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

[catnine]

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

I gave up on the fixed bias idea but I didn't want to start another post that I changed my mind . It's not worth the effort and I checked just what size the bias board would need to be and it's small but there is really no room for it if I want a pot to adjust it and I have one of the small 50KL cerment pots multi-turn but I would not be able to get to the adjustment screw . I should have checked before I brought this up ,on top of this I don't know if it will make that much of an affect on the amps tone , perhaps it will . Now I'm just trying to figure out what the max dissipation would be with the cathode bias. I want the screen to be on the safe side too. The amp has been like this since I built it in 2007 and I've had it cranked and it sounds pretty good and nothing has heated up or glowed . I just thought there was a way to get the amp a bit tighter with a bit more bottom end . As it is it does not breakup until I have the vol past 7, I leave the bass and treb at 10 . I'm not looking for more breakup . As far as I can tell the 6V6 is at 13 watts if I don't account for the screen current and that's really the part I'm stuck on now. So it could be at 12.? watts if I add in screen current. I am shooting for 14 watts max for a JJ 6V6. Yet if I go for the 14 watts then it will breakup sooner since the plate and cathode voltages will lower .

[jazbo8]

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.

Ok, I see what you are doing, please refer to Austin and my posts above, you should take the voltage drop of 1V and divide it by the resistance, do not using the voltage reading itself because you are tying to determine the current through the screen. Again, you are throwing us a loop - where did the earlier figure of 377 ohm come? Now you are saying the screen resistor is 463 ohm?

Jaz

[catnine]

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?

it's 1 volt drop on 463 ohms = 2.16 mA the 377 ohm was what i was reading at the ot primary .

[jazbo8]

I fired up the worksheet that Alan did awhile back to scale the plate curves per screen voltage. Here is what I came up with for the 6V6GT: Vs=350V, Vp=380V, Ip=36mA, Vg=-17.5V (pretty close to Fender's -19V). A few things jumped out: 1) the dissipation at idle is above 75% Pd Max, 2) Raa of 7K seems high and 3) the bias is no where near centered - a very none-textbook design to be sure. If it ain't broke, don't fix it...



Jaz

[Enzo]

At the risk of being late, a couple thoughts.

I suspect if you measured your mains voltage outside the house where it comes it, you would still see the voltage moving all over. It happens everywhere I have ever been. The 120v mains voltage is not regulated, it is nominal. Momentary changes in loads or switching in the grid or who knows what will cause this. In my commercialk building, in my home, it is everywhere. Some peoplke are at the end of a country road and the voltage is low, others have chronic high voltage, like 125-130.


Guitar amps are not precision circuits, all voltages are approximate. If you have 120v mains and 360v B+, then for every volt the mains changes, the B+ changes 3v.


And when you touched the plate pin to the heater pin? You didn;t send 3vAC through the OT primary, you grounded off the B+ in that primary through the very low resistance of the 3v winding to ground. If you took a clip wire to ground and just touched pin 3 of the output tube, you;d get the same pop noise.

As someone said, music is AC. That 350v B+ on that winding won;t really care if it wiggles 3v either way. In fact the music signal wiggles WAAAAY more than that.

[catnine]

Ok, I see what you are doing, please refer to Austin and my posts above, you should take the voltage drop of 1V and divide it by the resistance, do not using the voltage reading itself because you are tying to determine the current through the screen. Again, you are throwing us a loop - where did the earlier figure of 377 ohm come? Now you are saying the screen resistor is 463 ohm?

Jaz

I looked back through my posts here and the 377 ohm was a mistake on my part on post #19 , I guess I had too many numbers in my head or one drink to many . I had a glass of wine and my wife wanting me to be calm added more when I was not looking , I wondered why the glass was half full all the time.

I have to say with the line VAC drifting up and down I really can't be certain I have a 1 volt drop across the 470 ohm screen resister or not.

[catnine]

[QUOTE=jazbo8;285171]I fired up the worksheet that Alan did awhile back to scale the plate curves per screen voltage. Here is what I came up with for the 6V6GT: Vs=350V, Vp=380V, Ip=36mA, Vg=-17.5V (pretty close to Fender's -19V). A few things jumped out: 1) the dissipation at idle is above 75% Pd Max, 2) Raa of 7K seems high and 3) the bias is no where near centered - a very none-textbook design to be sure. If it ain't broke, don't fix it...

