Tube Amp Archive

Is everything grounded at one point on this amp? From the pix it looks like all 3 filtercaps are connected with a single ground buss. I always seperate the power supply grounds from the pre-amp and PI grounds. So theres 2 ground points powersupply grounds are at a ground lug near the PT and pre-amp...

You didnt reuse that cap did you? If it got fired up backwards its toast Yes i did reuse the caps. New caps on the way.. Actually i ordered all new parts except jacks and pots new fiber board as well just in case to replace the old turret board. I have a dealer relationship with mojo so not to cost...

For starters, I think you have a bad rectifier. A 5Y3 should produce 1/2 VAC * 1.1 = VDC. In your case, 355VAC * 1.1 = 390VDC. You are reporting 352VDC at pin 8. I wonder if you've got more than one problem contributing. Try swapping the rectifier. BTW, a Sovtek 5Y3 isn't really a 5Y3 and will outp...

For starters, I think you have a bad rectifier. A 5Y3 should produce 1/2 VAC * 1.1 = VDC. In your case, 355VAC * 1.1 = 390VDC. You are reporting 352VDC at pin 8. I wonder if you've got more than one problem contributing. Try swapping the rectifier. BTW, a Sovtek 5Y3 isn't really a 5Y3 and will outp...

cbass wrote:Are you getting any scratchiness on the volume tone controls or on the guitar volume?

No scratchiness on the pots the hum increases and decreases smoothly. No issues with the guitar controls. There is no increase or decrease ln the hum just by inserting the guitar.

You have swapped tubes right ? Post your voltages on the tube pins Tubes swapped Voltages V1 Pin 1- 107.4 vdc Pin 3 - 1.84 vdc Pin 4/5 - 3.19 vac Pin 6 - 118.6 vdc Pin 8 - 1.82 vdc Pin 9 - 3.15 vac V2 Pin 1- 148..4 vdc Pin 3 - 1.16 vdc Pin 4/5 - 3.18 vac Pin 6 - 185.3 vdc Pin 7 - 16.4 vdc Pin 8 - 4...

LATEST UPDATE As you can see from the new photo I have rewired the heater power leads. and have tried to dress the turret leads to come straight down over the tube socket. Hum remains: I did find an open in the jumper on V! pin3 to pin 8. fixed that now if I remove the 12AY7 in V1 the hum is almost,...

While not 100% certain, if pulling V1 does nothing and pulling V2 kills the problem, you can look for the problem between V1 and V2. All the ground connections between V1 and V2: -Tone and Volume pots -V2 cathodes should be tied together at the same ground point. I am thinking this is not the probl...

Dmac252 wrote:

cbass wrote:Does pulling out the preamp tube stop the hum

I will check this in the morning when I get back to the shop

Thanks

pulling V1 has no appreciable affect. Pulling V2 silience

M Fowler wrote:Looks like you have both a filament center tap to ground and the two resistors off the pilot going to ground (artificial tap). The artificial tap needs to go.

some updates
OK Removed the artificial tap - no if anything the hum sounds more familiar like 60 Hz.

M Fowler wrote:Looks like you have both a filament center tap to ground and the two resistors off the pilot going to ground (artificial tap). The artificial tap needs to go.

Cool, 1st thing in the AM...

Your PT filament (6.3 VAC) secondary has 3 leads. Two go off to power the tube filaments. The last lead (usually green with a yellow stripe) is the center tap for other two leads. This is typically connected directly to ground. Many filament secondary windings do not have a center tap. You can crea...

Your PT filament (6.3 VAC) secondary has 3 leads. Two go off to power the tube filaments. The last lead (usually green with a yellow stripe) is the center tap for other two leads. This is typically connected directly to ground. Many filament secondary windings do not have a center tap. You can crea...

vibratoking wrote:No need to get crazy and change the whole grounding scheme. These things work really well when the problems are fixed. I was to lazy to look much, but do you have the heaters artificially grounded?

Not sure what you mean by that. Please explain

On V2, the heater wires are laying on the signal wires. The twist is non-existent, so there is no hum-cancelling going on there. http://ax84.com/bbs/dm.php?thread=283738 Pratical stuff on grounding: http://www.valvewizard.co.uk/Grounding.html The Heater pins are 4/5 & 9 yes ? the Green & Red wires ...