Help with strange distortion MP3 clip

[paddy]

Hi guys,

I have just finished building a Marsh Amps 5E3 for a friend.It sounds great
but has a strange noise/distortion when playing the bottom E and A string together.Both strings played by themselves sound fine but open E and A
fretted at various positions have this buzz/warble riding along with the notes.
I suspect this may be intermodulation distortion but I'm not sure.
I have tried the following fixes but all to no avail.

1. Changed all tubes(haven't tried NOS 12AY7,just new Sovteks)
2. Reduced 1st stage coupling caps to .01uF
3. Changed O/P grid coupling caps to .01uF
4. Replaced PSU filter caps
5. Increased " " " to 100uF plates,40uF screens
6. Tried several grounding schemes including the one recommended by
Marsh Amps and Hoffman but haven't tried the brass plate.
7. Isolated speaker jacks
8. De-coupled all preamp stages with 22K and 22uF
9. Increased O/P grid stopper resistors to 47k
10. Decreased O/P grid leak resistors to 100K
11. Tried a different speaker cab
12. Fitted 1K screen resistors
13. Separated mains ground from circuit ground

The 1st kit I built was a Mission Amps and I don't recall this noise.
All voltages are very close to those on the Mission schematic.

I have attached a clip in the hope that someone can tell me what I'm
hearing(is this perhaps normal?) The clip is a bit muddy as I have the guitar tone pot all the way down and neck p/u selected to make it easier to
capture the recording.

Sorry for the long post.

Any help/suggestions will be greatly appreciated.

Best Regards,
Paddy

[paddy]

I just noticed that I couldn't play the clip by simply clicking on it.
I can,however,play it by right clicking and using "save as",sending
it to my desktop and opening it from there.

Cheers
Paddy

[Nemesys]

Mine did something that sounded quite like that when I first built it, it turned out that I had forgotten to tighten one of the nuts when I mounted the speaker (I finger tightened them, then went around again with a nut driver and tightened them properly, and I missed one on the second pass). The speaker wasn't tight against the baffle board, and the basket was rattling against it. I seem to remember that I had to tighten a couple of the screws that attach the baffle board to the rest of the cabinet, as well.

Something to check, anyway. Hope you get it figured out.

[MWJB]

Any pics of the build?

[paddy]

Hi Guys,

thanks for the replies.One of my first moves was to try a different speaker but
it was not the problem.My daughter is away on holidays with our camera,so I
can't post any pics just yet,unfortunately.If I haven't sorted out the problem
by the time she returns,I will post some pics.
I built the "stethoscope amp" as described on Hoffmans site,which is basically
a signal tracer.I probed around the circuit and found the first sign of this ugly
distortion at the output of the phase inverter.I then changed all resistors
around the P.I. for metal film types but the problem is still there.
I then disconected the grid coupling caps and probed the P.I.output and
found that it's output had no signs of the bad distortion.it's only present
when the P.I. is connected to the output tubes(could it be blocking distortion)
I am about ready to completely strip the amp and start again but I'd
rather not if someone here can give me any ideas.

Any ideas will be greatly appreciated and considered no matter how
crazy they may seem,I'm about ready to try anything.

Cheers
Paddy

[paddy]

Hi Guys,

Well I'm still going crazy trying to sort this problem out.Here are the pics you
asked for.Sorry the build is so messy but I have just about had every
component out and put back in.The extra resistors about the place are
droppers for the heaters and the lamp also you may see the diode mounted
to the chassis(used for reducing B+)
The coupling cap from pin 1 to pin 7 of the AX7 is mounted on the socket
to eliminate the long underboard wire(a tip from Gerald Webers book)
I have run the amp on a variac without all the added resistors and the
diode but it makes no difference to the problem distortion.

I noticed the 1 Meg attachment limit so I didn't post all the pics.
I can post more later if required.

Hoping someone can help,even if you can tell me what it is that I'm
hearing,it may steer me in the right direction

Cheers
Paddy

[MWJB]

Grid wire to V4 is long & a bit wildly routed, why not follow Gerald's advice on that subject too?

Why did you need to drop heater & b+ voltages, what do you have now & what did you have before?

[paddy]

Hi MWJB,

Thanks for the quick reply.
I did try Geralds method of turning the V4 coupling cap 90 degrees anti-
clockwise and shotening the wire but it had no effect,so I put it back to
standard.
I am in Australia(240 Volts) and the transformer supplied seems wound for 220
Volts.I can't remember the exact heater voltages but they were all high.
Nearly 7 Volts on the 6V6's and 12Ax7's and about 5.7 Volts on the rectifier.
With my resistors they are now bang on at 6.3V and 5V.
The B+ was around 400V and is now 360V
V1 Plates 123V and 126V
V2 Plates 163V and 191V(44V on PI Cathode)

At one point I even tried shielded cable for the O/P grids--still the same.
The one thing that keeps bugging me is that the noise is on the o/p of
the PI when the coupling caps are connected but not when they are
unsoldered--what does this mean?

Cheers
Paddy

[MWJB]

If your heaters are under 6.9VAC that's fine, you don't need the 6.3VAC dropping resistors. It's normal for amps to run somewhat over 6.3VAC.

What's the rating on your B+ dropping zener diode?

How's the integrity of the ground connection of the 220K 6V6 grid load resistors.

Preamp tube wiring - you really want the plate wires low to the chassis floor, the grid wires kinked over the top & away from the plate wires.

"I did try Geralds method of turning the V4 coupling cap 90 degrees anti-
clockwise and shotening the wire but it had no effect, so I put it back to
standard." Well, you still have the problem, in your position I would be incorporating and including any & all suggestions & eliminating possible causes one by one until solved...I wouldn't be so quick to put things "back to standard" (lets not forget, irrespective of wiring style, your symptoms are definitely not "standard") until the problem was eliminated.

I'd also try the stock 6.3VAC CT with the 100ohm resistors, have another look at DC biasing the heaters only if noisy.

Describe your grounding scheme.

[paddy]

Hi MWJB,

Thanks,once again,for the reply.

Well I think you are on the right track with the lead dress.I dressed the plates
close to the chassis and got the grid leads up and away from them.This has
reduced the problem noise by about 50%
I would like to reduce the noise further and will spend a day or two playing
with the lead dress.

My B+ dropping zener is a BZV15-47 rated at 15 Watts and is dropping 37.5
Volts( I don't know why it isn't dropping the full 47 Volts but I used one in my
last 5E3 and it did the same)

The ground for the grid load resistors is fine.
I have checked all grounds with a low ohms meter and I have nothing over
.01 ohms.

I will use standard 100 Ohm resistors for the 6.3V CT as you suggest.

As for my grounding scheme it goes like this:

Mains/Line ground going to one mounting bolt on the P.T by itself.
Bus wire soldered to the back of the pots and to the i/p jacks.
All preamp grounds tied to the bus wire using shortest route
220K o/p grid resistors tied to preamp filter and screen filter then to bus.
One single wire from bus to bolt on PT
B+ filter tied to o/p cathode resistor and wired to the same PT mounting
bolt as the preamp
The dropping zener has its own wire running to the same pt mounting
bolt as the preamp/B+ ground.

Thanks for the help,it is much appreciated.
If I have any further success I will post.

Cheers
Paddy