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Old 12th Jan 2022, 6:03 pm   #41
etherman
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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Originally Posted by Robert Gribnau View Post
Good to see you restarted your project.

About the Dansette schematic. If I'm not mistaken, both positive and (more) negative feedback are applied in it.

The negative feedback is created by the feedback loop from the secondary of the OPT, through R11 going to the cathode of the first triode section. The positive feedback is created by having R6 not return straight to ground but return to ground through R4. This sharing of R4 by the two triode sections creates some positive feedback.

My guess is that this positive feedback is there to create a high enough sensitivity (gain) to make up for the losses in the tone control, while still being able to apply negative feedback over the whole amplifier.

If your amplifier has more than enough sensitivity for a CD-player, you could try to reduce the sensitivity a bit by getting rid of the positive feedback. For that you have to connect R6 straight to ground (than it has no connection with R4 and R11 anymore).

Be carefull not to try a much lower value for R5 (470K). The first and second triode section are dc-coupled so R5 more or less sets the voltage at the grid of the second triode, which sets the voltage at the cathode of the second triode (that voltage automatically settles a tiny bit higher than the voltage on the grid). By choosing R5 high (as to create a larg enough voltage drop over it), the voltage at the grid and the cathode of the second triode will stay low enough to respect the maximum cathode to heater voltage of the ECl86/PCL86, which is 100 V.

Ofcourse the quality of your amplifier will depend mostly on the quality of your OPT, like others already wrote. But I think that the schematic you chose is a good one.
Thank you Robert for commenting.
I didn't know that connecting R6 the way it is creates positive feedback. This creates more areas to experiment. As for gain and positive feedback, I will inform you when I make some objective measurements rather than my current subjective impressions. May be I could use a PC or mobile app as a sine wave generator and measure output on the single channel Soviet oscilloscope that I have.
As for negative feedback, I could not detect any audible effect in the produced audio. Or is it mainly to guarantee stability?
Again, I appreciate your comment regarding the value of R5 and its role in setting bias conditions for the second triode. The circuits look simple but there is a lot of complexity and a lot to learn and understand.
My further plans are to continue experimenting and measuring with the PCL86 before checking on the PCL82, as you have suggested earlier last year, which I have stocked a few and were the focus of my initial plans before getting a fixation on the PCL86 after reading some history.
I might even leave this channel temporarily and construct a second channel with 2 PCL82s for the sake of comparison, although I think I will have to reduce B+ supply.
Regards.
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Old 12th Jan 2022, 8:32 pm   #42
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I love the Mechano, Pukka, British slang for proper, I think it comes from the day of India in the commonwealth. You rig reminds me of the Leo computers cards, photo of just one flip flop.
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Old 13th Jan 2022, 6:22 am   #43
Robert Gribnau
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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As for negative feedback, I could not detect any audible effect in the produced audio. Or is it mainly to guarantee stability?
I don't think it's mainly there for stability reasons but I could be out of my depth on this one.

You could temporarily try it out with a lower value for R11, at least to see if the phasing of the feedback is in order. With good phasing you should hear some drop in volume, with wrong phasing the feedback will be positive, which increases the volume and, if the feedback is strong enough, the amplifier starts to oscillate (howling-like sounds).

Just to be sure: Is one tap of the secondary connected to ground (and the other one to R11) like in the schematic? Without the connection to ground, the feedback from the secondary will not work.
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Old 13th Jan 2022, 6:28 pm   #44
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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As for negative feedback, I could not detect any audible effect in the produced audio. Or is it mainly to guarantee stability?
I don't think it's mainly there for stability reasons but I could be out of my depth on this one.

You could temporarily try it out with a lower value for R11, at least to see if the phasing of the feedback is in order. With good phasing you should hear some drop in volume, with wrong phasing the feedback will be positive, which increases the volume and, if the feedback is strong enough, the amplifier starts to oscillate (howling-like sounds).

Just to be sure: Is one tap of the secondary connected to ground (and the other one to R11) like in the schematic? Without the connection to ground, the feedback from the secondary will not work.
The secondary has one terminal connected to ground and the other to the negative feedback loop. The phase of the output signal is inverted relative to input (not sure about that). Attenuated output signal is fed back to the input sort of subtracting part of the input signal and reduces gain and therefore reduces distortion: this is how I understand it. Also, if negative feedback is properly executed it should contribute to stability. However in certain situations it may lead to oscillation. The frequency at which negative feedback has a significant effect is determined by the time constant of the RC combination in the loop.
I will try changing values. Right now I have 10k and 100 pF.
When I reach a reasonable result, I will post a schematic.
What would be the maximum B+ you suggest for a PCL82 pp amp like the Dansette Mk I?

