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Old 1st January 2013, 04:04 PM   #21
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Default Here's another amp

Very interesting input from everyone.
Here's the schematic of what I call an humble amp. It is very simple and sounds good. If anyone tries it out, just be sure to run the bias up from 0 V slowly until it settles. you need about .14 - .15 volts on the emitter. Then, you should not have to make anymore adjustments for awhile. I have thought of some variations, but for now this is satisfying. Use efficient speakers. Shown is one of my Jim's Audio P-610st drivers being broken in before measurement.
Happy 2013

Terry
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File Type: jpg Humble 1 001.jpg (768.1 KB, 167 views)
File Type: jpg Humble 1 002.JPG (956.8 KB, 162 views)
File Type: jpg Humble 1 003.JPG (894.3 KB, 166 views)
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Old 1st January 2013, 04:57 PM   #22
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Quote:
Originally Posted by Parasonic View Post
Antoinel,
Not surprising--but I have wondered what the effects are with plate and screen voltages in place with their bypasses, and sufficient bias applied to grid via cathode resistor or LED are. I will post the scheme of my amp soon, just having trouble finding the time anymore! I am making an assumption of course, that you observed voltages on the tubes pins without any of the above mentioned voltages in place---or perhaps you had applied voltages?
This is interesting to me as it will be instructive to learn the nature of what any feedback is, whether it be degenerative or regenerative, or possibly some wacky combination of both. All I know is, the amp sounds better than I thought it should. What amp? Well, I will try to post the schematic tomorrow.
What type of speaker are you using and how loud will it go? Is it driven well enough, or do you feel some pre-amplification is in order?
Thanks and Happy New Year!
Terry


Parasonic. All of us are on a roll. Thank you for the pictures of your amp. The LED glows mighty bright. Here are answers to your questions above:
  • The voltages on the cathode, grid and plate were a consequence of the modulated heater current. No external [bias] voltages to the tube's electrodes were applied.
  • A picture of the full range 8 Ohm Radio Shack [Realistic and not Archer brand] is attached. Its loudness was adequate when hung in free air mode, and not boxed.
  • Please go to the Headphone Systems Forum. Search for my username. Locate the topic "Does anyone make their own headphones?". I describe [with pictures] therein a bicycle helmet headphones using 4 " full range and 8 Ohm loudpeakers. In this current app, they were connected in series for a 16 Ohm load. Plenty of high fidelity SPL.
  • I also have Grado SR80i headphones
  • LM386 is the preamplifier. It has a voltage gain of 20-200 in solo application. In my app it has a gain of 20 before Pass loop feedback.
This amp has 3 possible power outputs. At LM386, at emitter and collector of power NPN.
  • I will characterize each output separately with the above transducers with and without feedback.
  • I will also remove one tube of the collector load so as to increase Vce of the power NPN which is now at ~6 VDC.
Best regards, and a happy 2013.
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File Type: jpg IMG_2660.jpg (278.8 KB, 111 views)
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Old 1st January 2013, 05:33 PM   #23
cotdt is offline cotdt  United States
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Quote:
Originally Posted by tubelab.com View Post
The sound heard coming from your amp is most likely produced by the OPT. I have some OPT's that are rather loud and some totally quiet. Loud ones can sometimes be quieted by jamming something between the bobbin and the core. Sometimes the windings themselves produce sound.

It is common for tubes to produce random sounds associated with thermal expansion. Sometimes these sounds wind up in the speakers at a low volume.

I have never heard music come from a tube, but if one had something inside that was free enough to vibrate, it would be a better microphone than speaker. I have a Motorola branded 6SN7 that is microphonic enough that you can talk to it and hear yourself in the speaker. Turn the volume loude enough and acoustic feedback occurs.
Very cool! Happy new year George!
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Old 10th January 2013, 06:42 PM   #24
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Default Performance of Amp

