18Watt.com

Hey

those voltages are really strange all over the board. Way too low for a 290-0-290 VAC secondary.
To help you with the voltages A-B-C-D is very simple, but you have to know which is which.
I would say A is the positive DC voltage on the 1st filter cap in the chain, B is the one on the 2nd filter cap and so on.
I would leave out the rectifier tube and go straight from each 290V lead to a series of 2 UF4007 diodes and join the last diodes together at the unstriped side. (stripes of all 4 diodes should point towards the 290V wires). Now that is your B+ that you should connect to the 1st filter cap and the OT primary side center wire.
What voltage do you measure now? They should all be higher, IE the plates of the EL84 and the PI.

That's backwards. Rectifier diode stripes point away from the source voltage.
Zener diode stripes point towards the source voltage.

Thanks,
Josh

Since visuals are much easier to understand, here's a couple full-wave bridge rectifiers from https://www.allaboutcircuits.com/textbo ... -circuits/.

This is what we commonly use with center-tapped power transformer secondaries:
If the secondary is not center tapped, then you can use this method instead:
In both cases, you can see the cathode line (also the stripe on the actual diodes), is pointed away from the source voltage.

See the link in my signature about dropping B+ voltage for more info on Zeners.

Thanks,
Josh

JMPGuitars wrote: ↑

Mon 09/08/25 8:20 am

That's backwards. Rectifier diode stripes point away from the source voltage.
Zener diode stripes point towards the source voltage.

Thanks,
Josh

Correct! it was just to see if anyone's paying attention

Hey 18Watters !

Sorry about the long delay in answers, life got in the way of my plans...
Anyway, I'm happy to report that after much head scratching, looking at the schematics, the diagram a bunch of times, I finally figured out my mistake.
On the printed version of the diagram I used during my build, one of the black wires jumping from one turret to the other was so dark I totally missed it until I was looking at both the printed one and the digital one on my computer screen. (see attached pictures for illustration....)
Now that I installed the missing jumper wire, the power tubes are getting 10V on pin3, 345V on pin 7 and 325v on pin 9.

So now that things are back in order, I would like to re-install the option to switch between SS rectifier or tube rectifier.
I have the 4 diodes and a switch, just need a little help as to how you think the best way to do it would be VS the version I had installed which I took from the Tube Depot manual for their 18W kit and only uses 2 diodes soldered on the spare pins of the tube socket.

Also, any suggestion on a nice set of tubes for this amp to replace the EHX 12ax7's that came with it and the used TungSol el84s I threw in for now ?

Thank you again for all your help !
I feel like such a dumbass for having missed that jumper wire for so long...
Max

Hi

There are more ways to achieve that, but here's an example. keep in mind to power off the amp when switching rectifier mode.

"power off the amp when switching rectifier mode"

And maybe rest it for a minute to let the voltages in the capacitors drop?

Dd

Take a look at the method I use in the Xtra TMB: files/JMPGuitars_18W_Xtra_TMB_Schematic.pdf

That's simple and doesn't require powering off the amp to change rectifiers.

Thanks,
Josh

JMPGuitars wrote: ↑

Mon 10/27/25 9:41 am

Take a look at the method I use in the Xtra TMB: files/JMPGuitars_18W_Xtra_TMB_Schematic.pdf

That's simple and doesn't require powering off the amp to change rectifiers.

That's really simple and very smart!

Q about Zeners... multiple lower drop diodes? e.g. 3 x 3.9V vs 1 x 12V drop?

I can see a benefit of multiple being the energy dissipation, and therefore heat, being spread when using multiple diodes. Is there another consideration?

Iain T wrote: ↑

Tue 10/28/25 4:10 am

JMPGuitars wrote: ↑

Mon 10/27/25 9:41 am

Take a look at the method I use in the Xtra TMB: files/JMPGuitars_18W_Xtra_TMB_Schematic.pdf

That's simple and doesn't require powering off the amp to change rectifiers.

That's really simple and very smart!

Q about Zerners... multiple lower drop diodes? e.g. 3 x 3.9V vs 1 x 12V drop?

I can see a benefit of multiple being the energy dissipation, and therefore heat, being spread when using multiple diodes. Is there another consideration?

Wattage rating. 12V isn't that significant as long as the wattage rating is high enough (5W minimum). Two 6V 5W is probably a good option too.

Quick update: Amp works but is very noisy on the lead channel.
I will pop it back open and try to improve the grounding and use shielded wire on some of the connections. And probably replace the resistors on the input jack with metal film caps instead of the carbon comp.
I read through a couple of the other Mojotone kit threads and will implement some of the suggested fixes.
Just for eye candy, here are a couple of pictures of the cabinet I built for it.

Thank you everybody for the help with this kit !!!