Guitar Gear Forum

A buddy of mine has a Jet City JCA50H that suffered a failed PT. He managed to get a great deal on a Mercury Magnetics PT, so I took it to my workbench to plop it in. It wasn't a direct bolt-in deal, but we went over the issue with Mercury and they will be making accommodations for the future units. I will explain the whole thing in this write up.

Here is the patient:



So I replaced this:



With this:



Now, here's where I ran into problems. The mounting feet that are supplied with the MM unit are not even CLOSE to where they should be. Hopefully the pic is clear enough that you can see the right foot is lined up with the bolt hole, and the left one is 1/8"-1/4" off.



My buddy and I both contacted Mercury about this issue, and they said they would be able to send brackets with an inward offset to correct the issue. I however, came up with another idea......

The endbells are fairly similar in size. In fact, the bolt pattern of the EI core is the same dimension.



The guts of the MM unit are a lot beefier than the JCA unit, but all I needed to do was make some spacers with the old MM mounting brackets by cutting the 'L' feet off.



Then, install them on the PT core, like this:



Then bolt the JCA endbells on like this:



Then, bolt it to the chassis like this:



The chassis mounting bolts are a bit tight, and MAY need a hair of grinding if anyone does this swap before the newer brackets are released. OR, maybe you just want something that looks stock and stealth.

Next is wiring!

Nicely done.

I would have probably just drilled two new holes but the workaround is great.

Nice fix

I also would have just drilled new holes in the chassis.

OT change?


PT isn't gonna change the sound much, is it?

~Abstract~ wrote:OT change?


PT isn't gonna change the sound much, is it?

He was replacing a failed PT

Now to the wiring:



Here's the layout for the PT:

Primary one:
Black: 0v
White/yellow: 100v
Red/black: 120v

Primary two:
Black/white: 0v
White/orange: 100v
Red/white: 120v

The JCA PT has a similar layout.

Primary one:
Black: 0v
White: 100v
Red: 120v

Primary two:
Blue: 0v
White: 100v
Brown: 120v

Here is the voltage selector with some wire remaining:



If you want to hook it up completely stock, then the white/orange and white/yellow wires from the MM unit will be installed where the white wires from the primary of the PT go to. Doesn't matter which one goes where, they aren't connected to anything. They are "floating" leads and the small board is a good anchor for them, rather than just cutting/sealing them and having them hang around inside the chassis.

The wires go like this for stock configuration:

JCA PT----->-----MM PT
Black--------------Black
Red----------------Red/Black
Blue---------------Black/White
Brown-------------Red/White
White-------------White/Yellow
White-------------White/Orange

Otherwise, just follow the wiring scheme from Mercury. In this case, the amp doesn't have a passport for travel, and will remain in the states. So, the white/tracer wires were "floated" on the switch board in the stock locations. Then, the two red primary wires were soldered to one switch terminal and the black primary wires were soldered to the other switch terminal. Oh! And be careful to solder the primary leads to the terminals facing INTO the the chassis. I used those terminals for the 120v hookup and the light in the switch stays on permanently. I need to hook them up in reverse. OOps. So, 120v leads from the fuse block go on the terminals that face the bottom of the amp.

After that, the 6.3v heater tap, 12v switching tap, center taps, HT taps and bias tap can all be soldered up.

Like this:





So you can also note: the switch on the right is the power switch. The visible terminals are where the black and white wires from the IEC socket SHOULD go. Otherwise, the power light just stays illuminated, switch on or off. So, I need to switch those. lol

Also, the MM PT is stout enough to be able to have some 6550s punish your speakers.



Later, I'll be doing the SLO mod, and maybe I'll try and embarrass myself by posting some clips. I PROMISE I'll practice before recording.

Toe-Knee wrote:Nicely done.

I would have probably just drilled two new holes but the workaround is great.

The problem with drilling the holes is that there are weldnuts in place, which are pretty wide. You would need to either drill them out, pop them out or something of the like, and THEN drill some holes. I didn't want to make additional holes, and those weldnuts are very stout locating pieces. Plus, in the above pics, you can see how wide they are.

And thanks for the compliments!

~Abstract~ wrote:OT change?


PT isn't gonna change the sound much, is it?

