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

[catnine]

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?

[nickb]

No. The current supplied by the bias circuit is very small and will happily drive two tubes.

[Jazz P Bass]

I thought the Champ was a 'single ended' amp.
Why would you put a bias on the grid of one tube?
That is for push-pull, isn't it?

[Enzo]

It seems unnecessarily complex to me too, but bias is bias. A negative voltage relation has to exist from grid to cathode. I don't think the tube cares how it gets there. Push pull amps can be cathode biased too, certainly.

Cathode bias subtracts the cathode voltage from the plate voltage for voltage across tube. If you convert that to fixed bias, you then have increased the effective plate voltage by the amount of the bias voltage. That amount may or may not make much difference.

I guess with fixed bias, you can make it adjustable, so you can toy around with various bias levels. But it is still just a Champ, so I don't know how far that takes you.

35v sounds like a lot to me, but whatever you like.

[catnine]

It seems unnecessarily complex to me too, but bias is bias. A negative voltage relation has to exist from grid to cathode. I don't think the tube cares how it gets there. Push pull amps can be cathode biased too, certainly.

Cathode bias subtracts the cathode voltage from the plate voltage for voltage across tube. If you convert that to fixed bias, you then have increased the effective plate voltage by the amount of the bias voltage. That amount may or may not make much difference.

I guess with fixed bias, you can make it adjustable, so you can toy around with various bias levels. But it is still just a Champ, so I don't know how far that takes you.

35v sounds like a lot to me, but whatever you like.

I was thinking -35 volts because that is what is about the mid range bias for a p-p 6G2 princeton using two 6V6 tubes so I thought -35 volts would work for one 6V6 tube as a mid point. Somewhere on this forum I can across some people talking about changing a SF champ to fixed bias Mission amps chimed in and said he did it and it sounded great , some said it allows the amp to not drop off when you hit a note on the guitar that it stiffens up the amp . I have no idea , I never tried it and don't know if it's even worth it . I know going to fixed bias on the P-P fender I built it changed the entire sound of the amp and made it louder and and I had a DPDT switch on it to compare and liked the fixed bias much better , I removed the switch and left it fixed bias and never looked back. but then that was not a SE amp.

[J M Fahey]

I was thinking -35 volts because that is what is about the mid range bias for a p-p 6G2 princeton using two 6V6 tubes so I thought -35 volts would work for one 6V6 tube as a mid point.

Absolutely unrelated.
You can not take normal bias voltage from a PP amp, specially a "normal" AB1 type one, and apply it to a single ended, pure class A (by definition) .
Sorry.

[Dave H]

Somewhere on this forum I can across some people talking about changing a SF champ to fixed bias Mission amps chimed in and said he did it and it sounded great , some said it allows the amp to not drop off when you hit a note on the guitar that it stiffens up the amp . I have no idea , I never tried it and don't know if it's even worth it . I know going to fixed bias on the P-P fender I built it changed the entire sound of the amp and made it louder and and I had a DPDT switch on it to compare and liked the fixed bias much better , I removed the switch and left it fixed bias and never looked back. but then that was not a SE amp.

If you want to try it you could measure the voltage across the cathode resistor then replace the resistor with a zener diode of the same voltage.

[printer2]

You want the same bias voltage as you have on a cathode biased amp. This will give you roughly equal swing in either direction. You might get a little tightened bass with fixed bias.

[kleuck]

You can use a fixed-bias with a SE amp, but i can't really see a good reason for that (fixed-bias is an easy way to get max power from a PP amp without going too high with voltages, but most of the times with SE, using a higher voltage gives the exact same result, with the benefit of a DC feedback)
Would be about -20 volts (voltage doesn't matter : what matters is the current trough the 6V6), but depends on the primary impedance of your OT.

[Austin]

I prefer to use fixed bias on my home made se amps because I like the sound. I have tried cathode bias both bypassed and unbypassed and the fixed bias gives a clearer more solid sound to my ears. It kinda depends on the supply voltage but for say 200 volts b+ 9 volts bias is about all you need for se 6v6 or el84 and you could even use a 9v battery if you wanted. If you have 400 volts b+ then you probably want to snap two nine volts together in series for -18v bias

[kleuck]

Depends on the OT's primary, and under 200 volts, you will have a hard time getting more than 2,5 or 3 watts.
And here, we are talking about a Siverface Champ, so about 350 volts one the anode of the 6V6...and almost 20 volts on the cathode.

[catnine]

Depends on the OT's primary, and under 200 volts, you will have a hard time getting more than 2,5 or 3 watts.
And here, we are talking about a Siverface Champ, so about 350 volts one the anode of the 6V6...and almost 20 volts on the cathode.

Are you saying that with cathode bias the champ will put out more power?

[kleuck]

Not at all, you can squeeze a little more for a given B+ (and OT) with a fixed bias of course.

[catnine]

Not at all, you can squeeze a little more for a given B+ (and OT) with a fixed bias of course.

What has the OT got to do with it? I don't get that part . the B+ is 384VDC the OT is 7000 ohm primary to a 4 and 8 ohm secondary. I can't at this point say if it's even worth trying to change it to fixed bias I just thought it would sound a bit more tight over all. I say this because with a cathode bias once a string is plucked there is that drop in the 6v6 plate voltage .

[kleuck]

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