Now another question on the sound of a SF champ

[catnine]

I built a SF home brew and it sounds fine yet somethings missing to me anyway. I like the simple tone stack like the 5F2A princeton has same one in the 6G2 .

I know if I do that the amp will have more gain which is not what I'm after. I went through everything a while back because for some reason this build had icepick highs . I got rid of most of that simply by taking out the weber Sig10S ceramic and installing a jensen C10R . looks like they have the same cone and frame the jensen just has a smaller magnet and I used it a lot more and I put a JJ 12ax7 instead of the ruby and sovtek 12ax7 and that took out the highs.

Now I have a high plate voltage on the 12ax7 about 230 VDC now if I knock that down below 200 VDC or even less by changing the plate load resister from 100K to something higher will this make it sound a bit thicker and browner yet it will cause the preamp to break up sooner is that correct ?

I don't know the best way to approach this . If I change the second dropping resister then this will lower the plates on both the 12ax7 triodes so I think I will loose the gain after the tone stack . I just want to retain the bottom end it does have with the 10" speaker yet have a bit darker sound to the amp . If I remove the bypass cap across the preamp cathode resister to ground I will loose gain too. I don't blast the amp really and even when I do it does not breakup until it's at 7 on the vol so I don't mind if it breaks up on 5 or 6 . I do like the headroom and champs aren't known for that really so I do feel the high plate on the preamp triode is the reason .

Any ideas would certainly help.

I know this is a lot of questions and I could experiment thought I would ask for some experience here.

[echuta13]

Take a look at this for a primer on values and how they affect the sound/feel of an amp: http://www.rh-tech.org/public/S2/docs/vj_kit_inst.pdf

Voltages can have a pretty significant affect as well, so that's also something to play with...

[kleuck]

If you want more headroom, lower the plate resistors to 47K, it will halve the gain.
Or, use a 12AY7 in place of the 12AX7.

[Dave H]

The 5F2A tone control will have more mids. To get an idea of how it would sound with more mids disconnect the 15k resistor at the bottom of the Silverface tonestack from ground. Disconnect the cathode capacitor on the first stage if it has too much gain.

[catnine]

If you want more headroom, lower the plate resistors to 47K, it will halve the gain.
Or, use a 12AY7 in place of the 12AX7.

It's got plenty of headroom now . It doesn't break up until the vol is on 7 . I already have 240 VDC on the 12ax7 plates so it's near the max . I fear if I install 47K plater resisters it will exceed the max plate.

[Austin]

It's got plenty of headroom now . It doesn't break up until the vol is on 7 . I already have 240 VDC on the 12ax7 plates so it's near the max . I fear if I install 47K plater resisters it will exceed the max plate.

What is your supply voltage to the 12ax7? The way I understand, you would want the plate voltage to be about half of the supply voltage for max headroom. So if you have 250v on pins 1 and 6, the supply voltage would be 500 volts. If your supply voltage is only 300 volts or so and you have 250 volts on the plate then the tube is biased too cold.

[kleuck]

You should give your actual schematic, anyway most of the times, Fender amps do not have their triodes "center-biased".
But it seems that your voltage is too high on the Preamp, measure the voltage at the last fitering node, not at the plate of the 12AX7, then the using a higher dropping resistor in the voltage path would cure the issue.

[catnine]

What is your supply voltage to the 12ax7? The way I understand, you would want the plate voltage to be about half of the supply voltage for max headroom. So if you have 250v on pins 1 and 6, the supply voltage would be 500 volts. If your supply voltage is only 300 volts or so and you have 250 volts on the plate then the tube is biased too cold.

I had the chassis cover off just last night . It's a tweed style chassis . Anyway this is what I have and the line voltage was 120 VAC , usually it's 118.2 to 119.8 , it always varies .

The B+ is 384 VDC
The plate of the 6V6 is 371 read off pin 3 to ground and across the cathode resister to get the true Plate It's 345 VDC . I always was never certain but i was told that to bias the 6V6 proper you need to subtract the voltage across the cathode bias resister from the plate so the cathode voltage is 26.0 and my cathode bias resister is 680 ohm so the 6V6 is at 13 watts.

the screen on the 6V6 is 380 VDC .

The first node is 1K ohm across it is 382 VDC and the 2nd node is 10K ohm across it is 359 VDC. So there is 356 VDC going to the 100K ohm plate resisters for the 12ax7 .
pin 1 is 242 VDC and pin 3 is 1.72 VDC pin 6 is 240 VDC and pin 8 is 1.86 .

[kleuck]

Well, 356 is high indeed, i find high voltage on triodes to make a more agressive sound, conversely lower=smoother.
You can use a higher resistor in the path (33K instead of 10K would drop about 27 more volts) or add a series zener to do the same.