In any amp where I've changed a PT or OT, the PT makes the most difference. Often times, you might not ever notice a change in tone or playability by changing the OT.

Jay wrote:

In any amp where I've changed a PT or OT, the PT makes the most difference. Often times, you might not ever notice a change in tone or playability by changing the OT.

Nice job on the PT change

I don't necessarily agree about the OT not making much of a change. A beefy OT can produce a lot of headroom if that is a desired result. Ampegs and vintage Sunns had huge OTs and lots of headroom, and big volume.


Looking forward to your clip.

And I found this loose on the board. Not sure if it caused the failure or not, but I wouldn't rule it out.

Switching supply filter cap



Looked like it failed at one point and then some butthole hacked up the fix by trying to solder the new cap to the legs of the old cap from the top of the board instead of lifting the board and doing the job from underneath. Nice.

Jay wrote:

~Abstract~ wrote:OT change?


PT isn't gonna change the sound much, is it?

In any amp where I've changed a PT or OT, the PT makes the most difference. Often times, you might not ever notice a change in tone or playability by changing the OT.

I've never needed to swap either..


But I've heard (from the big boys on TGP), that OT makes more difference...

~Abstract~ wrote:

Jay wrote:

~Abstract~ wrote:OT change?


PT isn't gonna change the sound much, is it?

In any amp where I've changed a PT or OT, the PT makes the most difference. Often times, you might not ever notice a change in tone or playability by changing the OT.

I've never needed to swap either..


But I've heard (from the big boys on TGP), that OT makes more difference...

In most cases, swapping to a "better" set of transformers is sometimes a lot of money spent for a small result, unless you are aiming for a very specific result (needing additional current for adding tubes or using different tubes, changing B+ rail voltage or current). If your transformers are really an inferior set, then there may be some kind of tonal benefit. In the case of the OT, changing for a unit designed for Hi-Fi use (like the SLO) may offer different results, or in the case of something like the Mesa Dual Rec. or some of the Mark-series amps, changing the OT for a larger core may change the tone.

In the case of this amp, the PT had failed, so replacement was needed. The added benefit with using the Mercury, or a spec'd Heyboer or some other similar "upgrade" PT, is that it has the additional heater current and B+ current to facilitate using other types of power tubes (KT-type tubes, 6550s) EL34s can also be used as long as you rewire the power sockets for the pin 1 connection, and ANY deviation from ONLY 6L6s should also have some changes done to the screen grids.

So if you're not interested in running different tubes, is upgrading the transformers in the JC amps a waste of time? I heard that Doug et al over at JC tried a JC100 with O'Netics and didn't think it was worth the effort. I like 22 enough that I've been thinking of doing upgrades. Not the mods that are intended to change the circuit, but upgrades that are better component quality etc. I thought transformers were the big deal there but maybe not.

I mean we're talking about a $350 amp here that is basically a copy of a $1500 amp. There's got to be some kind of 'split the difference' level of quality?

Casey4s wrote:

Jay wrote:

In any amp where I've changed a PT or OT, the PT makes the most difference. Often times, you might not ever notice a change in tone or playability by changing the OT.

Nice job on the PT change

I don't necessarily agree about the OT not making much of a change. A beefy OT can produce a lot of headroom if that is a desired result. Ampegs and vintage Sunns had huge OTs and lots of headroom, and big volume.


Looking forward to your clip.

Now, is the amount of headroom and volume due 100% to the huge OT, or is it the sum of the parts/design? There are certain application such as the Mesa Dual Recto and some of the Mark-series amps where the OT has quite a bit to do with the tone of the amp. In those cases, the OT is sized differently to saturate early and cause that sag and bark that defines the tones of those amps.

Then, there is the case of the SLO's OT, which is designed to have a very Hi-Fi replication of the amp's tone.

Like I said before, unless you are aiming for a SPECIFIC result, in most cases, you may never notice a difference with an OT swap that is designed as a stock replacement. But if you ARE aiming for a specific result, and you design the OT for that result, I agree with you that you will notice a change. Still, I say that changing the supply voltage and/or current of the PT will make more of an overall change.

