Retubing my Jet City

[marnold***]

I was toying with the idea once again of retubing my Jet City. Fortunately, I haven't had the problem that I posted about elsewhere again. Nevertheless, my plan was to put in better quality tubes and keep the originals as known-good backups. It would seem to me to be best to get a kit from Eurotubes. Here is their page for JCA20H kits:

https://ssl.eurotubes.com/cart/index...ategory_id=248

I was just wondering if any of you t00b experts had any advice. I've heard clips of my amph retubed with the standard set. It sounded like the stock amph, only moreso, if that makes sense. Basically it was a little smoother on the top end. I don't think the low-output EL84s would be what I wanted. While I don't need the full power of this amph, I also don't need to reduce its already limited headroom. My initial thought would be to go with the standard kit. The high gain option sounds interesting, although once again I'm guessing that it'd take the headroom with it. Also are the gold pins worth the upcharge?

[tunghaichuan***]

I was toying with the idea once again of retubing my Jet City. Fortunately, I haven't had the problem that I posted about elsewhere again. Nevertheless, my plan was to put in better quality tubes and keep the originals as known-good backups. It would seem to me to be best to get a kit from Eurotubes. Here is their page for JCA20H kits:

https://ssl.eurotubes.com/cart/index...ategory_id=248

I was just wondering if any of you t00b experts had any advice. I've heard clips of my amph retubed with the standard set. It sounded like the stock amph, only moreso, if that makes sense. Basically it was a little smoother on the top end. I don't think the low-output EL84s would be what I wanted. While I don't need the full power of this amph, I also don't need to reduce its already limited headroom. My initial thought would be to go with the standard kit. The high gain option sounds interesting, although once again I'm guessing that it'd take the headroom with it. Also are the gold pins worth the upcharge?

I've never played a Jet City amp, so I can't comment on the tone. I like Eurotubes EL84s, and have used a number of them in my amp builds.

I'd probably go with the standard set. I wouldn't pay extra for gold pins. All other things being equal, I doubt anybody could pick out gold pin tubes over ones with non-gold plated pins. The only advantage I can think of is that gold pins won't corrode, but I doubt the tubes will last long enough for that to to be an issue.

If you like the amount of gain your amp produces, I'd avoid the high gain sets. The tube itself may be capable of more gain, but what determines the amount of gain in an amp is the circuit; the resistors, caps, and B+ connected to the tube. If you need to push your amp over the top, I'd use the boost setting of your DVM OD/Boost pedal.

Also, keep in mind that your amp is a fixed bias amp, so you need to have a tech rebias the amp every time you change the power tubes.

[marnold***]

Michael at Eurotubes got back to me on the higher gain pre-amp tubes:

The high gain ECC83S's have about 10-15% more gain than standards which adds a bit of bite and a sharper dynamic but does not hurt the clean tone. The other attribute that these tubes have is a faster filament rise time which is great for an EVH type style and is also good for fast palm mute and metal styles. They certainly won't replace an od/distortion pedal but if you're looking to coax a bit more gain and attitude from the overdrive channel then they are a good way to go. They don't really breakup faster than the standard ECC83S's, they just have more gain on tap. Imagine if your gain knob went to 11 or 12 instead of 10...

Sounds like it could be a good option, although as you pointed out, Tung, I'm not sure how the rest of the circuit is set up.

[Zip]

I also don't need to reduce its already limited headroom.

I retubed mine with JJ's, but I also installed the BitMo mod kit. It now has tons of clean headroom, and can get really crunchy.

From their instructions: "I do recommend changing the V2 preamp tube to a 12AT7 or other lower gain tube and V1 to your 12AX7 flavor of choice."

[kiteman]

Fixed biased or cathode biased?

Anyway, ever consider some used (or new) old stock tubes?

[marnold***]

Fixed biased or cathode biased?

Anyway, ever consider some used (or new) old stock tubes?

It's fixed bias with a trim pot for adjustments. From what I understand they are shipped biased slightly cold from the factory. Here's a Youtube video on biasing it. If I were to do it myself, I'd rather get a bias probe.



I don't have the cash for NOS and I don't know that I really have the desire to research.

[tunghaichuan***]

It's fixed bias with a trim pot for adjustments. From what I understand they are shipped biased slightly cold from the factory. Here's a Youtube video on biasing it. If I were to do it myself, I'd rather get a bias probe.



I don't have the cash for NOS and I don't know that I really have the desire to research.

I've seen that video and it makes me cringe. That guy is sticking both hands inside the chassis at the same time. Not a good idea. At the very least you could get a nasty shock. Worst case scenario is that the current would stop your heart and kill you.

The information in that video is valid, but the technique is bad.

To do it right you need a retractable spring clip that attaches to the tip of your DMM. That way you can clip one lead where it needs to go and use one hand to probe inside chassis.

This is important enough to say again: never stick both hands inside a tube amp. You should always keep one hand behind your back or in your pocket to resist doing so.

