Marshall 18w clone tmb problem

[Skev]

Shinning tubes!

[johnnyreece]

A thing of beauty.

[Skev]

Here is a picture of the amp in its cab of massive niongon ( mahogany family)

[Skev]

Back of the amp...

[M Fowler]

The amp looks great all dressed up.

Mark

[Skev]

This circuit is so creamy ! A good and true Marshall crunchy sound!

[Skev]

Hello everybody!
Need your help, please.
I built a 18w clone last weeks and all work fine ( thanks for ampgarage members help!!) .
There is a last problem: when i turn gain/volume pot full throttle and that i try to add volume with master volume, the amp make a big hiss noise at about "2" ( for "10" printed on the face plate...)...
Here is what i ve already tested: swap primary wires of the output transformer, shielded guitar input, isolate guitar and speaker from the chassis with plastic type jacks, checked (and corrected sometimes) my "mv v tmb" pots , finally checked all my connections...
Nothing works ...

[Skev]

Here is the shematic i use:
http://www.18watt.com/files/18wattSuperliteTMB.gif

[Phil_S]

Hiss could be tube related. If you swap V1 for another tube without any change, then it is probably circuit related. If the tube swap fixes it, then, you have your solution.

If a tube swap does nothing for the problem, I'd probably bypass the tone stack and the MV (2nd volume) pot. Unhook the 470K from the wiper of the first volume pot and connect it to the 10n cap after the 2nd volume pot. See if the hiss changes. Depending on what you hear, it may point you to the input section or the tone stack-m/v section.

Finally, I might suspect the plate load resistors. Change them to 1W rated 100K. That might fix it.

I can't give you any assurance that any of this will move you in the right direction, but it's an honest try. Good luck.

[Skev]

Big thanks for answer.
-Ok so, i try to change all the preamp tube with other 12ax7 and then with 12ay7, no changes
-i already tried the 470k on the master volume after the 0.010uf cap, more hiss , i gave up this track.
- about the load plate 100k resitors ( on v1?) you mean change the values ? Lower them?

You know, i thought about this 820r cathode resistor on v1, shouldn t it be 1.5k instead ? I feel ( maybe i m wrong) the big amount of gain is the problem? What do you think about that?

[Phil_S]

- about the load plate 100k resitors ( on v1?) you mean change the values ? Lower them?

Replace typical 1/2W 100K with 1W 100K.

You know, i thought about this 820r cathode resistor on v1, shouldn t it be 1.5k instead ? I feel ( maybe i m wrong) the big amount of gain is the problem? What do you think about that?

On other 18W types 820 is typical on both cathodes. I'm not sure why this one has 1.5K. The names on the schematic are people who know what they are doing. I would not question their judgement, but you might want to change it to suit your taste.

[Skev]

Ok . I wasn t sure of what i had understood so , in fact those 100k are already in 1w actually.
Yes , i was suprised of those strange 820-1,5k cathode wiring?!

i know it s a minor problem...it drives me
Crazy !lol

[Skev]

After googling a big part of the evening lol, my problem really looks like a lead dress adjmustment.
I have been really careful about ironing but, i must confess that , for me, a wire is a simple conductor so it must have a good quality but i didn t figure how much lead dressing is important.
Any advice about that? Is there a particular place i have to search at first?

[Skev]

I have another possibility: the link between master volume and pin 2 of v2 , it should probably be shielded?
A guy had strictly same problem on his tmb and after hadding grid stopper everywhere he could without real effect he canceled hiss by shielding this pin.
It sounds totally realistic to me?

[rock_mumbles]

All of the control leads can be sensitive ...

One Superlite I built had a lot of instability issues until I changed out one lead and moved others around a bit ...

The 820 ohm cathode on the second preamp triode is to try to make up for some of the gain lost to the plate driven tone stack.

The first triode was changed in a revision to the Superlite done about three years ago, the cathode resistor was changed to a 2.7k with a 0.68uf bypass cap like a Plexi bright.

The circuit was also changed around the gain control ...

Ignore the tone control mods ... here's the revised schematic
http://rh-tech.org/public/18-Watt/Super ... tic_V4.pdf
It was developed on a pcb, but it has been built on the pcb and with turret/eyelet boards