If I wanted to change my SF champ to adj fixed bias ?

[catnine]

Um... I think you don;t have a 470 ohm screen resistor with 390 volts "through" it. I suspect you have 390v on one end of it.


How much voltage is dropped across the resistor? Use Ohm's Law. If you know the resistance and you know the voltage drop, that tells you how much current there is flowing through the resistor. And whatever current flows through the resistor, is coming from the screen. Now you will know eactly how much screen current you have.

I measured on both sides of the 470 ohm screen resister and saw no voltage drop at all. When I measure the plate and sreen VDC should they both be measured acrass the cathode or rather should I subtract the cathode voltage from both the plate and screen to determine what the 6V6 is actually at?

[jazbo8]

I measured on both sides of the 470 ohm screen resister and saw no voltage drop at all. When I measure the plate and sreen VDC should they both be measured acrass the cathode or rather should I subtract the cathode voltage from both the plate and screen to determine what the 6V6 is actually at?

Under what condition was the screen voltage taken? At idle, there will be negligible voltage drop, take it up to near clipping and see what you get. And yes, you need to subtract the cathode voltage from the plate and screen voltages, or roughly ~13V per the datasheet.

Jaz

[kleuck]

Idle, there is a voltage drop, and it's the on we need to know. Measure across the resistor, on a 10 volts scale, the drop will be a few volts.

[catnine]

I took it at idle and I used the auto range on my DVOM . I'L try to do it on manual range . All I know is I have read many posting in the past that they had a screen higher than the plate and many said for a SE amp this is normal .

The screen resister is 463 ohm and I have a 1 VDC drop across it. the screen is 374vdc before the resister.

[kleuck]

Not at all, anyway, that's only the voltage drop acroos the resistor that matters to find out the screen current.

[catnine]

I was checking the plate to ground to and screen to ground to see the VDC difference and the cat jumps up so I saw a spark because the probe of the DVOM for a split second touched the one pin of the 6.3VAC heater to the plate , so 3.15 VAC went through the OT primary and I heard a pop through the speaker . Hope this didn't hurt anything . I checked all the voltages again and nothing seems to have changed . I don't think the OT or speaker likes VAC but it was only 3.15VAC so a split second.

[catnine]

If I check the OT primary and the spec is 7K shouldn't it read 7K using a DVOM ? mine reads 0.377K measuring across the two primary leads with the OT installed or am I missing something here?

[kleuck]

No, impedance is not resistance, and a MM can only read resistances. The resistance is by far lower than the impedance in the case of the primary of an OT.

[Austin]

I was checking the plate to ground to and screen to ground to see the VDC difference and the cat jumps up so I saw a spark because the probe of the DVOM for a split second touched the one pin of the 6.3VAC heater to the plate , so 3.15 VAC went through the OT primary and I heard a pop through the speaker . Hope this didn't hurt anything . I checked all the voltages again and nothing seems to have changed . I don't think the OT or speaker likes VAC but it was only 3.15VAC so a split second.

AC won't hurt it. AC is what music is. If you wanna know what your impedance really is, connect 120 VAC to the primary and then read the output voltage on the secondary with your dvm on ac volts(disconnect the speaker). Divide the volts you get into 120 for the turn ratio. Take that number and multiply it times itself. Take that number and multply it times 8 or whatever ohms your disconnected speaker measures. Thats the impedance of your output transformer. Dont forget to reconnect the speaker..

So if you measure 4 volts on the secondary with 120 volt mains on the primary for instance, 120/4= 30
30x30=900
900x8=7200 ohms impedance

So you would have a 7200 ohm opt if you connect it to an 8 ohm speaker but you can see how if your speaker were 4 ohms, the impedance of your opt would only be 3600 ohms for that example.

[jazbo8]

What are you trying to do? Figure out the maximum power dissipation of the current configuration? or what you would like it to be, i.e., fixed bias operation, as the title of the thread suggests? Seems like we are veering off track (so what's new )...

Jaz

[catnine]

You're both right . I must say I don't know what I was thinking . I did some checking to jog my burnt out brain and realized that the 7K was not resistence but impedence . Like I said I don't know what I was thinking. I should know the diff simply because with a OT for the P-P amp I have and a build I make years ago they both can run either 6v6's or 6l6s and it's the ratio meaning the primary resistence does not change but the impedence of the output tube used so if I used a 6V6 then I could use a 4 or 8 ohm secondary load and if it was a 6L6 the secondary was a 2 or 4 ohm load but the actual winding resistence does not change . Man do I feel stupid .

