Tube DIY Asylum: Re: Latest version of my 7788/7721 LCR 600Ohm RIAA Stage by BBeck Do It Yourself (DIY) paradise for tube and SET project builders.

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Posted by BBeck (A ) on October 2, 2005 at 19:53:31

In Reply to: Latest version of my 7788/7721 LCR 600Ohm RIAA Stage posted by Thorsten on October 2, 2005 at 15:11:26:

Thorsten,

Interesting!(But I can barely read your second posted schematic, BTW)

A question though: How much grid current are you measuring in the first grid? In a recent similar RIAA project a friend and I abandoned such low input tube biasing due to concerns about grid current flowing through the moving coils (of a MC cartridge in our case), creating a slight mechanical force and an electrical/magnetic imbalance. We used 4 parallel tubes (4HA5s we had on hand) but with a small Vgk bias of several tenths of a volt. Still we had significant grid current in the several microamp range. Granted, our application was for a MC that would generate roughly/typically 20 times less signal current than a MM cartridge. Even with a MM cartridge, if it is loaded by 47K and plays at 5mV rms "full blast", there is only about 0.1 microamp rms of signal current. DC grid current (with low frequency noise) could be an order of magnitude, or more, greater than this signal value. Any measurements/thoughts with the E810F (a great tube!)?

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