Not arguing, just sayin'

Promit wrote:So if you're not interested in running different tubes, is upgrading the transformers in the JC amps a waste of time? I heard that Doug et al over at JC tried a JC100 with O'Netics and didn't think it was worth the effort. I like 22 enough that I've been thinking of doing upgrades. Not the mods that are intended to change the circuit, but upgrades that are better component quality etc. I thought transformers were the big deal there but maybe not.

I mean we're talking about a $350 amp here that is basically a copy of a $1500 amp. There's got to be some kind of 'split the difference' level of quality?

In the case of the JCA transformers, they are cost-effective parts designed for a cost-effective amp. If you don't want to swap different power tubes, it might just be a moot point. I did not get to hear this amp before the change because the PT had prematurely failed. Drastic differences in tone/playability would be noticed more by changing components in the circuit itself.

If you just want to upgrade the Transformers from a RELIABILITY standpoint, you MAY gain some longevity/durability with some better parts. I don't know the track record for Transformer failures, but this one failed. Not to say that they ALL will fail.

You may however notice a change in a more CONSISTANT tone if you play loud for extended periods of time, by adding a choke to your 22H. I've heard that the bottom end stays in check a lot better on the smaller JCAs at louder volumes with the addition of a choke.

Okay, here's few crappy clips with my Nikon camera, both of 6L6s and with 6550s. Left all the knobs where they were for all clips to get a raw comparison. Clips were done on the OD channel, gain at 5, TMB and presence at 5.....mids might be a little higher...6...maybe, and volume at roughly 2~ish. Bias on the 6L6s is about 35mA, bias on the 6550s is about 45mA. Cab is a stock PV 5150 cab, guitar is a Les Paul Custom with Duncan Seth Lovers. Standard tuning. Nothing fancy.

NOTE: NO modifications was done to the Screen Grid resistors. In order to run tubes OTHER than 6L6s, some modifications must be done to the SGs in order to expect any kind of longevity of the power tubes. This is just for a raw comparison.

Test with JJ 6L6s:

[video]https://www.youtube.com/watch?v=pFbJ5fjSuCw[/video]

Test 1 with TungSol 6550s:

[video]https://www.youtube.com/watch?v=v26YYrvCybk[/video]

Test 2 with TungSol 6550s:

[video]https://www.youtube.com/watch?v=7JGzzHhksms[/video]

A dollar to whomever can name the Sabbath song that I was butchering.

Lol. Sabbra Cadabra??

demonglory wrote:Lol. Sabbra Cadabra??

YOU need to do a clip of this thing when I give it back to you. I can't do it justice. lol

Great thread, this is extremely helpful. The same thing happened to my JCA50H but I am having trouble finding a suitable replacement PT... I really can't afford a MM PT and it seems like they're the only ones out there who make PTs for this specific amp. But there are some out there that I think may work. I have a question though if you can help me... If I'm not mistaken it looks like the 58V secondary isn't really used for anything? I've found several PTs with 350V, 6.3V, and 12V secondaries but none with 58V or close to.
Thanks!

Pete9 wrote:Great thread, this is extremely helpful. The same thing happened to my JCA50H but I am having trouble finding a suitable replacement PT... I really can't afford a MM PT and it seems like they're the only ones out there who make PTs for this specific amp. But there are some out there that I think may work. I have a question though if you can help me... If I'm not mistaken it looks like the 58V secondary isn't really used for anything? I've found several PTs with 350V, 6.3V, and 12V secondaries but none with 58V or close to.
Thanks!

The 58v tap is for the bias circuit. I had actually contacted Heyboer a while ago about trying to make a drop-in replacement that doesn't require selling body parts to fund and their prices were very reasonable. They also said they could make a PT that would fit in the stock location. My intent was to buy one of these amps and buy one of the Heyboer PTs and do a test install, but I became so busy with other things and never go to it.

Ok thanks. Yea it looks like I'll have to order something custom, or maybe just buy another head. I found a manufacturer, Edcor, with a PT that looks identical except has a 12v secondary instead of 58v. Closest thing I could find, American made but was affordable. But that was foolish of me because I really jumped the gun and assumed it would still work. So I'm regretting that decision but it hasn't arrived yet. When I checked the circuitry I didn't see any loose solders or anything that looked bad so that worries me if what caused the PT to fail is some needle in the haystack problem. Or maybe it was the PT itself. I played that amp pretty frequently.

Thanks for the ot pt info.