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

[catnine]

For max power, so a center-biased tube, you need the right OT, and you have to choose it considering the voltage (or conversely) The Valve Wizard -Single Ended

I built the amp using a PT and OT that are the ones weber uses for his champ kits and for fender champ replacements and I use the proper output tube and used all the exact same componant values for every part of the amp so what I have is a copy of a SF fender champ . I don't see how a fixed bias will affect this build . I did not design it just copied it. Valve wizard talks about design . Even with the cathode bias as it is now on any SF champ the stock 470 ohm bias resister biases the 6V6 to hot close to 17 watts or in some cases more which is to hot for a max 14 watt 6V6 so I had to apply the same logic to my build and ended up with a 680 ohm bias resister for 13 watts. Some people just let them run hot but to me they sound brittle and that high does not help the tube either. But some like the sound biased hot.

[kleuck]

I just wanted to point out that you can't have the full power of a 6V6 under 200 volts.
Conversely, using a fix-bias means that the anode to cathode voltage is 20 volts more, spo with a fixed-bias, you can obtain more power with a slightly lower B+.
Obviously you don't need this extra voltage
Anyway, you don't want -35 as a mid-point, but more between -15 and -20.

[Austin]

If you look at the tube data, the curves are more evenly spaced the closer the grid voltage gets to zero volts, when you allow more current through the tube it gets more linear. I started using batteries to bias when I first started learning how an amp worked because it was easy to understand and didn't require any parts other than a battery, there are better ways to do it but they have the same result. You just want to bias so that the max possible voltage swing (max volume) on the grid will not hit the zero mark or go so low that the tube goes into cut off or where the curves are all laying down bunched up together. You can get a ballpark figure how many volts you can measure this with your dvm on ac setting after the coupling cap, If your preamp can swing 18 volts then you could bias at 10 or 12 volts volts so the wave can have room to go 9 volts up and nine volts down. At 12 volts bias and 384 volts with 250 volts on the screen the 6v6 should be drawing about 50ma which is fine. Usually people say the sound loses some of the sparkle or chime when you bias too hot, not geting more harsh. If the sound is in your ears brittle then I would think the problem is the output transformer impedance is too high. 7k is imo a bit high for a 6v6, you could connect a 4 ohm load instead of an 8 ohm speaker load to get 3.5k impedance ratio but I think 5k would probably be best. 3k is better for a 6L6. The screen voltage being too high can cause the sound to be more edgy as well. Remeber that the 6v6 max screen voltage is 285 volts according to the spec sheet. You might try a 4 ohm speaker and a 6L6 which has a higher screen rating and see if you like the sound better. Should give more solid bass too.

[kleuck]

7K is not "high for a 6V6" ! The primary impedance is choosen according to the High Voltage, not anything else.
High voltage >> high impedance and conversely.

[Austin]

7K is not "high for a 6V6" ! The primary impedance is choosen according to the High Voltage, not anything else.
High voltage >> high impedance and conversely.

Your probably right, I didn't mean to mis-lead but I myself would go for 5k in a guitar amp anyways for more power and more second harmonic distortion though. The sound would improve in my experience..

[kleuck]

High voltages/high impedance does not sound the same as low voltage/low impedance, that's right.But more even harmonics means most of the time a "not center-biased" amp, and therefore a waste of power.

[catnine]

The more I read here the more I get confused. My OT is 7K primary which most champs with 6v6 have as the norm. How do you really know if a power / output tube is center biased ? I have a feeling this has to do with the tubes bias setting yet how do you really know. I assume my bias is set for close to max watts @13 watts on the 6V6 so wouldn't one assume it is center biased or can you assume nothing and what about all the fender SF champs out there built or exact copies?

[kleuck]

Merlin explains that pretty well here.
Your tube is about max dissipation, which is casual for a class A amp, BUT, it doesn't mean that it is center-biased ; "center biased" is relevant only for a given load and a given voltage.
And if we are after max power, we have to choose a suitable load if the voltage is given, or a suitable voltage if the load is given, and most of the time a compromise.
Example with your 7K/13watts :
Voltage needed is 7000*13^(-2)=301 volts.
With 5K, 255 volts would be theoretically enough, but as the plate of the power tube cannot really go down to 0v, the actual power is always less, and the lower the voltage, the more waste.
So to be close to the max power we can get from a given power tube, we must a somewaht high voltage (depending on the tube)

[catnine]