[Austin]

You could bias hotter by reducing the vlaue of the cathode resistor, that would bring the voltage down by making the tube conduct more and change the sound too, it seems most people agree that warmer bias is warmer sound.

[catnine]

Well, 356 is high indeed, i find high voltage on triodes to make a more agressive sound, conversely lower=smoother.
You can use a higher resistor in the path (33K instead of 10K would drop about 27 more volts) or add a series zener to do the same.

I think the high voltages are directly relater to the weber PT since that PT puts out 330-0-330 @100mA where a stock fender SF champs has 325-0-325 @70 mA . the sf champ schematic shows 200VDC on the 12ax7 plates and 350 VDC on the 6V6 plates and they allow + or - 20% so basically I am at the + 20% across the board.

The amp sounds fine , not brittle or gainy or grainy . It sounds clean with plenty of headroom . If I bias the 6V6 hotter by reducing the cathode resister value then it will exceed the 14 watt max for a 6V6 . My line voltage varies so much just checking the amp that most times the line is at 118 VAC so all the voltages drop 5 % this time it was at 120VAC . with the 100 mA PT what I think is that the tubes can't pull the current down enough as if it were 70mA like a stock fender champ .

I'm afraid if I start changing dropping resister values or plate resister values to supply the 12ax7 or rebiasing the 12ax7 then the amp may sound warmer but break up sooner , that is not what I want . I like the headroom it has now . on the fender champ schematic the droppers are 360 and the 2nd one 330 VDC compared to mine at 382 and 359 so the diff is 22 VDC and 29 VDC across the 1st and 2nd node

[catnine]

You could bias hotter by reducing the vlaue of the cathode resistor, that would bring the voltage down by making the tube conduct more and change the sound too, it seems most people agree that warmer bias is warmer sound.

I'm not certain what you are refering to as a warmer sound aren't my tubes already set warmer bias wise? Or are you saying since the 12ax7 voltage is high that if I lower the value on the cathode bias resister on the 12ax7 this will lower the plate voltage because more current draw = a lower plate just like biasing any cathode biased tube? But you said to have 150VDC on the 12ax7 plates I would need 500VDC on the supply. I have 354 VDC on the supply and 240 VDC on the 12ax7 plates . I must be missing something here. I think a 12ax7 plate max is 330 VDC.

To me sound is subjective and to me warmer means browner with more breakup .
I think the high voltages are directly related to the weber PT since that PT puts out 330-0-330 @100mA where a stock fender SF champs has 325-0-325 @70 mA . the sf champ schematic shows 200VDC on the 12ax7 plates with a 330 VDC feed. and 350 VDC on the 6V6 plates and they allow + or - 20% so basically I am at the + 20% across the board.

The amp sounds fine , not brittle or gainy or grainy . It sounds clean with plenty of headroom . If I bias the 6V6 hotter by reducing the cathode resister value then it will exceed the 14 watt max for a 6V6 . My line voltage varies so much just checking the amp that most times the line is at 118 VAC so all the voltages drop 5 % this time it was at 120VAC . with the 100 mA PT what I think is that the tubes can't pull the current down enough as if it were 70mA like a stock fender champ .

I'm afraid if I start changing dropping resister values or plate resister values to supply the 12ax7 or rebiasing the 12ax7 then the amp may sound warmer but break up sooner , that is not what I want . I like the headroom it has now . on the fender champ schematic the droppers are 360 and the 2nd one 330 VDC compared to mine at 382 and 359 so the diff is 22 VDC and 29 VDC across the 1st and 2nd node.

Now the weber PT in this build has a second PT secondary of 300-0-300 VAC right now I am using the 330-0-330 VAC . I tried the 300-0-300 VAC secondary and the amp sounded lame . I eamiled weber and he told me the 300-0-300 secondary was if I used a SS rect and the 330-0-330 was if I used a 5Y3 which is why I chose the higher seconday.

[catnine]

Well, 356 is high indeed, i find high voltage on triodes to make a more agressive sound, conversely lower=smoother.
You can use a higher resistor in the path (33K instead of 10K would drop about 27 more volts) or add a series zener to do the same.

Can you explain smoother . does this mean smoother breakup or earlier breakup or a browner sound. I don't want the amp to sound like a 5F2A breakup wise . As the amp is now I can crank it up and it is clean with punch and has a solid bottom end , it does not break up until I reach 7 on the vol pot. Perhaps in the original post I stated I wanted it to sound more like a 5F2A and if so that was my mistake. I was just asking what I could do to make the amp simple like a 5F2A.

[kleuck]

Just browner (well, not exactly the right word, but close)
If the preamp voltage is about 300 volts, you won't make the amp breakup sooner i think (the preamp generally stays clean or almost in a 5F**)
But can you provide your schematic ?
I must confess i don't really understand the point ? "I was just asking what I could do to make the amp simple like a 5F2A."