Mojotone 18w TMB experiences?

[Explorerman]

Thanks Josh, maybe a 6v 5w zener between the CT and ground? (I already use 2x10v and a 6v 5w zeners in-line with the SS B+)

I’m also putting in a VRR which can be used to lower the voltage as well so might see how that goes...

[JMPGuitars]

Thanks Josh, maybe a 6v 5w zener between the CT and ground? (I already use 2x10v and a 6v 5w zeners in-line with the SS B+)

I’m also putting in a VRR which can be used to lower the voltage as well so might see how that goes...

I wouldn't recommend that method. If you want to drop the B+, in your situation, I would add something between the PT and the rectifiers...

But, if you're putting in a VVR, I wouldn't bother. This way you can go to 100% for higher headroom, or keep it just under for the standard B+.

[Explorerman]

Sounds good, I'll go with that. Just need to finish the VVR now and should be all set!

Just a little disappointed with the MT instructions that didn't really explain any detail on what values to look for with the cathode resistor, all it mentions is to "measure pin 3 and make sure it is between 0-50mV" and that's it! Mine was within that range, only for some extra reading that I realized there was a little bit more to it.

[Explorerman]

Thanks for sticking with me on this, just had another question regarding the VVR - I’ve placed a 30uf cap and 220k resistor to ground after the standby switch between the standby switch and VVR. However i noticed in the downloads section liteIIb w/VVR the cap and resistor are placed before the standby switch. Will this matter? Should this be before the standby switch? - not easy in my situation where the standby switch is on-off-on used to toggle between SS and tube rec (see scan drawing below).

(note: the pdf is what i’ve wired to, but not shown on the drawing is the addition of a 13K 3W resistor between the B+ UF4007 diode and pre amp)

[JMPGuitars]

I'm not a VVR guy, but if the purpose is to drop the B+, then it probably doesn't matter if it's before or after the switch.

One thing though. I wouldn't use 1N4007s. UF4007s are better, faster, and supposedly more reliable with less noise.

[Explorerman]

I'm not a VVR guy, but if the purpose is to drop the B+, then it probably doesn't matter if it's before or after the switch.

One thing though. I wouldn't use 1N4007s. UF4007s are better, faster, and supposedly more reliable with less noise.

Thanks mate, yes UF4007 are what I have used (I think you originally recommended using these earlier on so I took it on board).

[Explorerman]

Ok so the VVR looks to be working as expected (except for the pot terminals 1 & 3 look to be reversed since all the way up clockwise is the lowest) however during initial power up using a variac i’m getting 260V at node D (pre amp supply not fed via the VVR) at 210VAC supply (I didnt want to go any higher on the variac in case i damage something). Before i installed the VVR i was getting around 260VDC at node D at 235VAC supply. I think i may need to increase the resistor between the VVR and filter cap from 13K to 15K.

[Explorerman]

After doing some further testing with the VVR i’ve discovered it’s doing something weird. When I wind the dial from 10 to about 8 a promenant hum is heard and the volts drop to about 235vc, the hum then continues down to about 3 then it disappears. It then does the same when you wind it back up, from all the way back the hum kicks in at about 5 then stays there all the way up to 10 and volts don’t go over about 235vdc. When i then wind it back to about 8 and hold it there is disappears and then when I wind it to up to 10 volts go straight up to 346vdc as the should. It like it’s getting stuck at a point and hums.

Could this be a faulty mosfet?

[colossal]

Can you post a photo of your implementation and the part of the schematic showing the circuit and the values used?

Did you VVR just the power amp, the power amp and the phase inverter, or the whole amp?

[Explorerman]

No worries, I'll try and get some pics today. I'm just in the process of moving the mosfet closer to where the gate stopper resistor and associated board are located as I read somewhere that it needs to be as close as possible or else there can be oscillation issues. Its on scaling the power amp.

The other interesting thing I found was that when it was in this "stuck" state ie. pot all the way up at 230vdc and humming, when I touch the multimeter probe on pin 3 of the pot and ground (pin 3 goes to Drain and supply from standby switch (after UF4007 diode)), the hum immediately disappears and goes to 350vdc. The multimeter is a fluke high impedance meter set on DC volts.

This is how I've implemented it:

18WTMB wVVR.jpg

[colossal]

Well, that is the correct implementation. I see that you are regulating the PI and Power Amp. Good that the preamp and power section are isolated by diodes.

Is the Mosfet "tab" bolted to and isolated from the chassis with a Thermo-sil pad?

[Explorerman]

Yep it’s bolted to the chassis with some heat sink thermal pad between the mosfet and the chassis. The mosfet IRFPE50 has an insulated hole as well.

[Explorerman]

Moved the mosfet away from where i had it near the output wires and AC heater wires to near the tone stack dials (see pic), runs a lot better now. The hum is now only apparent when the pot is turned to between 3 and 4 o’clock but then disappears. It no longer gets stuck humming and is now usable. I’ve learned that these mosfets are very sensitive to induced AC.

See below voltage chart with R220 cathode resistor. I’ve also needed to put on an 18K resistor between the un-VVR supply to the pre-amp node to match the voltage I was getting before I installed the VVR (around 260-264VDC). What are your thoughts?