Last edited by etherman; 13th Jan 2022 at 6:56 pm.
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Old 13th Jan 2022, 6:51 pm   #45
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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I love the Mechano, Pukka, British slang for proper, I think it comes from the day of India in the commonwealth. You rig reminds me of the Leo computers cards, photo of just one flip flop.
Thank you. I read about the origins of "Pukka" and the history of Leo computers, both were very interesting. It was vey kind of you to compare this mess with the computer cards.
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Old 14th Jan 2022, 5:41 am   #46
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What would be the maximum B+ you suggest for a PCL82 pp amp like the Dansette Mk I?
The datasheets for the PCL82 (and UCL82) state that Va max and Vg2 max are 250 V, while the datasheets for the ECL82 show 300 V for these voltages. However, I have my doubts about these differences. A link to a thread about this topic:

https://www.vintage-radio.net/forum/...d.php?t=178766

But since the impedance of the primary of your output transformer is 5K, I think that 210 V is the optimal B+ (see post #4 in this thread).
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Old 16th Jan 2022, 5:46 pm   #47
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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Good to see you restarted your project.

About the Dansette schematic. If I'm not mistaken, both positive and (more) negative feedback are applied in it.

The negative feedback ...... (than it has no connection with R4 and R11 anymore).

Be carefull not to try a much lower value for R5 (470K). The first and second triode section are dc-coupled so R5 more or less sets the voltage at the grid of the second triode, which sets the voltage at the cathode of the second triode (that voltage automatically settles a tiny bit higher than the voltage on the grid). By choosing R5 high (as to create a larg enough voltage drop over it), the voltage at the grid and the cathode of the second triode will stay low enough to respect the maximum cathode to heater voltage of the ECl86/PCL86, which is 100 V.

Of course the quality of your amplifier will depend mostly on the quality of your OPT, like others already wrote. But I think that the schematic you chose is a good one.
I reviewed this paragraph once more and remembered that I did not check the DC conditions on the triodes. I was so much focused on the output stage, that I overlooked that preamp stage.
I discovered that I had placed a 100k resistor in R5 position, probably copied from some other schematic. Va at the first triode (also the grid of the scecond) was 111 volts. Grid to cathode voltage was - 0.8 V.
I swapped a 470K resistor at R5 and all the distortion was gone. I could increase the volume of the output from the CD player (source) to maximum without audible distortion, the sound is loud and clear, though I wanted louder. Thank you Mr. Gribnau for the spot on comment.
Va became 83V (as far as I remember). Grid-cathode volyage is -0.75V.

The thing that was there and still is, but I did not previously mention, is that the input volume potentiometer (0.5M), was always left on maximum volume (max. resistance to ground). If I try to bring volume down a bit) I immediately get a buzzing sound in the speaker, sounds like it is picking EMI. Considering the state of chaos in wiring this contraption, may be it is too early to cure this. However I would like to explore it with you.
I would also like to mention that I had omitted the tone control part of the Dansette schematic, at least for the time being. I thought I would start with a minimalist design to get the least sources of trouble.
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Old 17th Jan 2022, 7:12 am   #48
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Maybe a grid stopper will stop the buzzing. So you could try placing a 1K resistor ('grid stopper') between the control grid of V1a and the wiper of the 500K volume pot. Solder the 1K resistor as close to the tube socket as possible, so that the lead between the resistor and the connector on the tube socket is as short as possible.

Although I'm not sure about this, I get the impression that you didn't build the tone control (yet) and that you only have a 500K volume pot before V1a. If this is true, and if you are going to use only line level sources like CD-players, which have low output impedance, than I think it is better to use a 50K (or 100K) volume pot. Lowering the input impedance of your amplifier could contribute to less noise issues.
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Old 18th Jan 2022, 11:31 pm   #49
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Maybe a grid stopper will stop the buzzing. So you could try placing a 1K resistor ('grid stopper') between the control grid of V1a and the wiper of the 500K volume pot. Solder the 1K resistor as close to the tube socket as possible, so that the lead between the resistor and the connector on the tube socket is as short as possible.

