JTM 45 first build, can' get a reading on V4 to bias, V5 OK

[norburybrook]

I think it makes sense to forget about the OT phase for the time being, and get the V4 bias issue resolved first. There is a possibility that there really isn't any current flowing through V4. I suggest you discharge all the caps, and measure the OT primary's DC resistance, one half at a time. Measure from the tube socket to the center tap. These two resistances will be similar, but usually not the same. Record the resistance for each half by wire color. Then, AND DO THIS WITH EXTREME CAUTION. Use clip leads to connect meter leads so that you do not have to be probing around in a hot chassis. What you want to do, is measure the DC voltage across each half of the primary with the amp at idle. Measure for each color (usually blue and brown), at the tube sockets. So, if your primary colors are brown, blue and red (center tap), measure the DC voltage from brown to red, and from blue to red. Then report back with the DC resistance and DC voltage for each color. Please be very careful as you will be dealing with 500 or so volts when measuring. If you find there is no voltage across V4's primary, swap in the 6L6's and try again.

Ok, I'll do that in the morning.

Of course I'll be careful, I'm very aware of the high DC voltages flying around inside.


cheers

Marcus

[norburybrook]

Morning all

Ok I've done a quick Dc resistance and got some strange results so here's the info I was given with my Dagnal transformers.

Output transformer VTN0017 JTM45 build
Primary: A-A 8K with CT. Leads red-brown(CT)-black
Secondary: Yellow (zero), Orange (4 Ohm), Green (8 Ohm) and Blue (16 Ohm).

Anode to anode should give around 120 Ohms, and the two anodes to CT should give around 55-65 Ohms

however anode to anode gave 63 Ohms, anode 1 to CT 61Ohms and anode 2 to CT 124Ohms.


would this suggest the transformer is wired wrong?

I didn't proceed with the voltages after finding this out.

Marcus


Edit...reading this back......I realise I read this as red,brown...(CT) Black....

when it should be Red- Brown (CT)--Black



Duh!!!!!!!!!!!!!! I've a funny feeling when I swap these wires everything will be fine and I'll feel such a fool

[norburybrook]

As expected;

problem solved

I had indeed read the OT wiring colours wrong.

But, I've learned more in the last 3 days than I would had I not made this mistake, so I suppose I'll take that as a positive.

I'm eternally grateful to you guys for all your knowledge and help in this matter, I knew we'd get there in the end.

This is a great forum, and one of the reasons I love the internet is in cases like this.

so many thanks again guys the beers are on me

cheers

Marcus

[dorrisant]

Great to hear that you figured it out!

Tony

[jelle]

Great!!

[dorrisant]

Great to hear that you figured it out!

Tony

[norburybrook]

Thanks guys,

Once I'd wired up the OT properly and the squeal stopped , I could then take bias resistor readings on the output valves in their now swapped position.

In doing so I could then ascertain that one of the valves is indeed not good as I was still unable to get a bias reading when it was now in V5.


So all in all we had a bad valve which fortunately led to me finding out the OT wires were wrong.

All good stuff .


Right , I'm going to fit the Metro zero loss FX loop to it now it's all done.


Marcus

[Reeltarded]

Excellent! I made similar advances in my own understanding while chasing a DOA tube vs bias reading on a brand new build.

I HEAR you.

Logic wins.

[JazzGuitarGimp]

Great news, Marcus! Keep up the good work!

Lou

[JazzGuitarGimp]

BTW, when you measure the voltage across the 1R cathode resistor, you are actually seeing the sum of the anode current plus the screen grid current. If you use the transformer primary method I was describing, you will see just the anode current. It works just the same from the standpoint of the mathematical formula: voltage divided by resistance equals current. So for example, if the black OT lead measures 65 ohms to center tap, and you measure 2.275V across it, then:

2.275
-------- = 0.035A. = 35mA
65

And you get true anode (only) current. It's just a bit more dangerous than the cathode measurement. And so most folks just Use the safer, cathode method and add a fudge-factor of three to five milliamps for the screen current.

Cheers,
Lou

[norburybrook]

BTW, when you measure the voltage across the 1R cathode resistor, you are actually seeing the sum of the anode current plus the screen grid current. If you use the transformer primary method I was describing, you will see just the anode current. It works just the same from the standpoint of the mathematical formula: voltage divided by resistance equals current. So for example, if the black OT lead measures 65 ohms to center tap, and you measure 2.275V across it, then:

2.275
-------- = 0.035A. = 35mA
65

And you get true anode (only) current. It's just a bit more dangerous than the cathode measurement. And so most folks just Use the safer, cathode method and add a fudge-factor of three to five milliamps for the screen current.

Cheers,
Lou

Cheers Lou,

That makes sense, I might give that a try while the amp is open just for experience. How much difference in real terms though does say 5mA either way make to the actual sound. Maybe I should just try it .

I presume the 1R resistors don't need to be included then in that case.

I also presume however that they don't actually affect the tone in anyway so no reason to ditch them if they're already there.

Marcus

[JazzGuitarGimp]

You're right, the 1R cathode resistor won't make a difference in tone, so just leave it in. I think a good way to approach biasing the amp is to start at 70% of plate disipation, and also note where 75% is, then adjust by ear, but not exceding the 75% mark. No worries going colder, except if you've got it too cold, you'll notice an unpleasant dustprtion (crossover distortion) when the amp is cranked.