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Old 30th December 2017, 05:51 PM   #1
DrAwesome is offline DrAwesome  United States
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Default Petite boutique HELP

Short version- am I an the right track/ any serious flaws?

A few years ago I bought a Nutone intercom at a garage sale. Maybe $10. All I knew at the time was the 12ax7 was a good thing. Now I want to complete my vision. Most or the power supply is copied from the Nutone manual (took forever to find) The pre amp is basic fender.

I know i will have to play with bias and what not- any suggestions? (Other than build something else)
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Old 31st December 2017, 04:57 AM   #2
PRR is online now PRR  United States
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You are missing a grid to ground resistor on the power bottle, which will run-away and melt.

"Basic Fender preamp" is usually 100K plate resistors. Gibson and others swung to 470K. There is a difference in treble response. Either may be valid. Fender sold a few amps so I'd lean to his values. And you don't really need the small extra gain from high value plate loads.

You want a fuse on the wall-voltage side. 1A just so the line-cord can't start a big fire. You will of course also smoke-test with a Lamp Limiter, 100W Incandescent (or halogen) bulb.

Ground the Chassis!! 3-pin plug. Green to secure chassis bolt.

You are taking a lot of loss from 170V AC down to 120V DC. I checked your design; yes, at 50+mA the 6X4 will drop about that much. Ah, I see you stole the design; good policy.

You have an OT? If the NuTone sported that obscure 6CU5, that OT is sure to work, though maybe not with wall-breaking bass. (Probably break no walls with 2.3 Watts anyway.)

Last edited by PRR; 31st December 2017 at 04:59 AM.
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Old 31st December 2017, 05:21 AM   #3
PRR is online now PRR  United States
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The 700r in the B+ filter *may* be excessive voltage drop. If that's how NuTone did it, fine. If you adopted it from another design, leave it, measure where the plate voltage comes to. The full 2.3 Watts of goodness needs near 120V. Of course you may be happy for less power if it plays well.
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Old 1st January 2018, 12:42 AM   #4
DrAwesome is offline DrAwesome  United States
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First- thank you!

Other first- grounding will be done, proper 3-wire and a fuse. I didn’t bother to draw it because it’s a ‘given’ but since I am a complete unknown and might be a lunatic you are wise to remind me.

100k fender plate resisters- I see that now. I took that value from the Nutone. I will start with Leo’s value.

I do have a Nutone OT- the original speaker is 3.2ohm- seems an odd value- I imagine a 4ohm will suffice.

And it is once again Nutones idea for the 700r in the B+, I will try to “breadboard” this before moving to a proper chassis

Thanks again, I know my blueprint is well beyond the the compression of the common man.
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Old 1st January 2018, 12:50 AM   #5
DrAwesome is offline DrAwesome  United States
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Oops- forgot the pics maybe I need monitors
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Old 1st January 2018, 07:13 AM   #6
Enzo is offline Enzo  United States
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3.2 ohm speakers were very common, especially in low power things like table radios, TV sets, and yes, intercoms. The system will never know the difference if you use a 4 ohm instead.
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Old 1st January 2018, 03:51 PM   #7
Tubelab_com is offline Tubelab_com  United States
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Quote:
any suggestions? (Other than build something else)
No need to build something else......I made a few similar amps years ago but used 6AQ5 output tubes because I had lots of them, and they eat more B+ voltage. They worked well.

Quote:
If the NuTone sported that obscure 6CU5, that OT is sure to work,
The 6CU5 is the 6.3 volt version of the common 50B5 which was replaced by the 50C5 to comply with the new UL spacing rules. Any OPT from an old radio will work fine. 2500 ohms was the most common flavor.

Quote:
Nutones idea for the 700r in the B+, I will try to “breadboard” this
This was designed primarily for intelligible speech. Depending on what kind of music, and the playing style, you may want to experiment with the 700 ohm and 500 ohm resistors in the power supply. I'm sure that Nutone never dreamed that someone would plug a stomp pedal into this and set it on KILL!