A happy, safe, healthy and prosperous 2013 to everyone. The attached picture is one of many which characterizes the performance of the shown amp [schematic]. I'll refer to this amp as A1. Please note the following:
  • A dual +/- 15 VDC regulated power supply was used. No hum or noise at the collector and emitter nodes.
  • One 12AX7 was used for the emitter load and another 12AX7 was used for the collector load.
  • The heaters of each 12AX7 were operated in parallel; meaning ~300 mA idle current flowed through each. The value of each load resistor [heater only] was ~20 Ohms.
  • Ve is the idle DC voltage at the emitter node = 5.6 V. It follows by oscilloscope measurement that the maximum unclipped peak to peak Voe was = 10 Vac in the frequency range of 30 -150 Hz. This is the same as saying that the useful unclipped peak to peak current flowing through the output stage was equal to 500 mA.
  • Voltage measurements were taken at the five nodes in A1; namely Vi, Ei, Eo, Voc and Voe. These measurements and further algebraic calculations thereof characterize the operation of A1
A1 was loaded with an 8 Ohm 20 Watt non-inductive load resistor [Radio Shack]. A sine signal [30 Hz via function generator] was applied at input port Vi and adjusted upward while Voe was simultaneously monitored on the oscilloscope until the onset of clipping was visible. Voltage measurements were taken with a Micronta AC voltmeter at each of the above indicated nodes. Started at 30 Hz, and continued at 50, 70, 90, 110, 130 and 150 Hz. The measurements were averaged and reported with their attendant standard deviation. At each measurement node I show a box containing 3 averages. Let us talk about the top average and characterize A1 as a working example [Experiment 1] .
  • A1 was operated open loop; meaning the switch in the Pass feedback loop was open.
  • The voltage gain of A1 is Voc divided by Vi. Or 1.05 V/0.183 V = 5.7
  • Mentally disconnect the load = 8 Ohms. It follows that the new Voc must equal Voe. Thus voltage gain of A1 when Z load = infinite is 3.84 V divided by 0.183 v = 21 which is the ~ specified gain of LM386.
  • The equivalent collector load is 8 Ohms in parallel with the 20 Ohms heater resistance = 5.7 Ohms. The numerical ratio of 20 ohms to 5.7 Ohms [3.5] is ~ equal to the ratio of Voe [3.84] to Voc [1.05] = 3.7. The numbers ~jive.
  • The voltage gain of LM386 is Eo [3.97 V] divided by Vi [0.183 V] = ~22
Let us talk about the the second average in the measurement boxes [ Experiment #2]
  • Close the Pass feedback loop.
  • The voltage gain of A1 is still Voc [0.81 V] divided by Vi [0.165 V] equal to 4.4. This is a 23% loss in the parent original gain; albeit put to good use via Pass feedback to lower distortion, and output impedance etc..
  • The voltage gain of LM386 is Eo [2.97 V] divided by Vi [0.165 V] = 18; apparently reduced from the parent 22 due to feedback.
Experiment #3, gave the bottom voltage averages at each measurement box.
  • Open the feedback loop
  • Adjust Ei to 0.135V like its value in Experiment #2
  • The voltage gain of A1 is Voc [0.82 V] divided by Vi [0.136 V] = 6.0; roughly similar to that calculated in Experiment #1 of 5.7.
  • The voltage gain of LM386 is Eo [2.98 V] divided by Vi [0.136 V] = 22 like found in Experiment #1. The numbers jived again.
Two future experiments to do like the above to further characterize amp A1:
  • Z Load equal to 32 Ohm non-inductive
  • Z load equal to that of a full range 8 Ohm loudspeaker in the frequency range of 30 -150 Hz.
Best regards
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Last edited by Antoinel; 10th January 2013 at 06:58 PM.
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Old 10th January 2013, 07:05 PM   #25
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Default Performance of Amp A1

Sorry about the picture above. Here it is rotated 90 degrees CCW
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Old 10th January 2013, 08:45 PM   #26
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Default Good

Thanks for the analysis Antoinel. I'd like to put one together when I have time. (work just tires me out to where I don't care if anything gets done when I get home!)
I'd like to try it with some AN214 I have. Have you tried auto lamps? (somebody I think mentioned those). I threw together a little NPN based SE amp a few years back and played around with Wagner 2004 (I think is what they were)as collector loads. Had to parallel a couple of them as I remember and was 12 volt supply. Worked --but really lit up the room.
Good luck in your experiments.

Just a side note on the humble amp--I currently am using RCA 15CW5 and a plate load resistor of 1.5K ohms works better there instead of 2.2K ohm. This will be somewhat tube dependent. I've not tried any grid stopper resistance--yet.

Cheers
Terry
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Old 11th January 2013, 04:12 PM   #27
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Quote:
Originally Posted by Parasonic View Post
Thanks for the analysis Antoinel. I'd like to put one together when I have time. (work just tires me out to where I don't care if anything gets done when I get home!)
I'd like to try it with some AN214 I have. Have you tried auto lamps? (somebody I think mentioned those). I threw together a little NPN based SE amp a few years back and played around with Wagner 2004 (I think is what they were)as collector loads. Had to parallel a couple of them as I remember and was 12 volt supply. Worked --but really lit up the room.
Good luck in your experiments.

