No millivolt reading at 1 ohm resistor

[Underwood]

  I put in some new tubes and biased my 1965 Bandmaster today and was playing for a bit and it started to lose power a little. I checked the primary voltage on the tubes and it was at a proper 434V. I checked the millivolt measurement at the one ohm resistor I have on pin eight and got nothing. I changed the tubes, rebiased and it is working fine. The PT was slightly warm(barely), and was wondering what anyone thought? Was it just the tubes?

[Platefire]

Were they brand new tubes or just different tubes? If they were new tubes, what kind were they?
Good quality new matched set shouldn't do that--normally unless one had a defect. But from my experiance, that's kind of rare. Don't you need a 1 Ohm for each tube to check with millivolts? As I recall for 6L6's 35mv on each tube for not to hot or cold setting. Platefire  

[Underwood]

The tubes were NOS GE 6L6GC. They were biased at about 34mv. The others I put in are RCA 6L6GC baised the same 34mv and I just palyed for over an hour and they sound great no problems. SOmething must have been wrong with one or both of the tubes. I got no reading on both of the 1 ohm resistors when the other tubes were in.Originally when I put the GE tubes in there was a reading when I biased them to 34mv. I played for a few minutes and they sounded good. Then it started to lose power a bit a I would hear a pop every couple of seconds. I turned off the power, replaced the tubes, played for over an hour, sounds great. No problems.

[Willabe]

A pair of NOS GE's died and now you have a pair of NOS RCA's in there.

I'd be worried that something might be happing in the output stage and might kill the rca's.

I'd check at least the grid return R's and screen R's.


             Brad     

[Underwood]

All the resistors check good.I did have a 12At7 in the vibrato tube socket by accident. The trouble seemed to start when I turned the vibrato on. The no reading at the 1 ohm R's I still don't get. Maybe I had my meter on the wrong setting. No idea. I have it on again and it is still fine. What reason would there be for no reading at the pin 8?

[plexi50]

I checked the millivolt measurement at the one ohm resistor I have on pin eight and got nothing

Which side of the 1r resistor did you probe to do your testing? I thought milla amps is what you are reading on the 1r resistors on pin 8 of each power tube. Milla volts is what you are reading on pin 5. Do you have an auto setting on your testing meter? Cheap meters will lie to you any chance they get

[DummyLoad]

measure DC millivolts across 1ohm to ground whereas, 1mV = 1mA. if 1R is not tied to ground then across the resistor. bias is measured in (-)DC volts - make sure sure your meter is set to DCV and on the lowest range (usually 200mV or 2V) if not an auto-ranging DMM.

--DL

[sluckey]

I thought milla amps is what you are reading on the 1r resistors on pin 8 of each power tube.

No, you're measuring millivolts across the 1Ω resistor.

Milla volts is what you are reading on pin 5.

No, you're measuring plain ole negative volts.

[eleventeen]

Maybe you have a heater-to-cathode short or near-short in one of the GE 6L6 that is conducting current around your 1 ohm resistor. Just throwin' an idea out there. H-K shorts are pretty darned rare, but they tell me they happen. Make sure there is no continuity between pins 7 and 8 on either/any 6L6 with the tube in your hand, out of the amp. Before you put the 6L6 back in the amp, I'd take ohms readings betw pins 8 and 7 on the tube socket and be sure those readings are the same.

Does a bias adjustment affect both 6L6 the same, meaning produce identical voltages on the pin 5s?

Can you put the amp in standby and check voltages at the pin 5s of the 6L6?

Are your 6L6 ground connections (that you had to interrupt to install the 1 ohm R's) re-made, good-plus quality solder connections?

And by the way, if you have a '65 Bandmaster that has seen use, replacing the 100K plate resistors is probably the cheapest thing with most dramatic effect you can do. I just rebuilt three blackface/early SF Fenders ('65 Pro Rev, '68 DelRev, 72 PrinRev) and replacing those 100Ks made all kinds of difference in hiss, in spurious noises, in those weird things that amps do where suddenly they are louder/quieter..... I just shotgun them outta there on old Fenders.

[Underwood]

Yes the bias adjustment produces the same voltage on both pin 5's.
Are you asking if there is voltage at pin 5 in standby? What would it mean if there isn't? I am going to recheck tomorrow.
The ground connections are what I am thinking. I am going to resolder them tomorrow. I bet that could be the problem. I was using a thin point on my solder pen and it was having trouble melting those solders on the frame. I'll use my other one tomorrow, it gets hotter.
I did replace almost all of the 100K resistors. Someone put in metal film and I wanted to put in some new carbon comps. WHo knows?
At this point I have it sounding better than it evr has, but I want to go over some of those solder points tomorrow. I also want to redo the way someone put the new 3 prong chord in. They left the death cap in and did not put the black PT wire from the fuse to the ground switch. I've always seen it done that way, maybe it is OK the way it is but I want to change it to the way I have always seen it done.    THANKS Guys!!

