A friend of mine owns one of these Bassman heads. It is an "all original" drip edge. He is not interested (yet) in "Black Facing" the amp. So I am just replacing the filter, bias, and cat bypass caps. I have also installed a 3 wire power cord. Fender had the amp out of phase, starting from the convenience outlet; so they had the fuse and switch on the Neutral....... probably not the first time.
Anyway....can one of you guys explain this "bias balance" set-up to me There is the normal negative bias voltage, but then the power tube cathodes are grounded via 150 Ohm resistors, with a 5/25 cap that runs cathode to cathode. How does this last part work ?????
Thank You
http://www.el34world.com/charts/Schemat ... _AC568.pdf
Fender AC568
Moderators: pompeiisneaks, Colossal
Re: Fender AC568
Well, its a obviously a combination of Cathode bias and fixed bias.
lets run through different operating modes.
Mode 1)
small signal (class A) : still both tubes contribute. In this mode the capacitor works as if you would have 2 separate cap to ground, the cathode resistor is bypassed for for audio signal.
Mode 2)
Now let us assume a large signal (A and B op mode). Assume a sinus as a signal, starting from zero (crossover). In the first phase we are in mode 1)
With the increasing voltage we cross the point when one tube switches off. Now there is no "opposite signal"at the cathode of the Tube switched off. As a result you have now no longer a bypassed cathode resistor but both cathode resistors parallel between the cathode of the driving tube and ground (from AC signal point of view). This is basicall a negative feedback which reduces the amplification. As a result you have a very smooth an soft "clipping".
I have built a guitar amplifier with a power stage with this behaviour. Its a very nice sound you get.
lets run through different operating modes.
Mode 1)
small signal (class A) : still both tubes contribute. In this mode the capacitor works as if you would have 2 separate cap to ground, the cathode resistor is bypassed for for audio signal.
Mode 2)
Now let us assume a large signal (A and B op mode). Assume a sinus as a signal, starting from zero (crossover). In the first phase we are in mode 1)
With the increasing voltage we cross the point when one tube switches off. Now there is no "opposite signal"at the cathode of the Tube switched off. As a result you have now no longer a bypassed cathode resistor but both cathode resistors parallel between the cathode of the driving tube and ground (from AC signal point of view). This is basicall a negative feedback which reduces the amplification. As a result you have a very smooth an soft "clipping".
I have built a guitar amplifier with a power stage with this behaviour. Its a very nice sound you get.
Re: Fender AC568
OK....Thanks
I do not have a whole lot of electronics knowledge, so I was embarrassed to ask a stupid question like, "is it possible for power tubes to be cat and fixed bias"? I did not realize it was a plausible bias scheme.
Like I said, I do not have much electronics know-how, but I have never worked on an amplifier (I don't think) that employed two different bias schemes. Not sure if I have seen a schematic, besides this one, that showed two methods of bias.
Thanks Again
I do not have a whole lot of electronics knowledge, so I was embarrassed to ask a stupid question like, "is it possible for power tubes to be cat and fixed bias"? I did not realize it was a plausible bias scheme.
Like I said, I do not have much electronics know-how, but I have never worked on an amplifier (I don't think) that employed two different bias schemes. Not sure if I have seen a schematic, besides this one, that showed two methods of bias.
Thanks Again
Re: Fender AC568
No problem at all!
This bias combination is very seldom. And it it took a while for myself to understand the circuit.
Please have a look to the attachment. This is the powerstage output signal when the powerstage is driven with a large clean sinus signal: smooth clipping.
This bias combination is very seldom. And it it took a while for myself to understand the circuit.
Please have a look to the attachment. This is the powerstage output signal when the powerstage is driven with a large clean sinus signal: smooth clipping.
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Re: Fender AC568
Could that bypass cap between cathodes, be applied to a cathode biased amp with a single bias resistor?
Re: Fender AC568
No, because with a single bias resistor both cathodes of the power tubes are connected always.
Re: Fender AC568
So I could do this if I had seperate bias resistors?es345 wrote:No, because with a single bias resistor both cathodes of the power tubes are connected always.
Re: Fender AC568
This is how I have done it in one of my amps:
- 2 separate Bias resistors
- 2 caps, switchable to ground
This allows clean up to hard clipping (caps connected to ground) or smooth clipping (switch open).
The 10K resistor in parallel to the switch defines DC ground when the switch is open.
- 2 separate Bias resistors
- 2 caps, switchable to ground
This allows clean up to hard clipping (caps connected to ground) or smooth clipping (switch open).
The 10K resistor in parallel to the switch defines DC ground when the switch is open.
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Re: Fender AC568
A brilliant idea and a great realization!
Thanks!
Thanks!
Re: Fender AC568
This is the first time Ihave ever heard anything positive said about that bias setup.
Usually I hear "it's plain goofy, get it outta there".
Usually I hear "it's plain goofy, get it outta there".
Re: Fender AC568
Smooth Clipping?es345 wrote:No problem at all!
This bias combination is very seldom. And it it took a while for myself to understand the circuit.
Please have a look to the attachment. This is the powerstage output signal when the powerstage is driven with a large clean sinus signal: smooth clipping.
I am not sure that I see it. Is it on the down side of the positive wave?
That is to say when the positive wave has peaked and then starts its descent, is that where you are seeing the soft clipping? It does seem like the sine wave is not quite linear in that area.....or am I imagining things?
Thank You
Re: Fender AC568
ok, let me define my wording.
What I mean with "smoooth clipping" is that an overdrive from sinus input signal does not does not create hard clipping at the sinus of the output signal. In the picture above you can see that the sinus is clean about half way up and then compressed increasingly. The result is a - how I call it - smooth clipping resulting in a warm tone when driven by a guitar.
just for more explanation some pictures of my understanding and naming conventions
- hard clipping with crossover distortion
- smooth then hard clipping without crossover distortion
- clean output signal
What I mean with "smoooth clipping" is that an overdrive from sinus input signal does not does not create hard clipping at the sinus of the output signal. In the picture above you can see that the sinus is clean about half way up and then compressed increasingly. The result is a - how I call it - smooth clipping resulting in a warm tone when driven by a guitar.
just for more explanation some pictures of my understanding and naming conventions
- hard clipping with crossover distortion
- smooth then hard clipping without crossover distortion
- clean output signal
You do not have the required permissions to view the files attached to this post.