Ampeg Jet J-12-B Clone Change

[neddyboy]

I just finished my first point-to-point build which is also my first time working from only a schematic. No layout drawing! It's an Ampeg Jet, the version that uses 2 6SL7s and 6V6s for power. The amp works fine, but I can't get the trem to work. Occasionally at certain tone settings I'll hear a faint wobble, but mostly the speed pot has no effect. I have plenty of other tremolo amps so I wondered if I could bail on the trem and use that half a 6SL7 for added gain. Possible? And a side question: what is the lead that goes from pins cathodes of V2 to the + side of the speaker output? Is that a feedback loop? The schematic seems to want a resistor between 5 and 10k between the tube and the speaker out. Right now I split the difference and put in a 7.5k. How would I determine what's appropriate?

[sluckey]

I built one a few years ago. Trem works OK but I did several mods to 'improve' it. Here's my docs file...

     http://sluckeyamps.com/RCA/Ampeg_J12B.pdf

The 5-10K resistor is the negative feedback resistor I used 10K. Value is not critical.

[neddyboy]

Sluckey, you are my guardian angel. If I understand your schematic correctly, you've substituted a 2-pole switch for the speed pot, and added an intensity pot. How does that variable resistor interact with the speed switch? It seems like the switch would either be dead slow or an adjustable faster speed. Is that how it works? Could I add the intensity pot without doing the rest of the mods? And how does doubling the cathode resistor affect the sound?

[Paul1453]

I recently built the Ampeg R-12, basically the same circuit as this, and also had issues with the Tremolo circuit.

I left out the NFB loop, and finally pulled the input/trem 6SL7.
I don't have many 6SL7s, so since I couldn't get the tremolo working properly, even with the switch removed and wired as always on.

I replaced the input tube with Joe Befumo's 6DJ8 totem pole input design.
I have a ton of premium Euro 6DJ8s to use.
The 6DJ8 has a very clean and articulate response.
It still easily drives the 6SL7 PI into nice tube distortion tones.

I'm not saying this is what you should do.
For me this was a way to utilize one of my excess tubes.
I went to just a volume pot between the 6DJ8 and 6SL7.
It simplified the circuit and sounds really good to me.

If tremolo is something you really want I hope Sluckey's info helps you get it going.
I'm saving my scarce 6SL7s for other builds.   

[sluckey]

you've substituted a 2-pole switch for the speed pot, and added an intensity pot. How does that variable resistor interact with the speed switch?

I ADDED a slow/fast switch and still have a speed pot. The switch simply provides a slower range of tremolo.

Could I add the intensity pot without doing the rest of the mods?

Sure. I think it's a good idea. But doing so will not fix your problem of no trem.

And how does doubling the cathode resistor affect the sound?

No change that I could hear. I changed that resistor so the 6V6s would run cooler with my PT voltages. I'm only driving a 6" speaker so loud was not a goal. This was an old RCA radio conversion. Here's the rest of the story...

     http://sluckeyamps.com/RCA/RCA.htm

Tremolo oscillators require a strong tube. A less than strong 6SL7 may work fine in the preamp but fail in the trem osc. circuit. My 6SL7 worked fine but the trem was not as strong as I wanted. The LED fixed that by providing a much stronger trem signal to send to the 6V6 grids. I suggest you try a 5mm Red or Yellow LED rated for 1.7v to 2.2V and about 20mA. You can mount the LED on the front panel for some eye candy. The LED will not fix a trem circuit that is miswired though, so double check your wiring.

[neddyboy]

Cool, I'll try swapping tubes. I have some new 6SL7s, was using an old one. The LED thing is cool! I never think of LEDs as being useful in tube amps. Thanks!

[neddyboy]

Hey Sluckey, I inserted a 1M intensity pot as shown in your schematic, and the tremolo went from not-at-all working to sort-of working. It's scratchy and not very intense, but it's progress! I did nothing else but insert a 1m pot for a 1m resistor and it fixed itself. Huh. More diagnostics tomorrow.

Thanks for the layout drawing -- it's the first I've seen for the Jet.

[sluckey]

Odd. That 1M resistor should act exactly like a 1M pot that's turned all the way up.

[neddyboy]

Yeah, weird huh?

[sluckey]

Can you post some pics of this build?

