Marshall 18w clone tmb problem

[Skev]

hello everybody!
I ve just built a 18w clone tmb ( this amp has a volume, master volume, treble , medium, bass, one input)
All behave normaly except that...the guitar signal does not arrive to the preamp ( to the pin 2 grid of v1...)
I have checked all my grounds, check the wiring of the output transformer, checked my tubes ( which are brand new jj's), check the continuity of the input from the jack to the pin two/grid.

About the amp. I said it behave correctly, i mean: all the tubes lughts are ok, the plates voltage is ok everywhere ( for both el84 and 12ax7) we can hear the treble, volume and gain pots work. All my caps and resistors have correct values...

I m a bit lost!!

[xtian]

Should be a simple problem to tackle, you'd think! Turn the amp off. Plug in a guitar cable. Use your multimeter to measure resistance from the tip of the guitar cable to V1 pin 2. You should read about 68K. Measure resistance from guitar cable shield to chassis ground. Should be about 2 ohms.

If these check out, the problem is not what you stated, but what happens AFTER the guitar signal reaches V1.

Let us know!

[Skev]

That s exactky the values i just measure

[Skev]

I have to say that i have a 22uf instead of the 33uf on the layout.
No other difference between my amp and the layout

[ampgeek]

You mention that you can "hear the volume and tone knobs work".

Please describe what you are hearing and how "loud" you are hearing it.

That might help to dissect the amp into "works" and "doesn't work" sections and provide some focus on what to do next.

Also, measure the cathode voltages of all tubes and let us know what you get.

Good luck,
Dave O.

[Skev]

So when i turn the volume knob and gain knob, both react by a upper level in the speaker, just like when you turn the volume of an amp without guitar. The treble pot , when you turn it , make the "noise" or "hum" ( filtering section is stock and could be better) is trebly. No special details about bass and middle pots. I use a log pot instead of linear one for the middle? Does it make a change?

[Richie]

A couple things from looking at the pic.. you have 2- 470k resistors that come together in the PI. the ground should be at that connection point. I'm not seeing those, are you using a post MV?

Also, the tail of the PI, 47k resistor.. does not look to have a ground connection. Maybe check those. or just that its not clear in the pic.

[Skev]

At first, thanks for your help, guys!
Ritchie very good observation but the 47 k and 470ks are connected to the ground at the back of the board.

[Skev]

Here is anotherpucture showing the preamp parts

[Skev]

I also have connected the 1M resistor between the positive negative of the guitar jack input? Is is correct?

[ampgeek]

Unusual to not be using a tip-ring-sleeve jack.....but...no reason why that approach wouldn't work.

Unless, of course, you have accidently wired the sleeve of the jack to the control grid of the tube as opposed to the tip of the jack to the control grid of the tube.

With two white wires from the jack disappearing into a section of opaque overbraid that could have easily happened.

Can you confirm which one of the white wires from the jack is going to the tube socket and which one is going to the ground?

The fact that twiddling the knobs generates an amplified signal at the speaker implies that the signal chain after the knob(s) is intact and likely functioning properly.

Cheers,
Dave O.

[Skev]

Shame on me!!! You were right Ampgeek!! I must confess i finished my wiring by the jack and was so happy to switch on the beast that i made that error!!!
So it was my second amp "Diy" , and three mistakes : a ground forgotten on a preamp stage ( corrected by myself) , an underrated output transformer ( 15w...correctes by myself), and that reversed jack!!!


Anyway, a big thanks to you guys!!
I will have totally finished this amp ( the cab') on staturday and will send you a picture!
Thanks

[ampgeek]

Right on Skev!

How does it sound? I am in the process of getting familar with an 18W Liverpool-Xits-Tone King mash up and am really starting to dig the aggressive but smooth, overdriven EL84 sound and feel.

I threw a VVR at the PI and power tubes. That final touch is what made it perfect for the small, dive bar venues that I typically play.

Rock on,
Dave O.

[Skev]

Not tested yet! But that was the only problem!
I made the tmb ez81 version. I live the valve rectifying , sounds a bit more " organic".

[Skev]

Here is pictures of the amp , the cab will come tomorow.
It sounds pretty good !! Realyy a nice creamy crunch but subtlle not a big amount of gain, just exactly what i waited from that circuit!!