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Old 3rd Feb 2022, 6:19 pm   #61
Al (astral highway)
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Quote:
Originally Posted by etherman View Post

...The LED seems stable to my eye, only until I start taking pictures with the iPhone, when it is flickering on the screen.
The 'flicker fusion threshold' for the human eye is 75Hz if I recall correctly. That's the frequency when a flickering light source appears steady-state to the human eye (brain, obviously).

But this is obviously idiosyncratic, since, for you, it's much lower -- you are registering a 25Hz flicker as steady-state.
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Old 3rd Feb 2022, 10:40 pm   #62
etherman
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Quote:
Originally Posted by Al (astral highway) View Post
Quote:
Originally Posted by etherman View Post

...The LED seems stable to my eye, only until I start taking pictures with the iPhone, when it is flickering on the screen.
The 'flicker fusion threshold' for the human eye is 75Hz if I recall correctly. That's the frequency when a flickering light source appears steady-state to the human eye (brain, obviously).

But this is obviously idiosyncratic, since, for you, it's much lower -- you are registering a 25Hz flicker as steady-state.
What I meant to clarify is that it seems steady for the naked eye, but with the phone camera it is seen as flickering may be due due to the shutter speed or frame rate per second. I think Nyquist limit applies here where sample rate (frame rate) should be at least double the observed frequency. Local mains supply is 50Hz and that would be the rate at which the led switches, I think.
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Old 10th Feb 2022, 5:52 pm   #63
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

A few days ago I have assembled the second channel and rearranged the first channel a bit. Listening to stereo sound on the Mordaunt Short MS 5.5 speakers is impressive, at least for me. The source was a SABA TS-2020 AM/FM tuner. I had to come down on the volume control. Here are the main changes compared to my previous one channel:
1. Slightly more tidy construction: I planned on a small copper clad board engraved using a rotary tool as a substitute for a tag board. But eventually gave up and used the same flying leads as before.
2. For the second channel I used high voltage film capacitors (yellow colored in the photos), for the signal input (2.2uF 100V, way higher than schematic, did the same for the first channel) and between the phase splitter and the output pentode grids (0.33 uF 160V).
3. Considering that the DC current will be doubled with addition of the second channel, resistor R1' was reduced from 620 to 330 Ohms 15W (the most suitable power resistor value I had at hand). With the 2 channels connected, B+ became 285 V, significantly less than expected. The 2 channels were drawing 145mA, again more than expected.
4. A 100K stereo pot was used at the input.
5. Feedback as before. 47k resistor.
6. The 2 valves for the second channel are from a different brand than the first. They all come from NOS from one Cairene shop.
7. Used an aluminum heatsink from a dead PC PSU as a bracket for the volume pot and RCA jacks. Also fixed two speaker connectors at the other end to hook up the speaker wires or the dummy load.

Things to do:
Check the cathode current of individual valves to find out the reason for increased current.
Use oscilloscope and 8Ohm dummy load to examine wave forms, gain, distortion, clipping, and output power.
Asking you about the effects of increased values of capacitors at signal input to first triode, and coupling from the concertina phase splitter to the grids of pentodes.
Regards.
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Last edited by etherman; 10th Feb 2022 at 6:04 pm.
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Old 12th Feb 2022, 6:00 am   #64
Robert Gribnau
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Higher values for coupling capacitors make the -3dB point at the low frequency end shift to a lower point.

The coupling capacitors between the phase-splitter and power stage are inside the feedbackloop. Output transformers cause phase-shift at the low and the high end of the frequency spectrum. Higher values for these coupling capacitors could have a negative influence on the stability of the feedback loop. But you are using decent output transformers and the feedbackloop only contains one set of coupling capacitors (capacitors also cause phase shift) because the first stage and the phase-splitter are dc-coupled so probably the higher values are no problem.

Maybe the designers of this amplifier, meant for a record player, chose the values for the coupling capacitors so that the very low frequencies got a bit attenuated to prevent rumble being amplified (too much).
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Old 12th Feb 2022, 12:58 pm   #65
Al (astral highway)
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Quote:
Originally Posted by etherman View Post
1. Slightly more tidy construction: I planned on a small copper clad board engraved using a rotary tool as a substitute for a tag board. But eventually gave up and used the same flying leads as before.
Hi there, I've been watching this thread with interest and curiosity and your tenacity stands out as very high.

I do wonder if you could consider the impact of a perhaps tidier construction? The footprint of your amplifier is very large, especially considering the outboard power-supply.

