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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Would this scheme work ?


2010-11-04 4:43 pm
So would this scheme basically work as an headphone hybrid amp ? I counted two triodes as double triode(6n23p; ECC88, 6DJ8). Output power should be 0.5W. VÕIMskeem.png
The outputs appear to be shorted to ground. Some problems in the power supply too I think, cannot have grounds in two different places off the same winding. Inputs appear to be shorted to power. The tube cathodes don't have any DC return to B-. And the tube plate B+ voltage distribution are messed up. Other than that, the amplifiers are configured as voltage followers on the pullup sides and as current amplifiers on the pull-down sides. This can be made to work, but requires some consideration of biasing, thermal stability, and current gain into a known load impedance. (the bottom side current gain x Rload needs to equal the voltage gain on the top side path, assumes a more or less constant load R) Thermal stability requires that T2 and T4 have some emitter degeneration (like 1 Ohm, but see below) to counteract the increase in transistor current gain with temperature. Otherwise thermal runaway current could occur between the pullup and pull-down halves. Linearity wants even larger emitter resistors, try like maybe 5 or 10 Ohm. Adequate heatsinking is also called for since this must run in class A.
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Paid Member
There are many mistakes in the schematic you have drawn, so many in fact that I can't begin to mention them all, but the grids of both triodes are tied together, and to the negative return of one of the supplies, the cathodes are shorted to ground for ac signals, the outputs are shorted to ground for ac signals, the primaries of all the transformers are in series when they should be in parallel, for the most part I can't make heads or tails of your power supply stuff, the output stages have no thermal compensation, you show ac power and signal ground connected. :hot: (electrocution hazard!!)

This is just electronics inspired art, not a workable schematic.. Sorry..

Please download and install LTSpiceIV from www.lineartech.com and get the tube libraries scattered around the net and install them. You may need to do some reading of older threads and I wrote extensively about how to add tubes and other parts to the libraries. (The library files may still be attached to these posts.) This program will run in WinXP and Linux under Wine. And it's free. Once you manage to successfully simulate a circuit you have a reasonable chance of getting it to work.

I'd say you have quite a lot of reading and studying to do before building anything. Please read the newbie and Safety threads here, and I'd strongly recommend getting a copy of Morgan Jone's "Valve Amplifiers, 3rd Edition" and its companion book "Building Valve Amplifiers"

Incidentally there are a great many OTL all tube headphone amplifier designs, you don't need transistors at these power levels.


2007-04-24 10:55 pm
The output stll seems shorted. And to me there seems to be a number of strange things going on but it is not very easy to follow your schem.
That might be why there is little response on this thread, it is simply too much work to find out what is going on.
I would suggest to simpify by show only one channel and break out the power supply as a separate shematic.



Paid Member
2009-02-11 12:58 am
Short answer: Your circuit will not work. Among other things, your output is taken from ground and the input to the BJT output stage appears to be grounded as well -- at least the base on the lower BJT is grounded.

You mention that your circuit is supposed to deliver 0.5 W, though, it's not clear what the load impedance is. Assuming it's 0.5 W into 4~300 ohm (small speaker or headphone amp), why mess with the BJT output stage? Surely an OTL (Output Transformer-Less) amp would do the trick. Or a SET (Single-Ended Triode) amp such as 2A3 or any other output triode.
Maybe, it's time to turn the project on its head and ask the question, "what is your goal?" rather than starting with a possible (or in this case impossible) solution and working your way back.

If you insist on using a BJT output stage, I suggest learning how those work before you attempt this project. Sedra/Smith would be a good place to start. Making a BJT output stage that operates safely and without self-destructing from a 200 V supply is not trivial for a beginner.

Once the output stage is biased properly and working correctly, look at hooking a tube stage to it. Morgan Jones is a user-friendly place to start for this.

To make your schematics more user friendly - and probably solicit more responses from users here - I suggest separating the power supply and amplifier parts. Show one schematic with the power supply and another schematic with one channel of the amplifier.

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