Mojotone 18w TMB experiences?

[Explorerman]

Update: Another Fluke Meter gives me the same results, I’m at a loss as to how this can be.

[Explorerman]

I took a scope reading:

Channel’s A and C are straight off the PT HV (EZ81 pins 1 and 7), channel B is the Recified output DC (pin 3). Do I have a faulty PT?

[colossal]

Hmmm....

Can you post a photo of your rectifier tube and the leads connected to it? That looks like the transformer is operating in the "additive" mode and it's possible they got the color code wrong with the windings. Heyboer winds transformers for Mojo (there should be a number like HTS-xxxxx on the sticker identifying the PT). If you can, I would desolder the leads from the tube socket and test them individually (energized/floating) with your scope and document again. Do this for each lead so that you can identify the voltage and the polarity of the waveform. You might think about sending that scope capture above ^^ to Phil at Heyboer.

[Explorerman]

The below one is with no rec connected, just the PT red leads on the tube socket with nothing connected, essentially floating with the centre tapped still connected to ground, no difference.

[colossal]

You should only have four connections to the rectifier socket from the power transformer:

0VAC Filament (pin 4)
6.3VAC Filament (pin 5)
290VAC Secondary (pin 1)
290VAC Secondary (pin 7)

You will probe Pin 3 for your rectified B+

[Explorerman]

Sorry, it’s probably hard to see in the pic, but the yellow wire on pin 3 goes to the standby switch (with nothing connected outgoing), the 5v yellow unused wire on the filament winding is heat-shrinked and cable tied under the other yellow wires.

[colossal]

(Puts on glasses...)

Ok, I see it now. I would take those photos and your scope trace and email them to Heyboer Transformers. I would also call them; Ask for Phil or Alden. Give them that HTS number and ask if they have had any QC returns.

[Explorerman]

What DC rms voltage am I expecting to see at pin 3 with nothing connected?
I have a feeling that my scope waveforms might be mis leading, I get the same result when i connect the scope to the power and preamp tube filaments AC connections (they have a centre tap connection as well). The AC waveforms on the scope look to have shifted since they don’t have a ground reference (just some capacitive coupling) internal to the scope meter, the rectified output also has a consistent peak magnitude.
300vac rms gives us a 425v peak, and would expect a DC peak a little bit less that that. If we shifted the above scope waveforms to the same zero crossing it looks about where it should be.
A full wave rectifier with a peak AC voltage of 420v would expect a 2 x diode rectifier to output 270vdc RMS (The Fluke MM gives rms value).
So, with the filters connected to the rec output, the DC RMS value should increase to the expected measured value?

[colossal]

So, with the filters connected to the rec output, the DC RMS value should increase to the expected measured value?

Yes! Sorry, I didn't think to say that your filters weren't connected and floating (you did say that B+ just sitting at the standby switch, not hitting the rest of the power supply). With my Fluke, I get 425VDC with all tubes pulled but the rectifier and filters inline. Take it out of standby and remeasure!

[Explorerman]

So, with the filters connected to the rec output, the DC RMS value should increase to the expected measured value?

Yes! Sorry, I didn't think to say that your filters weren't connected and floating (you did say that B+ just sitting at the standby switch, not hitting the rest of the power supply). With my Fluke, I get 425VDC with all tubes pulled but the rectifier and filters inline. Take it out of standby and remeasure!

Cool thanks, so what do you measure at the standby switch off and no filters online?

[colossal]

Cool thanks, so what do you measure at the standby switch off and no filters online?

With pin 3 fully isolated, no filters, I am seeing 279VDC on my Fluke; same as you. Mystery solved. Carry on, my wayward son!

[Explorerman]

That's a relief, I probably shouldn't look too hard into things, thanks for verifying that Colossal! I'll keep on going, no doubt I'll be back with a genuine problem at some stage lol.

