Ampeg Jet Weird Design

[Helmholtz]

Lowering the left 470k will increase feedback and reduce input gain.
Instead of lowering the right 470k I would try to increase the 10k grid resistor to increase modulator drive.

[Chuck H]

Have you considered trying a 6C10 tube in V2? That wouldn't fix the trem issue, but it would increase gain a bit. Slightly more affordable than the 6BK11 tube too.

As to the trem issue, I don't even have my head wrapped around the circuit sufficiently. As Helmholtz mentioned, raising the resistance of the 10k shunt resistor would seem to make more sense than decreasing the 470k series resistor.

EDIT: Nevermind, I see you have 6C10's now and don't want to purchase more.

[Justin Thomas]

Thanks Chuck,

Yeah, when the 7591s are the most easily sourced tubes in your amp, you have a problem!

I can sort of figure out the trem circuit by folks' descriptions above, and it sorta makes sense. But I'm sort of resigned to this thing just being what it is, and that there's a reason Ampeg abandoned it! (cuz it don't werk so good).

I've upped the 10k as Helmholtz suggested; tried everything from 20-50k. Settled on 22k because it gave marginal improvement but going higher didn't seem to do much more. The amp itself aounds quite nice, better when I get a proper speaker in it. I've learned the hard way, Ampeg OTs do not like a mismtch & being asked to play loud.

I think that this trem circuit is kinda like that "local negative feedback on the power tubes" in my AB165 Bassman: "well, that was fun, but we won't do it ever again..."

I'm gonna clean up the wiring a bit, put the back on it, enjoy it as is, & stay open to any more suggestions that don't involve new tubes or silicooties. Diodes okay.

It's also interesting to me that Ampeg never moved to the LTPI & stuck with cathodynes & paraphases for so long.

Justin

[Helmholtz]

The input triode is wired as an inverting amp with negative voltage feedback between plate output and grid input. Its gain is (mainly) determined by the ratio of feedback series impedance between plate and grid (essentially 470k+47k at audio frequencies) divided by input impedance (47k+guitar impedance). The modulator tube taps the impedance between plate and grid, shunting part of the feedback signal to ground and thus periodically reduces feedback thereby periodically increasing gain.

Consequences of this wiring are low input impedance (47k+ guitar impedance) and low gain.

[pdf64]

FYI the benefit of diode biasing is that they're effectively 'self bypassed', right down to dc (and so full gain down to dc), so no worry that the bypass cap isn't big enough or that its ESR is too high

[Helmholtz]

FYI the benefit of diode biasing is that they're effectively 'self bypassed', right down to dc (and so full gain down to dc), so no worry that the bypass cap isn't big enough or that its ESR is too high

True, but dynamic impedance determining local feedback and gain depends on current and is not very well defined (though mostly low enough). Also stabilization of the DC operating point is greatly reduced compared to a resistor and a bypass cap.

[Justin Thomas]

Y'alls just went right over my head.

I'll just say that the 250uF bypass cap made no audible difference except to make it take 2 whole seconds to get the trem on instead of less than half a second.

Maybe I'll still try some plain old diodes there. 2? 3? 37? I've actually got some quite ancient ones that would look pretty cool in there...

Justin

[Chuck H]

re: the 6C10 tubes. They can be found more affordably on *bay tested "strong" or "NOS" (in a tube tester at least). It looks like you can count on $20 to $40 bucks each. Seriously doubt you'll find two for $10 like the one listing, but you certainly won't be paying tube store prices that are typically over a hundred bucks. Here is a listing of sold 6C10's:

https://www.ebay.com/sch/i.html?_fro...p2045573.m1684

I know you want to avoid spending $$$ on a novel amp you're not even crazy about, but you'll probably need tubes for it some day and they ain't gettin' cheaper. Plus, I doubt Ampeg sold the amp with a trem so anemic that it sounds like a ceiling fan is going in the room (that was a good one ) And since swapping the tubes produces NO trem I'm thinking a probable tube issue. It's also possible you have a circuit failure (leaky capacitor, etc.) throwing things off. But I believe Ampeg would have seen to it that the design functioned before sending it to market. So sumpins off with yours.

[Justin Thomas]

The input triode is wired as an inverting amp with negative voltage feedback between plate output and grid input...

Consequences of this wiring are low input impedance (47k+ guitar impedance) and low gain.

Hey, wait a minute... I've seen something like that before. In my AB165 Bassman. Okay, for no other reason than shites & grins I'ma pull one end of that 47k resistor up just to see what happens. Don't anybody tell me!

