18 watt mojo kit tremolo woes

[sluckey]

The schematic shows the 2nd footswitch to switch a 1M across the speed control.  If this is typo, then forget it, but if you wired the switch on the intensity control, it'd be something to fix (maybe not the culprit).

The 1M is in series with the speed pot, not across it. The FS is across the 1M. When the switch is open the 1M is added to the speed pot resulting in a slower speed range. Closing the switch shorts out the 1M resulting in a faster speed range.

One end of the 1M is physically connected to a GROUNDED lug on the intensity pot. This connection is purely a convenience and is not part of the intensity pot circuit.

[phsyconoodler]

Ok.Changed all the trem caps.Nothing.
Here's what I've done so far:
1) tried several tubes
2) replaced the crappy chinese tube socket with a real belton
3) replaced the trem timing caps three times.
4) verified under board wiring three times
5) checked all circuit paths with an ohmmeter
6) tried a 1meg resistor instead of the 2.2meg
7) moved the .047yf cap from the board to V3,pin 6 several times
8) scoped the output and grid signals(not sure about this step)
9) moved grounds,verified grounds several times

 Still no trem at all. Exactly the same as square one.This circuit is really simple.What the heck am I doing wrong here???

[Ed_Chambley]

Try bypassing the footswitch jack.  Hardwire the trem so it is on to see if you have a problem here.  With as much as you have done, I suspect a bad part as I believe you know how to wire it up.

[phsyconoodler]

Did that early on.

[phsyconoodler]

Gonna try a red led and see it working before I go any further.

[sluckey]

How much resistance from V3-3 to chassis. How much resistance from V3-8 to chassis. I know they're tied together. Just want to be sure.

I'd also like to see some hi rez pics that are not chopped up. Something that clearly shows V3 tube socket, circuit board, int and speed pots, and wiring. Probably need several pics from different angles to see everything. I sure wish you had the jumpers on top of the board.

Try bypassing the footswitch jack.  Hardwire the trem so it is on to see if you have a problem here.

This trem will be on when the footswitch is not plugged in.

[phsyconoodler]

V-3 pin 8= 1.2k resistance.Same for pin 8(tube in socket)

I pulled the board earlier and put the power jumpers on top.And also removed almost all underboard wiring that I originally did and put it on top,namely the pot side and the PI wires as well.
  Pictures coming. starting to look pretty sloppy from all the fun I've been having!

[terminalgs]

The fact that the audible motorboat changes frequency with changes in the speed knob indicates that the oscillator is working (i.e. no need to change the .02, .01, .01 or the 2.2M, 1M, etc resistors). If you want, put an LED across one of the 1M's to see a visual indicator of the speed rate, or put your scope lead on the grid (make sure the scope and circuit have a common ground), and dial in the timing of the scope so you see a sine waveform sitting on your scope.   when you change the speed control, the waveform should get "shorter" or "longer".

the cathode to ground should range from 820 to 1500 ohms as you turn the intensity control from end to end.  This will change the Mu Gain of the signal gain stage.

for a moment, I'd focus on the other half on the tube: the gain stage.   If you plug guitar into the tremolo channel, and turn Tremolo off at the footswitch, do you have normal volume through the amp? (as compared to the normal channel)?

have you verified that the 50uf/50v cathode by-pass cap is connected to the shared cathodes on pins 3&8.

[phsyconoodler]

Both channels work normally with either the intensity turned down or the footswitch engaged.LOTS of volume.
  I will try the scope again and see what happens there.And also measure the resistance to ground on the cathodes as I turn the intensity control and report back.
  Still makes no sense why this bugger isn't working.

[phsyconoodler]

The resistance from cathode to ground goes from 820 ohms to 2.5k when the intensity control is turned up or down.
  I'm going to get an LED and see this damn thing oscillating.Back in a bit,the saga continues........
Thanks for your help dude!!!

[terminalgs]

The resistance from cathode to ground goes from 820 ohms to 2.5k when the intensity control is turned up or down.
  I'm going to get an LED and see this damn thing oscillating.Back in a bit,the saga continues........
Thanks for your help dude!!!

besides an LED or a scope reading, you can usually see a phase-shift oscillator (like this one) oscillate with a voltmeter set to DC or AC on the plate.   it'll fluctuate as it oscillates, even with a DMM it is slow enough of an oscillation to make the voltage reading move.   when you step on the footswitch, and turn the oscillator OFF, the DC voltage should stabilize at the plate, and the AC  between any of the caps will go to 0.

ah yes, your Rk ohm readings look good,, 820-2.5K::  when intensity is "off",, Rk=820, when "on", Rk=820+(2200||5000)=2348

I'm at a loss as to what to suggest at this point.


