Other Applications of Vacuum Tubes

[Antoinel]

Originally Posted by Parasonic

In keeping with the budget nature of humble, this PSU/ regulator sounds good with the amp. After trying several more complicated schemes, this one wins on sonics, no hum. Parts are readily available, maybe in the junk parts box. The reverse biased collector/emitter string works great, but I can't vouch for all species of 3906 that you may have. You can put them in series with a 330 ohm resistor and 36-37 VDC applied and you should see about 32 VDC across the string. Out of about 12 I had from the same vendor, you could interchange any 3 of them and obtain that voltage. Sometimes simpler is better.

Cheers
Terry

You have an effective and simple solution. On several occasions, I used the reverse-biased base to emitter junction of power and complementary bjts [TO-3 pkg.] as power Zeners.

Is it proper to say that an objective of your circuit is to introduce a characteristic tube-like sound in otherwise amplifying semiconductors; FETs or BJTs?
Best regards

[Parasonic]

[QUOTE=Is it proper to say that an objective of your circuit is to introduce a characteristic tube-like sound in otherwise amplifying semiconductors; FETs or BJTs?
Best regards[/QUOTE]

Hi Antoinel,

I wouldn't say that it is an objective, but it is a consequence of such a circuit. I don't have a large listening space, so I have had an interest in simple, mostly single ended designs. I listen at very moderate levels, and you can get away with little power in a small space. I like the idea of a tube driving a BJT for some reason, just a preference. With this recent amplifier, I am enjoying my music collection again very much, and that is with only one amp strung together on the desk, driving a Diatone "clone". I now have a stereo pair in the works. I don't like the idea of spending huge amounts of $$ on audio equipment. One of these days I hope to build a Nelson Pass design, when I can get the time and resources, but for now I am having a great time! (and isn't that what it is about?)

Terry

[Antoinel]

The classic hybrid designs of vacuum tubes and transistors have a common feature. The input signal is first amplified by tubes and then presented to a transistor power output stage. Examples are MOSKIDO, AIKIDO etc. The sound signature of the resulting hybrid amp is believed to be like that of an all tube design. The hybrids of Millett and Parasonic add another interesting feature to the classic designs by utilizing the amplifying tube heaters as loads to the transistor power output stage. Millett and Parasonic speculated about feedback from the heaters to the cathode etc..Both may have concluded that if present feedback was not relevant.



In this post, the reverse of the above practices was used instead. Please see the attached schematic. Here are the details:

• The input audio signal is amplified first by LM386 instead of a vacuum tube; be it that attendant to the emitter or collector of the NPN power output stage.
• The vacuum tube attendant to the emitter port is wired as a current source [cathode shorted to grid]. The twin triodes therein are connected in parallel. Their combined idle plate current was ~0.5 mA at idle.
• The cathode/grid of the equivalent triode was then connected to the output port of LM386. The signal output of LM386 modulated plate current; such that a signal appeared across the plate load [3.3K]. The emitter and base ports are also possible junctions to connect the cathode/grid of the equivalent triode.
• The plate signal was then fed back to the inverting port of LM386. This is the principal difference between the approach in amp A1 and the classic practices.
• The impact of the fed back signal was to decrease the power output signal e.g. at the emitter port. It was negative feedback. Clearly; I infused the solid state hardware with a triode signal; with a possible outcome of a net tube-like performace for amp A1.
• The connections at the non-inverting port of LM386 was that of Pass negative feedback as described in earlier posts. Its value is to recycle or rerun all signals one more time through the solid state devices and triode.
• Sounded great

I'll add objective results in a future post.
Kind regards.

[Antoinel]

The diagram in the above post showed the voltage output of the vacuum tube was fed back to the inverting input of LM386. This was negative feedback because it diminished the power out voltage at the emitter and collector ports. I asked myself what would happen if I connected the same voltage output of the vacuum tube to the non-inverting input of LM386 instead. First I disabled Pass feedback. I expected oscillation. Fortunately oscillation did not happen. But in this case of positive feedback the power output voltages at both of emitter and collector ports increased. I'll quantify the gain of amp A1 with positive and negative feedback ftom the triode.

I found a role or purpose for the triode attendant to its heater connected to the emitter of the transistor; namely this and the preceeding posts. What role can I expect the triode attendant to its heater connected to collector of the transistor ? I connected its grid to its cathode [one of the twins], and this joint was further connected to the output of LM386 like I did for the triode attendant to the emitter. Fortunately, this collector-triode performed like the emitter - triode for both negative and positive feedback. It is possible now to use a suitable light bulb and one 12AX7 [instead of two 12AX7s] at either emitter and collector ports. The light bulb can be incandescent [lit] or cold .

[Antoinel]

The attached schematic demonstrates the meaning of the term Other in this thread's title "Other Applications of Vacuum Tubes". The tubes are used around the solid state circuitry to first generate signals, and then infuse this circuitry with these signals via positive and/or negative feedback. Note the following in the schematic.

