Anyone know of a dual pentode suitable for a LTP.

[Jeb-D.]

Penny for your thoughts.

With a choke on the anode, it is going to steal voltage which could be used for the load instead. I really don't see an effective way to apply cross coupled feedback with cathode follower output. Having the feedback go to the local driver would really be a more elegant solution. If you run the output common-cathode, then cross-coupled feedback would be the right ticket.

Garter bias won't ensure that the cathode voltages are the same. They will be closer than they would be without it, but not an exact match. Also, having the transformer DC coupled between cathodes is going to ruin the affect of garter bias, unless the DCR is about 10x the value of the bias resistor. If the DCR is significantly lower than the bias resistors, it's going to behave similar to a single shared cathode resistor .

With the cathode follower output, you are probably going to need more supply voltage for the driver. The output gain is going to be about .4-.5 (perfect signal level for screen feedback, like UL). This means your going to need a peak voltage of around 60V(with bias of -30Vgk) to drive it to full power. I'd probably make a negative rail off the existing transformer (so you have and equal +/- voltage). You could then direct couple the the drivers screen grid to the output tubes cathode.

[Shoog]

I'm not certain I agree with you on the garter not producing a matched voltage. If it creates a matched current then there has to be a matched voltage since it is going through a simple resistor. If the current is balanced to within 1mA, in my example the voltage imbalance would be about 0.7V which for most tranaformers would create little problem. However I take your point on it shorting out the Garter - so its a none runner.

The whole idea of getting the cathodes current and voltage balanced is a none runner really. Since there are three contol elements and what i am trying to achieve is to fix all three - its impossable.
The only solution would be to ground the cathode transformer, fix the anode voltage and then servo the grid bias. This may be a path worth investigating - and I suspect that Gary Pimm may have a suitable circuit.

On the point of the plate choke. I think you are a bit wrong. The plate choke will consume far less voltage than a voltage follower, and will be able to swing above +B which will allow for greater overall efficiency especially when entering class B - which is where the benefits of cross coupled feedback would come into play.

Research - research - research - O what fun.

Shoog

[Jeb-D.]

If it creates a matched current then there has to be a matched voltage since it is going through a simple resistor.

It creates a closer-to, matched current and voltage, but they will only be an exact match if the tubes are. Think, if both voltages and currents are equal, both tubes will have the same grid voltage as well. This means they both would have identical Vgk.

On the point of the plate choke. I think you are a bit wrong. The plate choke will consume far less voltage than a voltage follower

Sorry, but it is true... that output, for the most part is a cathodyne phase splitter with mis-matched loads. For a given delta current, it will create voltage = I x R. The choke running with little shunt resistance will have a higher impedance than the transformers primary and hence create more voltage. However, the current drawn at the point of saturation will be much less (because the choke impedance is part of the loadline).

If you were to shunt the choke with a resistor, it would lower the amount of voltage dropped across the choke, but it is going to be stealing power from the load. Example, the primary impedance of the transformer is 1k. You shunt the choke with 1k. The tube will see a 2k load. They will see equal voltages in this case which means that resistor will get half the power and the other half will be distributed to the load. However, you are correct in saying the plate will swing above B+, but the cathode is no longer going to swing as far below ground potential, apposed to just putting a 2k load on the cathode.

Another example. You decide to shunt the choke with a low value resistor to minimize power losses to the output. We'll select 100R. You use a transformer impedance of 100R less in order to keep the same load impedance. The circuit has a delta I of 100mA, this would give 10V across the choke + shunt resistor. So, the 100R resistor will dissipate 1W which could have been going to the load.

[Shoog]

Thanks for the input Jed,
I have come on a bit of a brick wall on this whole concept at the moment. The thing I will have to look into is self adjusting fixed bias - but there doesn't seem to be a lot of information out there so it obviously not a walk in the park to implement.

Unfortunately I have more pressing matters to attend to in the next fortnight, so i'm going to put this to bed for the moment. Its been a great brain workout and learning experience so far.

Cheers

Shoog

[Jeb-D.]

Its been a great brain workout and learning experience so far.

That's the most important part. I've tried many ideas that didn't work, but a lot was learned from it. Some of which are concepts that can be applicable down the road.

BTW, The student edition of circuitmaker is a free download. It's a simulation program that is very intuitive (easy to use). The tube models it comes with to start are very limited, but I found a way to edit + add them. It is an extremely useful tool. You should give it a try.

P.S. The ECF80 is a hidden gem IMO. They aren't very linear in some configurations, but it always manages to sound good. I recently purchased a small quantity of the russian equivalents (6F1P). They are nice and dirt cheap.

[Shoog]

I'm really happy with my ECF80 SLCF implementation. I am running the three tube SLCF at about 430V which gives each element a good 130V to play with - I'm also running them hard at 10mA. The front end has a 39K load so it gets the best out of it, and its great to have 8V of bias on the input. Really your right, it runs hard and plays hard - great tube. Its cheap enough to (shush don't tell anyone).

Shoog