Sluckey,  how does this tremolo work?: the 2.2M and the intensity control resistance form a voltage divider and attenuate  the  oscillating AC single  to some level that goes through the 820R and appears on the cathode.  (this is were I start to fall off) that oscillating signal changes the bias of the gain triode? because the 50uf holds a bit of the AC voltage above the natural self-biased DC of the triodes and varies the DC of Rk, and thus the bias of the gain stage?

One thing that confuses me,, the oscillating signal at the grid of the oscillator triode will naturally appear on the cathode of that triode at some smaller voltage, even without the 2.2M.  is the voltage of this naturally appearing signal so small that they added the 2.2M to supply a larger version of oscillator signal on the cathode?

[phsyconoodler]

Ok I got an LED and cannot get it to do anything.It lights up across the cathode resistor but not anywhere else.I will try my ohmmeter as it has a graph and I should see something there if indeed this is oscillating.As I said before it motorboats ONLY when I put the .047uf cap in it's stock location.On the tube socket there are no noises.

[phsyconoodler]

Ok using my ohmmeter I get from 2vdc on the cathode and up 5v as I turn the intensity control up.Normal?
 Then nothing either AC or DC on anywhere on the timing caps.NaDDA,zilch,zero.

And the motorboating increases speed with the INTENSITY pot,not the speed pot.
  I'm one minute away from tearing it all out and building it the original 18 watt way.

[Willabe]

As I said before it motorboats ONLY when I put the .047uf cap in it's stock location.     motorboating increases speed with the INTENSITY pot

Maybe try putting the cap where it motor boats with the LED wired up and see if the LED lights up then when turning up the intensity pot?


            Brad    

[Willabe]

Then nothing either AC or DC on anywhere on the timing caps.NaDDA,zilch,zero.

There should be an ACV bouncing around on the timing caps if the LFO is oscillating.



           Brad     

[Willabe]

Did you try this in the 1st post from Sluckey's thread on increasing trem strength? You use an LED instead of the cathode R.

http://www.el34world.com/Forum/index.php?topic=15087.msg145033#msg145033

Also did you try changing the LFO plate R to a higher value?

Tubeswell posted this for me on my Warblers LFO thumping in reply #433. It's on reducing the plate R for less gain your LFO only has a 100K plate R maybe a larger plate R will kick it in to gear?

http://www.el34world.com/Forum/index.php?topic=10681.msg150571#msg150571


             Brad    

[phsyconoodler]

Yeah I saw that.I have the LEDS so why not? Nothing else is working here that's for sure.I just removed the board AGAIN,cleaned up anything under the board that looked like it may be an issue,removed all the trem circuit completely and reinstalled it all and still the same crap.

Put in an led. No change at all. Painful.

[Willabe]

Try kicking the LFO plate R up to 220K to 270K, or more.

I have a red LED for the cathode R and a 470K plate R on my Warbler LFO, turn it up and it thumps like crazy because of too much gain.


            Brad    

[phsyconoodler]

It's at 220k now.  Now it seems like the tone control isn't working on the trem channel.No bypass cap the culprit here?

The LED goes dimmer when the pot gets turned up.

[Willabe]

Try both the LED and a 470K plate R. If those 2 don't kick start it then?     

(I'm sure you know but, LED cathode (band) goes to ground.)


       Brad     

[Willabe]

No bypass cap the culprit here? The LED goes dimmer when the pot gets turned up.

I don't know about the bypass cap.

Which pot?

Where's the LED in place of the KR?

Some things going on with the higher plate R if the LED is changing brightness. Try a bigger plate R.



               Brad      

[phsyconoodler]

 Ok,I'm game for anything at this point. You mean make the 100k resistor a 220k or 470k right? The circuit has a 220k between the plates of V3 and is FED by a 100k from the power supply

[Willabe]

You mean make the 100k resistor a 220k or 470k right? The circuit has a 220k between the plates of V3 and is FED by a 100k from the power supply

Yes. The LFO's plate is fed from 100K, then it's taped off and goes to the 220K that feds the driver.


            Brad     

[Willabe]

Sorry, I have a chicken on the weber gas grill outside.   

Increasing the LFO plate R might starve the driver but for right now we don't care. You just want to see if you can kick start that LFO and get it to oscillate.

I'd hit it as hard as you can with the LED in the cathode and the 470K for the plate. Or even just the 470K might be enough?

I might even disconnect the trem driver from the LFO so I know that the driver is not messing with the LFO.


           Brad      

[phsyconoodler]

Ok on it!

[phsyconoodler]

Willlabe!!!!!!!!!!!! I love you man!!!!! But not like that!!

 It oscillates!!!! It has deep,bleatiful tremoo now. But......it has too much intensity even with the control off and the second footswitch engaged the trem goes away.So...I think I'm going to go back to the resistor/cap and see what happens.

  Wow! After all this shit willabe is my favorite dude of all time,well at least for this anyway!
A bit of tweaking and this puppy is done!!