• PF means positive feedback. By contrast NF means negative feedback. One may apply NF or PF only and NF plus PF simultaneously.
• I have 4 functional triodes in the circuit of the NPN power output transistor.
• The grids and cathodes of the 4 triodes are tied together, and to the emitter of the transistor. This connectivity generates plate currents.
• The plate of the triode for the upper 12AX7 [PFu] and the plate of the triode for lower triode [PFl] are tied together.
• Ditto for the plate electrodes labelled NFu and NFl.
• *In a demo application, set Ve = 3 VAC at 100Hz. Apply NF only by joining the respective tube plates to the inverting port of LM386. Note that Ve drops to ~2.6 -2.7 VAC,
• *Now apply PF by joining the plates of the respective tubes to the non inverting port of LM386. Note that Ve is restored to 3 VAC like in the absence of both PF and NF.
• I chose [by trial ] the resistor values at the inputs of LM386 to give the response above [* bullet points] with simultaneous PF and NF.
• Can play more games with this topology. All or some of the plate signal outputs may be joined together for NF or PF; many possibilities etc..
• Amp A1 is stable as configured with simultaneous NF and PF. There was no oscillation or latch up.
• Amp A1 sounded great as configured.

The circuit in this schematic is highly versatile. It will give its user diverse flexibility to tweak it by vaying PF, and/or NF, and let's also recall the additional option of Pass feedback I wrote about earlier.

Best regards

[Antoinel]

The title pertains to current feedback around a single power output semiconductor like a BJT, FET etc.. Please consult the attached schematic for its details. The NPN output stage shows half-shaded sine waves at its collector and emitter ports; which are out of phase. This is a prime candidate for a bridge amp; by simply connecting the load Z [or loudspeaker] between Vo and Ve. This is ANT Negative Feedback.



Here are the details of the attendant experiment and the resulting performance of this amp before and after bridging.

• Disable all positive and negative feedback emanating from the plates of the vacuum tubes [as shown in the previous post]. To simplify analyses. Can easily restore feedback from vac tube at any time/later.
• Connect the load Z [e.g. 33 Ohms] to ground [No ANT Feedback].
• Set function generator frequency to 100 Hz.
• Adjust the input signal Vi to the non-inverting input of LM386 to get 3.85 VAC at the emitter [Ve] of the NPN transistor. This and all voltage measurements were done with a Micronta digital voltmeter. Log voltage value in Table 1.
• Simultaneously monitor this output signal [Ve] on the scope. The scope read 10 Vp-p, and the signal was not and must not be clipped.
• Measure Vo across the load and log its value in Table 1.
• Apply ANT Feedback by connecting the load Z [33 Ohms] between Vo and Ve. The scope showed a clipped Ve output signal.
• Lower next the level of input signal [Vi] to LM386 until the signal on the scope is restored to the clean shape of a sine wave.
• Measure the voltage at the emitter of the BJT [Vef; f for feedback] and across the load Z [Vof-Vef]. Log their values in Table 1.
• Repeat the above protocol for the remaining load resitances to complete Table 1.

I'll stop writing at this junction. I'll discuss in the upcoming post the results [performance], and their practical value.

Best regards.

[Antoinel]

The attached jpg and identical pdf are magnified Tables 1 and 2 shown in the above post.
Q: Why did I choose this range of load resistors Z?
A: Ignoring Z= 4, this was roughly the range of impedance as a function of frequency for the full range MCM 15" musical instrument which I use to listen to music.



Focus on Table 1 on the the data for Z = 8 Ohms as a working analysis.

• Note the transition of Ve = 3.85 V [before feedback] to Vef = 0.7 V [after ANT feedback]. This loss of voltage gain is equal to 82%. One may see it as an investment in a new state of linearity, lower output Z and increased bandwidth relative to the parent before feedback.
• Note Ve' which was a new measurement made before feedback and made numerically equal to Vef.
• Compare the magnitude of the voltage after feedback [{Vof-Vef}] = 1.43 V]; i.e. across the loudspeaker with Vo [1.05 V] , and separately Vo' [0.21 V]. These three voltages directly impact the power delivered to the same loudspeaker; which is the subject of comparisons in Table 2

Focus now on Table 2 on the calculated results for Z = 8 Ohms as a working analysis.

• The ratio of power out after feedback [row #2 = 0.26W] to that before feedback [row #1 = 0.14W] is roughly 2:1.
• The ratio of power out after feedback [row #2 = 0.26W] to that of row #3 [0.006W] is roughly 4:1.

There you have it. ANT Negative Feedback enabled a single power output transistor to function as a pseudo-bridging power amplifier. And this resulting brigde amplifier sounded louder [expectedly] and was fully satisfactory.
Best regards

[Pano]

I use bare nichrome wire just below the current where it glows. It works well. Cheap, too.

[Antoinel]

Originally Posted by Pano

I use bare nichrome wire just below the current where it glows. It works well. Cheap, too.

Thanks Pano. Its good to hear from you.

[Pano]

You are welcome. I know the thread had gotten past that point a little, but I wanted to add my experience trying to do the same thing. Maybe it can help someone else.