If that's your style, lower the 700 ohm resistor a bunch. I tend toward zero. The plate voltage spec in these tubes was based on what you got from tube rectified wall outlet in the 1950's. The 6CU5 will eat 200 volts on its plate as long as you keep the screen voltage in the 110 to 120 volt range. This means adding some resistance to the 500 ohm resistor.

I think I used something around 2K ohms and no first resistor. My power transformers were liberated from some old RF signal generators and made about 250 volts of B+, and I used a 6AQ5.

The preamp is fed downstream of both resistors. The screen current stays in a relatively constant range, then shoots upward when the amp is driven to clipping. To maximize sag, sustain, and touch sensitivity you want the screen voltage and the preamp B+ to drop as you slam the amp into clipping. As the input signal dies out the screen current drops allowing the preamp B+ voltage to rise, increasing the total gain and helping that dying note to sustain longer.

Hence some experimenting with these two resistors will determine the amps "tone." The optimum values depend on your playing style, how hot your guitar's (or pedal) signal is, and the characteristics of the speaker. Most guitar speakers have a resonant peak somewhere in the guitar's frequency range. The impedance of the speaker hits 20 ohms or more at this frequency, so more plate voltage allows for more output around the peak. The impedance drops on either side of this frequency, causing more current to flow through the output tube, hence dropping the plate and screen voltage.

Also as stated put a 470K resistor from the 6CU5's grid to ground. The .02 cap across the OPT primary will kill treble, you may want to experiment with it's value or eliminate it. Fender and most others used 100K plate resistors on the 12AX7. These resistors affect the treble response, and the amps behavior when overdriven. They do not need to be the same value. Try anything from 47K to 220K or more. 100K works for amps with 200 volts or more of B+, but you might like something different.

The 43K? resistor on the grid of the first 12AX7 will also kill treble. It also helps stop interference from nearby AM radio sources. Anywhere from 1K to 68K is seen in guitar amps.
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Old 1st January 2018, 08:35 PM   #8
PRR is online now PRR  United States
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470K plates works OK for AM radio and intercoms, where you don't want much over 5KHz.

You might think the same would apply to e-guitar, where the high overtones are obnoxious and must be cut. But Fender's 100K passes out to 40KHz. There are two steep high-cuts in the system: the pickup top resonance and the speaker top cut-off. Apparently these are ample and maybe another low-pass in the amp is too much. I won't say this is Fender's biggest trick, but it may be a point. Also the 100K/1.5K values leave some gain and output behind but define a soft top-clip which may be musically pleasing.

3.2 is the DC resistance. 4 is the audio impedance around 400Hz. They are the same thing. W.E. tended to mark 4-8-16 being audio-heads. Cheap radio makers tended to use the number on their DC Ohm-meter, 3.2+/-.

You "never" see a 6.4 or 12.8 Ohm rating. If the speaker is in the same box, 3-4 Ohm is a part-penny cheaper (fewer turns of fatter wire). If the speaker is across the theater, 16 and 32 Ohm nominal audio impedance reduces loss in long lines. So there is a sharp division between box-radios(combos) and pro separates.
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Old 2nd January 2018, 09:25 PM   #9
DrAwesome is offline DrAwesome  United States
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Here is the update ~ most of the change it should be included in this schematic.

The data sheet for the 6CU5 states that the maximum plate voltage is 135volts maximum screen voltage 117V seems it would be wise to stay within those parameters.

The 34k resistor on the first half of the 12AX7 is copied from the fender ‘champ’ and other fendor designs (the parallel 68 on input resistors.) I also added a 22K negative feedback, again from fender fame.
And I eliminated the “hum balance. I’m not sure what that was doing- I took it out.

The other question I have is for the 5000micromicrofarad (oops- no value labeled!but that’s what those are) capacitor‘s in the power supply by the 6X4- I don’t see those with other rectifying circuits. Anyone know about that?
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Old 2nd January 2018, 10:01 PM   #10
Printer2 is offline Printer2  Canada
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A couple of resistors for the heater supply tied to the power tube cathode? I am guessing the output transformer will be the weak link in the system. I doubt they put more transformer than was needed.
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