Express oscillation solved

[dariez75]

Hi guys, almost 5 years ago I built a TW Express using ampgarage express files. The amp sounded amazing but I heard a strange hiss or edge oscillation on the the top of saturated sound. So I decided to left behind... after lot of time I talked with a friend of mine electronic engineer and he suggested me to put 2 gridstopper a 33K resistor on v1A grid, a shielded cable from volume pot and v1B with a 68k resistor. Now the amp sounds ok, so I want ask you if this mod could be ok in order to preserve the real Express sound
Cheers
Dario


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[martin manning]

There have been reports of small grid stoppers on Ken Fischer TW's. You could try reducing yours a bit to stay closer to the Spirito di Trainwreck... When I built mine with a 22k on the input grid and an 820Ω on the second stage, and both of those leads are coax with the shields grounded at one end.

[dariez75]

Thanks Martin I change the resistors. I like “spirito di Trainwreck”![emoji12]


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[Littlewyan]

Post some pics of the insides. We'll try to help

[Mark]

I thought 10K was the grid stopper of choice on an Express. As far as I gather K.F. preferred no stopper, but that didn't work out in many cases so he used the 10K. The coax on the wiper is a good idea as it is a sensitive spot on most amps. There is a picture of a Liverpool built by Dave Funk where there is a 820 ohm off the grid of the second stage, the centre core wire of the coax is attached to free end of the resistor and the other end goes to the wiper of the volume pot. The shield of the coax isn't attached to earth, it's attached to the plate of the triode the 820 ohm resistor is attached to. I would say it isn't the safest or the best option but it does work.

EDIT: This is the link to the picture.

http://www.amparchives.com/folder/1854/

[RockinRocket]

I thought 10K was the grid stopper of choice on an Express. As far as I gather K.F. preferred no stopper, but that didn't work out in many cases so he used the 10K. The coax on the wiper is a good idea as it is a sensitive spot on most amps. There is a picture of a Liverpool built by Dave Funk where there is a 820 ohm off the grid of the second stage, the centre core wire of the coax is attached to free end of the resistor and the other end goes to the wiper of the volume pot. The shield of the coax isn't attached to earth, it's attached to the plate of the triode the 820 ohm resistor is attached to. I would say it isn't the safest or the best option but it does work.

Thank for pointing that out! Seen Dave Funks photo dozens of times and never noticed

[dariez75]

I thought 10K was the grid stopper of choice on an Express. As far as I gather K.F. preferred no stopper, but that didn't work out in many cases so he used the 10K. The coax on the wiper is a good idea as it is a sensitive spot on most amps. There is a picture of a Liverpool built by Dave Funk where there is a 820 ohm off the grid of the second stage, the centre core wire of the coax is attached to free end of the resistor and the other end goes to the wiper of the volume pot. The shield of the coax isn't attached to earth, it's attached to the plate of the triode the 820 ohm resistor is attached to. I would say it isn't the safest or the best option but it does work.

EDIT: This is the link to the picture.

http://www.amparchives.com/folder/1854/

Very interesting! It’s a kind a stuff used by Randall Smith in Mesa. I use this technique (shield on plate) when I make high gain mode for instance in jcm 800 to #SIR 36 conversion.



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[dariez75]

Post some pics of the insides. We'll try to help

IMG_0177.JPG

Here my TW Express previously the mod, with 10k grid stopper in first stage and without coax and gridstopper in second stage.


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[andresound]

I thought 10K was the grid stopper of choice on an Express. As far as I gather K.F. preferred no stopper, but that didn't work out in many cases so he used the 10K. The coax on the wiper is a good idea as it is a sensitive spot on most amps. There is a picture of a Liverpool built by Dave Funk where there is a 820 ohm off the grid of the second stage, the centre core wire of the coax is attached to free end of the resistor and the other end goes to the wiper of the volume pot. The shield of the coax isn't attached to earth, it's attached to the plate of the triode the 820 ohm resistor is attached to. I would say it isn't the safest or the best option but it does work.

EDIT: This is the link to the picture.

http://www.amparchives.com/folder/1854/

Mark, I noticed TWO resistors at the presence pot. What’s up with that?

[Mark]

Hi Andre

It's the blonde/blackface presence control. A 4.7K resistor with a 25K pot (or less I suppose) with the 0.1uF
cap with no DC on the pot.

[Littlewyan]

How bad is the oscillation? As in what control settings did it occur at? Were the bright switches on?

Can you have every control in the amp on 10 with no guitar plugged in? If not then a good thing to do is to move wires around to try and stop that.

Things that jump out at me are......
- The bright caps are floating and there is a lot of wire there. I glued mine to the switch itself and used a DPDT switch so I had just two wires going to the volume pot. You could just glue the caps to the switch and connect them together near the switch to give it less length if that makes sense. Even with bright caps off they are still connected and act as an antenna.

- Shorten the input wire

- The bass and mid control wires need to be flat against the chassis and maybe even pushed back underneath the controls.

- The treble wire could be shortened a bit. This one is quite sensitive. Maybe even try pushing it against the controls to see if it helps or makes it worse.

- Try shortening the anode wire for the 1st stage that goes to the treble cap and slope resistor

Obviously be weary of shortening wires and do it as a last resort if you have very little wire left. Just in case it doesn't actually help the issue. Been there done that

I also see that the ground connection for the 2.7K resistor is just sitting on the turret? Is there no space to slide it inside? If not then wrap it around the turret.

[strelok]

I'd also add that you might want to pull the lead of V1B's grid stopper in so that the resistor is as close to the pin as possible. You might also try flying its plate wire over the top of the socket so it goes straight to the pin instead of wrapping around the socket.

[dariez75]

How bad is the oscillation? As in what control settings did it occur at? Were the bright switches on?

Can you have every control in the amp on 10 with no guitar plugged in? If not then a good thing to do is to move wires around to try and stop that.

Things that jump out at me are......
- The bright caps are floating and there is a lot of wire there. I glued mine to the switch itself and used a DPDT switch so I had just two wires going to the volume pot. You could just glue the caps to the switch and connect them together near the switch to give it less length if that makes sense. Even with bright caps off they are still connected and act as an antenna.

- Shorten the input wire

- The bass and mid control wires need to be flat against the chassis and maybe even pushed back underneath the controls.

- The treble wire could be shortened a bit. This one is quite sensitive. Maybe even try pushing it against the controls to see if it helps or makes it worse.

- Try shortening the anode wire for the 1st stage that goes to the treble cap and slope resistor

Obviously be weary of shortening wires and do it as a last resort if you have very little wire left. Just in case it doesn't actually help the issue. Been there done that

I also see that the ground connection for the 2.7K resistor is just sitting on the turret? Is there no space to slide it inside? If not then wrap it around the turret.

Very interesting your observations Littlewyan, little by little I’ll put in place your advices. I’ve just finished to make experiments: 1) I’ve removed both gridstoppers and I’ve left in place only the 2 shielded coax, with this mod I can hear only a little clue of oscillation. 2) I did the mod as suggested by Martin and I’ve put respectively 10k V1a e 820r on v1b, with this no audible oscillation.



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[martin manning]

Good suggestion above to shorten up the bright cap leads. The shortest possible would be something like this:

[Mark]

Is there real estate on the front panel?