Direct Coupled, Self Biasing, CCS Loaded, 300B Amp

[dsavitsk]

I have some old 300B DRD amps (D3a -> 300B) and I was thinking about removing the plate chokes and instead loading the driver with a CCS. It occurred to me that I could also load the cathode of the 300B with a CCS (bypassed by an ultrapath cap) and that, in theory anyway, the amp should find its own bias points, and that even as the tubes drift, the CCSes should compensate. And, theoretically, even with a failed driver, the output should not be able to run away as current is limited by the CCS. But, I thought I'd see if anyone had any concerns regarding the practicality of such a thing.

[polyphaze]

I like this topology alot. Elegant in its simplicity, its gotta be good.
I would propose a separate +/- power supply for the driver,
as shown in the sketch attached. Separate power for the driver is always a good practice IMO, and in this case would significantly reduce dissipation in the output CS.
I'm in the middle of a 300B project so I am in a position to give this a try.

[EvilMose]

Dvastik - if you remove the transformer from the amplifier in your schematic, it becomes identical to the Bottlehead Crack OTL headphone amp with the CCS upgrades. You will find a legion of people over on the Bottlehead forums who are happy with the double CCS upgrade; I am among them.

[garbage]

Hi Dsavitsk,

It looks like a plausible design. I built something similar recently, but without the CSS at the output tube. I used some Caddock resistor instead and had some hum feedback loop.



Cheers.

[Gluca]

Now, if you use a beefy driver (let's say biased at 20mA) you can arrange something like my headphone amp. Limit is the max dissipation into VR150 or if you need to waste more W to bias the 300B you can replace the gas tube with zeners. It won't go into A2. That looks like a monkey.

[ThorstenL]

Hi,

Originally Posted by dsavitsk

I have some old 300B DRD amps (D3a -> 300B) and I was thinking about removing the plate chokes and instead loading the driver with a CCS. It occurred to me that I could also load the cathode of the 300B with a CCS (bypassed by an ultrapath cap) and that, in theory anyway, the amp should find its own bias points, and that even as the tubes drift, the CCSes should compensate. And, theoretically, even with a failed driver, the output should not be able to run away as current is limited by the CCS. But, I thought I'd see if anyone had any concerns regarding the practicality of such a thing.

A few issues.

1) The CCS on the 300B may have to deal with the whole +B AND 80mA. This can be quite a bit dissipation and voltage.

2) The Amp will find it's own operating point, but you cannot ensure the voltage across the 300B that way, using a "Mu-Follower" with fixed Gate/Grid voltage for the upper device may be a better idea to fix operating points. This way you can also drive the 300B well into Class A2 if you like a little extra power.

All in all I cannot really see any signficant improvement over the classic DRD topology, implemented correctly with a suitable (Value) anode load choke, except possibly in the extreme low bass, but there the output transformer would normally be more of a limit.

Ciao T

[Shoog]

I use a similar approach on my PP DC7 amp.
It works exactly as you expect it to work - adjustment free.

The only issue to consider is that at switch on the output stage CCS can experience the full B+ so needs to be rated accordingly or protected with zeners.

Shoog

[tomchr]

I've been working in a similar direction. You can see the schematic in this thread.

I use a 70H choke for the plate load on a d3A driving the 300B. In terms of amplifier complexity, this is by far the simplest circuit I've come up with. But in terms of power supply complexity, quite another story... I use a zener regulator for the driver tube. The cathode ends up around -150 V and varies a lot from tube to tube. I ended up making the supply adjustable and just tweak the voltage (thus the tube current) for minimum THD.
I run the 300B at 400 V, 90 mA, 5 kOhm OPT primary. I'm getting about 10 W out (the driver stage saturates first). 0.06 % THD at 1 W out. Sounds good too.

The CCS in the cathode of the 300B is intriguing. It would be nice to not have the bias adjust pot there... I might play with that.

~Tom

[tomchr]

Originally Posted by Shoog

The only issue to consider is that at switch on the output stage CCS can experience the full B+ so needs to be rated accordingly or protected with zeners.

I'm trying to wrap my head around this. When the filament is cold, all you really have in the output tube is a spark gap. So there should be no current flow between anode and cathode.

When the filament heats up, some current will start to flow between anode and cathode. The current would be limited by the CCS, hence, the voltage across the tube shouldn't collapse completely to zero. Does this sound reasonable?

I'm not trying to argue, I'm just trying to understand. It makes a big difference if the CCS is to be designed to survive 475 V or 75 V...

In my design, I ended up with the choke load on the driver tube for two reasons; 1) simplicity, 2) better control over the grid voltage on the 300B. Having the 300B float between two current sources made me nervous. Not thereby said it wouldn't work, though.

DC coupling the 300B really rocks!

~Tom

[ThorstenL]

Hi,

Originally Posted by tomchr

When the filament heats up, some current will start to flow between anode and cathode. The current would be limited by the CCS, hence, the voltage across the tube shouldn't collapse completely to zero. Does this sound reasonable?

Does this sound reasonable? Not at all.

The driver is IDH. The Cathode there will heat up much slower than the 300B. The Driver CCS will attempt to push several mA into the 300B Grid directly from +B. Most DHT count as "high perveance", meaning they do not drop a lot of voltage when you link grid to anode...

So maybe you do not get the whole 700V or soof a DRD Amp, but you get most of it.

Originally Posted by tomchr

I'm not trying to argue, I'm just trying to understand. It makes a big difference if the CCS is to be designed to survive 475 V or 75 V...

Yes, hence my warning.

Ciao T