New amp build finally started. JTM45 with KT66.

I meant to include the B+ went to normal after pulling the power tubes.

Looks like there are some issues with the bias circuit. And maybe smoked tubes. Have plus voltage where it should be negative after the diode.

At least my B+ is where it should be without tubes.

Have real life stuff to deal with the next few days, so may not get to this until Saturday.
 
Hi SG John, I had to look back through pics that you had posted to check this. Here we can see the "tophat" type diode that you have in the negative bias supply circuit.
SG John said:
View attachment 49954
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If this is one of the "Valvestorm" supplied diodes, they have "opposite polarity" to those used in the original old Marshalls & so have to be installed "in reverse" as here in my JTM50 buildWP_20190413_18_26_47_Smart~3.jpg
Notice that the large flat part is towards the valve sockets, whereas on yours it's facing away from them.
Flip that diode end for end if it's a Valvestorm supplied item.
Pity about the Genalex KT66's. Positive voltage on the grids would have run them "flat out", flowing as much current as the cathodes could supply.
I don't think I was in attendance when you posted those pics, apologies for not picking up on it.

When we first power an amp like this up (no tubes fitted), we first check that all AC voltages are in the correct range & that the negative bias voltage is present at pins 5 of the power tube sockets, & set the bias adjustment trimpot to give the maximum negative voltage. This part about checking for negative bias voltage at the power tube sockets (pins 5) & set to maximum is of critical importance. It must be done before fitting the output (power) valves.
Cheers
Edit:
If that is a Valvestorm supplied diode, it should have this symbol in black on the side of the body16026576065393935221316116639192.png
You need the "anode" end towards the power valve sockets. Cheers
 
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ivan H said:
If that is a Valvestorm supplied diode...
Click to expand...
SG John said:
Yeah Ivan, they are from Valvestorm. Could not find the symbol when I installed it, so went by old photos of JTMs.
Click to expand...
This is probably not the first time this has happened with these types of diodes. Valvestorm should add a special note to pay extra attention to the "polarity" of the modern tophat diodes compared to the vintage tophat diodes.

From Valvestorm:
20201014_060034.jpg
 
syscokid said:
This is probably not the first time this has happened with these types of diodes. Valvestorm should add a special note to pay extra attention to the "polarity" of the modern tophat diodes compared to the vintage tophat diodes.

From Valvestorm:
View attachment 50244
Click to expand...
Yes, I imagine quite a few people have found out the hard way about them being of opposite polarity, compared the type Marshall originally used.
There really should be a notation to pay special attention to polarity, being that it's at the expense of a set of power tubes if oriented as they were in the old JTM's. Cheers
 
Ok, replaced to diode. Also found the stamp, and made sure it was on top.

Do I have the bias pot wired correctly? It looks backwards compared to Ivan’s last photo. It is correct to the Mojo plan, but seems wrong compared to the Mark Huss schematic and Ceriatone plan.

DCD6A2CB-E3DF-420B-BC75-4BF0B010CE97.jpeg
 
I've never seen a bias pot wired exactly like that before. Doesn't mean it's wrong though... I just don't know! However.... Why did you change that phase inverter plate resistor from 82K to 68K (blue/gray/orange)????
DCD6A2CB-E3DF-420B-BC75-4BF0B010CE97.jpeg
 
syscokid said:
I've never seen a bias pot wired exactly like that before. Doesn't mean it's wrong though... I just don't know! However.... Why did you change that phase inverter plate resistor from 82K to 68K (blue/gray/orange)???
Click to expand...

Damn! That was supposed to be 82K. The first resistor I found to be only 820 ohms.
I’ll have to do some more digging around.
 
The way that SG John has the trimpot wired will work just fine, as they are operating as a simple "variable resistance". I think it's preferable to mount them as in my JTM50 & the example that Sysco shows above, because it's more solidly mounted ("variable" lugs soldered directly to the turrets), but John's way should still work just fine.
Check that it IS varying the negative bias voltage through the proper range before putting output tubes into it John. Cheers
 
ivan H said:
I think it's preferable to mount them as in my JTM50 & the example that Sysco shows above, because it's more solidly mounted ("variable" lugs soldered directly to the turrets)
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Yes, that's a good point. A lot more support for that trimpot. I wonder why Mojotone produced a layout with this particular design feature?

And continuing with Mojotone's JTM45 layout, I'm also noticing that abnormality with the location of that 1K/5watt resistor and the missing 8.2K/1watt (minimum) dropping resistor...:
British 45 2.0 WD.png
 
syscokid said:
Yes, that's a good point. A lot more support for that trimpot. I wonder why Mojotone produced a layout with this particular design feature?

And continuing with Mojotone's JTM45 layout, I'm also noticing that abnormality with the location of that 1K/5watt resistor and the missing 8.2K/1watt (minimum) dropping resistor...:
View attachment 50301
Click to expand...

You guys are super smart!
 
syscokid said:
Yes, that's a good point. A lot more support for that trimpot. I wonder why Mojotone produced a layout with this particular design feature?

And continuing with Mojotone's JTM45 layout, I'm also noticing that abnormality with the location of that 1K/5watt resistor and the missing 8.2K/1watt (minimum) dropping resistor...:
Click to expand...


I just changed the trim pot, as it is much more solid.

As for the 8.2K and 1K resistors, that's why I asked before soldering the connections. It contradicted Mark Huss's schematic. I'm still not really completely conversant in schematics, but I am much more knowledgeable than I was. Good thing I asked a second time, because I almost switched their locations by accident.
 
Robert Herndon said:
You guys are super smart!
Click to expand...
Well, you can remove me from that statement because I think I can see where I was wrong about the missing 8.2k dropping resistor. Mojotone is using a 10k dropper instead, that is located between the positive lugs of the bottom 32/32uF filter cap. Therefore, having that 1K/5watt screen dropper located and wired like that makes sense... Very clever... (y).

I still think Mojotone's bias pot layout is not so clever. Plus: Mojotone's wiring of any ground to a transformer's mounting stud is supposed to be a no-no!
 
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@SG John
Now I'm thinking... With the exception of the backwards diode in the bias circuit, you probably had your amp wired properly if following the Mojotone layout.

Looks like you avoided grounding anything to the studs of the transformers... so your good here too.

You might want to put back that 1K/5watt where you had it originally and reconnect to the screen of V4. This will look cleaner

That also means you'll be discarding that 8.2K because you already have a 10K at the 32/32 filter cap. Or use that 8.2K instead of that 10K... your choice! Your voltages to the PI, V2, and V1 will rise slightly. As your amp sits right now, you have two dropping resistors with a total of 18.2K resistance... see what I mean?

This is so much fun... :fingersx:... :cheers:
 
syscokid said:
@SG John
Now I'm thinking... With the exception of the backwards diode in the bias circuit, you probably had your amp wired properly if following the Mojotone layout.

Looks like you avoided grounding anything to the studs of the transformers... so your good here too.

You might want to put back that 1K/5watt where you had it originally and reconnect to the screen of V4. This will look cleaner

That also means you'll be discarding that 8.2K because you already have a 10K at the 32/32 filter cap. Or use that 8.2K instead of that 10K... your choice! Your voltages to the PI, V2, and V1 will rise slightly. As your amp sits right now, you have two dropping resistors with a total of 18.2K resistance... see what I mean?

This is so much fun... :fingersx:... :cheers:
Click to expand...

Why not ground to a stud??? If the transformer is mounted to the chassis, the entire chassis is grounded, No???

Asking for educational purposes, not being a dick.
 
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