Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ndross475 on November 02, 2019, 05:46:40 pm

Title: JTM45 Build test voltages way off
Post by: ndross475 on November 02, 2019, 05:46:40 pm

I just built a JTM45, ran into some strange voltage readings during power up testing and wanted to see if someone could set me straight...

I followed the Valvestorm  / Metro Jtm45 plan.

Attached is sheet of readings  / targets.  No power tubes are installed, got here and stopped.

Here is a link to a gallery of hi Rez images... let me know if you need more detail. 

https://nickdross.smugmug.com/JTM45-Build/


I have been tracing the under boards and everything ohmed out.  I used the sluckey method and came through the tops so I don't think any fell out...

Presence pot shows ground at the capacitor all the way to V3 everything is connected.  I say another post where V3 voltage issues were related to no ground, I don't think that's the case here...

Maybe I swapped a value somewhere, been checking that and haven't found it yet..

Title: Re: JTM45 Build test voltages way off
Post by: ndross475 on November 02, 2019, 05:48:53 pm

This may be easier than the pdf...

Title: Re: JTM45 Build test voltages way off
Post by: sluckey on November 02, 2019, 06:06:28 pm

Voltage readings don't mean much if the power tubes are no plugged in.

Title: Re: JTM45 Build test voltages way off
Post by: ndross475 on November 02, 2019, 06:15:33 pm

Ok,,,, I was going by the Metro power up instructions... and got spooked by those numbers... I'll give it a go...

 As I have said before... if I smoke it,, Doug's gotta eat too...:-)

Thanks,,,

Title: Re: JTM45 Build test voltages way off
Post by: pdf64 on November 02, 2019, 06:30:08 pm

The purpose of the check you’ve made is that the tubes are safe to be plugged into the amp.
eg you don’t want to fit tubes if the plate or bias voltages are missing.

Title: Re: JTM45 Build test voltages way off
Post by: ndross475 on November 02, 2019, 06:53:32 pm

Pretty much similar on V3

Title: Re: JTM45 Build test voltages way off
Post by: ndross475 on November 02, 2019, 08:21:19 pm

Progress... User error,,, I has a 470k in place of the 470 on the cathode... readings are still high but not as high... Progress...

Title: Re: JTM45 Build test voltages way off
Post by: ndross475 on November 02, 2019, 08:45:55 pm

closer but not right...

There is most likely a user error that I am too tired to see...I looked at that 470k 3 times before I saw the problem... Will try with fresh eyes in the a.m.

Latest readings....

Title: Re: JTM45 Build test voltages way off
Post by: sluckey on November 02, 2019, 09:08:43 pm

Are you experiencing some problem with this amp?

Title: Re: JTM45 Build test voltages way off
Post by: JB on November 03, 2019, 04:45:33 am

You really only getting 5.6V AC on your 6.3V heater supply?  Measure across the heater pins on any of the valves.

Title: Re: JTM45 Build test voltages way off
Post by: ndross475 on November 03, 2019, 08:08:52 am

Its running, can't really give it a go yet... It's 9a.m. here thanks to EST kicking in last night... biased it and noticed all I could get it -40ma, which is right at the bottom of the recommended 40-47ma for Kt66 in the book...

gonna let it settle in for awhile and recheck numbers and hopefully wait long enough not to totally piss-off the neighbors...

Title: Re: JTM45 Build test voltages way off
Post by: ndross475 on November 03, 2019, 08:52:35 am

now for the next phase.... :l2: :dontknow:

Title: Re: JTM45 Build test voltages way off
Post by: Pietro on November 03, 2019, 12:43:14 pm

measure/monitor heater voltage and jank all the tubes out until you pulled the one that makes the heater voltage jump back up to 6.2 - 6.5 volt..

replace that tube in question. re-measure all voltages..

Title: Re: JTM45 Build test voltages way off
Post by: ndross475 on November 04, 2019, 05:16:35 pm

Got the heaters in line @6.3 so everything seems to be ok, got the bias set and have been playing it with some old 12ax7s need to find something a bit more appropriate but all seems to be well. Thanks for all the assistance, this forum is a great asset.. I look forward to learning enough to be able to contribute someday..

Nick