Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on November 18, 2012, 09:34:51 am
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My buddy dropped off a cheap little plastic AM radio/coffee maker timer to fix. I promptly informed him the thing isn't worth the calories he burned carrying it in the house. The usual series filament job w/ no PT. Oh well.
But for some reason I kept it laying around and eventually noticed it does have this teensy OT driving a little 4" alnico speaker. The tube driving it is a 50C5 w/ the following vital statistics:
Plate R 10k ohms
transconductance 7500 microhmhos
load R 2500 ohms
2.3 max watts
Since I simply must tinker with something at least once a month, and the 1949 Philco & AIMS Dual Twelve I just rehabbed have gone back to their happy homes, I'd like to see about making this little POS into a tabletop guitar amp of some kind. My wife loves how it looks, go figure.
Two questions:
Have we any sources for small PTs w/ low voltage & current ratings?
What kind of power tube should I use with this little setup? A 6BM8? I have tons of 6AQ5s also.
And yes, I will pay the guy something for it.
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I should clarify that I don't want to use the 50C5, but rather a 6v filament small power tube approximating the above characteristics.
And I suppose Doug's 6G15 PT will fit the bill.
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What kind of power tube should I use with this little setup? A 6BM8? I have tons of 6AQ5s also.
How about something like the 6GW8, used in some of the Hammond organ amps? I have an AO-44 that I added a 1/4" phone jack to. Doesn't sound bad for a small amp with these in a PP setup.
http://tubedata.tubes.se/sheets/049/6/6GW8.pdf (http://tubedata.tubes.se/sheets/049/6/6GW8.pdf)
It's a similar tube to the 6BM8. You'll get a pentode and pick up a triode in the package.
Jack
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Maybe a Firefly? Except a paralleled 12AU7 into the SE OT?
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Shameless plug here...have you checked my phono/radio project here http://www.el34world.com/Forum/index.php?topic=14602.0 (http://www.el34world.com/Forum/index.php?topic=14602.0)? I used a 12SQ7 and 50L6, but you could probably use the same values for the 50C5 (don't know for sure..haven't checked the datasheet). You can sub a 12AV6 for the 12SQ7 if that's what your radio had. I'm actually surprised by the volume of the resulting amp - through my BJ III 12" speaker, it really gets loud. You will need pedals for overdrive/distortion, but if you like it clean with slight breakup and mild compression at the max, it's not bad at all. I've read on other forums where people were pleasantly surprised by how good those "junk" radio tubes can sound. The other benefit is that you can re-use a lot of what you have (sockets, xformer and speaker, possibly volume pot). The Triad N-68X transformer can be wired for 60VAC output instead of just straight isolation. The AA4 is a similar variation that should have more gain, still using radio tubes.
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> What kind of power tube should I use with this little setup?
I like 50C5 and friends.
Use an Isolation Transformer, then run an LPB pedal to the top of the Volume control. Done. (For extra fun, re-jigger the IF tube as Triode and use it as a first audio stage before the volume control.)
The table-radio tubes and their transformers were designed for "Good" sound (at low-low-cost, of course). They overload mellowly.
If you realy can't live with 50V heater, bang a hole in the top so 6Y6 will fit. Same tube, more of it to cover industrial applications, and your 6V heater.
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As always, the simplest (and cheapest) solution is probably the best. Thanx for the reality check!
Where do I procure an isolation transformer?
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Mouser and Digikey should have them.
Brad :icon_biggrin:
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TRIAD MAGNETICS - N68X
ISOLATION TRANSFORMER
Secondary Voltages: 1 x 115V
Current Rating: 435mA
Power Rating: 50VA
about $12
http://octopart.com/n68x-triad%2Bmagnetics-996690 (http://octopart.com/n68x-triad%2Bmagnetics-996690)
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PRR, I don't want to hijack the thread, but for future reference...
If you want to strap the IF pentode as a triode and use it as the first gain stage, I'm assuming you need to disconnect the coupling xformer going to the IF grid and apply your signal there...should I also bypass the next coupling transformer and go direct to the audio amp using capacitor coupling? I'm assuming the frequency filtering of the transformer stages would make things a little funky.
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> I'm assuming
Nah. I like a little 445KHz zing on my strings.
Nah. As you assume: identify the pins, snip everything except heater off that socket, wire cathode grid and plate the usual (audio) way. Try 100K on plate and 1K-10K||5uFd on cathode, 0.01uFd to top of Volume control. B+ is available on one of the IF can pins.