[kagliostro]

Steve I don't see the Jet on your pages

and I'm sure you have draw a lot of layout that you published but not added to your site http://sluckeyamps.com/

Why don't you do a collection of those your works and you put it on your webpage? Surely there would be many interested

Franco

[sluckey]

K, the Jet is my radio conversion...

     http://sluckeyamps.com/RCA/RCA.htm

The board layout is not my regular style. The board I used is a cone board salvaged from a Hammond AO-41 vibrato unit. I adapted a layout to fit that cone board.

[kagliostro]

the Jet is my radio conversion...

My fault

However I'm sure you have other works that you published here and there but didn't put on the web page ....

one nice thing will be you publish a layout book (I'll get one for sure )

Ciao

Franco

[neddyboy]

Well, turns out I have a bad 6SL7 tube. I hadn't tested it before since it was fine in another amp. I'm waiting for a replacement and will keep you guys posted. The bad tube is an old JAN tube and the glass is loose in the base. I suspect all the plugging and unplugging during this build broke a lead inside, because it was fine for the first rounds of testing.

[kagliostro]

If is a continuity problem you can try to repair it

disconnect the tube from the base (use your iron on the pin)

verify the inside wires integrity, redo the connections and the soldering

usually the problem on doing that is that the base is firmly glued to the glass

but if the base is free it is possible to do with a bit of patience

Franco

[neddyboy]

Duh, I am so silly. Thanks for the reminder Kagliostro! I've taken 'em apart lots but never tried to fix 'em. It's the heaters.

[neddyboy]

OK, an update. I got the whole thing going, but I'm not into the tremolo, and it's a little lower in gain than I'd like. I have several other really sweet trem amps. Anyway, I found a schematic for an Ampeg M-15 amp. It also has a tremolo, but it's a separate tube. What I hope to do is use the front end of the M-15 on the back end of the Jet, so 2 inputs, 2 volumes, 2 tones, with each input going into either half of the V1 6SL7. I'm just not sure what to do with the leads that go to the trem circuit.

FYI I used the M-15 because I couldn't find a 2@6SL7 + 2@6V6 schematic without tremolo. I'm not an Ampeg expert, so if there's a better schematic out there for me to follow please let me know!

I've attached a standard M-15 schematic and a second one I've redrawn to show what I'd like to do. I get that the M-15 uses 6L6s rather than 6V6s, so I'm using the power section components from the Jet. Don't laugh -- this is my first attempt at this complicated a mod! Will this idea work?

[neddyboy]

Sorry, didn't attach the stock schematic...

[sluckey]

Take a look at page 4 of this...

     http://sluckeyamps.com/rocky/rocky.pdf

Or the entire conversion project...

     http://sluckeyamps.com/rocky/rocky.htm

[Paul1453]

If you have come to the conclusion that you want to eliminate the tremolo, like I did.

Then I had a choice of finding an amp schematic that used both triodes of the 6SL7 for it's input tube.
I chose a different plan, but think that if you search the Ampeg line-up you can find that 6SL7 input configuration.
Then you could just wire up your 1st 6SL7 like that and connect it to your PI/power amp section.
Something like that should have plenty of gritty 6SL7 flavored distortion, if that is what you are looking for.

I chose to save my input 6SL7 for another build.
I have a ton of premium 6DJ8's, so I just used Joe Befumo's cascode totem pole input design.
Connected it through a 1M volume pot to the output section, and am quite happy with the results.

I'm pretty sure you could use any number of 12AX7 input designs with good results too.
I have also used an EF86 as the input tube to drive this power section.
I think there are many tubes you could also try for this input section. (6AU6,6AV6,5879,etc.)
I guess it just depends on the sound you are looking for from this amp.   

[neddyboy]

Hi Paul, I'm going with 6SL7s because I have a few lying around. I scored 6 NOS JAN 6SL7s a few years ago and none of my amps use them, so this seemed like a good choice.

[Paul1453]

I'm not trying to steer you away from using your 6SL7, it is a great sounding tube IMO.

It sounds slightly different than a 12AX7.

If you are going with the 2 inputs, each using 1/2 the tube, you might be able to just disregard the tremolo connection of the M-15.

There is a reasonable amount of gain from 1/2 the 6SL7.
If that is not enough for your taste, then you could drop 1 input and use that triode as a 2nd gain stage.
That should up your gain significantly, but may also reduce your clean headroom in the process.

