Troubleshooting my Express combo build: noise and hiss

[vincenz]

Hi,
Here's my Express build into a special chassis (the goal is to put this into a Fender Twin Combo cab).
Custom chassis and boards are from Ceriatone.
Trannies Heyboer, filter caps and signal caps are from RJ.

[img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/1th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/2th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/3th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/4th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/5th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/6th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/7th.jpg[/img]


On this video you could hear the problems:
-a light hum coming from the filter caps (I guess)
-big noise all along
-some instability remains on the NFB... tricky to manage

http://www.youtube.com/watch?v=bEfqvDkZmXQ
(2x12 cab with a AirBrake at maximum attenutation)

At this point I have already:
-readed again the Express Build Guide Version 2.0, and made some changes to match the layout and lead dress as close as possible.
-followed the steps explained in the troubleshooting section.
-added a 15k grid stopper: it helps a little (tried 68k, not better).
-changed the wire to the v1 grid for a shielded one, and get it over the board: helps a lot.
-changed the position of the NFB wire: helps a little. Tried to change the NFB res to 150k: no change.
-and plays with a chopstick on all wires: no other particular noise.

Thanks in advance for your help

[leadfootdriver]

Looks good tho!

One thing I see that differes on your amp is the lead runs to your preamp tubes. They're long, and un twisted. Ken's amp has the board and tube pins in virtually the same plane, and very close together. That's why I think he could get away without twisting the leads.

The phase inverter tube has it's leads close to the heater wiring, and power tube pins. I, in my limited experience would try installing longer leads on the PI tube, and twist them tightly together which would allow you to route them away from the heaters, and power tubes.

[fishy]

If you have not looked at any of the combo style Express builds already, I encourage you to do so. There are differences but the one linked below does illustrate good lead dress in a combo style amp. It is different to yours in that the signal wires are close to the chassis and more in line with most TW style builds.

https://tubeamparchive.com/viewtopic.ph ... ress+combo

Suspending the wire on this amp is a good recipe for microphonics...been there.

I would look at the NFB wire again and run that close to the chassis. You can try a different tap on the impedance switch instead of switching the resistor which is equivalent to changing the feedback ratio.

You also might have a tube issue which should be checked out.

Do you have a voltage chart?

[vincenz]

One thing I see that differes on your amp is the lead runs to your preamp tubes. They're long, and un twisted. Ken's amp has the board and tube pins in virtually the same plane, and very close together. That's why I think he could get away without twisting the leads.
The phase inverter tube has it's leads close to the heater wiring, and power tube pins. I, in my limited experience would try installing longer leads on the PI tube, and twist them tightly together which would allow you to route them away from the heaters, and power tubes.

Suspending the wire on this amp is a good recipe for microphonics...been there.

and what about using shielded wires (like the v1 grid ones) for v3 connections?

If you have not looked at any of the combo style Express builds already, I encourage you to do so. There are differences but the one linked below does illustrate good lead dress in a combo style amp. It is different to yours in that the signal wires are close to the chassis and more in line with most TW style builds.
https://tubeamparchive.com/viewtopic.ph ... ress+combo

Thanks for the links, I haven't seen it before.
I note that the power stage lead dress is not so different than mine.
Actually the preamp signal wires are shorter and closer to the chassis. Note that my signal wires (except they are longer) follows almost the same layout, and also touch the side of the chassis.

I would look at the NFB wire again and run that close to the chassis. You can try a different tap on the impedance switch instead of switching the resistor which is equivalent to changing the feedback ratio.
You also might have a tube issue which should be checked out.
Do you have a voltage chart?

Ok I would try NFB on different tap.
There is no tube issue (tried other complete set, no change)
I will do the voltage chart tonight.


Thanks for these answers

[JamesHealey]

I would try shielding some of the grid wires to the pre-amp valves from the main board.

You're really going to struggle to get that layout stable whilst using unshielded wire.

Yes you're going to loose a tad of high, probably an inaudible amount due to capacitance in the shielded cable.

Also keep that shielded cable from the volume pot away from the input shielded cable, get it running under the main board and across under the tone stack.

Twist the phase splitter output grid wires to the 1k5 grid stoppers on the power tubes, this will frequency cancel any odd oscillations in this later part of the circuit.


The NFB wire from the impedance selector to the presence control needs to be down onto the deck of the chassis and play with it a bit, find the point where this will become stable, look very closely at how this is done on Francesca.

