Single channel AB763 White Noise (light Hiss)

[p2pAmps]

Single channel AB763, no trem build question for all you pros.  So, I have built a few of these beasts and they are a 4 – 6l6 config and I am pretty darn happy.  There is one thing I have run into that I would like some feedback on.

The preamp is LOUD as heck.  On 2 or 3 it’s to the point where you have to start making faces!  There is some white noise going on at idle, more than I like to hear.  I have already swapped out all the plates with some metal film 100Ks and also tested some other new 6L6s just in case, same issue.  Now it’s not that its super bad but I strive for perfection.

I have a few thoughts.  First with only one channel I wonder if I should drop V1 to an 12AT7?  Also, V3 (reverb recovery) now feeds directly into the phase invertor as there in no tremolo in these builds.  Could my white noise (hiss) be a combination of the HOT preamp and V3 -> Phase?

I would love to hear from anyone who has experienced this and hear what your solution or tries were to bring that floor noise down.

[shooter]

wonder if I should drop V1

I do that, I go with an AU, but that's personal choice.

Isolating the noise helps "understand" the reason.  scopes are handy for that, knobs work pretty well getting in the ballpark.
My 'ol life signal to noise SNr was a big deal

If V1 can get you a good "drive" for the next stage, and NOT amplify the noise much
that's worth "varying from the clone/original" design

[tubenit]

I will offer two things to consider.  I find a 5751 far more "musical" sounding then a 12AT7. I'd try that.  This is a tube where I prefer Old Stock or NOS.  However, the newer JJ 5751 is a decent tube.  I have a 12AY7 in V1 in several amps that I have.

The other thing you might try is an "enhance cap" on the LTPI entrance triode plate.  Try something between 120p to 390p.  I usually use something around 220p.

You can do a search on some threads about this on the forum.  The best way I can describe it is that this "enhance cap" eliminates the "harshness" of the treble highs and smooths them out.  Sort of the difference of a kid's top spinning in a wobbly fashion vs. a smooth spin.  I have found this cap to help with removing some unwanted idle noise in the past on my amps.  It goes on every LTPI build I've done in the last few yrs.

https://el34world.com/Forum/index.php?topic=12723.msg153636#msg153636

You can safely use two insulated alligator clipped wires and caps of different values until you find what you like.

With respect, Tubenit

[sluckey]

Open up your pocketbook and get some good NOS 7025s.

[p2pAmps]

Thanks fellas,

I got to thinking about this more so I started checking voltages again.  The plates on V1 were 250+ which helps explain why the front end is so strong.  I popped a 12AT7 in and the voltage dropped to 177vDc.  A 12AU7 was 100vDC :(.   Went back to the 12AT7 and the floor noise dropped quite a bit.  Amp still sound strong and bold as ever.  I do have a little secret stacth of NOS RCA 7025s which will go in next (Steve).  I think I will experiment around with the "enhance cap" too Tubenit and see what changes.  The highs are pretty right so far and the Jensen PIOs are pretty great. 

Thanks guys

[tubeswell]

12AY7 is a good alternate V1 tube for guitar. Good bandwidth and very 'bluesy'


EH12AY7 is a good current production version

[ac427v]

The ab763 original trem/reverb channel has the 50k trem pot connected to ground. Deleting the trem circuitry requires replacing that pot with a resistor to ground to preserve the original gain structure. I would verify that before any other mods.

[p2pAmps]

The ab763 original trem/reverb channel has the 50k trem pot connected to ground. Deleting the trem circuitry requires replacing that pot with a resistor to ground to preserve the original gain structure. I would verify that before any other mods.

I used a 51K resistor for this...  Thank you!

[p2pAmps]

Ok, this morning I put a 12AX7 back in the V1 and pulled the dropping resistor (4.7K - was getting 252vDC on the V1 plates) out and played around with some different values while listening.  I ended up with an 18K on the supply and now my voltage is 220vDC on the plates of V1, much better.  The floor noise has almost vanished and the tone and feel remain very strong and bold.  Thats my story

Mike

[tubenit]

The floor noise has almost vanished and the tone and feel remain very strong and bold

Hooray!  I love a success story!  THANKS for taking the time to let us know what the resolution was.  That is always helpful
to others.

With respect, Tubenit

[shooter]

A 12AU7 was 100vDC :(.   

 

My V1 plate rarely exceeds 80vdc, after all, I'm just taking a 100mV signal to 2v, how much extra do you need 

[Willabe]

My V1 plate rarely exceeds 80vdc, after all, I'm just taking a 100mV signal to 2v, how much extra do you need 

BF Super Reverb's, very clean Fender. One of the reasons was 270dcv on the preamp tubes.

https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab763_schem.pdf

[shooter]

any gain stage that just amplifies without distorting is clean
my only point on V1 (or 1st gain stage)  It's typically used to get the input signal to a useable, or abusive range, then maybe more plate vdc becomes "important"

worked on a nice one awhile back, was real genital with tech tools 

[brewdude]

was real genital with tech tools 

😮😬

[JB]

was real genital with tech tools 

😮😬

He's right, never dick around inside an amp!   

[p2pAmps]

was real genital with tech tools 

😮😬

He's right, never dick around inside an amp!   

HAHAHA