86-Special ready with noise problem

[mat]

The build (183clean/EF86drive channel into 2xEL84) is ready (the footswitch is not) and everything works as supposed.

The sound is very sweet but there is a noise (does not increase much with the volume) that goes away with cap to ground-probe just after (not before) the 270k entering the PI. The noise goes away also after next component (C5) on schem: http://www.ampmaker.com/images/ak03kit/ak03sc2.jpg

So it seems the problem is around the PI I suppose ? I tried new fresh PI tube but no help from there.

Next I'll try to run the plate wires from different route to the PI tube.

I poked the probe to heaters and basicly all around the build but the two locations were the only spots that got rid of the noise complitely.

Any suggestions what to look around PI or elsewhere ?

Thanks,

[10thTx]

Mat,

Always great to see you on the forum with your innovative and toneful ideas!

I had something like that issue on an amp sometime back and came up with the "enhance cap" which is simply a cap across the plate resistor going into the PI. It has worked extremely well for me on every amp I have tried it on.

I think on the original amp I tried it on, there was some type of oscillation going on with the phase invertor tube? Well, the enhance cap resolved that for me and I could not tell any loss of highs. It seemed to simply smooth the highs out.

This seems to have worked for some other forum guys on EL34 world. I think it may be a reasonable approach when you have a high gain amp.

This may not work for you, but it's a very easy quick try with some insulated alligator clipped wires and caps from the 120p-330p range.

http://www.el34world.com/Forum/index.php?topic=12723.0

With respect, 10thtx

[mat]

Hi Jeff,

Tried the cap but no help. I'll try moving the wires later. I made my own layout (not so bright idea..) and there are maybe some basic lead dress problem in there because I cannot find a wiring/component mistake.

I was just wondering if there are wires between PI and the rest of the power amp that should not be close to each other. I'll keep studying/hunting to find the solution.

Soundclips as soon as I nail the noise source.

thanks,

[Tillydog]

I was just wondering if there are wires between PI and the rest of the power amp that should not be close to each other.

Nothing coming out should be anywhere near anything going in!

The grid wire into the PI is probably most vulnerable, and the OT primary wires are probably the biggest hazard, but everything in between is a suspect.

Is there NFB to the PI in this design?

Pictures & a schematic would help.

[mat]

Sorry, I don't have a complite schematic but the preamp is like on schem (to the EL84): https://tubeamparchive.com/download/file.php?id=17531

Power amp (from the C5) as in: http://www.ampmaker.com/images/ak03kit/ak03sc2.jpg

I'll check the wires You suggested.

Thanks,

[10thTx]

This isn't exactly like what Mat is using but probably has some similarities.

with respect, 10thtx

EDIT: deleted schematic and reposted a corrected version later in thread

[10thTx]

Mat's an incredible guitarist, IMO. I love his sound clips!

For those of you who have not heard him play ............ check this out.

http://www.soundclick.com/player/single ... i&newref=1

http://www.soundclick.com/player/single ... i&newref=1

With respect, 10thtx

[mat]

Is there NFB to the PI in this design?

No NFB on this one. I moved PI pin 1,2,6,7 wires to another location but no help. OT primaries comes thru chassis very near the output tubes.

The noise increases after all with the volume pot some.. I'll try to add some more shielded wires next.

I'll take a photo later (at work now..)

[mat]

I've tried to move every single wire on the chassis (and it shows ) - no help for the noise problem.

3 not so good iPhone pics: http://www.flickr.com/photos/fuzzface1966/

Anything obviously wrong ?

Will be away from the build for the weekend.

[Tillydog]

If I am reading it correctly: From the photos, it looks like the outputs from the PI run across the top of other preamp components, and almost parallel to the input to the PI, similarly the connections to the output tube grids. To me, that's not good

I've attached a brightened up version of one of your photos - what I'm talking about is the two pairs of yellow wires in the centre of the photo and the single one just above them.

Assuming I've interpreted it properly, and you can't / don't wish to move the coupling capacitors and grid leak resistors for the EL84s (or the PI itself!), I would try the following:

Run the wires from pins 1 & 6 of the PI south (down) towards the back(?) of the chassis then carefully steer east (right) towards your bridge rectifier, keeping well clear of the output tubes & OT connections, then north (up) to the tag board. Twist these two wires together and run them flat against the chassis.

Run the wires to the output tube grids back the same way, again with the pair twisted together and run as close to the chassis as you can.

It's going to be difficult to run the input to the PI clear of all harm. Maybe you can get away with running it back diagonally under the tag board, tight against the chassis, as directly as possible to the PI, but you want to steer it clear of both the output tubes and the preamp. It may be simpler to make this run in shielded cable.

Hopefully others will chip in with suggestions, too.

[mat]

I'm ready to move every wire/component to get rid of the noise.

Big thanks Tillydog for Your answer. I'll follow Your instructions precisely on next week when I get back to the build.

Will get back,

[mat]

Hi Tillydog,

I finally got time to make the changes You suggested and it helped some but the main noise still stayed.
I was thinking of the transformers being so near the other components so I did put a metal box over the power amp section and the noise dissappeared almost complitely ! I then tried with only a medium size nose pliers and moved it on top of the power switches and the 12V transformer always touching the back wall of the chassis and eaven the pliers helped to get rid of some of the problem ?!
I'll cut proper size metal cover for the chassis and see what happends. Reports follow..

Thanks again for Your help,

[Tillydog]

Did you do anything with shielding or re-routing the input to the PI?

(What I thought was a bridge rectifier must be your 12V transfomer - between the fuse holder and the power(?) switch?)

Good luck with the shield - are you covering the whole of the bottom of the chassis?

The only other thing "up in the air" that I can see are the connections to the speaker jacks - can these wires be run flat against the chassis?

[10thTx]

I did put a metal box over the power amp section and the noise dissappeared almost complitely ! I then tried with only a medium size nose pliers and moved it on top of the power switches and the 12V transformer always touching the back wall of the chassis and eaven the pliers helped to get rid of some of the problem ?!
I'll cut proper size metal cover for the chassis and see what happends. Reports follow..

Watching this with great interest!

With respect, 10thtx

[mat]

Did you do anything with shielding or re-routing the input to the PI?

Yes, I did put in a shielded cables for the pins 2 and 7 for the PI and rerouted them.

(What I thought was a bridge rectifier must be your 12V transfomer - between the fuse holder and the power(?) switch?)

Yes, the bridge is on the left side of the chassis.

Good luck with the shield - are you covering the whole of the bottom of the chassis?

I have to check what size aluminum sheets I'll found. Would like to cover all of it.

The only other thing "up in the air" that I can see are the connections to the speaker jacks - can these wires be run flat against the chassis?

That would be hard. The area is very tight. Would some kind of shield (copper tape) do any good around the speaker jack wires ?

Another question: I have two speaker jacks 4 and 8 ohms. How could I insure not to have an open circuit (no speaker connected) with those two jacks ? A 270 ohm resistor between + and - on both of them ?

Jeff, I'll record something asap when the shield is in its place

Now I have to start making the box for the chassis. I got all hardware and tolex ready