Grid Current

[pre65]

what the hell is with the A3. Only person i know who tried to invent that stupidity was Thorston i assumed for marketing purposes. The world does not need another class. In this case if its good enough for rdh its good enough for me.

OK, so is A2 360 degree better ?

[Paul Barker]

I'm looking at that last picture, trying to work out how the DC magnetisation is cancelled in the interstage . Both grids are grounded, both cathodes are positive, the current is going the same way in each half... but is it just down to which way you connect-up the secondary winding ?
Do you have that choice, which just inverts the driving signal , which is no problem ?

you probably missed what I said in the first post. It still applies.

In the circuit below don't copy the transformer connection. I haven't paid attention to making the connection so that the two currents cancel. Simply connect both ways and the way without obvious distortion due to 200mA of dc current is the one you want.

[Paul Barker]

Only if you bias it as such and we would then be starting from scratch a whole new design. That is not what I have designed. If you concluded the above I would suggest your reading is in advance of your ability to assimilate the knowledge into successful cogniciense.

Most probably Paul.

I'll leave my 811a mutterings for another time, when (hopefully ) brain cells A & B have assimilated the required knowledge.

the 811a lends itself to direct coupling into it's grid from a choke loaded CF for the exact same reason but conversely that the 211 suits this connection.

So in simple terms if you need positive quiescent grid bias connect to the grid if you need negative quiescent grid bias connect to the cathode. Work it so that the CF choke load DCR or the interstage dcr provide your bias voltage in both cases and stay well clear of any further complexity.

Se Ren Dipity! However that is spelt.

[Paul Barker]

By the way I didn't exhaust all connotations. It is quite possible that one of the IDH pentodes could fit the driver bill triode connected.

I was just starting at the top in terms of quality. But it could be cheapened. Though 300b's are cheap enough these days if you don't buy into the snake oil and feel you absolutely must have cryogenically treated titanium coated gold grids or whatever is sold these days at three times the normal price for a mrginally different sound most likely heard clearly by the one with the investment.

[Paul Barker]

what the hell is with the A3. Only person i know who tried to invent that stupidity was Thorston i assumed for marketing purposes. The world does not need another class. In this case if its good enough for rdh its good enough for me.

OK, so is A2 360 degree better ?

I think you were understandably misled by something I have not seen and won't bother to go looking for, but if it exists it isn't something which is helpful to our case. so forget about it. It might possibly be one of those sales techniques of bamboozle to impress, but in the words of Shania Twain

"That don't impress me much
"

[Mike H]

in the circuit above I forgot to mention the transformer is connected 3.75k : 3.75k so DC resistance is 200 ohms. I had to minimise that as explained in the text.

For 25 watts power output we need 150v peak to peak into the 211, we can get that from 1v RMS with headroom.

That could work, max. power is 5 Watts I'm told, r.m.s. I believe.

Is that 200R of the interstage a value you have measured and is the transformer wired 1:1. Are Mikes interstages not interleaved ?

You can arrange the 4 windings however you like, interleaved would be my recommendation.

All four in series is a 1600 ohm dc resistance choke. I no longer recall the inductance. Plenty for an anode choke not nearly enough for a grid choke.

I believe I measured one of mine as 32H for 2 windings, so 128H 4 windings in series.

I'm looking at that last picture, trying to work out how the DC magnetisation is cancelled in the interstage . Both grids are grounded, both cathodes are positive, the current is going the same way in each half... but is it just down to which way you connect-up the secondary winding ?
Do you have that choice, which just inverts the driving signal , which is no problem ?

That's how I interpret it, if AC polarity is an issue we can invert it somwhere else as required. Most simply, at the OPT secondary.

He hasn't put in phase dots, so the way it looks can imply that the ends of the two windings are in phase as drawn, but they're not. Imagine one of them is reversed.

[Paul Barker]

He hasn't put in phase dots, so the way it looks can imply that the ends of the two windings are in phase as drawn, but they're not. Imagine one of them is reversed.

I just wanted to keep it simple. It is the simplest thing in the world to try one way then the other and then you have your answer without having to understand what is only after all a picture and interpret it into the transformer leeds in front to you which you also have to then interpret from their picture.

Do it my way trial and error and it is very quick and easy and absolutly clear when you have it right.

Regarding the power. By balancing the currents we are free to extend the capability of the transformer. I may have some Nickel lams which would fit the bobbin and may actually try to fully balance the core to try pinstriped 50% Nickel steel alternated with silicon steel, but assembled in the Purvine method. The dc flux would have to be pretty well balanced though. The ear will soon advise when the balance is offset. If the ear remains happy, don't worry about anything.

If the case is that the transformer cannot deal with 150v peak to peak with no DC flux, then the 801a option is as far as it goes.

But the 4 watt 801a option looks very tasty to me sat hear listening to 750milliWatts of 6EM7.

I have now inserted the 801a version into the first post. here it is also. Mark will note I have paid attention to diagramatic correctness.



i suppose the simplest form of balancing method would be a regulated supply for the driver, adjust driver B+ for perfect balance.

[Mike H]

There you go.

Wasn't getting at you just trying to help Mark see what might be going on.

[Nick]

what the hell is with the A3. Only person i know who tried to invent that stupidity was Thorston i assumed for marketing purposes. The world does not need another class. In this case if its good enough for rdh its good enough for me.

OK, so is A2 360 degree better ?

A2 is all you need to call it.

[pre65]

Did you look at GM70 at all Paul with regard to this topic ?

[Paul Barker]

good point, will have a look.

[Paul Barker]

Using this data sheet it has possibilities.

Restricted by the 200 ohms lets first stick to the 100mA. For the gm70 that results in 9 watts.

So the operating point is 400v -20v 100mA into a 5k load,
and we can use
the 300b CF driver option or the pair of 4p1l common cathode driver option.

But we can get 20 watts if we oush the current up to 150mA which biases us at 30v anode volts 575 but be cautioned it depends which curves you read to decide on anode voltage. Adjust on test.

But now we need to run 150mA through the driver also. I suppose as they are cheap a GM70 CF is the most obvious choice at the same operating point.

[pre65]

Excellent Paul.

100ma into 5K opens up another possibility.

[Paul Barker]

Sorry Phil I have done my thing of adding more info while you were typing, go back please?

[pre65]

Will this GM70 setup be A2 as in some grid current will flow, or A2 where grid current flows full cycle ?