[JamesHealey]

Since you've got the PI tube off to the right slightly.

You're going to struggle with getting that stable.

my suggestions are to twist the anode and grid wires to the PI tube and get them down to the deck of the amp chassis and away from the power tubes.

Almost at a 90 degree angle across the floor of the chassis away from the power tubes.

this will help with stability.

the rest of the amp seems reasonably well laid out

[vincenz]

thanks James for your advices.
Obviously this custom chassis layout is not good at all.

Maybe I could relocate the board and preamp sockets. Like this, the lead dress between the board and the preamp tubes will be closer to Francesca's layout, but the wires to the pots would be longer...
What do you think about it?

[img:712:400]http://www.clipperton.ch/vincenzo/pub/g ... /modif.jpg[/img]

[M Fowler]

It's a high gain circuit your going to have hiss and more when you crank the volume knob. From the video I can't tell anything other then normal hiss.

I built an express combo a few years ago. I did end up shortening my tube wiring after this picture was taken.

Mark

[vincenz]

Very interesting Mark !
Your picture demonstrate that it is possible to have long leads to preamp tubes... Unfortunately, I do not have the same magical recipe

The hiss you hear on the video is, in fact, is very loud, even with Airbrake at max attenuation ! Not normal at all

[M Fowler]

You could twist together the wires that go to each separate triode to help cancel out the noise. THe PI can all be twisted together.

Some times shielded leads add to problems!

The NFB circuit is also a noisey area. So much gain in an Express change to 5751 V1 and 12AT7 PI.

Mark

[vincenz]

Just one other question:
There is a steel bar all along the chassis (to bolt it on the cab).
Visible on these pics:

[img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/5th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/6th.jpg[/img]

Would it be a problem regarding hum and hiss, I think in particular the part on the back of the filter caps ???

[rooster]

I think you could improve things if cut that ground buss off at the bass pot. Then establish a separate ground path for the Presence pot - right in front of the Presence pot, on the chassis.

Next I would look at your input jack. I don't know what it is about these things but typically they tend to be RF attractive. I dig the Euro thing, but try the Switchcraft version instead.

The build looks good otherwise, BTW, although I have my concerns about putting an Express in a combo because of V1. If you use the typical 12AX7 here I, well, maybe you have a better bag of tubes than I do? At any rate, I dig that you have pursued this. Don't stop now.

[geetarpicker]

Make sure the pots have a good connection to the front panel. Try pulling them a tad and cleaning the area with rubbing alcohol. Also it seems you have some black lock washers on the pots. Make sure these aren't causing grounding issues or resistance there from the coating on them. I've had random noise at times with both my original and clones and it was always either a loose input jack or loose tone stack pot. On one of my clones I had a bad solder joint on one of my pot lugs that looked perfect, but a chop stick test found the flaw. From the random nature of the noise in your video I would suspect something along these lines, or perhaps a bad solder connection as opposed to the layout changes being the cause. I work at it a bit more before taking drastic measures with a layout change.

[guitarsnguns04]

I was thinking the same as Glen..take a jumper with alligator clips...clip one end to the chassis and clip the other end on your ground rail on the back of your pots..see if it helps..its a quick way to check.You can also check all your grounds quickly to make sure they are good...good luck

[vincenz]

Thanks Glen and other people for your interventions. I really appreciate your help.
Well, I have made a "couple" of mods/improvements:

-I have all dismantled and:
-drill new locations for preamp tubes
-new location for the preamp board
-new location for the OT (it was too close to the PT)
-rewired the power section, with shorter and twisted leads
-removed the long steel bar within the chassis
-change the plastic jack input for a switch craft.
-properly wired (new location as well ) the heaters
-properly wired the preamp and pots section. With preamp sockets closer to the board, I am now able to best match the real Francesca layout.

[img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/10th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/11th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/12th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/13th.jpg[/img] [img:120:68]http://www.clipperton.ch/vincenzo/pub/guitar/twinwreck/build/14th.jpg[/img]

Now the amp seems to be stable, but it remains a big amount of cccchhhhhhhh.

This chhhh noise appears on operation mode (not on stand by) and it increase a lot with the volume knob (presence knob as well) and the bright switch.
When I remove the V1: no noise, dead quiet.
I have tried different tubes, which are quiet on my other Express clone.
The less noisy V1 seems to be a NOT JAN GE 12AY7...