Although I'm not sure about this, I get the impression that you didn't build the tone control (yet) and that you only have a 500K volume pot before V1a. If this is true, and if you are going to use only line level sources like CD-players, which have low output impedance, than I think it is better to use a 50K (or 100K) volume pot. Lowering the input impedance of your amplifier could contribute to less noise issues.
The grid stoppers were there for the grids of the first triode and both pentodes. I did not build the tone control circuit as I had mentioned in the previous post. I temporarily omitted the volume control and soldered a 220K resistor between signal and ground instead. I control volume from the source.
I attach here a hand drawn schematic of how things are right now. It is far from final but it might help the discussion, instead of describing it in text.
I separated the supply to the final stage B+, from the supply to the triodes C+, to be able to adjust their values independently. I became brave to increase B+ again (for more power), after getting rid of distortion that originated from the low value resistor at R5.
The photos were taken earlier when R1' was 1K, so B+ was 300V. After some resistor swap / voltage measurement / listening exercises, I got the impression that the sweet spot for the B+ is 280V, where output is loud enough and bass is strong, but without nasty, audible distortion. Again, will have to do some measurements.
Only as I was sketching the schematic, did I notice that I had departed from the Dansette design where I had placed significantly larger resistor(s) instead of the original R4 in the cathode of the first triode, probably inspired by another schematic. I will have to modify and test again.
Your suggestions are welcomed?
Another difference that I noticed earlier concerns R6 and R7 in the concertina phase inverter. I used 100 k (rather than 180K), and R6 is (absent mindedly) connected to ground rather than to the cathode of V1 triode. I tried to correct this last error connecting R6 between the 2 resistors in the cathode of V1 triode, but found that it resulted in excessive distortion (positive feedback) as Mr. Gribnau had anticipated.
Until now, the filaments have not been grounded. Both are A/C powered and the LED needs a rectifier diode, though I only notice its flicker when I see it through the cell phone camera when taking photos.
When I will build and connect the second channel, I expect that B+ will drop further due to double the current draw, so R1' will have to be reduced and may be R2' as well.
Measuring voltage drop across R1' (1K), at 52-55 V, yields total current draw of 52-55 mA. Screen grids draw 3.5 mA each measured across respective resistors. So each anode draws less than 25 mA. This is significantly lower than the data sheet.
What should I do next?
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Old 19th Jan 2022, 7:47 am   #50
Robert Gribnau
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

I suggest that you try out the input stage and phase splitter using the original values first (so 180K instead of 100K, only 470 Ohm in the cathode lead of V1a and 15K for the feedback resistor; what is at the grid of V1a can stay like it is now). If that works OK, you can always try out modifications.

You could try 270 Ohm instead of 330 Ohm for the cathode resistors of the output stage. See page 58 of the book I linked to in post #8 (B+ on the centre-tap of the output transformer there is 317 V so as long as you don't exceed that the PCL86's will be fine; see page 62).

Filaments should have a proper path to ground. When the filaments are being fed from the same transformer that provides the B+, than there is some capacitive coupling between the filaments and the rest of the amplifier (that's probably why you get away with it now) but good practice is to ground them properly.
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Old 19th Jan 2022, 7:01 pm   #51
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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Filaments should have a proper path to ground. When the filaments are being fed from the same transformer that provides the B+, than there is some capacitive coupling between the filaments and the rest of the amplifier (that's probably why you get away with it now) but good practice is to ground them properly.
Would you kindly explain how the filaments should be grounded?
The other suggestions are on my list of mods to try, plus measurements of grid voltages. Understanding working point of tubes and proper use of characteristic curves, is still a mystery to me. I tried to read many times but did not get an understanding.

Regards.
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Old 20th Jan 2022, 5:47 am   #52
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If the winding for the filaments on your transformer has a centre tap, than connect that centre tap to ground. If the winding has only two connections, than make a connection between one of them and ground.

On the site of The Valve Wizard you can read some more on the subject, like how to make an artificial centre tap (which by the way doesn't seem necessary in your amplifier because the level of signal in it will be relatively high, unlike in a guitar amplifier).

http://www.valvewizard.co.uk/heater.html
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Old 23rd Jan 2022, 10:36 pm   #53
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I suggest that you try out the input stage and phase splitter using the original values first (so 180K instead of 100K, only 470 Ohm in the cathode lead of V1a and 15K for the feedback resistor; what is at the grid of V1a can stay like it is now). If that works OK, you can always try out modifications.

You could try 270 Ohm instead of 330 Ohm for the cathode resistors of the output stage. See page 58 of the book I linked to in post #8 (B+ on the centre-tap of the output transformer there is 317 V so as long as you don't exceed that the PCL86's will be fine; see page 62).

