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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps

Post Subject: DC coupling and killing 13db and driving 6e5p miller and....Posted by cv on: 4/22/2009

Sorry I'm a little late here, but anyway... here's something that may worth trying. Apologies that I can't provide a schematic at the mo - I can sort this out later if you like.
In the meantime, you'll need a pen and napkin to help visualise this:

The input filter is a small isolation transformer with 4:1 stepdown (or whatever gain reduction you need). It has limited inductance, so you can use it as the high-pass filter. Say the series resistor is 8.2k; with the placette drivintg that, the 6E5p grid will see a driving impedance of 8.2k/(4*4) is less than 0.5k. So that takes care of the Miller capacitance there.

Now, the 6e5p is choke loaded, but the B+ is *0V*. The 6e5p anode is direct coupled to the output tube which uses cathode bias. The 6e5p cathode sits at -160V or whatever, and fixed bias is applied via the secondary of the input transformer. I know this won't use your beloved "Newton" bias scheme but it may bw worth trying.

One issue is you need a very quiet -160V (or whatever) supply as its noise will be coupled directly into the output valve grid (think of the choke and 6e5p as forming a potential divider).

A plus is that even if the 6e5p fails, the bias on the output tube should still be correct (other than the voltage drop on the loading choke in normal operation).
OTOH, if you wanted to ground the output valve cathode, you could have a resistor in series with the loading choke of the 6e5p and derive the output bias there; not as safe, but a purer circuit. (Similarly, you could replace the choke with a large loading resistor to the positive supply - with even more risk of frying the output tube and different sonics)

Make sense? This is a variation on the stacked supply amp - you could as well have the 6E5p at ground (with Newton bias) and stack the output stage supply on top of the 6e5p supply - but I don't think you have that luxury in your existing setup?
As I say, I will be in a position to sketch some schematics in near future.

A while back I wound some (IIRC) 2.5:1 nanocrystalline toroids for just this application; whilst that amp project is on hold, I do remember measuring a -3db of 2 or 3 Mhz; small is beautiful for MF amps...

cheers

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