Metropoulos Forum

Hi I am new to amp building but have built a few pedals.I just finish a tube depot jtm 45 kit and in the final testing Iam getting on pin 3 of pre amp ture 3 V3 111.4 volts not 35volts like they say I should .What to look for ,any help will be great I want to do one of thisother kits people are taking about on this form .Iam gald I found this form in a google search.Thanks

One of the most common mistakes with the PI (v3) is to put a 470k (between the (2) 1megs) ... that should be a 470 ohm instead... pin 3 should be connected to pin 8, so the voltage there should be the same.

Hi I do have pin 3 and 8 link together on V3 and I check the resistor like you said but it is 470 ohms .

Hey could it be a bad tube

Welcome to the forum! Are you taking the voltage readings with the power tubes installed or not? We will most likely need some clear, up close pics of the board and wiring to help out further.

OK I will work on some good pictures ,No power tubes on that 111.4 volt reading on V3 just rectifier and the 3 pre amp tubes 12ax7 size I am also testing all of the resistors too.

I never put pictures on a form before so,,,,, I tested the resistors and the only ones that did not test out to what they where to be are the two going from t14 to t15 and the one going from t14 to t17 both measured 235 ohms not 470 each.All knobs are on zero wall ac is 117 the first test just having the rec.tube in only I got 495 volts on the big cap and that is with in the range .how this helps you to figure out my mess.

First... your work is very sloppy... it's easy to have something wrong with that build quality..

second: when you put the power tubes the voltages will go lower.

I will clean if up ,but as the testing way of testing with only rhe rec. tube in and the pre amp tubes in no powers yet is how tube depot said to test it in there instructions.I don't know this is my very first build I have never seen the inside of a jtm45 so I will be learning on how to do it better. Just wanted to know why Iam getting 111.4 volts on v3 pin 3 not 35volts like they said that's all that is why I put my pictures up (not for grading)

I think Carbia was mentioning the neatness of the wiring because it can be the cause of parasitic oscillation and odd noises in the amp. Just moving one wire can be the difference between a great sounding amp or a noisy amp. Have you tried measuring pin 3 of V3 with a different tube in place. A bad tube can cause unusual readings in some cases and it is not impossible that a new tube can be bad. Try rotating the preamp tubes around and take the measurement again. Another good option for the first power up of a new build would be to use a "light bulb current limiter". It will protect your amp from any possible wiring mistakes by limiting the current that the circuit can draw. If you do a Google search you can find info on building one with simple parts available at any hardware store. If you have not yet tried to power up the amp with the power tubes installed then it would be wise to build one and give it a try.

I have built a bulb dimmer and tried they reading getting that 111.4 v on v3 but I more pre amp tube I got to put in my peavey amp of the same 12ax7 so I will swap them put and see. I have read up on the wires issue and will try to neaten up things..I have talk to a couple of repair guys and they too think that is a odd way they have me testing for voltage with the power tubes out.But being my first build I am going by there rules so if it messys up ( blows up ) may be they will stand behind there kit.Could there be any thing else that is maybe going to tube number 2 that could be the cause of this higher reading .I don't have any info on what the voltage is to be on the pins on V2??????? I really do like any help.

Hi so I was wondering if any body has any other things similar happen to them with their builds of like me having a higher voltage reading on V3 pin 3 111.4 volts instead of 35 volts what could do this

I pulled the power tubes from one of my 50watt builds and checked the voltage at v3/pin3 for you. My voltage doesn't read quite as high as yours but I am only measuring a little over 400v at the first filter cap. Being that you are getting an unloaded B+ of 495vdc without the power tubes installed, it is quite possible that your 111v reading is ok. Once the tubes are installed and loading down the circuit that high reading should settle down to it's normal range. If the amp passes the light bulb limiter test, I would install the power tubes and give it a try. Be sure to set your negative bias voltage, at pin 5 of the power tube sockets, to its highest level using the bias pot. Watch everything closely and be ready to power down at the first sign of trouble. I am not sure if you mentioned this previously, but have you taken all of the necessary voltage readings at the power tube sockets already?

Thanks for pulling your power tubes on one of your builds to see if you get what I am getting .I have not done any of the testing on the power tubes yet I didn't want to blow any thing up .So I will put in a 100 watt bulb in the dim bulb tester and put the power tubes in and plug in with a extra cord so I can unplug it with out handling the amp it might be hot if the bulb does not go dim so I will play it safe.I will try it out.

Also what should the voltage readings be on the power tubes