Schematic that matches the actual JTM45 Metroamp kit

[Slowhand428]

Hi guys,

Someone brought me a JMT45 Metroamp kit they had tried to build. It was put together so bad I had to take everything apart and start over.

I was given the build instructions V2.1 and a Marshall schematic with Drawing # JTM45.DGM issue 7 which does not match the build instructions.

Is there a schematic that matches the build instructions?

There was also a Magnetic Components PT installed which died soon after I turned it on. It had an output of 370VAC from the center tap and 731V end to end on the high voltage secondary. This high voltage would put the plate voltage up over the normal limit for a 12AX7. Also it has a 3H choke and not a 20H choke which would have less resistance.

Because I have to order an new PT anyway, what do you guys think is best that will make this amp stable?

I have not been able to get log on to this site with my own name, so I am using someone else log on ID.

Cheers,


Billy

[danman]

If you were measuring the secondary voltages with no tubes installed, the voltage will drop once the tubes are installed and the power supply gets loaded down. I don't think the voltages you posted are to far out of the ball park. Here is a Classictone PT which is very close...http://www.classictone.net/40-18033.html I believe user are reporting something in the neighborhood of 450vdc with this model.

What is it about the schematic that is different? Most of the Metroamp schematics are very close to the original circuits, but I have never built the JTM45 so maybe some of the other members can chime in with their experience. Wire colors on the PT and OT may vary from original Marshall's but this can be worked out if needed.

[Slowhand428]

Hi danman,

Thanks for the info.

The Magnetic Components 40-18033 is the transformer in the amp. Everything checked out until I got to the pre amp tubes except the transformer was putting out 367VAC at the secondary and it should have been 345. The voltage on the inverter tube was over 350V. I am 99% sure everything is wired correctly and all the resistor values are correct. I did not check every cap for value..well not yet.

The Marshall schematic shows the signal path from V2a grid through the plate directly to the grid of V2b with no decoupling cap which obviously could not work. No B+ voltage was noted on the schematic. The metroamp is not wired that way but I have not traced the exact wiring yet.

Does anyone know what the normal inveter plate voltage is when using a Magnetic Components 40-18033?

Thanks,

Billy

[danman]

Your listed voltage is very close to what you noted from the schematic. Most voltages are a bit higher these days because of the higher line voltage. People have also reported slightly higher than normal voltages with the Classictone iron but they are great transformers that perform very well. Most tube amps run the tubes well above spec anyway. Can you post a link to the schematic that you are referring to with the mistake? V2 is a cathode follower circuit which drives the tone stack. It does have a different setup than the other preamp tubes and the voltages run pretty high in most Marshalls. Some newer production tubes have a problem in the v2 spot because of the elevated voltage but it is a tried and true circuit that is used in many amps including the Bassman 5f6-a. The circuit did change slightly over the years as did the transformers and power tube setup. I'm not sure which version that Metroamp used in their layout.