[HotBluePlates]

  I put in some new tubes and biased my 1965 Bandmaster today and was playing for a bit and it started to lose power a little. I checked the primary voltage on the tubes and it was at a proper 434V. I checked the millivolt measurement at the one ohm resistor I have on pin eight and got nothing. I changed the tubes, rebiased and it is working fine.

The tubes were NOS GE 6L6GC. They were biased at about 34mv. The others I put in are RCA 6L6GC baised the same 34mv and I just palyed for over an hour and they sound great no problems. ... I got no reading on both of the 1 ohm resistors when the other tubes were in. ...

All the resistors check good. ...

These statements are all mutually exclusive unless you have a wiring error or shorted resistors.

If the tubes pass current, which cause the plate voltage to drop to the correct value, and they pass sound, they must be functioning right and your meter or resistor is deceiving you.

Which makes me wonder why you know you have 34mA of idle current when you couldn't obtain a reading across the 1Ω resistors? Because you used the OT shunt method? If so, did you remember to move the red meter lead back to the volts socket and switch the meter from the mA's range to the mV's range?

Regardless, if the resistors were really 1Ω and were connected correctly, if the tube passes any current, it would have to flow through the resistor.

[Underwood]

At first I did get a proper reading, then I was playing and lost power, rechecked and got no reading . I have no idea what happened. My only other thing is I am woundering about my bias adjustment pot. I was checking my negative voltage and was turning the pot to increase it and it did not do anything. WHen I put the other tubes in and started to adjust the bias current, the pot worked. The negative voltage should go up and down when you turn the bias pot right?

[eleventeen]

What HBP said is a good point. Are you sure your test leads were in the right jack on your meter or the meter properly range-set when you changed from measuring volts to millivolts?

Yeah, you want those ground connections under the 6L6 cathodes to be righteous. Those have to be top-notch connects. Many people don't have sufficient firepower in their soldering irons to do a really good chassis connection on a BF Fender.

I'm not too embarassed to say that I have made those connections by grabbing a long 1/4-20 bolt in a set of vise-grips or channel-locks, heating the head of the bolt with a propane torch, then using the bolt-head as my soldering iron. Works quite well. If medieval soldering techniques do not interest you, you could make a heavy ground connection to the lug on the PT. but one way or another, those 6L6 have to have real grounds.

Yeah, you twist the bias pot, bias volts should move. This is the thing you can test with the amp in stby, 6L6s out. Inspect the resistors that form the voltage divider around the bias pot, make sure they're not cracked in some invisible way. And yes, check the bias pot for smoothness over full rotation.

[plexi50]

I thought milla amps is what you are reading on the 1r resistors on pin 8 of each power tube.

No, you're measuring millivolts across the 1Ω resistor.

Milla volts is what you are reading on pin 5.

No, you're measuring plain ole negative volts.

My bad. You would think i would have this memorized by now. 

[Underwood]

I just thought about it. I would not see a fluctuation in negative voltage at the point after the bias cap because the bias pot is after it in the circuit. I should have been measuring pin 5. Right? No big deal, I measured correctly on the pin 8 for my bias.

[Willabe]

If the bias cap is feeding the pot you would.

But it's important to make sure that the -bias voltage is on pin 5.


             Brad     

[HotBluePlates]

Then how do you know you had 34mA of idle current if the measurement across the 1Ω resistors wasn't showing anything?

Either there's something in the story you're omitting, you're mixing up mV and mA, or the tubes are working and something about your measurement is not working properly.

I also want to redo the way someone put the new 3 prong chord in. They left the death cap in and did not put the black PT wire from the fuse to the ground switch.

If you add a 3-prong cord, you should not have the ground switch connected to anything. The cap can stay where it is, but there should be no wires to/from the switch and power cord or transformer.

Did you install the 1Ω resistors? How did you install them? If you attach your meter's black lead to the chassis where the PT center-tap is grounded, and measure resistance from pin 8, what do you get? Do not use a point right at the socket for your ground when making this measurement.

[plexi50]

I was using a thin point on my solder pen and it was having trouble melting those solders on the frame

You need a good old 100-150 watt solder iron to solder on a chassis. A 40 watt gun will not be able to supply enough heat. The chassis sucks up all the heat your 40 watt or so gun will produce leaving a slag of solder that is really not soldered to the chassis as you may think.

[HotBluePlates]

Right. The chassis is a big heatsink, and even a 100w solder gun will have trouble making a good solder joint on the chassis.

If you press a screwdriver against the chassis solder joints and hit them like the screwdriver was a chisel, a good solder joint won't budge even with a lot of force. A poor solder joint will pop of the chassis.

If you have to try to make your own chassis connection for the 1Ω resistors, you should crimp a ring terminal to one end, then flow some solder in the resistor lead/ring terminal joint to ensure a good electrical connection. Bolt the ring terminal to the chassis at the socket mounting bolt. Solder the free end of the resistor to pin 8.

[Underwood]

That's a great idea about the ring terminal bolting to the chassis. I think i will try that.

[Colas LeGrippa]

a good chassis solder can be performed with a heat gun with no problem. be sure to scratch the surface to remove all oxide for the solder to ''stick'' properly