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

 

[Spencerleehorton]

There is a weird thing that I can audibly hear, when I play in the mid position on G string I can hear a 4th below the note and when I go down to the D string 3rd fret it’s an octave below!!!
It sounds really full and I’ve never heard an amp be a harmonizer before!!??

 

[Spencerleehorton]

 

[Spencerleehorton]

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.

The loop I’m using has the 4 x bc549c in it.

 

[Spencerleehorton]

 

[Spencerleehorton]

 

[Spencerleehorton]

I’m using a mains transformer for JTM45 I’m also using an output txf from a 100W amp. So it’s the mains txf which is getting warm not the output txf, 35 degrees on lams and on covers its 24 degrees so just warm.

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.

 

[StarTrucker]

Like the videos, amp sounds great. Nice Feedback swirl going on the A chord during the Loop test.

For the heat, like Brian said check your grid stoppers and lead dress to prevent RF that you can't hear. Remember the tube can amplify these frequencies or oscillate which requires the tube to pull current through the PTX.

Add shielded wire for any signal runs over a few inches and gnd to chassis at only one end. The red leads from your tone stack and green leads to that switch look long and might be worth shielding. PPIMV leads run right by the Output tube sockets so could be picking up RF.

There are a couple long wires around the master volume that look like they go over the backs of the preamp tube and output tube sockets. Tube magnetic fields include the backs of the sockets too, so maybe check those. Clean looking wiring!


Q: Is 35*C (95*F) really a problem? My Marshall Super Lead's transformer gets warm too, especially when it's turned up. Does yours get warm if you just idle it without signal for an hour?

It sounds really full and I’ve never heard an amp be a harmonizer before!!??

Someone with more sage wisdom chime in, but I believe it is normal for tubes to add frequencies below the fundamental when they distort. I read somewhere that it's unique to tubes, don't remember why.

 

[Spencerleehorton]

So I’ve disconnected fx loop and tested again, the mains txf seemed fine?
Reconnected fx loop to last 10k on B+ and seems fine, here are some voltages.
B+ at first node 32uf = 403v
After 12k it’s 307v
After 1st 10k it’s 252v
Last 10k it’s 220v

So from that I get 8mA current draw at 2nd node, 5.5mA at next then 3.2mA.

So it must be on the heater side?

It gets warm on idle but if I give it a bit of beans it gets hotter, to the point after 30 mins I can’t keep my hand on there for very long!!! About 40C

 

[Spencerleehorton]

Would it be worth changing that first 12k for a 10k?