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The non passive pre becomes interesting

Posted 3rd August 2013 at 09:03 AM by hbc

Lots going on, my making things mojo has been busy. I have been reading 6 moons, and was interested by the idea of the directly heated triode preamp which sounds quite well apparently, and also repelled by the thought of using a huge output tube, slightly. So after some thought, I realised that the old battery radios used the directly heated ones, so a few weeks later some 3A5s were winging them selves to me.

Oh, (sigh) I had spent some time tweaking it first (the not passive pre) the most exciting feature was that the copied guru circuit pulled the output cathode to the HT rail when the heater is turned off! (I have big reservoir caps and small bleed). To be fair guru circuit is direct coupled so things are all over the place, but Anyway.

I decided to graft in the 3A5 in to this circuit, which was simple in principle, but ended up with a complete redesign of the power supply several times, as the addition of that one little tube hugely increased the heat generated...
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Little Big Amp

Posted 3rd August 2013 at 04:55 AM by homeskillet (Journey to the Center of the Circuits of Homeskillet)
Updated 5th August 2013 at 05:34 AM by homeskillet

Hi. Here continues the tinkering continued from the forum post:

I took out that EF184 and the plate to plate feedback loop since I read that devices with a more stable performance without feedback sounded better once it was applied than devices that rely on feedback for their very operation. This seemed about right and I was tiring of the sound a bit.

The plate to plate feedback was the only way I could find to stabilize the EF184 and 6550 in this basic configuration and give any reasonable results and wiring it satisfied my curiosity so apart it came.

I replaced V1 with a 12AT7 and wired it in parallel since I figured it could use all the help it could get to drive the 6550.

The global feedback is my first attempt at current feedback. It's basically part of my first attempt at feedback at all.

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