me while re-reading the whole thread. 11K for a 2 ohm load is an awful lot closer to the typical, so I figured it was worth asking.
Aside from that, whether the OT is fried or not, I think you are still going to get the funky reading across the cathode resistor. The reason I say this is because your bias probes are in series with the cathodes. This should take the OT out of the equation. The currents through the 2 cathodes must both flow through the common cathode resistor. Whatever the bias probes tell you is flowing through each cathode, the sum should agree with the current through the common cathode resistor.

[jasonguitar]

That's what struck me while re-reading the whole thread. 11K for a 2 ohm load is an awful lot closer to the typical, so I figured it was worth asking.
Aside from that, whether the OT is fried or not, I think you are still going to get the funky reading across the cathode resistor. The reason I say this is because your bias probes are in series with the cathodes. This should take the OT out of the equation. The currents through the 2 cathodes must both flow through the common cathode resistor. Whatever the bias probes tell you is flowing through each cathode, the sum should agree with the current through the common cathode resistor.

I ditched the probes when I took the latest cathode readings and I got the same readings whether the probes were plugged in or not. As far as the shunt not agreeing with the cathode reading I don't understand what's going on there unless the OT is affecting the reading? I'm still confused about this. Should I still be considering the OT bad?

[MWJB]

Yes, because your plate current measured by any of the methods you have used is way out of whack.

At your plate voltage, you would normally find around mid 30's per plate with a 200ohm shared resistor, you were getting 20, shunting the OT shows current disappearing accross the primary that isn't being dissipated by the tubes. Finding a 2ohm OT for 2x6V6 might not be that easy, might be easier to rewire the speakers.

Mercury do a BF Deluxe OT with multiple taps, but it's big, I don't know if it will fit on your speaker basket.

[g1]

"I ditched the probes when I took the latest cathode readings and I got the same readings whether the probes were plugged in or not."
Sorry, I was unaware of this. I think you are saying the probes were in agreement with the reading across the cathode resistor? If so, sorry about the wild goose chase.
Before getting into the mounting of OT to speaker, wire up the new OT (or your triad) and put the speakers in series. Your shunt reading should agree with your measurement across the cathode resistor.

[jasonguitar]

Ok. We're in business. New OT hooked up to a 8ohm load.

So now using shunt method I get:
44.8ma Left plate
42.9ma Right plate.
333v both plates
44.8 * 333 14.91 watts

Cathode resistor method I get:
18.8v/200ohm resistor = 94ma
94ma/2 tubes = 47ma per tube
47ma* 333v = 15.6 watts static dissipation

So now the two methods are in relative agreement.

Since the baffle board is simply mounted to the grill face with 8 wood screws and since the original 6x9s are in such poor condition, I decided to make a new baffle for a 10" 8ohm Alnico. This allowed me to use an existing hole in the chassis to mount the new OT horizontal directly below the terminal strip where the ct and plate connections are and I only had to drill one hole. Offset the speaker on the new baffle and it JUST fits. Sounds very healthy! This way I can keep the original baffle/speakers/OT/lead wires stored intact.

[jasonguitar]

This thing sounds AMAZING now!! Really fun to play!
Thanks for your help gentlemen!

[g1]

Sounds like a good solution with the new baffle & 10". Never thought much of 6x9's in guitar amps myself.
If it's not too much trouble could you get readings with the bias probes and by using the OT resistance method?

[jasonguitar]

Sounds like a good solution with the new baffle & 10". Never thought much of 6x9's in guitar amps myself.
If it's not too much trouble could you get readings with the bias probes and by using the OT resistance method?

I'm dropping it off to my friend tomorrow morning. If I have a chance tonight I'll crack her open again. Sorry, I meant to do it before I buttoned hr up but I was anxious to be done with this thing! =P

[jasonguitar]

Here's a couple more shots:

[jasonguitar]

Sounds like a good solution with the new baffle & 10". Never thought much of 6x9's in guitar amps myself.
If it's not too much trouble could you get readings with the bias probes and by using the OT resistance method?

OK. So it looks like we are in relative agreement with all methods now. Here are the measurements I got this morning:

LT plate to CT DCR 145ohms / Voltage drop 6.75v = .046ma * 335plate volts = 15.4 watts
RT plate to CT DCR 150ohms / Voltage drop 6.66v = .044ma * 335plate volts = 14.7 watts

LT probe 49ma * 335plate volts = 16.4 watts
RT probe 47ma * 335plate volts = 15.7 watts

[WholeToneMusic]

Sounds good, Your tubes are matched, new OT, AlNiCo speaker. It's always a plus to use as much as the original amplifier as possible. Sometimes space and other constraints does not permit it. however, it sounds like you were able to come up with some clever and creative mounting techniques.