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Ha...sounds like an interesting marketing line.
"We use authentic high-frequency modulation to add zing to your strings! Just like Grandpa used to do! The ultimate in mojo tone."
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I love how these threads take on a life of their own.
I've ordered the iso tranny and a couple of filter caps. Hopefully this project won't cost much more...
You guys make it sound so easy. It probably is, but I'll have to do it the hard way; i.e. tracing the audio signal back from the 50C5 grid to a couple stages before it and trying to work the signal in there somewhere. I hate not working from a schematic even if it is a simple circuit. I don't understand radio.
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> AM radio/coffee maker timer
i.e., a Clock-Radio, perhaps with an extra external timered outlet.
That complicates the re-wiring slightly.
The clock has to get power ALL the time. (I know you don't need another clock, but you can't have a clock that's wrong most of the day.)
The clock is fairly safe. (Little finger-exposure, teeny fire risk.)
OTOH, the isolation transformer *probably* should not be live 24/7. It's probably OK, but it does vampire a little power, it won't live forever, it may stink when it fails, and leaving it on hastens that day.
So we have to work-out how the clock and radio are inter-wired, so we know where to put the isolation. Also how the new ground wire goes.
Wires may be P-2-P or all Y-ed together. They may all be black or they may be black/white or who-knows. The trigger and switch will be hidden on the clock motor, but the wires will run about like this.
You also have a percolator timer? (No Mr Coffee drip when this radio was made!) This may be the same or a separate switch. There may be a switch to chose radio, coffee, or both (or neither). If you are not needing a coffee timer, you can probably ignore the outlet and leave any switching on Radio.
IAC, a Lamp Limiter will be very wise.
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> I don't understand radio
Don't need to. These rigs are all the Same!
There's a tuner to pull weak HF waves from thin air and detect any audio.
There's an audio amplifier to boost the modest detector output to fill the room.
There's a power supply to feed tuner and audio.
One small problem: for simplicity, the last part of the tuner and the first part of the audio are in the same bottle. That means the schematic shows the interlink (including volume pot) all scrunched-up sideways under that bottle. So it looks different from a straightforward guitar amp schematic. But it's very much the same.
Image below has the "tuner" part blocked-out in pink. For now, ignore all that stuff.
The detected audio is applied to the top (left side) of the Volume pot. Coupling cap, grid resistor, a triode very much like a half 12AX7. Plate resistor, coupling cap, grid resistor, a power bottle very much like a short fat (lo-volt hi-current) 6V6. OT, speaker.
Notable differences from the 2nd 3rd stages of a Champ or Jr: grid-leak bias on triode. Works fine at this signal level. Plate resistor higher than Fender would use. Added caps on both audio plates to kill treble and radio squeal.
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And feel completely free to ignore anything I say...I'm mostly wrong and a complete noob compared to PRR and Sluckey, etc. The stuff I was asking about was how to use the 12sk7 in the "pink" section to add another gain stage - you should completely ignore that stuff for right now. Looking forward to see how your project comes out - maybe inspire me to buy some of these lousy radios I come across all the time.
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It's like you saw inside my brain while I puzzled out why the 12AV6 (12SQ7) cathode was tied directly to ground! And, for all intents & purposes that schem. describes what I have. So far the only mod I can really see or understand doing, is to bypass the 50C5 cathode resistor. But these are preliminary findings, I have too many honey-dos to get deep into it at the moment.
I have not perused the clock connections.
BTW that is a lovely clock powerpoint.
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One thing to be aware of, from my reading, that grid-leak bias arrangement on the 12SQ7/12AV6 works well as it is, but can be very lousy if you try to use pedals/boosters in front of it. If you plan on using pedals, you will probably want to re-arrange that to the typical cathode-bias setup like PRR suggested earlier.
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> grid-leak bias ... can be very lousy if you try to use pedals/boosters in front of it.
As-shown, it is after the Volume control. You "will" keep that turned-down if you want "clean", or even "clean reproduction of dirty fuzz".
And as-is, it takes 100mV input to bring 50C5 to its full Watt output. It will feel tame. You will want LPB or similar not-so-crazy (and controllable) boost in front.
When turned-up, the 50C5 will overload before the 12AV7. By the time 12AV6 starts squooshing, the 50C5 will be beyond-bent into holy-smoke overload.
It would be different if we ran it guitar/booster -> 12AV6 -> Volume -> 50C5. Now an input can overload 12AV6, before signal gets to any control. (You could turn-down the guitar, but who does?)
