805 Power stage.

[pre65]

Hi-i am wanting to build a "power stage" for an 805 and drive it from my 300b amp (6CG7 + 300b).

I seem to remember asking this ages ago but cant find my posts (on t'other forum i think) - can i take the signal to 805 from the 300b anode and do i leave the output transformer primarys in circuit with the secondaries isolated ?
Or is there a better way ??

[andrew Ivimey]

You'd need to add a coupling cap for safety's sake, something like 4mfd at at least 630v working...... I'd have thought.

[pre65]

Hi-in order not to emulate Steves pyrotechnic display on his PP amp (now sadly deceased) what is (are) the recommended diode(s) to use in a bridge from a 360-0-360 transformer.

I have several 900-1000v power supply circuit diagrams that list 1N4007 in multiples of 2 or 3 per leg but is that a good idea.

I would favour a single diode that was rated correctly.

As the bridge will be followed by a valve rectifier (FW or 2 HW) do i need Shottkies,or fast recovery types ?

[Nick]

you won;t find many SS diodes over 1kv, and they will be internally several in series. I would use a string of 3 UF4007 for each leg, that should give you enough leeway.

Looking at the curves for a 805 looks like you will be in A2 land. What sort of op point were you thinking of working at?

What transformer are you planning to use?

[pre65]

Hi-looking up A2 i found more to do with Sir Nigel Gresley (great man ! ) than amps.

I have to go out for a few hours so will post more later.

[Cressy Snr]

Hi Philip

I was using two 1000V diodes in series per rectifier leg. The transformer feeding the bridge was only putting out 150-0-150V at 200mA. Why they blew I don't know. All I can think of was that there was a dodgy one somewhere within the four or a cap went short circuit in the PSU.

I'll not be going high voltage with anything. That's not of course to say you shouldn't, but as the voltage goes up the bangs get bigger. Take care and keep in touch with Nick at every stage of your project.

Steve

[pre65]

Hi-if i go with the 813 i would use the driver stage from Steve Bench but with a more "simple" PSU (not into microprocessors !) and if i go with 805 i have a couple of circuits from Chinese versions (gleaned from the web) so i dont know if they are A2.(how would i tell ?)

I have "lusted" after a pair of 805 monoblocks driven by 300b and am attempting to use mostly what i have to hand to accomplish this.

As i don't need 40 WPC the 813 might be more practical !

http://www.pmillett.com/813_se_triode_amps.htm

[pre65]

Hi-i've got 50 off EM516 coming from Germany.

They are similar to the 1N4007 but 1800v 1A rating,so two of those for each leg should be enough.

If they are of particular use to anyone else i will have a few spares !

[pre65]

Hi-in answer to Nicks question re transformers.

The output transformers i use with the PT15 are 6.8K 8 ohm and were rated at 25/35W so given my speakers (VoFos) should be up to the task.

I see from chinese amp circuits with an EL34 or 6L6 (or even KT66) in triode mode drive the 805 from the cathode with an earth connection via a resistor or choke.

Now as i have some spare KT66 (inc a couple of GEC) would i perhaps be better to go for the complete Chinese circuit ?

[pre65]

Or----should i be really BOLD and try to design my own circuit ?

I have to hand C3g,KT66,6cg7 & 300b valves and 360-0-360 fairly meaty toroids + 12H 150ma 1.5kv chokes +6D22s diodes and several types of directly heated FW bum friers.

But,i would need help ! The thought is making me nervous !!

[Nick]

The first one has got some very worrying (ie I can't understand just whats going on at the moment) DC feedback via g2 of the first valve. There is no way I would turn that on without a variac. The second looks more sensible, the use of the cathode follower with chike load is nice, but I am not sure just whats controlling the current through that stage.

Both of those a A2 designs, as I would expect looking at the curves. I personally don't think a A2 output stage is the place to start.

Any reason why you want to use the 805 or 813 instead of (say) a 211 or 845, both of those will give good results with a more conventional A1 driver stage?

[pre65]

Hi Nick-i would certainly consider a 211 or 845 !