Filaments should have a proper path to ground. When the filaments are being fed from the same transformer that provides the B+, than there is some capacitive coupling between the filaments and the rest of the amplifier (that's probably why you get away with it now) but good practice is to ground them properly.
Over the past few days, I had worked intermittently on this single channel PCL86 PP amp. As suggested by Mr. Gribnau, I readjusted resistor values and connections as close as possible to the original Dansette schematic: R6 and R7 are 180K, R4 is 470 Ohm with R6 (positive feedback) and negative feed back from R11 connected to the upper end of R4 at the cathode of V1 triode, and the center tap of the 13 volt heater secondary winding of the power transformer was connected to the ground bus.
This resulted in less buzz when I connect my old iPhone as a music source, and output volume is slightly louder. Total current draw from B+, as measured by the voltage drop across the first 1K power resistor after the bridge, was about 50mA. So I started reducing the cathode resistors (a single R12 in Dansette schematic) of the output pentodes, from 2 x 330 to 300 Ohms. This brought a small increase in current, so I changed to 270 Ohms. Now it is drawing 60-65 mA. Screen grid current has increased from 3.5 to 4-4.5 mA per tube. I have measured voltage across the cathode resistors but I can't remember now. There was a slight disparity from calculations done on global current draw. I will measure it once more.
After connecting as above, the amp started to oscillate at a low frequency: at first I disconnected the filament ground but it didn't work, then I disconnected feedback, and oscillation stopped. I removed the 100pF capacitor that was paralleled with the 10K resistor and reconnected feedback, the oscillations were absent.
Now negative feedback had a significant effect when I connect and disconnect it. When connected the volume is reduced with more effect at high frequency sound. The bass is full and powerful. Treble is low. I tried to reduce the magnitude of negative feedback by increasing the value of R11: from 15K in the original, in steps up to 100k. Treble is louder and the reduction in volume with the NFB is less, and still no audible distortion. I hope I can make some "measurements" in the near future.
I also got more daring and reduced R1' to 1K, to increase B+ to around 285-290 V, which also contributed to increased current draw.
Should I reduce cathode resistors more, to try to increase current? May be try also to increase B+ to 300-305V?
Regards.
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Old 24th Jan 2022, 5:47 am   #54
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I would try to increase B+ to 300 V and keep the cathode resistors at 270 Ohm each.

The maximum anode dissipation of the pentode section is 9 Watt (anode dissipation is the voltage between cathode and anode times the anode current). From the datasheet it follows that with a common cathode resistor of 132 Ohm (which translates to two seperate cathode resistors of 264 Ohm each) and a B+ of 300 V (so the voltage at the centre tap of the output transformer), the tubes operate very close to their maximum anode dissipation (how close depends on the dc resistance of the primary of the output transformer, so on the voltage drop in the primary).
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Old 24th Jan 2022, 1:07 pm   #55
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This reminds me of when I built my first amp a few months ago. I ran into the same issues, how much smoothing, how to stop the damn thing buzzing, how to apply negative feedback (if necessary), what size cathode resistor etc. It's all part of the learning curve

A couple of things which you may know already but which may help if you dont.

1. The position and orientation of the output transformer is crucial to avoid magnetic coupling. Google the "headphone test for output transformers" to work out where best to out it.

2. Usually a smaller volume pot will give you less hum. As has been already mentioned 47k will probably be quieter. If you connect a low impedance device to the input eg phone it may short out the hum and the amp will be silent which may solve the problem for you (sort of)

3. Make sure the negative feedback is connected right way round for negative feedback . There is 50% chance of getting this right first time and it's unpredictable (to me). If you get oscillation it may be that you're getting positive feedback instead. Flip the ground and signal wired on the output transformer secondary and try again.

4. 100hz hum is a pain. As others have said it is mostly down to smoothing especially in V1 and V2 but it's a balancing act if you don't have enough volts. Once you build the amp put your ear to the speakers you intend to use and see if the hum you can hear (if there is any) is acceptable to you or not. If not you may need to up the resistor value or add capacitance. As such you're building an amp to match the speakers you're using with it, if that makes sense.

Good luck, it's great fun once you get it to work as it should. Your next build will be easier.