Oh, BTW, you can do guitar-in and still keep the AM radio function. Could be as simple as a switching jack-- the wire from the detector to the top of the volume control goes to switch-finger, the plug-finger goes to volume control.
> use the 12sk7 in the "pink" section to add another gain stage
Yes, if you do not want radio the IF tube is idle. Leave the RF IF heaters alone, you need them (or pointless added parts) to keep heat on the other tubes. Look-up and clip K G1 G2 and P. K goes through say 3K to ground. G2 is your new input. G2 and P strap together to 100K to B+ (which is one leg of the IF can next to this). A 0.01uFd coupling cap to the Volume pot.
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> 12AV6 (12SQ7) cathode was tied directly to ground
Aside from being a penny cheaper:
We need a diode (or two). A cathode-plate structure is expensive, adding a plate near an existing cathode is cheap. The other tubes work hard, but the 1st Audio tube doesn't. Its heater-cathode can easily handle a diode plate (or 2) also.
But for several reasons, this diode cathode should return to Zero volts DC. Shared with the triode, that forces triode cathode to zero V. That can be awkward for large signals. But signals are really quite small. 50C5 plate needs abot 7V peak audio signal. 12SQ7 at 90V 0 220K 470K gives gain near 45. 7/45= 0.15V or 150mV. That's just about what the gridleak bias stage can do clean.
And in normal home use, you didn't turn-up within half of max loudness. So largest normal (non-party) level at 12SQ7 grid is maybe 70mV peak, 50mV RMS.
With the two tubes at these levels there is some distortion cancellation between them. Even though individually they run 4% and 9% THD at max, the sum may be closer to 5%.
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would someone mind describing how an "isolation trans" gets wired into the circuit please?
Jack D
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Refer to PRR's first pic... Using a 3 conductor power cord, connect the black and white wires to the iso transformer primary. Connect the green wire to the radio chassis. Cut off the 2 prong AC plug from the original power cord. Connect the secondary of the iso transformer to the orig AC power cord.
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PRR you are a saint for taking the time to explain what's going on in this little throwaway radio.
Guys I am not looking for a tone machine, I've already got some of those. I'll see what I've got going on with the ckt as is and if it sounds OK, I have a nice little convenient amp right there in the house. I don't use pedals so grid leak bias may be a nonissue. My neighbors have called the sheriff out on me a few times, even with the Vibrochamp, so it'll be nice to get a little tube distortion without the hassle...
When the parts come in I'll see if the thing is even capable of making sounds; it was DOA when I got it. Stand by for updates.....
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That was the impetus for my little 50L5 project...something small and cheap, grab-and-go, use pedals for crunch. It's mostly clean, works well with pedals, and is a manageable volume for home use. One minor drawback is that it takes a decent amount of boost to start getting good overdrive, which also increases the overall output volume. I guess I need to breadboard some fuzz or Electra-style distortion circuits...
You might want to consider adding a speaker-out jack....it's amazing how much difference a real speaker makes. My little amp with a small, inefficient speaker is about as loud as a Ruby 386-N1 amp (but without the distortion). Hooking up to my Blues Jr. III's 12 inch speaker makes it easily twice as loud and well into "annoying-the-wife" territory. Hooked into an efficient 12", I could easily use it for church, or mic it for larger stuff.
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> well into "annoying-the-wife" territory.
These radios could always annoy the parents.
Hook into a good Full Stack. It won't deafen a rock drummer, but will give a good account of itself in most venues. 40-Watt amps are mostly about carrying one/two 10"-12" speakers instead of an eight-pack truck-full of speakers.
Yes, the next-up mod is to convert the IF to a 1st-audio. Now it has serious gain, you can overdrive without heavy hands or a booster. (Not so easy if you have 50V+12V heaters on a 62V winding; a JFET booster avoids heater complication.)
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It's done, I just hooked up the 1/4" jack via a 0.1uf cap, to the middle volume terminal, per PRR's original advice. I also bypassed the 50C5 cathode with a 47uf cap. It's nothing special but a definite step up from playing into nothing at all! And there's just the barest hint of growl at max volume.
This radio is so small & cheesy I can't bring myself to do anything else with it. Maybe some old '40s tabletop radio will come my way, that will lend itself better to a makeover!
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Regardless of the cheese-factor, I would be intetested in pics and a soundclip.
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For whatever reason I'm completely unable to transfer my phone pics and audio files to my computer, even with a cable. I've got an old-school cell phone plan that charges max $$$ for data usage so that's out.
Nothing's easy!