Just a side note on the humble amp--I currently am using RCA 15CW5 and a plate load resistor of 1.5K ohms works better there instead of 2.2K ohm. This will be somewhat tube dependent. I've not tried any grid stopper resistance--yet.

Cheers
Terry
Parasonic. Thank you for your reply. Maybe in the long haul, the title of this thread will morph to " Incandescent Lamp Loads" so as to accomodate lamps in general. Fluorescent and Neon types are another matter and will be studied in good time. I have not tried auto lamps. A typical Halogen headlamp is specified at 12 V and 50 Watts; 4A at full brightness. Smaller wattage bulbs are rated at 7 W; ~0.5 A. Workable.

I'll briefly digress from my plan above, and connect voltages to one of the twin triodes in the emitter leg; max plate to cathode voltage will be 30 V for the time being.

An output stage like that of A1 maybe useable as the driver stage in a JLH and/or a PLH type amplifiers. Infuse their great parent sound with incandescent sonics; be it from lamps or vac tube heater loads. I' ll work a schematic on paper.

Best regards.
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Old 12th January 2013, 08:34 PM   #28
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Default Energizing the vac tube connected to emitter

I imposed voltages on the "bottom" 12AX7 and operated it in the "common grid " configuration. The attached picture shows the partial schematic of A1 amp with particular focus on the operation of the vacuum tube. Note the following :
  • Grid was grounded [at -15 Vdc regulated].
  • The voltages at the cathode were +0.37 Vdc and 0.015 Vac [100 Hz] as measured by the AC multimeter. The value of the cathode resistor was 0.56 KOhms
  • Plate voltage was +15Vdc regulated. The attendant calculated idle plate current was 0.124 mA
  • Plate voltage was = 0.1 Vac by AC multimeter and simultaneously equal to 0.3 Vp-p by scope. Furthermore, plate voltage as seen on the scope was constant to 10 KHz.
The above data showed that the tube expectedly operated within the constraint of its bias voltages. Its voltage gain was tentatively calculated as Plate Vac [0.1] divided by Cathode Vac [0.015] ~ 7.

Suppose a high power tube was used in a common grid configuration such that its cathode is connected directly to the emitter node [+5.6 Vdc] and its plate voltage at >100 Vdc. In a previous post, Voe was shown to swing 10 Vp-p. Will this be useful in power amplification?
Best regards
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Old 14th January 2013, 06:09 PM   #29
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Default A1 amp driving a full range loudspeaker

In Post 24, the performance of A1 was characterized with an 8 Ohm non-inductive power resistor in the frequency range of 30 -150 Hz. The test equipment worked very well, and the results appeared to be reliable. In this post, amp A1 energized a 15 inch, full range musical instrument [8 Ohm; MCM Electronics part # 55-1310]. The results of this study are shown in the attached picture. The top portion shows the power output voltage data [Voc] before and after Pass loop feedback as a function of frequency in the range of 30 - 150 Hz. The bottom part of the picture shows a simplified schematic of the circuit used. Please note the following results:
  • Without feedback [pull 22 uF capacitor from proto board], the collector is a current source output port. Because it gave a voltage profile like that of the typical impedance curve of an woofer. The resonance peak occured at 40 Hz, and the voltage ratio of peak to the average in the plateau [90 -150 Hz] is 2.1.
  • With Pass loop feedback, [install 22 uF capacitor back in circuit], the attendant graph showed that the collector is still a current source output. However, the voltage ratio of peak [at 40 Hz] to that in plateau decreased to 1.7. With and without feedback are expected to sound different.
  • Loss in voltage gain [due to feedback] may be tentatively taken to also mean that utilized constructively to improve the linearity and lower the distortion of A1. Numerically, it is {Voc before feedback [1.2] minus Voc after feedback[0.91]} divided by Voc before feedback [1.2]. The answer is 0.24. This is a 24% reduction in voltage gain. Since the result is a ratio of voltages, then 20 x Log 0.24 = 12 dB. Or one may see it as simply the ratio of Voc after feedback [0.91] divided by Voc before feedback [1.2] = 0.76; or the gain after feedback is down to 76% its no feedback value.
I'll characterize next the performance of the Emitter output port Voe.
Best regards
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Old 17th January 2013, 12:36 AM   #30
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Default Here is a very simple and cheap PS and regulator

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
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