ceriatone jtm 45 kit

neikeel · Post by **neikeel** » Wed Oct 24, 2012 1:18 pm

Yes it could well be, or a dodgy bias pot, quite a few odd things...........

cmb · Post by **cmb** » Fri Oct 26, 2012 4:13 pm

Replaced the mentioned cap.voltage doesnt move through cap as before , but humm persists. Im getting something weird on the .1uf cap to the left of it . when i check V on it the hum quiets. Could that one be bad too. or may it be agrounding issue? Or mabey niether. Im lost at this point.

neikeel · Post by **neikeel** » Sat Oct 27, 2012 6:46 am

cmb wrote:Replaced the mentioned cap.voltage doesnt move through cap as before , but humm persists. Im getting something weird on the .1uf cap to the left of it . when i check V on it the hum quiets. Could that one be bad too. or may it be agrounding issue? Or mabey niether. Im lost at this point.

Almost certainly a grounding issue (maybe in the bias circuit given the other issues).

cmb · Post by **cmb** » Sat Oct 27, 2012 10:17 pm

still chasing the hum. Tried chasing groundes . grounded everythinbg back stock ceriatone way. No change. Thinking of trying the larry grounding, if that doesnt work dont know what to do.

arledgsc · Post by **arledgsc** » Mon Oct 29, 2012 9:47 am

Let's recap so we can sync thoughts together. The hum persists when V1 and V2 are pulled... Correct ?

cmb · Post by **cmb** » Mon Oct 29, 2012 5:57 pm

Correct,it stops when you pull V 3.

cmb · Post by **cmb** » Mon Oct 29, 2012 6:41 pm

Also replaced bias caps because i was getting weird voltage occilations(dont know if thats the right word . the voltage was moving up and down 5-15 v on the Hv side or tube side and the same on the -dc side as i understand caps are supposed to block the Hv and let the other pass through coupled with the signal, so ididnt think this was right so ichanged the caps out .)This stopped the movement of V up and down,but hum persisted.Dont know what this ones called for sure , next to the bias caps is another .1uf cap when i test V on it the hum quited when i pull the test lead it comes back .I changed this cap out also but it behaved just like before so idont think thats the problem.However im sure that its in this circut somewhere...
thanks Clint

arledgsc · Post by **arledgsc** » Fri Nov 02, 2012 11:05 am

Is there any way you can supply pictures with more resolution? All detail is lost zooming them to 400%. Just a few good pics that show the V3/ bias supply area of the board, output tubes, presence pot section, and grounding.

I may have asked this before but what is the value of the resistor that bridges from the 8.2K 2W CC resistor to V4 pin 6? Where is the schematic/ layout with this component? This bridging resistor seems to be non-standard for a JTM 45.

I think more people will jump on board with help if better pictures are provided. But it is makes my eyes hurt trying to figure out the circuit from the fuzzy blown up pictures. Thanks, Scott

cmb · Post by **cmb** » Fri Nov 02, 2012 6:25 pm

Will try to get some more pics this weekend, although i dont know how mutch better they will be with the camera i have . Going to try moving some grounding points and see if that makes any difference. I ll keep you posted
thanks clint

cmb · Post by **cmb** » Sat Nov 03, 2012 12:34 am

Revised all my grounding using the larry scheme tonight, made no diffeerence. Found something strange though , when i hooked up the case groung for the OT common on the speaker jack i gota very loud buzzing noise, doesnt do this w/o it just hums. Diddnt have an earth on the OT common/ speaker jack before. Does this mean anthing to any one??

Still trying to get some more pics .

thanks clint

neikeel · Post by **neikeel** » Sat Nov 03, 2012 4:35 am

You must have the OT common grounded to a lug (usually an output tube socket) via the output jack ground which is grounded at the same point.

It is also worth making sure your mains ground is dedicated to that and nothing else, a lug with star washer near the mains input (a mains cap lug or chassis side wall).

