Jet City JCA 22H OT replacement and minor mod

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PKI

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Joined 2011
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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)
 

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PKI

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Joined 2011
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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
 

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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).
Best regards!
 
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, it´s not worse than playing a couple chords full tilt,.

It is annoying, of course, but if you can stand it ....
 

PKI

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Joined 2011
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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.
 

PKI

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Joined 2011
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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!
 

PRR

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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.
 
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 :eek:. 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!
 
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