Kendrick Champ bias circuit

[Mick Bailey]

I don't see many Kendrick amps - is the bias board in this Champ-type amp a later add-on, or were they made this way?

[Jazz P Bass]

That looks to be an add on board.
Single ended (adjustable) fixed bias.
Here are two pics of a Kendrick 5F1.

Note: both images have a reflection off of the chassis showing the 12AX7 cathode resistor & capacitor again.)

[Helmholtz]

The cathode network is there but shorted by a wire.
Doesn't look like a professional solution. Fixed bias for a SE class A power stage with transformer load is no good idea at all.

[mozz]

Could you explain that reasoning?

[Helmholtz]

Could you explain that reasoning?

Cathode bias has several advantages:

• It stabilizes the idle current by DC feedback against variations of tube parameters (caused by aging or different tubes) or supply voltages.
• As it's self-adjusting it reduces the risk of thermal runaway and overdissipating in case of some failure.
• The cathode cap allows to tailor the bass response.
• The cathode resistor helps to limit the DC plate current in case of failure and may protect the OT.

I consider the self-stabilzing feature essential when a tube is operated at or even above its max dissipation and with a transformer load that can't safely limit the DC plate current.


The only „benefit“ of fixed bias I can see is a slightly increased output power of maybe 0.3W.

[Mick Bailey]

I have it here because of repeated output tube failures - even with JJ6V6s tubes, which I find to be the most reliable. Maybe time to revert it to cathode bias and see how it goes.

[mozz]

With that reasoning, all fixed bias is bad? A properly designed fixed bias circuit should have no disadvantage over a cathode biased output stage. There are many disadvantages to a cathode bias circuit also so it goes both ways.

Fixed bias is also more easily adaptable to be adjustable than a cathode bias setup. I see that as a bonus also. It would make switching output tube types much easier.
In a circuit like a champ, most you see follow the same old cathode bias, added fixed bias may be out of the norm but I think adds a bit more versatility.

[Helmholtz]

With that reasoning, all fixed bias is bad?

No. As said above, its bad with single ended class A outputs that typically run at max. dissipation.

There are many disadvantages to a cathode bias circuit also so it goes both ways.

Please elaborate.

Fixed bias is also more easily adaptable to be adjustable than a cathode bias setup.

But it's not self-adjusting by principle, so it needs to be adjusted more often and more carefully. And it's prone to drift and sometimes thermal run-away.


Cathode bias can be combined with a little fixed bias to facilitate easy adjustment if necessary.

[Chuck H]

I agree with Helmholtz here. In class A single ended there would, ideally, be no current increase with signal conduction UNLESS operating parameters move the class of operation. In which case the cathode bias (self adjusting) keeps things somewhat regulated. For this type of circuit it's a win/win. There's no down side (well, other than a third of a watt as pointed out) because the bias point doesn't shift while operating in proper class A. And you can design around any power loss if you have room in the spec for more plate voltage. I don't see any way that fixed bias is better for a Class A single ended circuit. In fact I only see the disadvantage that the bias will remain fixed if/when the power tube starts clipping and there's a risk of overdissipation.

[Dave H]

Cathode bias can be combined with a little fixed bias to facilitate easy adjustment if necessary.

Cathode bias could also be made adjustable as below.

[Helmholtz]

Cathode bias could also be made adjustable as below.

Great idea!
As the positive grid bias will make the tube run hotter, the cathode resistor needs to be larger (as shown).

[Mick Bailey]

I contacted the owner today and he said the fixed-bias mod was indeed a later change, taken from a Gerald Weber article. I couldn't find it, but it may be interesting to read what the claimed benefits are. Perhaps it's contained in one of his books.

[mozz]

I didn't say fixed bias is better than cathode bias nor did i say cathode bias is worse than fixed bias. When in a small class A, cathode bias is more of the norm, therefore, any straying from that is bad and a thousand people will all agree to that. Manufacturing cost is the major driver of using simple setups. As i said, cathode bias is a lot harder to implement any adjustablility, yes there are examples of how to do it but none are really commonly used. Fixed bias you add a simple variable pot and that id fairly common and proven to work well. You can argue the pot may go bad and you lose your bias, transformer, tubes, fire, etc. Wire it correctly and you can't have that problem, anyway that pot is not turned daily and i can bet the odds of a bias pot going bad are really slim.

Cathode bias uses a power resistor, known to go bad/open from the heat, value change. You can't swap tubes (sometimes brands) and have the exact bias you want without changing the resistor. The electrolytic capacitors used are often right next to the power resistor so they get cooked and have a shorter life. Yes you can fine tune the sound by using different cathode capacitors but who actually does that? You solder one in and forget it.

I'm just saying because 99% of Champs have cathode bias does not mean fixed bias is terrible in that amp. Maybe the owner went from 6k6 to 6f6 to 6y6 to 6v6 to 6l6 to 7027.

[The Dude]

......the fixed-bias mod was indeed a later change, taken from a Gerald Weber article......

[Helmholtz]

Sorry to ask, but what is the meaning of this taped mouth icon ? Tried to google it without success.