diytube.com

Can the input impedance be lowered on the ST-70 with a stock circuit without any other ill effects to the amplifier? I have one preamp that does not do well with such a high input impedance and I would like to lower the value; if possible.

My ST-70 is rebuilt with new components, but the values and circuit are stock.

Thanks,

Dave

Dave,

I'd assume you could simply change the 470k ohm resistor (circled on the schematic) to a lower value. I wouldn't expect the 7199 to mind at all. I've seen maximum permissible values specified for the grid leak resistor, but never seen a minimum value. It's perfectly safe for the tube even if you dead short the control grid directly to ground. Of course, you won't get any audio signal into the tube that way. :)

For what it's worth, the RCA datasheet for the 7199 seems to suggest a 270k ohm resistor for grid No.1, with a maximum value of 1 meg if you're running the pentode section with cathode biasing.

Just out of curiosity, exactly what kind of upstream device are you using that doesn't like a high impedance load? Generally speaking, preamps prefer high impedances...

Hey Ty,

Thanks for the response and I hope things are well.

The preamp is a unit that I built from Bruce Rozenblit's book 'Beginners Guide to Tube Audio'. You probably have seen some traffic about the preamp discussion recently in this forum... When you were over, I forgot to show it to you.

I tried it with my ST-70 last night and the combo seems to have hi frequency roll-off and when you turn the preamp volume control down to almost no signal, some oscillation occurs. The preamp does not do this with my other tube and non-tube amps, but their input impedances are 75K or 100K.

I want to lower the input impedance if possible.

Dave

Problem found...good shielded cables. I have nice 5 foot long braided-shield cables that I tried and they caused the oscillation. I put a pair of 4 foot super cheap red and white 'came with the $50 DVD player' cables on between the two and the problem disappeared.

Bruce talked about this in his book and he was correct about capacitance and oscillation in cables.

Dave