RH84 Push pull?

[ThSpeakerDude88]

Has anyone tried this? I came up with a circuit that I thought might work while at school one day. It is based off of a RH84 stereo circuit with shared cathode resistors and a cathodyne splitter to drive the amp.

This achieves a few things:
1) allows each side of the amp to have local feedback to the driver tube
2) isolates the drivers and their lowered gain from the cathodyne
3) the cathodyne has a very accurate signal at the cathode as a replica of the anode signal only inverted, and not loading the cathodyne with the output tubes makes it easier to balance.

It basically looks like a LTP PI with local feedback. The reason I chose to use seperate driver tubes and phase inverter tubes is because it allows me to use the local feedback, which happens to lower gain, without affecting the phase inverter. This keeps symmetry in the signal and since the phase inverter is in front of these stages and only needs to simply invert the signal there does not need to be as much gain.

I am not sure if I am crazy or not, but it is an idea to play with. What do you guys think? Calling Mr. Kitic!
Excerpt from a thread on partial feedback amplifiers:
"ANY RH amp that was published to this moment can be made into a PP amp (with feedback and all...) provided you make it double... and you have a phase splitter in front of the two symmetrical amps per channel. It would work great -- actually, as great as the phase splitter is." ~Kitic
Well that is what inspired the idea anyways, so..

[DigitalJunkie]

The one thing that has me wondering,is the cathodyne's grid. It's going to want to be just a few volts below the cathode voltage,which will be somewhat significant. 100K grid leak might be kind of small,and you won't want to use the pot as the grid leak.(DC on the pot,etc.)
You'll probably want to AC-couple the input,and fiddle the biasing arrangement for the cathodyne.

Just my first late night impression.

[Shoog]

Looks promising.
We could argue about the choice of phase splitter. I personally would choose a transformer and use a mains toroidal ( but thats just me).
You have no provision for balancing the output stage and you are not making best use of your output stage with the simple 200R resistor. I have built a very similar circuit and would recommend turning the output stage into a differential stage. This will address your output stage balance issue and considerably improve the sound quality.
Here's what you do. Take two Constant Current Sinks (which can be as simple as a LM317 and a single resistor), place one in the cathode of each of the output tubes. Take a 1000uf cap and attach the anode to the cathode of the output tube. Connect the two cap cathodes together and attach a 300R resistor to the node. Connect the other end of this resistor to ground. Bypass the two valve cathodes together with a 0.1uf film cap of your choice. Hey presto, you have a differential output stage. You wont regret the extra effort.

With the drivers why not just drop the cathodyne and stiffen up the LTP and use it for phase splitting. I think the result will be superior. You will need a little bit of a negative rails - which you can create by rectifying a spare heater winding - and place a CCS in the tail rather than that sad little 120R resistor. If doing this you lose half your gain though, this is where a little input transformer comes in as it would allow you to drive both LTP inputs and maintain full stage gain.
Shoog

[Geek]

There should be no voltage developed across the grid resistor, so the pot there will be kind of useless.

You'll also need a bigger resistor in the 12AT7 differential amp cathode. Actually, a CCS in there is phenomenal (ask chrisb)

Cheers!

[Yvesm]

Originally posted by Geek

. . .
You'll also need a bigger resistor in the 12AT7 differential amp cathode. Actually, a CCS in there is phenomenal (ask chrisb)

Cheers!

Indeed, this cathode resistor will adjust the FB ratio by altering the effective 12AT7 Rp.
I would try a partial (adjustabe) by pass cap accross it ? ?

Just my guess.

Yves.

[SY]

The input stage won't work. It is not set up for a useful operating point. The grid will be at zero volts, so the cathode will be at maybe 1-2V. That will run the stage current at a few hundredths of a milliamp (highly nonlinear) and limit the swing to a few tenths of a volt.

[DigitalJunkie]

^^^ Yea,what SY said.

Scroll down this page a bit,it gives some info on biasing a cathodyne:
http://www.freewebs.com/valvewizard/cathodyne.html

[audiousername]

Is the cathodyne/split load/concertina really necessary?

The circuit will work with a CCS in the cathodes of the driver stage to use them as a differential pair.

Edit:

The 220 ohm resistors in series with the output stage control grids are 'grid stopper' resistors - the idea being that they work in conjunction with the output stage input capacitance to form a low pass filter to reject spurious high frequency rubbish that could cause instability or oscillation. To work best, they have to physically be placed as close as possible to the socket (some even have advocated using SMD components for this task, as they have no lead...) I know it's just a rough schematic, but the way it is drawn makes one think that the importance of their position has been lost.

[SY]

Not to mention that they're too small. Given the pentode mode here, the input capacitance is low, so 10k or even bigger would be more effective.

The other issue is that, without higher feedback, the output impedance of the amp will be quite high, and the LF distortion will not be a happy thing.

[audiousername]

Originally posted by Geek
There should be no voltage developed across the grid resistor, so the pot there will be kind of useless.

You'll also need a bigger resistor in the 12AT7 differential amp cathode. Actually, a CCS in there is phenomenal (ask chrisb)

Cheers!

Gregg,

Do you mean the 100k pot at the input or the 2M pot at the inputs of the driver stage? Either pot is going to see the full control grid input signal to the its respective valve, but the 2M pot is unnecessary - the balance of the split load phase splitter will be sufficiently good with equal load resistances.

I don't think split load phase splitter is necessary as long as the output stage is working in class A (class B could be a problem since as one valve cuts off, the load resistance of one half of the differential pair rises by a factor of the feedback ratio, causing the output impedance of the other half to change, which in turn alters the feedback ratio of the other half, altering its gain - and gain altering with signal level implies distortion).

Agreed that a CCS would be prefereable to the 120 ohm resistor.

I also don't think there's enough feedback. It can be increased without that much hassle with the use a pentode driver and completely dispensing with the separate load resistance, leaving the input valve anodes fed only from "Rfb" (in this case 100k, but the value would have to be recalculated). I'm also a fan of direct coupling the output stage to gain immunity to blocking distortion, so let's dispense of the coupling capacitors there and feed the cathodes of the input stage from a negative supply - this is trivial to create with the power supply drawn. DC balance can be gained from altering the screen voltages of the input stage.

Oh, wait. Someone has already built an amp with all that.