Merlin explains that pretty well here.
Your tube is about max dissipation, which is casual for a class A amp, BUT, it doesn't mean that it is center-biased ; "center biased" is relevant only for a given load and a given voltage.
And if we are after max power, we have to choose a suitable load if the voltage is given, or a suitable voltage if the load is given, and most of the time a compromise.
Example with your 7K/13watts :
Voltage needed is 7000*13^(-2)=301 volts.
With 5K, 255 volts would be theoretically enough, but as the plate of the power tube cannot really go down to 0v, the actual power is always less, and the lower the voltage, the more waste.
So to be close to the max power we can get from a given power tube, we must a somewaht high voltage (depending on the tube)

My build has more voltage than that. the B+ is 384 VDC that's what is at the OT primary of 7K ohm . I got the bias of 13 watts based on a plate of 371 VDC - cathode resister of 680 ohm cathode voltage 26.0 VDC = 345 VDC actual working voltage. So 680 ohm / 26.0 =38.2mA x 345 vdc= 13.2 watts.

[kleuck]

Right, but your amp is not center-biased "to the letter", as the voltage is more than needed, you have the max power, and probably more even harmonics when the amp is pushed.
BTW, you forgot to subtract the screen curent, you tube must be in the 12 watts ballpark, not 13.
26/680=38 mA, we can assume about 4 mA for the screeen, so plate current is only 34 mA, so 11,75 watts.

[catnine]

Right, but your amp is not center-biased "to the letter", as the voltage is more than needed, you have the max power, and probably more even harmonics when the amp is pushed.
BTW, you forgot to subtract the screen curent, you tube must be in the 12 watts ballpark, not 13.
26/680=38 mA, we can assume about 4 mA for the screeen, so plate current is only 34 mA, so 11,75 watts.

I have a 470 ohm screen resister with 390VDC through it . I used a calculator on webers site and just plugged in the numbers here's the link.
Weber Bias Calculator

on this they claim 5% of the cathode current is assumed to be screen current . I used to last colume with the 6 boxes .

[Austin]

Right, but your amp is not center-biased "to the letter", as the voltage is more than needed, you have the max power, and probably more even harmonics when the amp is pushed.
BTW, you forgot to subtract the screen curent, you tube must be in the 12 watts ballpark, not 13.
26/680=38 mA, we can assume about 4 mA for the screeen, so plate current is only 34 mA, so 11,75 watts.

So if 26 volts is what the bias ends being, is that close to cut off for a 6v6? It kinda looks that way according to the data sheet. Why not just put a potentiometer on there and adjust the bias the way you think it sounds best. As a bonus, then you could try any power tubes you wanted and adjust the knob to suit. I think that would be fun, 6g6 6f6 6l6 6550 6k6 6w6 6y6 el34 etc all have there own sounds.

[kleuck]

I have a 470 ohm screen resister with 390VDC through it . I used a calculator on webers site and just plugged in the numbers here's the link.
Weber Bias Calculator

on this they claim 5% of the cathode current is assumed to be screen current . I used to last colume with the 6 boxes .

This one gives 13 watts when the one just above give 12
In my 6V6 amp, the screen current is about 3,7 mA for a plate voltage (total) of 350 volts, so i think you can't have less.
You can measure the voltage drop across the screen resistor if you want to be sure.

With my datas, the two Weber calculators say 14 watts where i have "only" 13.4.

[Enzo]

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.

[soundguruman]

I would have to take the AC off one of the PT secondary leads since the PT has no Bias tap . My question is this.
I am looking for a -35 VDC does it matter if I am using just one 6V6GT or am I just looking for -35 VDC as a mid point? I have done this with push/pull amps with or without a bias tap and had no problem so if I use the same values for the bias range resister and bias resister and add a 50 kL pot does the bias care if there are one or two output tubes?

You should use a divider, a "Y" made out of two resistors, to supply bias for two tubes.
Most commonly the "Y" is two 220K resistors for 6L6 tubes or 6V6 tubes.
The bias is just a voltage, it doesn't need current. So, you can tap it from lots of places.
If you want to use a capacitor to couple the bias rectifier to the PT, this will block the majority of current.
Yes, you can have a bias supply that does not draw (significant) current...
SEE: OLD Vox schematics. You can (course) adjust the bias voltage by changing the value of the coupling capacitor. (instead of a series resistor)
You can use the pot for the fine adjustment...
This method was used on a bunch of old Vox amps. It's a cooler method than the Fender method, I think.