Re: $29.99 amp revisited...


"Mat Nieuwenhoven" <mnieuw@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:zavrhjqbagvapyhqrguvfmncnay.iq5ea90.pminews@xxxxxxxxxxxxxxxxxxxxxx
> On 12 Nov 2005 20:07:58 -0800, Mick wrote:
>
>>Hello Phil,
>>
>>Here are my pin connections, as far as I can tell:
>>
>>Pins 2 and 7 are not bridged. Pins 3 and 8 are bridged, but that is
>>because those are my cathodes. These go to ground through what was
>>supposed to be a 2.7K resistor. I noticed this beast was sort of
>>bloated. It measures more like 7.5K. If I short it to ground, I get
>>more volume, but lose the sweetness of sound. Gets a bit muddy. Perhaps
>>I'll replace it with an accurate resistor.
>>Pins 1 and 6 are signal to my driver tubes, typical voltage divider I
>>believe. If you check out the Super Reverb voltage divider it is almost
>>identical. That circuit also takes a single input from signal, then
>>divides and phase changes it.
>>
>>Pins 4 and 5 are connected to my heater (12.3 volts).
>>
>>There is just no preamp here as far as I can tell, but it seems that
>>there is one voltage divider into a push-pull power amplifier circuit.
>>
>>So, back to your suggestions. The current setup has the signal going
>>directly to Pin 2, then a resitor back to ground. Are you saying I
>>should connect the signal through a resistor to pin 2? Won't I lose
>>signal, and thus volume? I was looking at several other amps (Moonlight
>>amp is a good example), and they are about the same as this one, from
>>the phase splitter on. The main difference I see is that there is
>>normally a cap between the input and pin 2, and I don't have one. Maybe
>>that would get rid of the rest of my hum?
>>
>>Anyway, this thing is the perfect desktop practice amp with no preamp,
>>and when I add the Bug, it's loud and grindy. I guess I could put a
>>preamp in it, mounted sideways at a 90 degree angle to the current
>>12AX7. Maybe one day.
>
> If you add another tube, won't the power supply be overloaded? One more
> heater, several more watts from the high voltage?
>
> I'm not a musician, but if I were to have this problem, I'd put in a
> low-noise FET (=high impedance) opamp as preamp (e.g. a TL071) with a gain
> of
> 5 or thereabouts. I don't know what the max voltage is you get out of the
> guitar, but if you feed the opamp with 12 V from a voltage-tripled heater
> voltage with a 78L12 voltage regulator (small transistor-like thing),
> you'd
> get like 10 V peak-to-peak undistorted output swing, which is probably
> more
> than enough to drive the tubes fully into distortion, thus avoiding SS
> distortion. If none of the heater wire is connected directly to ground,
> you'd
> have to make a floating ground with a second opamp (use a dual package
> like
> TL072) .
>
> Mat Nieuwenhoven
>
>

Hi Mat,
It sounds like you are an engineer! These old amps often have overrated
transformers and a 12AX7 only requires 0.3A for the heaters if @ 6.3v or
0.15A if wired at 12.6v.
Your SS idea seems like a very compact solution, though.
As for the heater ground, I'm not sure I follow, but with tube amps we
typically make the artificial center tap ground by using one 100 ohm
resistor on each leg of the circuit and grounding the two resistors at the
same point. This is one instance where the ground point can pretty much be
anywhere on the chassis without creating a problem.
Phil


.

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