PLEXI GAIN 20 with BUZZ

[TIMBO]

Hi guys,Another build with a bad BUZZ.I know that these types of builds are prone to this problem but i thought that i had covered all bases.

V1 & V3 have DC Heaters
V2,4,5,6,7,8 & 9 elevated heaters
V1,3 with own Cap grounded at PT bolt
V4 with own Cap grounded at PT bolt
V2 with own Cap grounded at PT bolt
All preamp grounds grounded at corner of circuit board support
Input and pots grounded at preamp grounds

Removed V1 - No buzzing when turning plexi vol
                 - buzzing when switched to YJM
                 - more buzzing when blackmore sw. engaged
Replaced V1 removed V3 - No buzzing when sw. to plexi
                                  - buzzing when switched to YJM
                                  - More buzzing when blackmore sw. engaged
Replaced V3 removed V4 - No buzzing

Any thoughts, Thanks

[stratele52]

Timbo I can't open your loyout/ shematics. So I can't see your amp , just the picture on your post. I don't know this mod or this building project  but for one thing I 'm pretty sure, you must use shielded wire to your switch. Shielded at one end only.

[tubenit]

Look at Hoffman's grounding scheme.  All preamp and LTPI are buss grounded.  Power tubes grounded differently. Heater wires someplace else.  I use Hoffman's grounding on every amp and feel like it works great. IF I am understanding how you've grounded things accurately,  I think you have multiple ground loops causing buzz.

Doug grounds the preamp filter caps on the buss wire.

http://www.el34world.com/charts/grounds.htm

Beautiful job on the layout board!  Very nice.

With respect, Tubenit

[Tone Junkie]

Timbo when I did mine i used one similer to hoffmans grounding scheme. i used valve wizards here
http://www.freewebs.com/valvewizard2/grounding.html
Exept i grounded the first filter caps and rectifiers at the first ground stud on my PT, right next to the incoming ground quite as a church mouse exept for standard 4 gain stage hiss. then I used a built in noise gate from the 5150 tweeked by Joey from sloclone forum, got rid of the hiss.
 works great if you would like I could put it up, I never go high gain without it now.
Thanks Bill

[TIMBO]

Thanks Bill that would be great. I have not had much time to investigate the problem. Thanks

[stratele52]

Timbo, your switch look to have 2 shield wire, but what about the two other wire ? They look not shielded. Maybe they need a shield ?

If the come / go to a signal path, those long wires are like an antenna. But I can't look a your schematic, I can't open the file.

What do you think about that ?


IMO new grounding sheme is not an option for now, unless you make a big mistake on the ground

[Boots Deville]

Tone Junkie Bill, I read what I think is the relevant thread over at the sloclone forum, which included Joey's textual description of the circuit but no drawing. I'd be interested to know what you ended up with as a parallel resistor to the diodes, and also what comes before and after.  A drawing would be nice if you have one, thanks!

[Ritchie200]

Hey TIMBO,

Is this a home-grown, brand new, straight off your drawing board design?  Did a quick Bing search on the name and could not find anything, so I assume that it is yours?  Looks really cool!  I want to hear some sound clips when you get this thing rockin'!

Jim

[Tone Junkie]

Boots Ive been away for the weekend just got back I will get a drawing up real soon i tried to pin Joey down. Im kind of a pain in the ass that way i want to understand it before I build it.
Then he said its only 4 or 5 pieces just do it so I did and it worked perfectly although it made the channel a little brighter but it was a high gain channel on the 3rd gain stage so brighter was good.
Thanks Bill

[Tone Junkie]

I placed it between my second and third gain stage . If I understood it better I could change the attinuation of the stage and make it not as bright although on this one it was perfect, after going through one more gain stage.
Bill

[Boots Deville]

Thanks for drawing that up Bill.  So he *did* mean to parallel those resistors (1M5 and 470K).  I was curious, if that's what he meant, why he didn't just say to use a single 360K resistor, so I thought he might have meant something different. 

Cool. That's a good one to keep in the bag of tricks - thanks again!

John

[Tone Junkie]

yea it would have been nice if he would have answered my last question it would have removed the confusion for all I know he might have meant something else this is just what I used and it worked nicely in two amps now.
Bill

[Tone Junkie]

Timbo hey how are comming on the buzz.
Bill

[TIMBO]

Hi guys,Sorry that i have not replied,my internet has been on the down and conputer problems have had me  ( i thought these things were to make our lives simple  )

I have looked at the grounding that i have and can't see that this is the issue, as there is no change when different grounding are used.
This took me back to the MSJ buzz that was in the relay circuit and 51 ohm resistor was added for the fix.
I am going to try the same trick with the dc circuit. Thanks

[Tone Junkie]

Yea that one sure helped me keep us posted my friend.
Thanks Bill

I hate computer problems. I can eventually fix any thing, even if its shootin it full of holes with my shotgun and going and buying a new one (LOL).

[TIMBO]

Hi guys, I've have been able to tame the buzz a little with the help of a 51r resistor on the - side of the DC circuit but there is still a lot of noise. I am going to try Bills noise gate but i will draw it up first because i am not sure of its location, as my build is a bit different.

This thing is LOUD and i can't turn it up much past 4 on the two gain pots, i'm thinking that having the parallel front end is giving way too much gain even though i have a 12AY7 plugged in V1.

Is there anyway of turning the gain down on this stage. Thanks

[tubenit]

i'm thinking that having the parallel front end is giving way too much gain even though i have a 12AY7 plugged in V1.

That would surprise me. Paralleled 12AX7 is about 30% more gain with almost no noise increase.

You can put a resistor across the gain pots to lower their value so instead of "4" you can turn it up to "7-8" giving the psychological perspective that it's more balanced. It essentially will be the same, sorta.

With respect, Tubenit

[stratele52]

Yes 12AX7 have more gain than a 12AY7 , but a noisy 12AY7 is worst than a good 12AX7.

 Better to use a GOOD 12AY7

[TIMBO]

Thanks guys, Ive got some different tubes so i'll try them. So if i put a 470k resistor across the pot it would give me something like a 500k pot?

Just for the records if anyone is interested, the power amp section as it is setup at the moment can be switched to one pair EL84 and one pair 6V6, to my ears (deaf in one,can't hear in the other  ) sound so similar that i will possibly remove the ELs unless anyone has a better idea.Thanks