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Old 2nd August 2008, 02:09 PM   #11
GordonW is offline GordonW  United States
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With regards to current in the input stages- another limitation I'm working with, is the original power transformer.

I am sure I can get enough power to run the 12AT7/12AU7 stages (3ma on each section of the 12AT7, 3-4ma on each section of the 12AU7)... but I would hesitate putting any more load on the power transformer.

So, that given... I've seen plenty of designs that worked well with these tubes at these current levels... some at MUCH lower, in fact...

Regards,
Gordon.
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Old 4th August 2008, 01:50 AM   #12
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Gordon,
One of the Baby Huey variants I did was a modification of a Chinese Music Angel PP EL34. It was very close to what you have described. Details are here:

Pentode Push-Pull

You can use that for a few "ideas" to be applied to a 6V6 variation.

I have been "itching" to try the Gary Pimm PP47 arrangement with 6AU6. It fits the "Theoretical" requirements.

The clue to the Baby Huey arrangement is that the diff amp needs to be low current (say less than 1mA per side and preferrably about 0.6mA per side) so as to not load the output stage excessively.

That diff amp also needs to be high rp. The shunt feedback voltage is being divided and the "lower arm" of the voltage divider is the load resistance presented by the next stage in parallel with the impedance looking down into the anode of the diffamp sides. That means the tube stages of the diff amp need to be high rp. That means a 12AX7 or a penetode (6AU6) or a cascode. It also means that you cannot simply connect fixed biased 6V6 with Rg1 value of 100K max. directly to the diff amp anodes but must impedance buffer them with mosfet source followers. The ZVN0545A that was recommended above is a great device for this (VERY low input capacitance and Crss).

The 6V6 Baby Huey variant I did is at post #54 here

EL84 Amp - Baby Huey

Good luck - keep us "posted".

Cheers,
Ian

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Old 4th August 2008, 09:35 AM   #13
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The 6AU6 does everything you need with only a single gain stage. Gary Pimms implementation has excellent bandwidth and sounds great.
I built a little headphone amp using the ECL82 and coupling caps on the same principles as the Tabor. The front end passes around 1mA and is equivalent to an ECC81 (I think), it works very well with good bandwidth.
In both cases I had to use step down transformers to limit the gain to reasonable levels. Also with the range of LTP valves you are thinking of, the bias point is usually less than -1V which in my experience invites overloading and distortion (remember in partial feedback the feedback is reducing the output voltage gain and not the grid drive to the front end. If you supply 2V to a -1volt biased front end then the valve is in overload). The 6AU6 is capable of sensible bias points of a few volts. Really its the only sensible choice of partial feedback driver.

I would be inclined to try all I could to build it as a two stage design and only add a further front end if absolutely forced.

Shoog
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Old 4th August 2008, 09:56 AM   #14
Svein_B is offline Svein_B  Norway
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As Shoog says, the partial-feedback topology a'la Baby Huey present a difficult load for the driver stage.
This is discussed in some detail in this thread, supported by some measurements, and loadline illustrations:
Partial Feedback EL84PP - Not quite as expected (yet)

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