Anderton '64 reissue JTM145 - any info?

[Littlewyan]

Ok what is your heater voltage without the .22R resistor?

That 220pf cap needs to be at least 300V. With the pre amp valves the anode voltage could swing up to the HT Voltage. So as long as your cap can take the voltage on the other side of the 100K resistor you're fine.

Also does the amp hum with V1 and V2 removed?

[arjepsen]

I think it's around 6,7 without, 6,4 with the resistor.
And yes, even with the first two tubes removed, there's a bit of hum.
Not a lot, mind you, but it's there.
I wonder if it's caused by how close the OT, PT and 12au7 are together.

[Littlewyan]

I believe 6.7V is safe but might be borderline so might be best if you leave that 6.4V Resistor in. The 12AU7 is very close to the PT but being a Torodial there shouldn't be much hum. Now what if you remove all of the DC Elevation and go back to just two 100R Resistors to ground what is the hum like?

Also what is that big coil of orange, green, yellow and black wires?

[arjepsen]

The current arrangement of elevating the heater is the quietest so far, also compared to just going to ground.
That big coil of wires, are the OT-out leads. I haven't trimmed them down yet.

[Littlewyan]

Ah, you dont want them anywhere near your input. Whilst the amp is on try moving those wires about to see if the hum changes at all.

[arjepsen]

Yeah, I thought about that too, and did some chopsticking.
However, that didn't change anything. Also, the hum is present even with the 12au7 tube removed, so I don't think it's coming from those leads.

[Littlewyan]

O dear. That means the hum is being picked up by the OT. It could either be the OT itself picking up the hum or the wires. You will always get some hum though. Have you tried having the volume on full with all the valves in and then chopsticking? The low level hum could be the OT picking up hum and the increased hum when turning up the volume could be the input picking up hum from the output wires. Try lifting those output wires right up out of the chassis. Also try putting the bottom plate on the chassis.

[arjepsen]

Yeah, I did try lifting them out - but moving them didn't really change it.
Isn't it likely to be picked up simply beause the PT and OT is quite close to each other on the top?

[Littlewyan]

It is but it wouldnt increase with the turning the volume up. That means the hum is being picked up by the input as well. Try putting some grounded kitchen foil between the OT and PT to see if that makes any difference to the hum with volume on 0. Did you try chopsticking with the volume up?

[arjepsen]

kitchen foil between the trafos didn't do anything.
Wonder if it would? Wouldn't you need a ferromagnetic metal like iron, to shield the magnetic field?

[Littlewyan]

The foil should have been fine, was it grounded? If not then it wouldnt ave done anything.

Have you chopsticked with the volume up?

[arjepsen]

The foil was grounded - used a wire to the chassis.
And no, chopsticking the OT wires didn't change anything.

[Littlewyan]

Hmm all I can suggest for the moment is to try and keep every wire as short as poss. Also I forgot to ask, does it hum with volume increase without a guitar plugged in? If so what about with a guitar plugged in?

[arjepsen]

Those two are about the same.
Anyway, it's not that critical - it's a cramped layout, and I can live with a little hum.
But I'm thinking about messing a bit with the filter over the 12au7 plates.
Matec: I'm not sure I understood your description.
If I want to lower the brightness a bit, do I put in a higher cap - say 2n2?
Or do I lower it?
Also I'm a bit curious about the 10k resistor.
I actually think I read a description of such a filter somewhere, which suggested a resistor value of around 1,3 times the OT impedance. Am I wrong in this or?

[Littlewyan]

You're right, that 10K Resistor SHOULD be 1.3Times the plate-plate impedance, which in this case is 24K I believe (depending on the OT used). However using 10K isn't harming the amp in any way I don't think, I believe you can actually run a 12AU7 with a 10K Plate to Plate Impedance anyway. If you want to mess with the filter you could increase the size of the capacitor, however don't go too big otherwise it just kills the tone.

What you'll achieve by using Matec's Suggestion is increased bass/midrange so it will seem like there isn't as much high end. Give it a go, you might like it.