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Jet City JCA 22H OT replacement and minor mod
Jet City JCA 22H OT replacement and minor mod
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Old 17th January 2019, 05:17 PM   #21
JMFahey is offline JMFahey  Argentina
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Location: Buenos Aires - Argentina
I wouldn´t worry about 3mA difference.
Any decent transformer must take that and more, say at least 5mA difference, wouldn´t be surprised at them standing 10mA.

"In the old days" , I guess amp manufacturers matched tubes, but not so sure about that, I bet workers just pulled needed tubes from the proper shelf in the warehouse and installed them as-is ; then if amp worked as expected , fine.

I see NO tube matching or even current measuring notes at all in any old Fender/Marshall/VOX schematic.

Fender just spec´d "-52V" for 6L6, Marshall/Hiwatt and others "-38V to -40V" for EL34 ... period.

Smaller tube amps were often self/cathode biased so that tends to self-adjust.

Of course tubes were more consistent but mainly: bought in large batches from same manufacturer, so ...

As of "Joe Public", he just bought random tubes from his friendly "Radio and TV Shop" and plugged them in.
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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Old 18th January 2019, 02:15 AM   #22
PKI is offline PKI  United States
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Jet City JCA 22H OT replacement and minor mod
Originally Posted by Kay Pirinha View Post
I'd rather be worried about the vendor's carefulness in matching tubes . It appears that they measure and match at only one operating Point - or they don't match at all and just charge their customers.
Best regards!
Actually I am pretty upset with Tube Depot. I got all new tubes for the head, 2 Tung sol 12ax7 for first two tubes in the preamp as I remember I used to like them before, 3 rest are JJ and 2x JJ el84. I already commented on matching of el84, but what is most upsetting that it looks one of the tung sol tubes is bad. I had this fizz and weird foreign sounds coming of the amp and I've checked everything. So neighbor of mine had two old tung sol in his closed. I swapped mine pair and weird sounds went away. I swapped JJ into same place and sound I good again (more gain and compression with JJ, but with no fizz or foreign sounds). I guess not buying from them anymore...
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Old 6th February 2019, 06:41 PM   #23
PKI is offline PKI  United States
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Jet City JCA 22H OT replacement and minor mod
Just to bring up to this discussion (monologue mostly, but whatever hehe). I finished with my amp. Re-did some chassis wiring as it was a bit messy, changed poly caps to Wima(most I end up having in my stash), brought all values in the preamp section to the ones in SLO and added some switches from SLO schematics.
Amp sounded great! Next I what I did I increased those caps in PSU filtering. First 47uF —> 150uF and two 22uF —> 47uF. The result was controversial I must say. The bottom end and overall sound became more tight, even. I absolutely love it on hi-gain settings, so great. However, in lower gain/crunch-clean sound amp lost some of the dynamic range and that tube character to react to how hard you pick. I does lack some of the sponginess on crunch channel now. Do you think it is possible to have both? what if I change values of those 47uf back to 22? Or anything else?
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