Best way to reduce noise when reducing gain, triode pre-amp stage?

[jimboyogi]

I got the nomimal drive current figure from the the Accutronics site ::::::::: Accu Bell Sound Inc :::::::::

The driver is a constant current, cap coupled single 12AU7 triode, modified from the circuit on Merlinb's website The Valve Wizard
Tank is a 8F with an input impedance of 1.925K at 1 KHz. I calculated the drive current using the AC load line rotated around the DC load line bias point. Being very new to this sort of thing, I may well have got it wrong! I suspect that I am getting a fair bit of drive to the coil though, because with a 1V pp, 1KHz sine wave into the amp input I am getting ~ 20mV output from the output coil, which is as high as any published figures that I have seen for tank outputs.

[Steve Conner]

Yep, I like Merlin's reverb driver circuit. I only did what I did because I already had an 8 ohm tank and the OT.

20mV is a factor of 10 less than a guitar output, so I guess that explains why the reverb subjectively seems noisier than the dry signal. When you dial in surf-worthy amounts of it on a clean tone, there may well be more gain from the reverb return to the speaker, than from the guitar input to the speaker.

[Amp Kat]

Jim I had a similar situation in a cram design and was getting noise and oscillations when the Reverb pot was cranked past two. Ended up being the reverb transformer didn't like being around the input RCA jacks and moving it away cured the problem. Not sure it's any of you're prob but thought I'd mention it.

[jimboyogi]

Thanks for the tip Amp Kat. This particular design is transformer-less, so not the problem in this case, but good info nonetheless.

I tried a couple of values of reverb pot and tested gain at 1Khz. 500K pot, Vgain ~ 13.5. 100k pot, Vgain ~ 13.0. I stuck with the 100K.

Then I tried subbing a 1n4007 diode in place of the red LED on the stage cathode. New Vk = 0.65V, Vgain ~ 14.7. I stuck with the 1n4007.

[Wombaticus]

Here's a link to a handy thermal noise calculator: Noise calculation calculator calculate Thermal noise Johnson noise voltage level Nyquist dBu dBV signal-to-noise ratio S/N temperature bandwidth noise figure - sengpielaudio.

It's instructive to play around with the various parameters. The influence of bandwidth may be especially surprising for those unaccustomed to thinking about this.