[ATTACH=CONFIG]21253[/ATTACH]

I fired up the worksheet that Alan did awhile back to scale the plate curves per screen voltage. Here is what I came up with for the 6V6GT: Vs=350V, Vp=380V, Ip=36mA, Vg=-17.5V (pretty close to Fender's -19V). A few things jumped out: 1) the dissipation at idle is above 75% Pd Max, 2) Raa of 7K seems high and 3) the bias is no where near centered - a very none-textbook design to be sure. If it ain't broke, don't fix it...

[ATTACH=CONFIG]21253[/ATTACH]

Jaz

Thanks Jaz : Are those voltages off the fender champ schematic? If so the plate and screen are both 350VDC and the voltage across the 470 ohm cathode bias resister is +19 volts instead of - 19 volts. That is if these are where you got the voltages . Or are these arrived at from the tube worksheet?

All I know is todays 6V6's cathode biased can run at 14 watt max and that is at idle . Many old champs well SF and BF champs had a more powerful PT so the early champs the 470 cathode bias resister was ok and some think that fender missed this and continued to install the 470 ohm cathode reister value , I can't say I buy that . Many also claim the still use the 470 ohm cathode resister and the amps sound fine and I know if you use that value watts at idle are 17 to 20 watts . I have had a few old champs and they were all stock nothing ever replaced after over 30 years and they still worked fine and had stock tubes . Now what I don't know is if the amps were used much most of them looked in good shape so for all I know they may have sat for decades before they were sold on Ebay or through a local music shop.

On my home brew champ copy I have a cathode resister actual resistence of 677 ohm, a plate to ground of 371 VDC - 26 volts across 677 ohm = plate 345 VDC. 26 / 677 = 38.4mA x 345 vdc = 13.24 watts . now if assuming i have a 2.15 ma screen current 38.4 - 2.15 = 36.35 mA = 12.5 watts. Now since the 5 watt wire wound resisters like I have and mine is 680 ohm printed actual 677 ohm measured the next value lower is 500 ohm and if I use that value the IPD will have to be above 14 watt max per 6V6 GT / JJ 6V6S so I would need to get more 5 watt resisters depending at a guess and wire them in series so say I needed 650 ohm or 550 ohm a 500 + 150 or 500 +50 because there is no way to calculate the needed resister value and that brings me back to my thought about using a adj/fixed bias in my champ because it's either find the proper value by trying or need one heck of a huge 5 watt variable resister or just leave it alone as is and find another 10" speaker that can produce tighter more pronounced bottom end and there is probably one out there that would but then that would be just like swapping cathode bias resister values = spending money on a guess.

[catnine]

At the risk of being late, a couple thoughts.

I suspect if you measured your mains voltage outside the house where it comes it, you would still see the voltage moving all over. It happens everywhere I have ever been. The 120v mains voltage is not regulated, it is nominal. Momentary changes in loads or switching in the grid or who knows what will cause this. In my commercialk building, in my home, it is everywhere. Some peoplke are at the end of a country road and the voltage is low, others have chronic high voltage, like 125-130.


Guitar amps are not precision circuits, all voltages are approximate. If you have 120v mains and 360v B+, then for every volt the mains changes, the B+ changes 3v.


And when you touched the plate pin to the heater pin? You didn;t send 3vAC through the OT primary, you grounded off the B+ in that primary through the very low resistance of the 3v winding to ground. If you took a clip wire to ground and just touched pin 3 of the output tube, you;d get the same pop noise.

As someone said, music is AC. That 350v B+ on that winding won;t really care if it wiggles 3v either way. In fact the music signal wiggles WAAAAY more than that.

I figured that might be the case . I find late at night I have a more steady line VAC and this old apt building with 10 units all drawing off the same mains and most have all sorts of electronic stuff plugged in.

Well it's nice to know I didn't screw up anything touching the plate to the heater 3.15 VAC. One question though since the B+ off the rect tube feeds the entire amps circuit in filtered VDC and the primary of the OT is fed off VDC where does the AC part come in are the OT tranny's AC or DC?

