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-   -   Jet City JCA 22H OT replacement and minor mod (https://www.diyaudio.com/forums/instruments-and-amps/329660-jet-city-jca-22h-ot-replacement-minor-mod.html)

PKI 6th November 2018 07:50 PM

Jet City JCA 22H OT replacement and minor mod
 
3 Attachment(s)
Hey,

as been an long time fan of Slo-100 (played it a couple of time like 10 years ago and still cant forget), I picked up JCA 22H. Not Slo-100, but if you played the beast you would know that that amp cant play even moderately quiet. Also, JCA 22H schematically pretty similar to the SLO-100 with a number of cost cut solutions and cheaper components. So my thought are that it can be brought pretty close to the actual slo-100.

The amp I picked up had blown OT transformer, so I did some online research, contacted the ZM all-mighty and ordered Classictone Transformer OT Marshall JTM 45 Output Transformer and this choke: Marshall 3 Hy Plexi Style Choke

However, as I am tube kinda-noob ZM suggested me to start a thread before I fire it up as there are quoting him: "it could be easy to introduce separate biasing of output tubes", and most important this: "if you're not sure exactly how to set Iq and everything (secondary phasing is important , due to feedback) , start new thread before powering on ....... just to avoid more smoke" :-).

Here is are schematics I found tp start. (There is a slo-100 clone thread here with slo 100 schematics)

Zen Mod 6th November 2018 08:09 PM

wrong pdf

I see some particles , of which I'm even more clueless than in electronics

:)

regarding changes to separate biasing of output tubes , if possible , post little better schematic (higher res) and I'll do edit for you

PKI 6th November 2018 08:18 PM

Hehe that a structure of Ryanodine receptor 1 :-). Changed the pdf. I will look more for better resolution schematics and if needed will upload pictures of actual PCB. Thank you

PRR 6th November 2018 11:00 PM

Quote:

Originally Posted by PKI (https://www.diyaudio.com/forums/instruments-and-amps/329660-jet-city-jca-22h-ot-replacement-minor-mod-post5594651.html#post5594651)
"...separate biasing of output tubes"...

Why?

Especially on an amp that "cant play even moderately quiet"?

djgibson51 7th November 2018 09:23 PM

Do you mean to replace the output tubes, or keep the EL84's?
I am not sure why you bought a JTM45 op transformer instead of a Marshall 18 watt one.

PKI 8th November 2018 02:45 AM

I was planning to leave E84 for now why?

Because I like my amps big and heavy :)

djgibson51 8th November 2018 05:41 PM

Just making sure I understood what you are trying to do.
Enjoy your project.

PKI 17th November 2018 03:43 AM

1 Attachment(s)
Quote:

Originally Posted by Zen Mod (https://www.diyaudio.com/forums/instruments-and-amps/329660-jet-city-jca-22h-ot-replacement-minor-mod-post5594665.html#post5594665)
wrong pdf

I see some particles , of which I'm even more clueless than in electronics

:)

regarding changes to separate biasing of output tubes , if possible , post little better schematic (higher res) and I'll do edit for you

Still waiting for the transformers to be delivered...
here Is the schematics of the output stage and PSU

PKI 11th December 2018 05:23 AM

I would really appreciate any guidance on how to swap the OT transformer in the head. Anything I need to do or it is just plug and play?

thoglette 11th December 2018 07:49 AM

Quote:

Originally Posted by PKI (https://www.diyaudio.com/forums/instruments-and-amps/329660-jet-city-jca-22h-ot-replacement-minor-mod-post5629007.html#post5629007)
I would really appreciate any guidance on how to swap the OT transformer in the head. Anything I need to do or it is just plug and play?

Read your Rob Robinette, particularly Tube Amp Safety, Tube amp start up and (hopefully unnecessarily) Troubleshooting.

But there's probably a reason the old OT popped. :scratch2: So you might be in for a little fun. :D
Finally, spend some time reading and considering R Keen's Immortal Amp articles


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