Pentode driven PT15 trioded for Stuart

[James D]

Here is the pentode driven PT15 design I promised. Its not fully detailed and shows a C3g, my favorite driver. The EF180 is almost a direct swop for the C3g. We can add detail to it as the thread progresses...

I've used a light bulb for the cathode resistor on the PT15 about a 60W one should do the trick - they sound good and get the heat out from under the chassis - still need a cap bypass though.

It is pretty simple but it does use a VR tube on the sg. This could be replaced with just a resistor and cap....

ciao

James

[Stuart Dean]

That looks really interesting James.
I must get around to building something again.
Problem is although I 'retired' two years ago my boss dosn't seem to believe me and I just can't seem to get away from work.
I read up on your Low noise PS thread and came to the conclusion that what I might do is build a stand alone version of this PS before doing anything else.
I could then use it on other projects.
I have some 70uf motor runs . Use these prehaps?
(like Paul I was a bit confused by your power TX primary arrangement)
Re the PT15 design I take the C3G are Loctals. I will call in at my friends shop as a few week past he had a large box full of un-loved Loctals. See what I can find and get some Loctal bases and some B5's for the PT15's.

I don't have any OA2's either. I discoved that the one example I had wouldn't bounce.
STU

[Global]

Hi James

What are the values of the components round the OA2? The resistor above and the bypass resistor and cap.

The data sheet for the C3G suggests that the suppressor grid go to earth rather than cathode. What do you reckon on this?

Just trying to understand pentodes after beginning to understand triodes!

Thanks

Mark

[Andrew]

Hi James,

As you know I've biten for an E180F driven version. I'll let you know if and when I get anywhere with the pentode curves...

cheers,

-- Andrew

[James D]

Hi Stuart,

Sorry for the delay in replying - work got me big time!

Yes the C3g are loctals. Nice valve though. E180F is close to the same Gm at the same Op point so an easy substitute. Any of the 150V VR tubes will do nicely.

The 70uF motor runs would be super for the non BG design.

Mark,

The resistor above the VR tube is calculated as follows (from a post I made on Diyaudio covering this:
Rg2 follows standard pentode calculation practice. So

It is based on the voltage drop between the B+ it is connected to and the required Vsg i.e V(Rsg)= B+ - (Vsg+Vk) where Vk is the voltage on the cathode. And the screen grid current under static conditions (i.e. no signal). Isg can be found by examining the datasheets for the valve and finding the screen current at the operating point (Va, Vsg, Ia, Vgk). Alternatively it can be estimated from the anode current Ia. As a general rule (their are exceptions) Isg is about 25% of Ia.

So Rsg = {B+ - (Vsg+ Vk)}/Isg.

The decoupling capacitor should work with the screen grid input impedance and Rsg to decouple the screen grid with a 1Hz time constant. Ideally to the cathode (not to ground) but it can be to ground if the cathode is also bypassed to ground...

The zobel network (R&C) across the VR tube is calculated to neutralise the VR tubes inductance and 470R and 100nF (from memory) will do this nicely.

Both Dr. John and I reckon you should always return the suppresor grid to the cathode as it helps prevent oscillation at rf frequencies.

Andrew,

Good luck! We can chat it through at the phono shoot out.

ciao

James

[Andrew]

Originally Posted by James D

Andrew,

Good luck! We can chat it through at the phono shoot out.

ciao

James

Ready when you are James, had a stab at this....

I'll bring the laptop, see you Sat.

cheers,

-- Andrew

[Andrew]

Hi James,

Hope the family is recovering well.

Pity about Sat, I think I speak for all when I say we really missed your input.

I can post this, if I'm feeling brave, just let me know.....

cheers,

-- Andrew

[James D]

Hi Andrew,

Thank you for your thoughts. My family are recovering as expected. I have told them both that their timing is awful

I would have loved to hear all the different phono stages - oh well...

Post away and lets see what we can work out.

James

[Andrew]

Originally Posted by James D

Post away and lets see what we can work out.

James

Good to hear they are on the mend.....

Here it is.....

-- Andrew

[James D]

Neat! I haven't had time to analyse it yet but first look suggests the positive feedback loop values are not optimum but the rest looks good. I think the grid resistor needs reducing to about 100K and the feedback resistor needs to be about 220k ish. C2 will need tweaking if you do this.

Not sure what the 125V at 90mA refers too?

I find the grid stopper on an C3g unnecessary and it robs some life from the music but you might need it with an EF180. Oh I trannie couple into the pentode as well which probably helps remove the need for a stopper.

Thats all tonight more tomorrow once I've looked at the plate curves...

ciao
James

Biasing of the PT15 looks about right. As a tip the PT15 in triode mode has almost identical plate curves as the KT66 in triode mode :-)