JTM45 Build with 6P3P output tubes and GZ34

[Spencerleehorton]

I have done all the list apart from get all the voltages.
Everything sounds so much better now!!
When I do the bias to 45ma all the hum is completely gone!!
The PPIMV works great.
I’ve moved the fuse onto the HV CT.
Moved the standby switch position to the correct place.
The only thing I can’t quite get working is the fx loop so far, part of the signal is getting through just not a lot?
Ive drawn out the schematic of the fx loop and it’s exactly the same as the Ceriatone/friedman version.
Must be a missing contact somewhere on the pcb, just gotta find it.

 

[Spencerleehorton]

This I think is the fx loop

 

[Spencerleehorton]

Pretty sure I’ve just used the wrong side of the jacks as I had to take them off the pcb to get the jacks through the chassis.
I’ll fix tomorrow and it should be all good.

 

[Spencerleehorton]

This was before I sorted out a fair bit but you should be able to see the heater wire redress, this has helped a lot.

 

[Spencerleehorton]

Here is after, something I did noticed was that the mains txf now doesn’t get warm at all? So something was off!!
Loop still isn’t working and it wasn’t what I thought? Getting small signal through?
Might make my own now I’ve got the schematic

 

[Spencerleehorton]

Well looking at that schematic and comparing to this fx loop pcb, this pcb is completely different and wrong!!
I’ll see if I can swap the components around and correct it, if not I’ll build a new one!!

 

[Spencerleehorton]

I also wonder whether I’m giving it the correct voltage as only seeing 47v on the first lnd150 drain?
B+ is going through a 10k then into a 33k? Before then feeding the 100k and the 110k?
What voltage should the Q1 drain be seeing?

Looks like it should be about 180v on the drain of first LND150?
I’m only getting 47v as I’m taking B+ down by having a 10k then a 33k, I will take out the 33k and see if voltage improves.

 

[Spencerleehorton]

So now the problem is the mains transformer is getting hot again, about 35 degrees, warm touch, doesn’t seem to get any hotter but before the fx loop was working and installed it ran cool for several hours.
I’ve got a working fx loop now so it must be pulling the extra current?

 

[StarTrucker]

Someone else will have to comment on the FX loop voltages and current. Check datasheet to see what each of those FETs are pulling and add it up. Transformer should be able to deliver more current than it's spec'd for, and I would assume that 2 Fets wouldn't pull more current than the B+ line is able to deliver. If you're using a transformer from a 100w amp, I think people would agree that your heat sounds like a wiring or circuit problem.

You could also consider using Marshall's own FX loop from the 1959SLP 100w Plexi. Someone with more experience than me with solid state circuits would have a better answer about the merits of the different circuits.

 

[Brian Roth]

Maybe the amp is oscillating at some ultrasonic frequency?

Bri