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1/4" phone jacks, grid leaks, grid stoppers...


Erik Mandaville

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Just to confirm with some of you familiar with this:

I've removed the volume control from a small, single-ended 6BQ5 amp in order to use it as a headphone amp. Since I don't want to drill a new hole in the chassis for the phone jack, I've taken out the existing stereo pot, and only need to enlarge that hole slightly for the headphone jack. So, this leaves the amp without a volume control.

In place of the volume control, you can see toward the bottom left and right of the picture, two brown resistors. They are connected at one end to the shielded input cable, and the other to ground. They establish the 100K input impedance for the amp in place of the volume control. The blue resistors are the grid stoppers, without which there is a slight amount of oscillation. This looks correct, doesn't it?

I'm going to be using my tube preamp with this for volume and channel balancing.

One more question regarding the phone jack follows in the next post.

edit: this file is a little too big. Let me reduce and repost the picture.

Erik

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Craig:

I have no idea what brand they are -- just had piles of the 100K ohms in my bin and the 10K grid stoppers were in some other bin they shouldn't have....bin in.

So. I think I'm on track with this. The volume control was 100K ohms, so I just yanked that out and put the fixed resistors (same value) in its place. Actually, I could probably go to 50K ohms, which (might) help with noise a little bit, or maybe not at all. I'm pretty sure the stoppers are the last in line before the grid, and in series with it. 10K is high, I know, but it does the trick without any HF roll-off or other problems I can detect.

Erik

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Here's the ultra high-end, audiophile grade phono jack I'm going to use. I just can't remember whether the tip of the jack is the left channel or the right!

I've done this bunches of times in the past, but I simply can't remember, and don't want to pull the cover on the Moth amp to see how I made the connections there. So, if any of you know, please share!

Thanks,

Erik

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Mark:

Okay, that's what I did. I don't know where you get all of these perfect little diagrams, but they always are exactly what's needed. I had a 100K ohm pot, and replaced that with 100K fixed. Honestly, since I have the hood off on this thing, maybe I should experiment some more with that grid stopper value.

Thank you both for having a look!

Erik

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Mark:

"The "tip" is the left channel. As for the diagrams I draw them as needed-----MD"

Great, thanks!

Okay, it's the drawing part I want to know how to do, then! What I always resort to are pencil drawings on paper, and then a picture of the diagram I drew, and then load and send via email or whatever. Are you drawing these with a computer program, or something?

Erik

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Mark:

That's neat. I really need to learn how to use CAD. All the industrial/mechanical drawing I did in school was by hand. RIT had (has) a very good design program (as in courses of study), but I had a teacher (outstanding, too!) who I think was worried that all the manual skills of the past would be eclipsed by CAD software. It seems that is actually the case, too. Anyway, it would be great to learn to use some of what's available.

Erik

Oh: This amp uses 1/2 of 12AX7 for each channel.

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Well, Erik, your circuit certainly looks good to me (as if I would know...heh).

I think my dad also uses some sort of CAD program for schematic diagrams, but I don't recall who it's by. Hell, he still draws 'em up on napklins or scrap pieces of paper on his workbench (the ol' traditional ways are hard to beat). He just recently cleaned out the garage and found his old '88 Generic AutoCAD software on 5.25" floppies that were just donated to The Salvation Army!

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All my schematics are permanently etched into my brain or hand drawn and very unreadable to all but me[;)]. Erik try a 1K grid stopper. I believe you do not have a scope to check for oscillation so maybe you would be better off just stay a bit higher though. I'd say 10k is kind of extreme and should result in some excess HF roll off. But hey you may like it better that way? Use your ears...

Craig

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Craig:

Betcha I could read your schematic! ;) No, I don't have a scope, but I'm going to talk my dad into letting me borrow/buy one of his. 10K is higher than I've ever used before, and I think it was actually on the schematic I used to build the amp. I actually tend to like more in the way of HF information. Some people equate that with hash and grain, which is completely wrong. I'm talking about good quality HF extension, not brittle playback. I know what that sounds like and don't like it.

Mark: Thanks, too for that information. I've got a big assortment of common resistor values, and can give between 1K and 5K a shot tomorrow. I've even built designs that used a couple K or so, but sounded fine without anything (which doesn't mean that a small amount of parasitic oscillation wouldn't show up on a scope)

This project is done, and it's working really well with my tube preamp. It's actually quieter than the Moth was as a headphone amp, and that's good.

Jim: I can see your dad drawing out radio schematics on napkins! But, I have to say that the presentation and collection of information in the user's manual he sent me with the radio was really impressive. Speaking of which, I unfortunately can't get BBC anymore because they aren't broadcasting to North America. Lots of other stuff to listen to out there, though.

Erik

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Erik, you got a user's manual with your shortwave receiver? When my dad built my tube shortwave radio, all I got was a few verbal instructions on how to use it, and then was left on my own to figure out the rest (LOL).

What's this? BBC isn't broadcasting to North America any longer? Whassup with that? Did our country's government piss off our biggest (and only) European ally?

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I'll tell ya: New (orange drop) coupling capcitors. Please note I'm not about to go off on a rant on how I think orange drops hung the moon or anything. It wasn't in the original plan for the headphone modification.

The chassis is so small and tight, it's really very hard to work with. My one concern from the beginning was the close proximity of one coupling cap (blue rectangular thingy) to the hole that need to be drilled out so the headphone jack would fit. It's about an inch or even less from the hole, and right in front of it.

So, try as I might to brace my arm and drill slowly and carefully, those of you who have done chassis work with hand-held drills probably know where I'm going with this. Aluminum is softer and easier to work with, but as soon as the bit broke through the other side of the steel front panel, it pulled, very hard, all the way into the chassis. Ugh. In the process, knocked out that coupling cap and one B+ lead to the output tube. Both of those were fortunately very simple repairs, and I replaced the caps in both channels so they'd match. Works perfectly, and is really just about dead quiet in terms of background noise, where the Moth amp had a very low level background hum (with phones).

Erik

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I appreciate that, Craig. Thanks. I built this little guy the first time 'round about....uhm.....8 years ago, 9 years ago? Something like that, and decided to take it completely apart and rebuild it a couple of summers ago.

Jim: You're lucky though, because you get to talk to your dad fist hand (first voice?) all the time. That kind of radio is NOT user friendly, and takes some very fine control and practice to get the hang of it. That's also NOT a design issue, it has to do with the kind of radio it is. My dad worked with it as if he had been doing it his whole life, which is actually the case.

I love that radio, Jim. I'm really proud to have it!

Erik

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From the looks of those photos, Erik, it surely seems to me that you're more than capable of building a radio similar to ours as long as you have the schematics right there in front of you. If you can get into tight chassis underpinnings like that amp and still have the patients to straighten out the wires and line up the components, then there's nothing you can't do IMO. You impressed Craig...that's a vote of confidence if I ever heard one!

You're right about those shortwave receivers not being user friendly. When Dad was showing me how to fine tune a foreign broadcast thousands of miles away, he was twirling knobs left and right with both hands at the same time...I still can't find 'em all the way he does it (and yes, our dads have been doing that their entire lives, haven't they?).

PS - Micheal likes your macro work too...you could be a professional photographer and not even know it.

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