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Old 25th Jan 2022, 9:52 pm   #56
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This reminds me of when I built my first amp a few months ago. I ran into the same issues, how much smoothing, how to stop the damn thing buzzing, how to apply negative feedback (if necessary), what size cathode resistor etc. It's all part of the learning curve

A couple of things which you may know already but which may help if you dont.

1. The position and orientation of the output transformer is crucial to avoid magnetic coupling. Google the "headphone test for output transformers" to work out where best to out it.

2. Usually a smaller volume pot will give you less hum. As has been already mentioned 47k will probably be quieter. If you connect a low impedance device to the input eg phone it may short out the hum and the amp will be silent which may solve the problem for you (sort of)

3. Make sure the negative feedback is connected right way round for negative feedback . There is 50% chance of getting this right first time and it's unpredictable (to me). If you get oscillation it may be that you're getting positive feedback instead. Flip the ground and signal wired on the output transformer secondary and try again.

4. 100hz hum is a pain. As others have said it is mostly down to smoothing especially in V1 and V2 but it's a balancing act if you don't have enough volts. Once you build the amp put your ear to the speakers you intend to use and see if the hum you can hear (if there is any) is acceptable to you or not. If not you may need to up the resistor value or add capacitance. As such you're building an amp to match the speakers you're using with it, if that makes sense.

Good luck, it's great fun once you get it to work as it should. Your next build will be easier.

Gabriel
Thank you for your comments. All of them are going to be considered, especially when I get done with the experimental setup and start a final form of construction.
The principles of feedback learned from tinkering with regenerative receivers, helped me understand NFB and importance of phase for proper connection.
For the volume potentiometer, the only stereo pots I have now are either 10K or 100K. I will use the latter till I can procure a 50K pot, which means I have to drive to downtown, an experience ususlly associated with psychological trauma.
I will soon report the results.
Regards.
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Old 25th Jan 2022, 11:55 pm   #57
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I would try to increase B+ to 300 V and keep the cathode resistors at 270 Ohm each.

The maximum anode dissipation of the pentode section is 9 Watt (anode dissipation is the voltage between cathode and anode times the anode current). From the datasheet it follows that with a common cathode resistor of 132 Ohm (which translates to two seperate cathode resistors of 264 Ohm each) and a B+ of 300 V (so the voltage at the centre tap of the output transformer), the tubes operate very close to their maximum anode dissipation (how close depends on the dc resistance of the primary of the output transformer, so on the voltage drop in the primary).

I will try to describe the changes and observations from last night:

1. B+ ~300 V (297-298), by reducing R1' to 620 Ohms (7W)
2. Cathode (P) resistor 220 Ohms bypassed by 100uF
3. V drop across R1' 620 Ohms = 50 V, so total current 80 mA
4. Screen grid current = 5.1mA (0.24V / 47Ohms)
5. Cathode Current (no audio input): 8V/220 Ohms = 36mA
6. Cathode current (louder music) 8.5V/220 Ohms = 38mA
7. Anode of second triode = 171V
8. Cathode of second triode = 101V (idle), 106V (maximum music).
Audio was good, tubes were operating near maximum.

Concertina:
Anode 171V
Cathode 101V (no input)
NFB increased R11 47K (instead of 100K)

more changes:
1. Cathode R 250 Ohms (2 x 500 Ohm)
2. B+ 301 V
3. V drop across R1' 47 V, i.e., 76mA
4. Screen grid 4.6 mA (0.22V/47Ohms)
5. Cathode current (pentode) 9.0V/250 Ohms = 36 mA
I will add a modified schematic.
Any ideas?
Going to build the second channel.
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Old 28th Jan 2022, 7:47 pm   #58
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The LED may be destroyed by reverse voltages if you use that circuit design.
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Old 2nd Feb 2022, 10:11 pm   #59
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

For pentodes, the optimal load is very close to the DC resistance of the tube reckoned as Ebb/Ib, not confuse with the anode resistance, several times larger. In place of alter the DC voltage to an abnormal or suboptimal value you can combine speaker impedances to reflect into the tube plate the above DC resistance seggested.
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Old 3rd Feb 2022, 1:46 pm   #60
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Originally Posted by Silicon View Post
The LED may be destroyed by reverse voltages if you use that circuit design.
Thank you for your comment. The LED seems stable to my eye, only until I start taking pictures with the iPhone, when it is flickering on the screen. I am planning an antiparallel diode in the final circuit, which will eliminate the flicker, and protect the LED from damage by reverse voltage. The current circuit is still experimental, and I am modifying it as I go, with the help of many other members.
Regards.
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