Input Resistor Noise

[zendragon63]

I usually get my questions answered by a search but I can't find this one. Thanks to dummyload, I resurrected my JTM 4.5 a few weeks ago--it went into the I'll-get-back-to-this-one-sometime bin because it had a background hum demon I couldn't exorcise at the time. I finally figured out that hum was caused by the input resistor (a 22K) and as soon as I ran it direct, it was quiet as a church mouse. I tried 10K and 4.7K but still had that 120 hz hum going.

I don't really want to run without for all the regular reasons like rf interference, potential oscillation and miller capacitance advantages but I can do without the annoying background hum. Any thought on why this is happening?

Great little practice amp by the way. Thanks in advance. Regards

[stratele52]

I can't read your schematic download Your amp is a JTM 4.5 ??

[Fresh_Start]

I do not know why it would make a difference, but have you tried a separate 22K grid stopper for each of the 1st stage triodes?  How about mounting them right at the tube?  I've seen that in a number of 18-Watt layouts.

Hope that helps,

Chip

[zendragon63]

Thanks Chip. I don't know why it makes any difference either. There is some interaction between the trimming the heater center tap pot, amount of hum (the 120 hz variety) and the amount of hum that I am getting using an input resistor. These are those subminature missle tubes and the heaters/filaments have given me trouble before. Probably a layout issue but just curious. I will takes what I gots now.

The 'JTM 4.5' is kind of a nickname I gave this one jes because it has circuit simliarities a clean Marshall tone and I figure it probably gets close to 4.5W in PP when crankin. I however, reserve the right to be wrong.  Thanks again. Regards

dennis

[PRR]

Resistor layout is probably the issue, not resistor value.

[stratele52]

I'm curious to know more about this problem.

Can somebody can give me the way to find this schematic on the web ? When I download zendragon schematic my Mac said this file is defective and cn opened it.
I understand this not a "real" Marshall amp, an "homemade project ?

Thank's

[sluckey]

...

[stratele52]

Thank,s slucky it works , now I can read the schematic.

zendragon,  the 22 k resistor should have their wire very very short. Solder resistor as close as you can on the input jack and use shieded wires to pin 2 and 7 , tube's grid. Shielded wire must be grounded at one end only. Long wire could induce noise in grid

But you probably know that.

[zendragon63]

OK, so then after I put it back in the cab, I then had some 120 hum when I had the strat plugged in. Guitar cord? Fluorescent lighting?  Check. Now I am thinking I have a ground loop issue. After several frustrating hours trying to star ground and moving the filament supply, I changed out the Hammond PT with a 0-135v out of the recycle bin. Very quiet. Put the 22K back on and...very quiet. I lost about 15V in the preamp which is about 10% but the quiet is nice and still sounds great.

It sounded too strange to start with; just a noisy tranny. Appreciate the response stratele and all. Regards

dennis

[jojokeo]

How close together were the PT & OTs mounted? Aluminum or steel chassis? Unrelated to issue - what type & size speaker are you using? ps - I like those Hammond 269s and haven't had this issue.

[stratele52]

Good news zendragon63, enjoy your amp