[Tig***]

Yes, the retractable hook clips work great. Here's a set on eBay for $2.99.
http://cgi.ebay.com/NEW-Test-Hook-Cl...-/280576916134

[Ch0jin]

At the risk of going too OT, as Tig and Tung (sounds like a kids TV show) have alluded to, both this style and the regular alligator clip style test leads should be a part of every electronics toolkit from hobbyist to pro. If you don't have some, you should get some. At least a red and a black one.

[Commodore 64***]

Duly noted. Thanks.

[marnold***]

Yeah, I've heard that you want to keep one hand in your pocket when working on these amphs. I'd like to learn to do it myself, but I don't think my wife wants to be a widow just yet.

That guy has probably done that a lot, but he does seem to be a bit cavalier.

[marnold***]

I called just about the only place that does repairs in Green Bay today and asked how much they want for biasing. Their tech said that it was about 3/4 hour's worth of work at $50/hour, so roughly $38. That seemed like a very reasonable price to me. They have Sovtek and JJ tubes in stock. It would be a bit more than ordering them from Eurotubes, but I think I'd rather support the local guys. I also was impressed that he didn't try to "sell" me on tubes. He suggested just trying a JJ in V1 at first and seeing how I like it. Sometimes mixing and matching works wonders.

He mentioned that when he sets the bias he runs it through an oscilloscope to look for some kind of distortion that I was unfamiliar with. He also said that he likes to keep it and play through it periodically for a couple hours because if a tube is going to fail prematurely, it will likely do it right away. I was pretty impressed talking with him. Sounded like a knowledgeable guy. He also said he wanted to talk with me to see what I was looking for tonally.

[Ch0jin]

Crossover distortion perhaps?

I think that'd make sense for power tubes. With one tube amplifying the positive part of the wave and one tube the negative; crossover distortion occurs where those two points don't line up correctly, distorting the waveform. (I was struggling to explain without a diagram).

[FrankenFretter***]

I put some JJs in my Night Train (power valves only, left the preamp valves alone). The amph seemed to have a more "open" sound to it after the tube swap. I really feel like there's a noticeable difference, and it was a cheap upgrade. Lucky for me, Eurotubes are the local guys.

Good luck with your toobs, Rev! Btw, the tubes I took out of mine were Sovtek.

[marnold***]

Well, I decided to revisit this thread because I took it into Dick's Music in Green Bay to get a full set of JJs put in and get it biased. I just got off the phone with the tech. The amph is done, but I can't go get it because my vehicle needs new intake manifold gaskets (sigh). The tech couldn't stop singing its praises. He said he was going to try to get one himself. He praised the layout (saying it would be easy to service for a PCB amph), the transformers, and the quality of the resistors. It seems better constructed than the current crop of low-power overseas amphs. That was nice to hear. He did say that by design the screen voltage is relatively high, so I should look at new power tubes every 2-3 years or so. Thankfully those EL84s are cheap. He said you could tell by looking at it that it was a Soldano design--it's just like the Astroverb/Atomic 16. He biased the JJs a little warmer, about 70% or 8.5 somethings (volts? watts? help me out here amph gurus). I said I wanted it warmer but not at the expense of tube life.

Now I'm even more excited to get it home. My cab is getting lonely.

[Robert****]

Good to hear. Is that the same amph I have? I got mine from bcdon. I haven't used mine much.

[marnold***]

Good to hear. Is that the same amph I have? I got mine from bcdon. I haven't used mine much.

Same thing, yes.

[marnold***]

I've got my Jet City back! I haven't had a ton of time to play through it, but here are my observations thus far:
1) The amph is quieter--considerably less noise.
2) I may have a pinch more clean headroom. That was unexpected. I'm going to test that more.
3) The full range of the preamp gain knob is now usable. It gets a bit wooly at 9 (the top end of the range), but previously it became a bit of an indistinct mess. Even at 9, the noise isn't mind-bending.
4) The presence knob is now usable. I can turn it up a ways and not enter "rabid bees in a tin can" territory. I still would never turn it up past six or so, at least not in my office. My guitar is pretty bright, so it might be more usable with a very dark guitar.
5) I can see the tube glow better through the metal baffle, so it looks cooler. That has to count for something, doesn't it?

It turned out the way I hoped--it's still my amph, but moreso. I kept the Chinese tubes in case I'd blow one or if I want to swap around preamp tubes or something. I'll try to post some clips soonish here.

[marnold***]

OK, here's some clips. I _thought_ I did a lot more recording than I had. I think I've got the same settings for both. I know I do on the amph. Not so sure about the pedals, but the settings really haven't changed over the years. I apologize for playing In My Dreams again.

In My Dreams original tubes
In My Dreams new JJ tubes

I got the same string noise as before, but I wanted to keep everything as similar as possible.

The noise chain remains: Rev. Rawk->DK2M->Thesis 96 OD->DD-6->JCA20H

I think the differences should immediately be noticeable. I really wish I would have recorded more without the pedals since that colors the tone quite a bit.