Metropoulos Forum

Hi guys,

please help me out. I got this JTM45 clone and started testing, but I don't get any voltage on pin 8 of my rectifier socket. Pin 2 shows 5V AC. What's goin on there? I already resoldered pin 8, but still - nothing... PT is brand new and tested.

Thanks

The 5V AC reading is across pin 2 and 8 (not relative to ground).

Thanks toner. Just another newbie mistake.

Just for clarification: In George's instructions I read:

...place the meter's black lead in one of the chassis mounting holes for a ground connection. ... Using the red lead and AC voltage setting, test the following:

Rectifier tube socket:
Pins 2 and 8 5V

Output tube sockets:
Pins 2 and 7 3,15V

Preamp tube sockets:
Pins 4,5 and 9 3,15V

According to this, there should be 5V between ground and pin 8. Or am I wrong here?
Checking the heater voltage of the KT66's and preamp tubes, you go from ground to pins 2/7 and pins 4/5/9, right?
That's what I did, and that was alright.
All voltages are AC, directly from the PT. Why this exception with pin 8?

The rectifier heater winding is not center tapped so it's not referenced to ground like the heater supply for the other tubes. All other voltage readings, including heaters on the other tubes like you listed, are referenced to ground so you are correct. The JTM45 instructions and voltage chart notes do say to check AC between pin 2 and 8 of the rectifier.

Very nice looking build!

Thanks very much toner, I did like you told me, and the readings are ok.

Before I go on, I want to mention that I'm well aware of the danger of high voltages and act very very careful.

Next step: Using the 350V tap of the PT, I put in a new Sovtek 5AR4 and measured between ground and pin8:
140VDC - slowly dropping to 130VDC within a few minutes, further on down now to 94VDC. Georges instruction says 340VDC on pin 8 and one terminal of the standby switch. Is this a bad tube?
Please notice that this buildt has other switches than Georges and that they are wired different. (see pic)

This is a beautiful amp with all the right components, as you can see, but something seems to be still wrong...

...or just another newbie mistake? The person who build it said that it was running fine.

I can't see exactly how the standby is wired but it looks like the builder wired pin 8 of the rectifier straight to the first filter cap, rather than through the standby switch first. In other words, the filter cap is before the standby rather than after it like in the Metro kit. This will cause DC voltage on pin 8 to drop while still in standby as the cap charges but I'm not sure how much the voltage should drop or for how long. The drop you mentioned is probably typical as the tube warms up and the cap gradually pulls current from it.

Putting the filter cap before the standby switch like that is actually better because it puts less stress on the rectifier tube by allowing it to slowly supply current to the cap as the tube warms up, rather than all at once when flipping the standby. I don't use the standby switch at all in my 18w for that reason (I haven't changed the wiring).

That's a very clean build so I doubt anything is wrong with the wiring. It doesn't hurt to test it though. I think you can move on to checking the other tube socket voltages, bias, etc.

Toner - man, I made it! It’s running!
I guess you’re right concerning the filter cap. And I guess that the reason for the dropping voltage was me, accidently switching both switches on one time, so the cap had been charged and what I measured was the dropping voltage afterwards, while ‘Stand By’ was off again. I went for a walk this afternoon and thought that this voltage drop is typical for a discharging cap, not for a tube. So I checked again, went on testing and everything worked out fine. I have 436V on the plates now. All other voltages are alright too – except on V1 where I have 196V on pin1 and 211V on pin6… well. I hope this is no serious problem.
The amp is running now with 3 ECC83 Phillips Miniwatt – which I found in an old bandecho together with some mustards-, 2 Shuguang KT66 and the Sovtek 5AR4. If everything stays fine with it, I’ll put in my GEC’s and a Siemens GZ34 and start tweaking.
It is a very clean build, yes, but to be honest - it had some issues too. The OT was mounted too close to the KT66’s, so I had to drill some new holes to put it further away. While doing this, I recognized that the cables of the OT had been glued to the chassis under the board… You never know…
But after all it is a fine amp now. Thanks so much for your help. I wouldn’t have made it all by myself.

I thought some more about that voltage drop and there's no way it would have been that low just because of how the cap is wired. I was too focused on the wiring at the time I guess...

Anyway, glad it's up and running. Enjoy!

Mynameisfritz i have bought a igmt-m45/50r transformer the same as you have. could you let me know how you wired it up. i cant make see it in the photos properly.

thanks peter.