Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders.

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ]

---

[ Alert Moderator ]

---

Noise is back...

Posted by Ivan303 (A) on June 9, 2021 at 13:09:25

Maybe never went away?

So the first 300B monoblock I rebuilt was dead quiet, albeit with very little gain. Something one doesn't necessarily pay much attention to when one has an active tube pre with tons of gain that can be cranked up and one is only playing one side.

But now the second mono-block is rebuild it plays MUCH LOUDER (has a LOT more gain). But sadly it is also MUCH noisier that the first.

Turns out the culprit is in the 'quiet' amp (with very low gain) and is a faulty National Union 6SL7 in the driver circuit (which I have a few of) and when replaced with a NEW NOS 6SL7 BOTH amps now play MUCH louder (and the same volume), have MUCH more gain but, sadly, have more noise than I would prefer.

Much more.

The fact that a bad 6SL7 driver tube would cause low gain is not a surprise. That it would make an amp DEAD quiet and still play fine (loud) when the gain on the preamp is cranked up is a bit surprising.

Unless the noise is coming from the pre-amp and I never noticed it before because the other amps I was using at the time possessed fairly low gain as well?

Clearly the noise isn't coming from the 300B nor could it be the High Voltage PS powering the 300B nor the 300B heater supply as a weak driver tube should not fix a noisy High Voltage PS or a noisy 300B circuit.

Seems the noise HAS to be coming from somewhere withing the driver circuitry or BEFORE it?

But I tried a SS preamp as built into my Audio-GD DAC/Preamp/Headphone amp and NO difference, and it is dead silent with cans or in any other application I've tried.

Not the premap, not the 300B or its associated circuitry but somewhere in BOTH driver circuits of both amps something is generating noise and amplifying it.

Seems it HAS to be in the amplification part of the driver circuit somewhere as it is prevalent in BOTH monoblocks and silent with a weak but working driver tube.







First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2025, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---