I find that trying different configurations is entertaining and enjoyable.
I then see which one sounds best to me and doesn't have any other issues and go with that.   

[neddyboy]

Hi guys, sorry for the radio silence. Work has interrupted my hobbies again. Sluckey, I went through your Rocky schem and tried to make a fusion between it and my 6SL7 & 6V6 amp. It was challenging because I'm not sure what I need to change from Rocky to accommodate 6SL7s instead of the 6SN7s. I also didn't want the more elaborate tone controls in Rocky, so I tried to graft in the Ampeg Jet tone circuit. I mostly left the power section alone.

I've attached a schematic that show's the work I've done. Again, don't laugh, first attempt at a schematic. I've made the changes shown and I'm getting hiss from the speakers but no change as I rotate the volume control, even with a plug in the input. No signal passing, but no explosions, no magic smoke. I drew the schematic to see if I'd made any bad assumptions that would cause it to not work. It's likely I did, but even more likely I made a mistake.

Would you be so kind as to give the schematic a look and tell me if anything looks wrong enough to fit the symptoms? Thank you!

[Paul1453]

I'm not sure V1 is configured properly with it's output coming off the cathode.   

Maybe check some of the other Ampeg designs, looking for a 6SL7 input with the 2 triodes cascaded.   

[Paul1453]

Maybe something like this will give you something to work with?

[PRR]

> give the schematic a look

Ummm. Cathodyne V2b should have =equal= plate and cathode resistors.

> 6SL7s instead of the 6SN7s

These bias quite different. And there's two DC-coupled pairs. You gotta get pretty close. You probably want to reduce gain-stage cathode resistors at-least in proportion to the different Mus, 20 and 60. While that would work for a solo gain-stage, in this case it is a poor approximation for the direct-coupled stages. Below is my estimate of where you want to land. Leave gain-stage plate resistors as-is, reduce cathode bias resistors a LOT.

V2b cathodyne plate AND cathode resistors may both be taken from the RockOLa values; they are good.

V1b cathode follower was designed to slam long wire to the bar and back (barkeep's remote volume control). Here you all in one box. V1b cathode follower can use something higher than 10K, save power and reduce filtering needs. {edit} Also 6SL7 is a less beefy tube, can't pull the 7mA asked in the original.

In between all that, use the Ampeg tone/volume network.

22K NFB resistor actually works out despite huge change in V2a cathode resistor. RockOLa was voiced for good control of speaker in general music, guitar amps usually want less NFB for more cone tone. This can be adjusted to taste after initial play tests.

[kagliostro]

This is something about I'm thinking

and my proposal for a higher gain second channel on the Ampeg Jet

using one more tube, a 6SJ7

(design based on materials kindly made available by Steve)



Franco

[kagliostro]

This is one other version

No tremolo no added tubes, based on B-15N preamp with the Jet Tone Control



EDIT: The green X is to be connected to ground

Franco

[neddyboy]

Wow, thanks guys! Lots to think about, as long as I can avoid work long enough to play around with tube amps...

[neddyboy]

Hey kagliostro, what does the green X signify in the no-tremolo Jet schematic? Where would that go? It ends up at the tremolo circuit in the original Jet. Is it even necessary if the circuit doesn't have tremolo?

[sluckey]

Green X should connect to ground.

[neddyboy]

Thank you sir!

[neddyboy]

I've been futzing around with this chassis for so long I forgot to look at voltages. Turns out the PT only puts out about 280v. FYI this thing started life as a mono hifi along the lines of a Grommes. I have an extra 5E3 Tweed Deluxe PT in the shop that puts out 355v. The no-tremolo Jet schematic shows 310v going into the rectifier. Is it possible to change out the power supply resistors to soak up that extra voltage? How would one figure out how much resistance would drop the voltage the correct amount? This is the first project where I haven't been able to buy the correct transformer off the shelf so this is new to me.

[PRR]

> PT only puts out about 280v.

So? Were you needing to play Huge Stadiums? Lots of good music has been made without huge voltages.

> Tweed Deluxe PT in the shop that puts out 355v.

On the face of it: if it fed two 6V6 in a Fender, it'll probably feed two 6V6 in an Ampeg-copy.

[neddyboy]

I'm not worried about playing stadiums for sure. I wasn't sure a lower or higher voltage from the PT would cause problems. Looks like not.