[jazbo8]

The figures were not based on any actual amp, but were pretty close to the numbers on Fender's schematics - in the ballpark for discussion purpose, my point was that even if the loadline looked "off", since these amps have worked for many years so why mess with the design. Perhaps the Fender experts can jump in, but I think these old amps worked well because 1) the mains voltages were lower and 2) the tubes were better made back then.

If you want a better low end, changing the speaker is going to make a lot more difference then playing with the bias on the power tube, and if you are still worried about the power dissipation, you can try lowering the screen voltage, e.g., change the B+ dropping resistor and/or screen grid resistor to higher value.

Jaz

[catnine]

The figures were not based on any actual amp, but were pretty close to the numbers on Fender's schematics - in the ballpark for discussion purpose, my point was that even if the loadline looked "off", since these amps have worked for many years so why mess with the design. Perhaps the Fender experts can jump in, but I think these old amps worked well because 1) the mains voltages were lower and 2) the tubes were better made back then.

If you want a better low end, changing the speaker is going to make a lot more difference then playing with the bias on the power tube, and if you are still worried about the power dissipation, you can try lowering the screen voltage, e.g., change the B+ dropping resistor and/or screen grid resistor to higher value.

Jaz

You know where I get lost most . there are in affect three stages of gain on the SF champ. Now on the power tube if you raise the bias hotter the amp breaks up sooner and the higher the voltage on the plate the less breakup yet it's a pentode . On the preamp and second gain are both cathode biased and the higher the plate the less breakup so that part applies to both and they are both cathode biased except the 12ax7 is a dual triode. There is no screen on the pentode . I know if I lower the plate on the 12ax7 then the 12ax7 will breakup sooner, or sound what they title browner. You can also change the bias setting on the 12ax7 so instead of a 1.5k cathode resister I could go with a 1k which would bias the 12ax7 hotter and at the same time lower the plate because of the extra current draw on the cathode. So either way wouldn't this be the same affect?

I am not sure how high to raise the screen resister which I would prefer because if I raise the B+ dropping resister this will lower the 12ax7 plates too. I suppose there is no way to calculate the screens current draw using ohms law as a guide is there? is this because the screen has to draw current to calculate the actual voltage drop?

[jazbo8]

You know where I get lost most . there are in affect three stages of gain on the SF champ. Now on the power tube if you raise the bias hotter the amp breaks up sooner and the higher the voltage on the plate the less breakup yet it's a pentode . On the preamp and second gain are both cathode biased and the higher the plate the less breakup so that part applies to both and they are both cathode biased except the 12ax7 is a dual triode. There is no screen on the pentode . I know if I lower the plate on the 12ax7 then the 12ax7 will breakup sooner, or sound what they title browner. You can also change the bias setting on the 12ax7 so instead of a 1.5k cathode resister I could go with a 1k which would bias the 12ax7 hotter and at the same time lower the plate because of the extra current draw on the cathode. So either way wouldn't this be the same affect?

I am not sure how high to raise the screen resister which I would prefer because if I raise the B+ dropping resister this will lower the 12ax7 plates too. I suppose there is no way to calculate the screens current draw using ohms law as a guide is there? is this because the screen has to draw current to calculate the actual voltage drop?

No I think you have got a good handle on things, we have limited our discussion so far on just the power tubes but you are right that changing the operating points (by varying the B+ voltage as I suggested), will also affect the preamp stages thus the overall sound the amp produces. Again, you only need to do that if your sole purpose is to tame the power tube dissipation, as oppose to tightening up the low end.

There are tools that can model the screen current, but these tools are not precise (the pentode models in particular are problematic...) and the tubes themselves have wide manufacturing tolerances, so the only way to know for sure is still the old fashion way - swapping resistors until you get exactly what you want... My suggestion is for you to find the right speaker first, after you have the tone that you are looking for, then do some tweaking on the screen grid resistor/supply.

Jaz

[Austin]

I figured that might be the case . I find late at night I have a more steady line VAC and this old apt building with 10 units all drawing off the same mains and most have all sorts of electronic stuff plugged in.

Well it's nice to know I didn't screw up anything touching the plate to the heater 3.15 VAC. One question though since the B+ off the rect tube feeds the entire amps circuit in filtered VDC and the primary of the OT is fed off VDC where does the AC part come in are the OT tranny's AC or DC?