All I want to do is figure out this champs bias and leave it as a cathode biased amp . One thing would really help and that is if my line voltage here in this very old apt would stay steady for just long enough to take readings , one minute it's 117.5 vac the next it's 120.5 then it's 119.2 so I have a high of 120.5 and a low of 117.2 . The 6V6 jj seems to be somewhere between 12.5 and 13 watts and the screen well who knows it's 10 volts higher than the plate at idle . I can't check it with the vol up or it makes this odd noise only with the DVOM connected to it so I have a 1 volt drop across the 377 ohm screen resister so it's 374 VDC before the screen resister I used ohms law but the figure I get can't be right can't be .8mA . I am no good at math and ohms law and I have a book right here telling me but when I look at the DVOM I read 0.8 but now I'm not sure if it's mA or amp if it's amp that means 800 mA and that can't be right. It should be I= V/R right? I don;t want to stick a 1k ohm screen resister in there like some do or I may lose some power output .

[Austin]

You're both right . I must say I don't know what I was thinking . I did some checking to jog my burnt out brain and realized that the 7K was not resistence but impedence . Like I said I don't know what I was thinking. I should know the diff simply because with a OT for the P-P amp I have and a build I make years ago they both can run either 6v6's or 6l6s and it's the ratio meaning the primary resistence does not change but the impedence of the output tube used so if I used a 6V6 then I could use a 4 or 8 ohm secondary load and if it was a 6L6 the secondary was a 2 or 4 ohm load but the actual winding resistence does not change . Man do I feel stupid .

All I want to do is figure out this champs bias and leave it as a cathode biased amp . One thing would really help and that is if my line voltage here in this very old apt would stay steady for just long enough to take readings , one minute it's 117.5 vac the next it's 120.5 then it's 119.2 so I have a high of 120.5 and a low of 117.2 . The 6V6 jj seems to be somewhere between 12.5 and 13 watts and the screen well who knows it's 10 volts higher than the plate at idle . I can't check it with the vol up or it makes this odd noise only with the DVOM connected to it so I have a 1 volt drop across the 377 ohm screen resister so it's 374 VDC before the screen resister I used ohms law but the figure I get can't be right can't be .8mA . I am no good at math and ohms law and I have a book right here telling me but when I look at the DVOM I read 0.8 but now I'm not sure if it's mA or amp if it's amp that means 800 mA and that can't be right. It should be I= V/R right? I don;t want to stick a 1k ohm screen resister in there like some do or I may lose some power output .

Ohm's Law Calculations With Power

1 volt drop on 377 ohms is 2.6 ma or .0026 amps
2.6 ma at 374 volts is 0.97 watts.
Remember that the max screen voltage on a 6v6 is 285 volts and if you have 374 volts on the screen, that would be almost 100 volts too high according to the datasheet. If you have almost a watt of power going to the screens with no signal, I wonder how many watts the screen will draw when you crank it into overdrive?

[jazbo8]

You got it - the winding resistance of the OPT is fixed... Anyway, you have a bunch of numbers here: 1V drop across 377 ohm (I assume this is referring to the screen resistor, right?), and .8 mA or V? I am just guessing here - you are reading .8V (to 1V depending on the mains voltage) across the resistor, so I=V/R=0.8/377=0.00212A or 2.12mA, seems about right.

Also is there a dropping resistor from the B+ before the screen grid resistor as I am still not quite sure which schematic you are working from...

Jaz

[kleuck]

Bah, most Fender amps are built this way, my SP6 has the screen at 350 (static) but the screen resistor is 820, and there's 1K5 before the node.

[catnine]

You got it - the winding resistance of the OPT is fixed... Anyway, you have a bunch of numbers here: 1V drop across 377 ohm (I assume this is referring to the screen resistor, right?), and .8 mA or V? I am just guessing here - you are reading .8V (to 1V depending on the mains voltage) across the resistor, so I=V/R=0.8/377=0.00212A or 2.12mA, seems about right.

Also is there a dropping resistor from the B+ before the screen grid resistor as I am still not quite sure which schematic you are working from...

Jaz

I am working off a SF fender champ schematic , I think I posted it near the middle of this thread. There is a 1K dropping resister before the screens then I have a 470 ohm screen resister on the tube socket. So the voltage is the same after the 1K dropper before the 470 ohm screen resister . I have a 1 volt drop across the screen resister. The screen resister reads 463 Ohm . I have 374 volts before the screen resister and it drops to 373 volts across the 463 ohm resister = 1 volt drop. So 374/463= 0.808 rounded off so what is 0.808 can't be mA. I must be missing part of the math here.