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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 11th December 2018, 05:18 PM   #11
PKI is offline PKI  United States
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Jet City JCA 22H OT replacement and minor mod
Thank you! Will start reading.
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Old 12th December 2018, 12:28 PM   #12
Kay Pirinha is offline Kay Pirinha  Germany
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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?
As there's NFB to the PI, there's a 50/50 chance for you to get it right. So, after having the new tranny installed and connected, attach a dummy load instead of a speaker. Turn on the amp. Do you hear strange sounds coming out of the OT after the tubes have warmed up? If so, shut down and swap both plate connections. Turn on again. Now the OT should be quiet. Then you can connect your speaker(s).
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Old 12th December 2018, 01:27 PM   #13
PKI is offline PKI  United States
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Jet City JCA 22H OT replacement and minor mod
Thank you! Any way to know that before firing up the amp?
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Old 12th December 2018, 01:50 PM   #14
JMFahey is offline JMFahey  Argentina
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If you are series producing amplifiers, you test it once and then you know for all others "before assembly" but for just one, practical way is doing the test.

If you want to keep it simple, do it with the normal speaker installed.
Howling (which is a 50% possibility) will either not happen or be heard, but it must not damage the speaker, itīs not worse than playing a couple chords full tilt,.

It is annoying, of course, but if you can stand it ....
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Old 12th December 2018, 04:54 PM   #15
PKI is offline PKI  United States
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Jet City JCA 22H OT replacement and minor mod
Thank you, got it!
Another thing I have the choke and Soldano replace 1k R51 with it. Rdc of the choke is almost 10x times less than that resistor. Should I change other resistors to keep the voltage to the tubes the same or it should be fine?

I mean R52, sorry.

Ok so I finally had some time to work on the amp today. First of all I found that original OT was ok and some wiring was just broken off. I fixed that and made sure the head works. After that I swapped the new OT (as I already bought it and it suppose to be a better one). First time I did not get the primary phasing right. I started bringing voltage up with the variac and heard loud whistling/whining sound. Turned it off and changed the polarity. After that amp was quiet and I plugged my guitar to test it. There were no sound and only slight changing in hum level when I was touching the strings or some metal parts of the amp, but still was very very low level even when I cranked it all the way up. So there I left it seeking for help. What should I try next?Thank you.

Not that anybody is interested, but... I had a bad tube socket, now amp works and tone improved so much with the new transformer. I am very pleased.
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Old 16th January 2019, 03:32 PM   #16
PKI is offline PKI  United States
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Jet City JCA 22H OT replacement and minor mod
So I've been playing it for a couple of days and doing some minor measurements and testing meanwhile. That is what I found.
1. Initially the voltage on the central tap of the primary was 370V and bias was around 30mA which is completely unacceptable for EL84 from what I found. So I trimmed down the bias to 19mA. Plate voltage went up to 385-390V. Too hot imo.

2. Even though the output tubes were matched pair from TubeDepot the bias is around 18mA on one and 21mA on the other. Should I worry about it or it is ok?

3. I did some experiment with my Variac and found that sound wise I like plate voltage at 350-360. I cannot attest for going lower as I think the filament heating go down as well.

So I did some searching, but can not get any good answer and would appreciate your help. Is it possible to lower the plate voltage to 350-360? Change resistors in PSU maybe? Thanks!
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Old 16th January 2019, 11:06 PM   #17
PRR is offline PRR  United States
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These as-found values are very much like one of the Traynor 7199 amps. Yes, 18mA idle, and +/-20% unbalance never made any difference. (18mA and 0mA played less well.)

Modern "EL84" seem to survive OK and are the lowest-price power tubes around.
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Old 17th January 2019, 03:25 AM   #18
PKI is offline PKI  United States
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Jet City JCA 22H OT replacement and minor mod
Thank you! In other words there is no easy easy to lower the voltage? What if I increase R52?
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Old 17th January 2019, 12:20 PM   #19
Kay Pirinha is offline Kay Pirinha  Germany
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2. Even though the output tubes were matched pair from TubeDepot the bias is around 18mA on one and 21mA on the other. Should I worry about it or it is ok?
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.
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Old 17th January 2019, 03:30 PM   #20
PKI is offline PKI  United States
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Jet City JCA 22H OT replacement and minor mod
I see. Another non related question to PSU question. I see there is an input electrolytic cap on the input and return, but original Soldanos do not have those. I'd imagine those are just fool proof and not actually need, right?
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