Help debugging a JTM45 build

OnTheFritz · Post by **OnTheFritz** » Mon Oct 25, 2010 9:02 pm

Lift one end of your NFB resistor and try it again. If the motorboating goes away, your primaries are reversed.

francis · Post by **francis** » Mon Oct 25, 2010 10:24 pm

For some fun you can check it with another set of tubes (to rule that out), and with the standby already set to on when turning it on.
http://geekrants.com/francis/ax84/jtm45/amp_noise2.mp3

It sounds as if its laughing at me

Yep, it works without the NFB..
Reversed primaries... WORKS..

Thank you guys so much

now I have to tidy it up a bit, bias, and set it up in the head cabinet.

Francis

axeman · Post by **axeman** » Mon Oct 25, 2010 10:52 pm

Wow, I had reversed primaries once and it made thise load sound, like a thick buzz, you had some guitar sound.

I was wrong

good work so start to

.

OnTheFritz · Post by **OnTheFritz** » Tue Oct 26, 2010 2:39 am

francis wrote: Yep, it works without the NFB..
Reversed primaries... WORKS..
Thank you guys so much now I have to tidy it up a bit, bias, and set it up in the head cabinet.

Francis

Glad you've got it worked out! :rockon:

arledgsc · Post by **arledgsc** » Wed Oct 27, 2010 11:21 am

Excellent! I reversed the primary on my first build which POed me to no end. It would be nice if transformer datasheets would mark the positive polarity of both the primary and secondary. Then assembly would be a breeze. The old school dots would do just fine. Scott

j.christopher · Post by **j.christopher** » Sat Oct 30, 2010 5:49 pm

why doesn't the transformer I just received match the build instructions?

novosibir · Post by **novosibir** » Sun Oct 31, 2010 1:40 pm

j.christopher wrote:why doesn't the transformer I just received match the build instructions?

Ask Jesus, he probably knows!

Or ask with more concrete info!
Nobody knows or can imagine, what you got!
Nobody knows, are you speaking about PT or OT!
Nobody knows, why you haven't had at least a small "hello!" in your very first post!

Some minor behaviour always is helpful on this board!

Larry

j.christopher · Post by **j.christopher** » Sun Oct 31, 2010 4:08 pm

sorry slight panic

PCollen · Post by **PCollen** » Sun Nov 14, 2010 10:40 am

francis wrote:Hey

I just finished my JTM45 and have a problem.. this is actually my 2nd build ever.(First was an AX84 October)

Basically my voltages in the preamp section are way off.. way too high. Basically I have 1/10 the voltage I should have on the PI cathodes, and about 150V too much on its anodes..

V1 and V2's anodes are all off too. Cathodes seem fine.

I took the following readings: http://geekrants.com/francis/ax84/jtm45/jtm45_debug.png

Any clues? My anodes are all off.. I've been trying to trace everything going to the anodes but haven't found anything yet.

Thank you!

I looked at your pics, and noted that I can't se the power tube grid-stopper reistors, between the 220K bias splitters and pin 5 of the power tubes.

Francis

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