Plexi project with issues... Please help!

[slickdickrick]

Greetings all,

I'm a long-time lurking and tube amp enthusiast. I've got myself in a bit of a pickle here with a Ceriatone plexi project. I didn't order a kit and have made it mostly from parts I already had from building a few dozen fender tweed style amps. This is my second marshall style circuit other than a 59' bassman i built several years ago.  So i'm in a jam here and there is something super obvious that i'm totally missing and i'm hoping to get some insight from someone here.

Here's the issue:
After doing my preliminary testing and setting the bias, i plugged in and was ready to rock! Unfortunately I am getting no signal from any of the inputs! If i plug into channel 2 and jumper with channel 1 i get a very faint signal. Here is where it gets interesting, if i jostle the input jack on the jumper on channel one i get a volume swell and a slow fade out of the volume back to nothing. I also measured the voltage at pin one of V1 and see that while i move the jumper in and out it goes from 20VDC to just north of 100VDC when the volume kicks in and then quickly fades back out and the voltage drops. I hope this makes sense to someone.

I've obviously got something wrong somewhere. Any help or support will be greatly appreciated. Please see related links and picture below for reference:

I'm using Hammond transformers as follows:
Choke = 194C
Output = 1750U
Power = 290HX

Link to layout:
http://www.ceriatone.com/ceriatone/wp-content/uploads/2016/08/Ceriatone-67-Plexi-Super-L.jpg

Thanks in advance for any support!

 

[nashkato]

i´d try and  test the circuit with a frequency generator directly at the input of V1 , but with the Jacks not connected .
Maybe put the 68k/1M network in place .
(i always found that input-switching-jack wiring very confusing )

i cannot see any of those 1M , 68K directly at your input jacks where they should be , but they maybe hidden .

 

[slickdickrick]

Nashkato,

Thanks for the reply! I will try disconnecting the jacks and running the signal generator into V1 and report back.
I'm not quite sure i follow you on the 68K/1M being in place? Sorry for the high perspective photo, i will attach a better photo of the input section below. Everything on the input jacks is wired as per marshall and the 1M resistors are in place but not visible in the photo. I like to use 1/4 1M resistors on the inputs so that they will fit between the tip and sleeve lugs on the Cliff jacks and keep the input section tidy.

Eric

 

[johnheath]

When there is no signal or as you say a signal when you "jostle" it can indicate poor contact.

So this is what I would do:

Unplug the power cord and discharge the filter caps and run a check with your "buzzer" on your ohmmeter and look for poor solderings and make sure everything is connected according to the schematic… it is very easy to miss something along the way.

Then when you have it all checked and perhaps fixed hook it up and measure voltages all over according to the schematic… allow differences of +/- 10%.

Ususally this is when you will find some errors if they still exists.

It looks neat so it should not take too much time


Best regards

/John

 

[dmp]

You have the grounds hooked up wrong on the input jacks.
You have the ground attached to the pin that switches out when you plug in.
See how you have the grounds on the opposite side as the signal?

 

[slickdickrick]

Hey guys, thanks for all the support and suggestions so far. I'm a little confused about having the grounds on the switching side of the cliff jacks. Sorry maybe i'm just a little slow but when you insert a male 1/4 it lifts the it switches on the left side of the jack viewed from the top (the bottom being the solder lug side). So if i changed the ground to the other side of the jack of the sleeve portion i would not have sleeve making contact to ground when plugged in. I will do some more in depth continuity and voltage measurement testing tonight although i did quite a few tests prior to tubing it and firing it up off the dim bulb test. I'm realizing i may be weak in describing my exact problem so i have created a short youtube clip of the amp demonstrating it's unique ability, link below.

https://www.youtube.com/watch?v=_dc5tQaCNDQ

Also i've attached a photo of one of my marshall reissues input jack section for reference on jack wiring.

 

[dmp]

It's hard to see from the pictures... but as long as you've checked that the jacks are hooked up right
Maybe doublecheck with a continuity meter with a 1/4" plugged in to make sure the signal goes to grid and the sleeve goes to ground??

 

[slickdickrick]

dmp,

Thanks for the advice. I've checks for continuity and i have zero resistance from sleeve to ground and 65.5k from tip to pin 2 of V1 for channel two. For channel one i have 0.1ohm resistance to ground and 64.6k to pin 7 of V1.

 

[dmp]

Sounds right.
Very pretty build, btw.
Getting the bugs out can be the hardest part. Take a methodical approach and eliminate one thing after another until you find the problem.
Haven't checked out the video yet, but will let you know if I have any other ideas.

 

[CJ]

check B+ voltage feeding the two 100K resistors on V1

try one input jack with 68K feeder resistor and 1 Meg to gnd.

 

[slickdickrick]

CJ,

77VDC on the turret feeding the two 100k resistors to V1.
I get very low volume using the 1 meg jacks and on channel 1 the sound is crackly or like heavy block distortion on it but channel two is clean and quiet. If i measure back to the positive side of the pair of electrolytics on the turret board (usually a can cap mounted on top of the chassis on a marshall) i get 77V that feeds the pair of 100K resistors to V1 and on the other side of the 10K resistor that runs between the caps positive side that goes to pin 6 of V2 i get 167VDC. I hope i'm explaining this clearly enough to make sense. I will include a picture of that area of the board for perspective.

Thanks again,

Eric

 

[fazer]

my marshall has about 190 volts on the plates.    B+ to the 2- 100k resistors at 77 volts on your build could mean your drop string resistor is a wrong value.    Isn't it a 10k ?  Make sure the RC network of droping resistors and filter caps are correct value.  In fact you should check all the nodes for B* voltage.  Should be 390 to 460 on the center tap of the output transformer .  Work with one hand in your back pocket so you don't zap your self or turn power off,  discharge caps and then clip on voltage clips and turn back on without touching high voltage points.   

Something is wrong with only 77 volts.

 

[CJ]

pull the preamp tube and measure the voltage feeding the 100K resistors again.

check for wrong polarity on filter cap,

 

[Bo Deadly]

I think fazer is probably spot on. But with the amp off, use an alligator jumper to connect your DMM so that you can test each point in the supply without accidentally causing something to blow up. Then turn it on, read the voltage, turn it off, move the clip to the other side of the next dropper resistor and repeat.

Another possibility would be that the grid pull down resistors (1M) or networks related to the grid / ground are not actually grounded properly. At least that is the sort of thing that would explain why the volume drifts slowly when you mess with the input jack. Make sure you test using different ground points. If you connect the ground of your DMM to a bad ground, the measurement will be meaningless.

The most mysterious problems are usually multiple problems. It looks like a very clean build but there is a lot of work that cannot be tested until it's basically done.

 

[Whoops]

I recommend you also this forum for tube guitar amps,
a lot of people already build that project over there:

http://el34world.com/Forum/index.php?board=13.0

 

[ijzerpro]

I have take a look to the video and your first picture, that input you are using are in short to ground.
please check the wiring on the jacks .