[colossal]

I'm sorry if I confused you regarding testing just the isolated rectifier versus the rectifier seeing the rest of the filters. Usually we just test the amp with the rectifier in but no other tubes. That will produce the 425VDC on Pin 3 expected with VACrms x sqrt(2) "unloaded" conditions.

Glad you got it sorted. The kind of diligence and stepwise progression you are taking through startup is very good! You will know your amp inside and out.

[Explorerman]

Thats ok, I misinterpreted the B+ voltage of around 340-350 being direct from the rectifier. I guess being an electrician its natural for me to test components and circuit sections as i build to limit fault finding later on. But yeah this is a learning experience for me for sure!

[BlueBoozer]

I have my MojoTonr 18W TMB all buttoned up for the mean time. Clean side of V1 is perfect. Barely any noise and low/high inputs work flawlessly. I’m of course having issues with the high gain TMB side. Hum and hiss with slight buzz when at 8 or so on the volume and gets worse as I add treble and gain.

I’m going to take the advice of many posters and use mini coax for the instrument leads and stand all the tube leads up instead of lying them down per the diagram supplied by MT and see if that will make a difference. Redoing my grounding scheme per recommendations in the grounding doc.

This has been a definite learning experience.

[colossal]

Definitely go with a shielded lead for the input to both channels and ground the shield on the input jack. I also agree that redoing your ground scheme to match the one posted here should help as well. Look for any grid leads that are next to plate leads. The mojo layout shows cathode, grid, and plate leads the preamp paralleling each other. That is a bad idea. Grid leads should stay well away from plate leads and if they must be near, then they should cross at a ninety degree angle. Grid resistors should, ideally, be placed directly on the tube socket. The second stage on the TMB channel should have a shielded lead as well.

[Explorerman]

Definitely go with a shielded lead for the input to both channels and ground the shield on the input jack. I also agree that redoing your ground scheme to match the one posted here should help as well. Look for any grid leads that are next to plate leads. The mojo layout shows cathode, grid, and plate leads the preamp paralleling each other. That is a bad idea. Grid leads should stay well away from plate leads and if they must be near, then they should cross at a ninety degree angle. Grid resistors should, ideally, be placed directly on the tube socket. The second stage on the TMB channel should have a shielded lead as well.

Great info! I’ll definitely take all of this into account if I end up with oscillation problems as well.

Which TMB stage 2 lead do you recommending shielding?

[colossal]

Which TMB stage 2 lead do you recommending shielding?

I would use shielded coax for both the leads from the TMB input jack and from the wiper on the Gain pot to the input of the second stage. Mount the 68k input grid resistor for V1b and that 470k grid resistor for the second stage input, V2a, right on the tube socket. They should not be on the board. Ground the shields for both coax leads on the input jack and Pin 1 of the 500kA Gain pot respectively. The shields are only grounded in one place.

That layout of theirs is a mess. They are confusing the builder by showing that the input to V1a is shielded but have coded the wire green. Same with the TMB channel. They are showing shielded leads to and from the Gain pot as grey leads. You can see they are indicating coax but it is not obvious to the unindoctrinated

[Explorerman]

Ok cool.
If you’re referring to the layout that I posted earlier, that’s the M’tone one that I’ve modified for my own use. I marked on it the shielded cables that I planned to use but didn’t bother to change the wiring colouring. The M’tone layout doesn’t indicate any shielded cable nor do they include any shielded cable in the kit...sorry for the confusion. Yes I agree the layout is a bit of a mess.

[colossal]

Ok cool.
If you’re referring to the layout that I posted earlier, that’s the M’tone one that I’ve modified for my own use. I marked on it the shielded cables that I planned to use but didn’t bother to change the wiring colouring. The M’tone layout doesn’t indicate any shielded cable nor do they include any shielded cable in the kit...sorry for the confusion. Yes I agree the layout is a bit of a mess.

crazy.jpeg

Ok, thank you for indulging me that.