JTM 45/50 Build: Issues with high voltages

[bt2513]

updated my last post with full voltage readings with tubes installed, in play

[bt2513]

Should I be seeing any voltage at pins 2 & 7 of V1 with no input source or audio signal present?

[martin manning]

V1 grids should be at zero volts. The other voltages look reasonable too. What about the idle current on the EL34's?

[bt2513]

I'm relatively new to this - would you be kind enough to explain exactly how to measure this? Most web articles I've found are not to the point.

[bt2513]

[IMG:280:186]http://imagizer.imageshack.us/v2/280x200q90/840/ivfy.jpg[/img]

[IMG:280:186]http://imagizer.imageshack.us/v2/280x200q90/850/okqd.jpg[/img]

[IMG:280:186]http://imagizer.imageshack.us/v2/280x200q90/841/twurk.jpg[/img]

[martin manning]

Probably the best option for you right now is to use the DC resistance of the output transformer primaries to sense the anode current.

With the amp off and the caps drained, measure the resistance from each EL34 pin 3 to the OT CT at the standby switch and record the Rprimary values. From above, the estimated idle current for 70% plate dissipation is 41mA of anode current* flowing through each side of the primary, so the voltage drop across each side when the amp is idling should be 0.041 x Rprimary. Calculate these voltages and record the desired value for the "blue" side and the "brown" side per the wire colors.

Power the amp up, put the standby switch on play, and measure the voltage from pin 3 to the CT on each side using the same test points, and adjust the bias trimmer until the desired voltage is reached. One side with reach its desired value before the other (the tubes will not be matched exactly), so you'll have to stop there or split the difference.

After doing this, you should remeasure the anode voltage. If it moved more than a few volts, you should recalculate the desired idle current, and the voltage drops, and repeat the adjustment.

* (Pamax x 0.7)/Va, where Pamax for EL34 is 25W

[bt2513]

Thanks so much for this!

OK - I measured 39.9-40 ohms across both OT taps to the CT at the standby switch.

40 ohms x .041mA = 1.64vDC

is this correct?

I adjusted the bias pot and am getting 1.67-1.64vDC at both pin 3 connections now.

[martin manning]

Ok, good, except 40 ohms x 0.041A = 1.64VDC. Is the anode voltage still about 425? If so, you're good. This thing should be making noise by now.

[bt2513]

Yes - well 428 but close

[bt2513]

I'm beginning to doubt the output jack wiring. I followed the diagram exactly.

[sluckey]

I'm beginning to doubt the output jack wiring. I followed the diagram exactly.

They are wired wrong. Wire them like this...

[Cameron]

I'm beginning to doubt the output jack wiring. I followed the diagram exactly.

You have the speaker jack + going to ground.

[Cameron]

.

[cbass]

Is that a wire going from the fuse holder to the speaker jack.
I'm scared

[sluckey]

Is that a wire going from the fuse holder to the speaker jack.
I'm scared

That side of the fuse holder is connected to ground. Not a good idea IMO to fuse the PT HV center tap when you have a negative bias circuit that also relies on that center tap. Funny things happen to the bias supply voltage when the fuse is open. Bias caps may suffer.