The original "plan" was to use my 6CG7/300b amp to drive a "transmitter" valve of some sort.

Can you advise if this is a "sensible" idea ?

I want to try and use what i have to hand !!

[Nick]

Well, as to it being "sensible", I am not the one to ask about that. It depends on what you want to get from it, if just the experence of usong thorated tungsten valves, then a 211 at about 800v sounds good, as does a 845. With a 10k transformer, you could use either of those, with cathode bias, and use the 300b outout TX as a chock, and cap couple from the 300b to the output valve grid. At that sort of voltage you will get about 8w or so from a 211 in a1, and a bit more from a 845.

It will work, it can be done, hum from the 300b may be a problem. But you can just about as simply drive the 211 with a 6em7, at about 800v on the 211.

Depends on the goal.

You could use the 805 as you said, but the driver stage would have to be different, as shown by the Chinese amps you posted. It would be fun to try and drive the 805, but I suspect you would be hard pushed to learn much from it.

Given you don't need the power, why not take the route Steve did, and start from first principles and design and build your own 2a3 amp, or any other similar valve you have. I think you stand to learn more and won;t have to keep looking over your sholder with the high voltage, and the problems that brings.

[pre65]

You could use the 805 as you said, but the driver stage would have to be different, as shown by the Chinese amps you posted. It would be fun to try and drive the 805, but I suspect you would be hard pushed to learn much from it.

Hi Nick-i will give it some thought.

What is the "problem" with A2 ?(if i was to replicate the 2nd circuit with KT66 and 805).

I just get a headache thinking about the theory,have tried reading MJ but it goes over my head,just like it did when i was an apprentice at Marconi in the 60s.That's why i left and joined the motor trade !!

[Nick]

Both the Chinese circuits are A2.

A2 is defined by grid current flowing during part or all of the conduction cycle, Normally we like to think of the grid as a high impedance thing that can be driven via a cap, or transformer, and the only thing to worry about is a small amount of capacitance that make life a little harder for the driver stage.

Once the bias on the grid, starts to wander near 0v (from the -ve direction) the nice quiet world I described above ceases to be, and the grid becomes the anode of a diode. And just like any other diode, if its forward biased, then current will flow. So our nice simple driver stage finds its looking into a source of electrons, and the impedance its having to drive plumets. And another problem is the impedance seen by the driver will be far from constant, being hight for part of the cycle, and very low for the rest.

In many cases A2 operation is not within the designed area of operation for the valve. But in the case of some valves, A2 is where they were originally designed to operate, and they will have a grid constructed to pass significant current. And typical examples of this sort of valve are the 211, 805, 811 and 833. If you look at the plate curves for any of these valves you will see grid lines both above and below 0v.

Why is this a good thing, you ask? Well if you look at the A2 part of the curves, you will see that you can draw a load line that goes a lot closer to the 0v line, and because of this you will get more power from the valve.

In A1 the most you can expect from a value id 25% of anode dissapation, in A2 that becomes 50%, you won't get that far but its a lot more than is available in A1

For example my A1 211 produced about 20w, it ran at 1200v @ 70ma, so thats about 85w, so 25% of that is 21w, so its about right.

The A2 211 I have now, runs at 950v @ 80ma, so its actually working the valve less hard. And I get 32w from that, so you can see more power.

But the downside is to get that power I have to drive the grid of the 211 up to about +40v, at which poinbt the grid is passing about 10ma, so to get to that point, I need to use a cathode follower that can sink that current.

And of course, you can't use a cap to couple to the grid, as the grid current would cause the voltage on the grid to increase being blocked by the cap, to the point of run away if the grid resistor was not big enough to loose the charge. Thats why the bigger valves need smaller valve grid resistors, to catch them when the go into or near a2.

Any valve will pass grid current once its grid is +ve, but not all are designed to do this. The onset of grid current doesn't happen at exactly 0v either, it starts before that, so it can happen in what would normally be considered a1 amps, and the extra charge created by a transiant into a2 might be one of the problems that makes driving a 300b with a high impedance driver like a 6sl7 sound comparitivly naff.