 Thank you so much for following this thread and helping me out. I am at a loss for words right now and I think there is a tear choking it's way out. Well not really,but I am very grateful to you guys!!
  You are on my Christmas list!!

[Willabe]

It oscillates!!!! It has deep,bleatiful tremoo now. But......it has too much intensity even with the control off and the second footswitch engaged the trem goes away.So...I think I'm going to go back to the resistor/cap and see what happens.

Alright!!!!

Not enough gain for it to oscillate. Thank Sluckey and Tubeswell, I learned it from them.    

Yeah, now that it oscillates just tweak it for the amount you need so it doesn't thump.

I've learned a lot from you so just glad to help.

The chicken turned out great too!   


            Brad      

[phsyconoodler]

Well....not quite over yet...It doesn't oscillate with the footswitch disconnected.But it works beautifully with it plugged in the jack.Turns it on and off no issue at all. But it's too slow so the one .02 cap is coming out and a .01 is going in.
  Then I think the .047uf cap has to go back on the tube socket.
 Overall this has been a royal pain in the butt. It should not be this difficult to get a kit amp working properly.
  Mojotone is getting some feedback on this!

[Willabe]

It doesn't oscillate with the footswitch disconnected.But it works beautifully with it plugged in the jack.Turns it on and off no issue at all.

It's down hill now, you'll figure that out.

From Gerald Webers "Tube Amp Talk for Guitarst and Tech" page 78,

"This tube circuit is actually a feedback circuit where the output is fed back into the input. When sufficient energy is fed back to more than compensate for the loss in the grid circuit, the tube will oscillate."

So if he's correct, an oscillator needs enough gain to get it to oscillate.


           Brad    

[plexi50]

Wow! I have been working on one of these for 3 days now. Oscillation city. A mess. I have the tremelo working after shortening a bunch of wire leads that were too long. All squealling issues are gone. But now i have a hum in the amps preamp signal that wasnt there before i got the trem working. The mere notice on the MJ layout of choosing where you may locate the .047 cap either on the tube or on the board for oscillation issues says a lot about this amp not being well designed.  The amp just left 30 minutes ago. I recommended just gut the dam preamp and build a November plexi circuit. The intensity as well on this amp is so deep that it overtakes the volume and guitar signal. Im glad it's gone. Wish i had seen this thread a while back. I may have known what to expect before it got here/

One more thing: On the one i have the face plate has the intensity and speed pots in opposite positions from the layout. This is the British 18 watt MJ amp kit

[Willabe]

Here's my layout from my Warbler, it has an LED for the LFO cathode. I put the 12AX7 right next to the controls to keep the wiring as short as possible and to isolate the circuit from the rest of the amps circuits.

Where you see a small "t" is where I used turrets to raise parts/wires (.1 cap) up in the air above other parts/wires and for the 2 small radial filter caps, the rest are eyelets.

The "A" across the 10K R is for the SR coil, you'd have to change that.

No noise, no ticking, no problems. The 470K LFO plate R (pin 6) should be smaller so it doesn't thump from too much gain.


                 Brad        

[phsyconoodler]

Ok,just so people know what I had to do to get this amp's tremolo working properly:

1) increase the plate resistance to the LFO from 100k to 330k
2) definitely move the .047uf cap off the board and onto the socket
3) DIDN'T need to remove underboard wiring but it's done above board now.
4) reduce 1meg resistor for dual pedal to 470k(stopped oscillating with 1 meg)
5) reduced .02uf timing cap(2-parralleled .01's) to .01uf(way too slow)
6) change grounding scheme to more conventional like original 18 watt layouts

 Overall the amp sounds great,the transformers give correct voltages and the overdrive is great.
The experience was a learning curve for me but really? It is a poorly thought out layout and I would suggest going the original layout for the trem version corrected and improved by Richie at 18watt.com. I've built several of those in the past with zero issues.
  Thanks again to Willabe and the other's who make me think.Sluckey you are awesome too.Your pupils share their knowledge freely and that's a big part of what makes this forum so awesome!!!

Problems with Mojotone kit parts:
1) tube sockets are NOT Belton,they are obviously cheap chinese replica's-they suck bigtime!
2) the layout is wonky to say the least.Why is the big question here.
 
The rest of the components are great,way better than Weber's kits,however Weber's layout works better.
The diagrams are excellent but what's a diagram if it doesn't work easily?

[TIMBO]

Good work boys 

[Tom_Hull]

Good work boys 

yes that is great.
and
great team spirit .congratulations


tom

[plexi50]

Congrats phsyconoodler. At one point i had the tremelo working with the ground wire disconnected from the intensity pots wiper. In one instance if i waved my hand over the ground wire the intensity would increase. I thought i had some black magic going on for a minute. 
Just a weird experience all together