Zero Loss Effects Loop Help

[lgehrig4]

Before I start to install this in my JTM45 build I have a couple questions.

1. My voltage reading at the PI supply is 462V. The instructions stated that an ideal voltage at the loop is 290-210V. According to the formula provided I would need about a 100K resistor and they only sent me up to 68K. I find it odd because aren't most Marshall voltages in the same range? Am I doing something wrong? I would think that at least 1 resistor would be 100K unless I just doing something wrong.

2. Does anyone have directions geared specifically for a JTM45? I no it's close but I only have 1 build under my belt and it helps to have directions geared specifically for this model.

Thanks!
Jeff

[lgehrig4]

Man this forum is dead. Never mind that no one replied. No one even created or started a new thread since I started this one.

Anyway, I calculated that a 100k should suffice in bringing the voltage down to a recommended level for the loop. Can someone confirm this, based on the voltage stated in my 1st post if this is correct?

I hope radio shack has these 2w resistors. I would hate to have to purchase things I don't need and pay shipping all for one resistor.

Thanks
Jeff

[SDM]

A PI voltage of 462V would translate into a plate voltage of around 560Vish in a JTM 45 (assuming tubes are in), quite high. If no tubes are in though (or no power/preamp tubes at least), there's nothing to drop the voltage along the B+ line, so guessing you perhaps did not install all tubes here when measuring? If not, put the tubes in and check/measure again.

With tubes in, all tubes, current will flow as desired, voltages will drop where needed. Should then see that the voltage at the PI node falls well within the loop instruction chart range, be good to go from there hopefully.

By the way, can also contact me via PM or email on questions regarding the loop if I don't respond here in a timely matter here.

[lgehrig4]

Thank you. I take it by your last sentence that you are either the builder or expert on this loop.

I actually posted on GTP as well and it turned out that I was just measuring in the brong place. My PI voltage is actually 386. I was using the photo in the instructions, which is not a JTM45. They told me to measure from the lug closest to the pots and the instructions tell you to measure the lug nearest the tubes. I guess it's different for a JTM45?

[SDM]

I guess it's different for a JTM45?

Yeah sometimes it is, layouts vary so can get a bit confusing. Often in 45/50 watt layouts, the resistor string (or lone resistor typically in a 45) separating the PI node from the previous (screens) node is swapped around physically on the board vs. typical 100 watter layouts (or even other 45/50 watt layouts). In the pic below can see the red circled turrets, in the "typical" 100 watter, can see the PI node is at the top of the pic, red dotted line shows wire headed to PI plate resistors for easier verification . In the 45/50 style wiring, things are opposite here, PI node is at the bottom now.

This reversal of how parts are hooked up here can occur in any Marshall style layout, so if in any doubt, best bet (assuming everything is hooked up correctly according to the layout you use, all tubes are in, amp running properly) is to actually measure both of the red circled nodes in the pic. The one with the lower voltage will be your PI node in a Marshall clone build, hence will be the turret to use for the loop instructions.

Hope that makes sense, and again sorry for the late reply.

[lgehrig4]

Thanks Steve! I sent you a PM with one more question

[lgehrig4]

All hooked up and tested. I'll give it a try tomorrow. Thanks!