All transformers work on ac, and can only pass ac signals. Single ended amps have a steady flow of dc that varies with the music so there is still always some dc flowing in the primary but the ac wave happens there made from the dc as soon as you pluck a string. The se transformer has an air gap to help keep that dc from saturating the iron core and reducing the inductance of the windings. It would work better and not need an air gap if it had no dc offset tho. The ac comes from the voltage of the plate voltage going up and down with the music. It is ac of a varying frequency. Because transformers only pass ac signals (like a capacitor), the secondary where you connect the speaker only has ac voltage on it and zero dc. Hope that helps.

Obviously the most important thing is to be careful and not shock yourself, especially with both hands because it could go across your heart which might stop it, kinda like those paddles they shock you with in the hospital to attempt to restart a stopped heart.. Those same paddles might stop an already beating heart... Its the dc power filter capacitors in that you gotta really worry about because they can release all there stored energy instantly and can hold a charge for a long time if you have no bleeder resistors.

[catnine]

No I think you have got a good handle on things, we have limited our discussion so far on just the power tubes but you are right that changing the operating points (by varying the B+ voltage as I suggested), will also affect the preamp stages thus the overall sound the amp produces. Again, you only need to do that if your sole purpose is to tame the power tube dissipation, as oppose to tightening up the low end.

There are tools that can model the screen current, but these tools are not precise (the pentode models in particular are problematic...) and the tubes themselves have wide manufacturing tolerances, so the only way to know for sure is still the old fashion way - swapping resistors until you get exactly what you want... My suggestion is for you to find the right speaker first, after you have the tone that you are looking for, then do some tweaking on the screen grid resistor/supply.

Jaz

Thanks for your help . I was doing some reading and it seems the reason the plate is lower by 10 volts than the screen is because the plate for one draws more current , much more than the screen and the only resistence on the plate voltage is the resistence of the primary OT winding . The screen draws less current and there is the voltage drop across the 1k dropping resister plus the 470 ohm screen resister. The jensen C10R I have in the amp is not known for bottom end at all just bright tone same as the weber sig10s ceramic which really only has a larger magnet yet it was brighter than the jensen . Now jensen makes what they call MOD speakers that do state they have a tight bottom end and I have a MOD 12 50 and it sounded great in another amp. They have a larger voice coil and different felt instead of paper voice coil cover and a different cone. I emailed David Allen from Allen amps and he said the first thing he would try is a speaker saying the jensens will never get bottom end .

[catnine]

All transformers work on ac, and can only pass ac signals. Single ended amps have a steady flow of dc that varies with the music so there is still always some dc flowing in the primary but the ac wave happens there made from the dc as soon as you pluck a string. The se transformer has an air gap to help keep that dc from saturating the iron core and reducing the inductance of the windings. It would work better and not need an air gap if it had no dc offset tho. The ac comes from the voltage of the plate voltage going up and down with the music. It is ac of a varying frequency. Because transformers only pass ac signals (like a capacitor), the secondary where you connect the speaker only has ac voltage on it and zero dc. Hope that helps.

Obviously the most important thing is to be careful and not shock yourself, especially with both hands because it could go across your heart which might stop it, kinda like those paddles they shock you with in the hospital to attempt to restart a stopped heart.. Those same paddles might stop an already beating heart... Its the dc power filter capacitors in that you gotta really worry about because they can release all there stored energy instantly and can hold a charge for a long time if you have no bleeder resistors.

Yes thanks Austin: It does help . I always drain the caps before working on any amp and I found better probe lead ends I had so thet are covered and you have to push the end button and a hooked metal cantact comes out and there are spring loaded so they latch on to leads or tube tabs so they will not touch anything even 1/4" away form them , should have used them before so I dug around and found them.

[jazbo8]

I fired up the worksheet that Alan did awhile back to scale the plate curves per screen voltage. Here is what I came up with for the 6V6GT: Vs=350V, Vp=380V, Ip=36mA, Vg=-17.5V (pretty close to Fender's -19V). A few things jumped out: 1) the dissipation at idle is above 75% Pd Max, 2) Raa of 7K seems high and 3) the bias is no where near centered - a very none-textbook design to be sure. If it ain't broke, don't fix it...

Jaz

Moderator,

There were some mistakes in the chart that I posted, how do I remove it?

Jaz