Will have a look at the new pics (my tired eyes had the same problem), although arledgsc seems to have a plan to get you through this.

arledgsc · Post by **arledgsc** » Sat Nov 03, 2012 10:29 am

Will have a look at the new pics (my tired eyes had the same problem), although arledgsc seems to have a plan to get you through this.

No plan other than verifying the circuits are wired correctly. But it seems Clint borrowed ideas from many designs so not sure what is going on really.

With the OT common connected to ground hum issue are you sure the OT primary is wired correctly to V4 and V5? Among other things the OT ground is needed to ensure the negative feedback signal is DC referenced correctly to do its job. Are you using the Metro 784-103 OT? I think you mentioned you are using Metro/Heyboer transformers. If so it seems you had it wired correctly in the original pics (V4-Brown, Red-CT, V5-Blue).
http://metroamp.com/wiki/index.php/784-103

Better pics and knowing what designs you are trying to copy would move this along much quicker. Thanks, Scott

cmb · Post by **cmb** » Sat Nov 03, 2012 11:54 am

Not really trying to copy any design in particular. althoughn mainly metros .try pics later today thanks

cmb · Post by **cmb** » Sun Nov 04, 2012 1:44 am

Hey everyone, have finally had some luck though i feel rather silly once i figured out what was going on.

earlier today
neil wrote

You must have the OT common grounded to a lug (usually an output tube socket) via the output jack ground which is grounded at the same point.

arledsc wrote

With the OT common connected to ground hum issue are you sure the OT primary is wired correctly to V4 and V5? Among other things the OT ground is needed to ensure the negative feedback signal is DC referenced correctly to do its job. Are you using the Metro 784-103 OT? I think you mentioned you are using Metro/Heyboer transformers. If so it seems you had it wired correctly in the original pics (V4-Brown, Red-CT, V5-Blue).
http://metroamp.com/wiki/index.php/784-103" onclick="window.open(this.href);return false;

I had the OT wired correctly, but i had the grid wires backwards. Without the ground to the OT common/speakerjack ground hooked up when you had the grids wired the right way, you got a terrible squeel , the other way you just get a very bad hum..

Among other things the OT ground is needed to ensure the negative feedback signal is DC referenced correctly to do its job

Hello clint ...As soon as i hooked the ground back up and swiched the grid wires again I powered up and flipped the standby on..........my amp was totaly quiet . Had to plug into it to make sure it was working. It works fine and is totaly noise free( With the vol and precence dimed i can only hear a very low hiss with my head by the speaker.

Heres the part that makes me want to kick my own ass .Had i grounded the OT like it was supposed to have been everything would have worked right to begin with . It was just something i overlooked , nobodys fault but my own (its right there in the instructions and on both layouts I checked dont know how i missed it). Bad thing is i spent all kinds of time changing caps and grounding when all i had done was missed one ground to begin with. The larry grounding wasnt a bad thing to do anyway though. Hope lots of people read this ,dont care if they laugh at me or not if it will keep them from making the same mistakes I have.

may have asked this before but what is the value of the resistor that bridges from the 8.2K 2W CC resistor to V4 pin 6? Where is the schematic/ layout with this component? This bridging resistor seems to be non-standard for a JTM 45.

Still have questions about this . R value is 1k 2w , dont know if this is needed or not .Ctone layout showes it , but also showes this wired different than metro . Metro showes form32uf filter cap to pin 6 V5 to pin 6 V4 to 8k2 2w on board. Ctone showes the 32uf and choke wired to the board at the 8k2 2w bridged by the 1k 2w to pin 6 V4 bridged to pin6 V5. I used metros wireing on this so need to figure out which way is right and if i need this 1k2w or need to remove it...

Right now Im just happy that the hums gone.

hope to hear from everyone soon thanks again clint

arledgsc · Post by **arledgsc** » Sun Nov 04, 2012 9:53 am

Good job Clint! Reversal of the grid wires is the same as wiring the OT primary out of phase. You hung in there when many would have giving up. All of this is valuable knowledge and even I learned something (like don't assume V4/V5 control grids are wired correctly). Enjoy the amp and start thinking about your next build since you are a certified amp tech! Scott

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