Jusrin

[Justin Thomas]

Hey Chuck,

It's "better" than the ceiling fan thing now, just not near "swamp." It's definitely an improvement but when you're used to a 62 Concert or BF bug-trem, weeeeell...

I agree that Ampeg didn't generally issue stinkers, but they also never did this again, and the Jet was not exactly their "professional" model (actually their cheapest one - this was Ampeg's "Champ"), and I think this only lasted maybe a year? I'll just call this one their tremo-dud.

And to put it a bit more bluntly, I just don't HAVE any money to throw at it! I'll choose to keep my phone on. Although... I could throw them on the ol' tube tester cuz it has a Compactron socket.
Justin

[Helmholtz]

I'll just say that the 250uF bypass cap made no audible difference.

Sorry that it didn't help. The negative result indicates that you most probably don't have an LFO problem.

except to make it take 2 whole seconds to get the trem on instead of less than half a second.

That's what I meant with "turn-on delay".


A 250µ cap has an impedance of 64 Ohm at 10Hz. Two silicon diodes in series typically have a higher impedance than that at 1mA, so I don't expect an improvement.
Of course the diodes will not produce delayed trem turn-on.

[g1]

Sorry to jump in the middle here about tubes you don't want to talk about ( ), but since the issue was brought up...
Did this circuit work correctly for you (or others) with the 6C10 instead of the 6BK11 ?
If it turns out that this is the problem, it might be a lot easier if the question was simply "how do I convert a 6BK11 trem to work with 6C10".

I think it's important to determine whether this is possibly the problem, or if tube type can be eliminated as the cause.

[Chuck H]

Sorry to jump in the middle here about tubes you don't want to talk about ( ), but since the issue was brought up...
Did this circuit work correctly for you (or others) with the 6C10 instead of the 6BK11 ?
If it turns out that this is the problem, it might be a lot easier if the question was simply "how do I convert a 6BK11 trem to work with 6C10".

I think it's important to determine whether this is possibly the problem, or if tube type can be eliminated as the cause.

As far as I know the two tubes are very similar, except that the 6C10 has more gain on one of the triodes (6BK111 70-100-100, 6C10 100-100-100). I might be missing something, but I don't know how this might negatively affect anything in the circuit shown. Unless the biasing and current capacity of the higher mu triode is weak in current for the task at hand? I didn't examine the tube data and schematic to see which triodes in the circuit would be lower mu.

[Justin Thomas]

G1,

It never had 6BK11s to begin with. It came with a 6K11 & a 6AV11, both of which are mostly lower gain triodes.

Chuck is correct on the differences between a 6BK & a 6C. They are direct plug & play replacements, EXCEPT for pin 8. The 6BK11 lists an internal shield; the 6C10 lists an internal connection. The Jet had pins 8 for both tubes wired to ground. I checked my 6C10s and can verify that there is absolutely no connection to pin 8 on either, so I yanked the wiring associated with the shields. It cut the hum a decent bit, I might add. Ireally don't like having useless wiring in my amps, and I'm pretty sure nobody will ever put 6BK11s back in it. And even if they do, they'll find the discrepany. It's not like putting B+ on a heater.

In short, I have no idea if it worked with 6BK11s, the amp was super-quiet & had zero trem with the 6K11 & 6AV11, & I picked up the 6C10s @ the shop I bought the Jet at. The owner said he'd bought the 6Cs for Super Champs back in the 80s & never replaced a single one, so he let me have them.

I've got plenty of good 7591s on hand - every time I buy an Ampeg I get Sovtek replacements & keep the good oldies.

Justin

[Justin Thomas]

So, just to screw around... First I tried lifting the leg of the .01/47k that was going into the input grid. My trem died conpletely & the overall volume of the amp went up & it was clearer. If I never used trem I'd say it makes it a kickass little rawk recording amp. But, I like trem, so...

I put it back & lifted the leg of that .1/470k. The trem came back and a bit stronger than with it in place, AND the amp is clearer with more pleasant highs. It's like the tone control is more useful & I'm not wishing it "went to 13 instead of 12." So I'm going to just gank that .1 so there are no loose ends floating about & play the amp a while. May slow down the trem a bit with the usual cap change.

It's not "swampy" yet, but definitely more pleasing than it was. And a lesson in why you shouldn't change a useful circuit just because you can and then sell it in a commercial amp!

Justin