Ceriatone Forum

guys i have handwired JTM45 which i took to my tech a couple of yrs back needing more volume, he switched the valve rectifier tube for a s/state rec, replaced the KT66 with EL34's and i'd got a 50 watter,  the ohm selector does have to be mis-matched to opperate correctly, the 16 ohm setting is now really 8ohm & 8ohm is now 4 ohm etc etc basicly i have to set the amp at 16 ohm to match my 8 ohm cab.....i don't remember anything else being done as i was there.....

oh!! i do remember a resistor being added to the bias pot!!! could that be due to the change of tube????

anyhoo, i'd like to return it back to it's original JTM45 form, and i'd quite like to do the job myself...eversince i got my Ceriatone plexi 50 i've shown more interest in how my gear works.....

oh!!! one last thing, i've searched 'n' searched for any info on this mod 45 to 50 etc etc and can't find a thing, have any of you guys herd of this mod????
                                   thanks for your time....

What are you asking for exactly?  Never heard of the 45 to 50 mod you are talking about, but that does not mean there isn't one.
You do not say you need help with your JTM45, so I assume you are clear where that amp is concerned.  Regards the resistor being added to the bias pot, I assume there was insufficient bias adjustment when the EL34 tubes were installed.  The tech either could not get enough negative voltage, or he had too much negative voltage on the Power Tube grids.  The added resistor increased the swing of the bias circuit in the direction that your tech needed.

hey mate........i'm asking for help with regards returning the amp back to it's JTM45 form....

i guess what i need to know is.....

1...do i just pull out the ss rectifier and install a valve rectifier??? and it's back to being a jtm45???

2...does it matter much what output tube i use, el34's or kt66???
 
 Denis Cornell builds a plexi 45/50 with a switch to sellect either the jtm 45 (valve rec) or the 50 watt (ss rec)... but i can't find any other info on the mod, i though it would be quite common given the amount of mods there are these days....

thanks again buddy............ray.

I have never seen a schematic for the  SS/Tube rectifier mod.  It would be easy to find out, but how important is that to you?  Not sure what your tech did, but I assume he left the tube rectifier in place and just by-passed it to the SS Recto.  Can you pull your chassis and take a look?  What I am thinking is your tech made the SS mod easily reversible.  You probably just need to disconnect the PT from the SS and hook it up to the Tube Rectifier, and the same with the output of the rectifiers would be true.  Just connect the path of travel back to the Tube Rectifier, the SS will just sit in the chassis unused.  You will need to re-bias of course. 
The "difference" in power tubes can be pretty substantial, but it can also seem subtle.  It really depends on the playing style of the musician.  I think you would appreciate the change back to the KT66 tubes.  That is a pretty nice amp you have, I would return it to its original design if it were me.
Good Luck

hey myfriend......i'm not interested in being able to switch between the ss and the valve rectifier....i just want to return it to the jtm45 (30 watts) rather than the 50 watts it's running at now....

so am i right in thinking that i just put in a valve rectifier and bias to whatever tube i use????...

will this also correct the ohm's selected??? ie the 16 ohm setting is 16, 8 is 8 etc etc...the miss-matching must have been a huge factor for the mod to work without re-wireing or replaceing the trani right????

if it helps you at all to see pic's of the amp i'll be more than happy to send some if you wish????

thanks for your time/help mate...........ray

Like I said, I do not know what your tech did.  I assume the High Voltage secondary was diverted to the SS Rectifier.  You will have to rewire the PT secondary High Voltage and 5.0 Volt to the tube rectifier, and also make sure the tube rectified DC  is now the source of the B+ supply to the SB Switch, HT Fuse, Choke, Filter Caps, etc.
I assume the impedance mis-match to the OT was from the change in power tubes.  To compensate for the different impedance between KT66 and EL34, your tech had you select a different ohm setting via the Impedance Selector Switch.  With the change back to KT66 you will now be able to use the impedance selector in its "normal" settings.
Then of course, a check of/re-bias will be necessary. 
Do  I understand you correctly now?

That would be doing it the hard way.

99% chance he used a SS plug in the tube recto spot.  That's what must people do, heck for years Fender shipped the Bassman RI with one.  These things ahev been around since tubes became obsolete in the '60's, so most techs are familiar with them.

(http://www.tubesandmore.com/cemirror/inv/T-SSR01.GIF)

If so, the mods are easy, remove the plug, put a 5AR4 in there and rebias. 

Chances are the extra resistor because of the extra plate votlage the SS rectifier provided.  It may or may not bias with the resistor in there.  If you can't get the bias in the right range for either the El34's or KT66's, then you'll need to go in and mod the bias circuit.

The mismatched secondary remains the same if you stick with the EL34's.

Hey Wyatt -
I should have known better.  I was under the impression his "tech" had "modded" the amp in some way and he wanted to undo it.  I guess removing a Weber Copper Cap would definitely qualify as undoing it..... ::)

sorry for any confusion guy's.....i did list eveything that had been done to the amp to the best of my knowlege... a jtm 45 running el34's and putting out 50+ watts (not switchable 45/50) would class as a mod, i'm sorry for this...you must understand that i'm asking questions because i don't know how these things work, right??????

Nothing to be sorry for.  You did not do anything wrong, I did.  As wyatt pointed out, just remove the SS device from the tube socket and reinstall the Recto Tube.  If you go back to KT66 you may need to deal with the resistor change in the bias circuit, and follow proper use of the impedance selector 
Good Luck