The circuit is not complicated and doesn't have a high component count. As things stand, you've got flying leads all over the place, which aren't ideal from a safety view-point (although I'll stop there as safety hints are easily overdone).

But they are also not idea from another viewpoint. Routing wires in the signal pathway away from power supply (AC and DC) is done for a reason. Having wires as short as possible is also done for a reason. What I'm seeing here is a bit of a rat's nest. Supposing (very reasonable) that you are close to completing this iteration of your amplifier, how will you actually use it? You'd need to have an absolutely huge enclosure to carry it around safely.

It's true to say that a some people on the forum have insanely good machining and construction skills and even more can put a circuit into a tidy-looking enclosure that is also designed to avoid problems.

Others have to try a bit harder and it doesn't come naturally. But I honestly think that if you try to build this from a mechanical layout that has the smallest footprint, you will be a lot happier with the results. That means practically (carrying it around), aesthetically (looking at it) and in performance (minimal hum). I hope this isn't too intrusive, but you've put so much effort into this that it would be a shame to not see it looking its most beautiful. I hope this sounds helpful, not critical.

As I say, it takes a lot of patience and trial and error to do what you're doing, but there are certainly many ready examples of good and not-so-good practice out there already. Dansette amplifiers, for example, aren't complicated. They also don't have perfect innards -but they represent an easily-achievable standard of construction if you wanted to inspect something of similar complexity.

Cheers
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Old 12th Feb 2022, 1:46 pm   #66
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Here's an example from my own work.

Here is a Dansette chassis after I restored it. I made twisted pairs of the heater wires (laced together here) and the component leads to the valve bases are as short as possible. It was easily possible to improve on the original standard of wiring.

Mod please merge threads if you see fit. Thank you
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Old 13th Feb 2022, 9:43 pm   #67
etherman
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Quote:
Originally Posted by Al (astral highway) View Post
Here's an example from my own work.

Here is a Dansette chassis after I restored it. I made twisted pairs of the heater wires (laced together here) and the component leads to the valve bases are as short as possible. It was easily possible to improve on the original standard of wiring.

Mod please merge threads if you see fit. Thank you
Thank you Al for the complement.
The link above does not open the image. I searched for it, and I think this link works:
https://www.vintage-radio.net/forum/...pictureid=4573

The lacing is beautiful. When I mount the circuit on a proper, neat chassis (I hope in the near future), I will try to organize the wires as neat as in the above image. As I have mentioned before, the current construction is not intended as the final shape or as a portable device, but rather as a quick and inexpensive way to test the circuit. For me it was a "proof of concept", a crude prototype, just to make sure I can assemble a properly working amp with the parts available to me, with the great help from forum members.
But I have to confess that after a few days of enjoying listening to the amp for a few days, I may find it difficult to take it apart.
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Old 13th Feb 2022, 9:55 pm   #68
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Quote:
Originally Posted by Robert Gribnau View Post
Higher values for coupling capacitors make the -3dB point at the low frequency end shift to a lower point.

The coupling capacitors between the phase-splitter and power stage are inside the feedbackloop. Output transformers cause phase-shift at the low and the high end of the frequency spectrum. Higher values for these coupling capacitors could have a negative influence on the stability of the feedback loop. But you are using decent output transformers and the feedbackloop only contains one set of coupling capacitors (capacitors also cause phase shift) because the first stage and the phase-splitter are dc-coupled so probably the higher values are no problem.

Maybe the designers of this amplifier, meant for a record player, chose the values for the coupling capacitors so that the very low frequencies got a bit attenuated to prevent rumble being amplified (too much).
Thank you for the info. Would you kindly elaborate on the -3dB point shift to a lower point? Does this mean more (louder) bass?
I have changed to these capacitors assuming they are better quality than the older ones, hoping the change will improve the sound. Despite lack of objective measurement, I think it sounds better: I mean stronger bass and better high frequency response.
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Old 14th Feb 2022, 8:05 am   #69
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

I don't think you will hear more (louder) bass because the -3 dB point was already low.

Here's some info on (how to calculate) the -3 dB point or "cuttoff frequency":

https://en.wikipedia.org/wiki/High-pass_filter

With C = 47 nF and R = 470K the -3 dB point would be at 7,2 Hz.

With C = 330 nF and R = 470K the - 3dB point would be at 1 Hz.

I only just saw that your 330 nF coupling capacitors are rated for 160 V. That's too low. I see that you measured 171 V on the anode of the phase splitter, so that's already over 160 V (since the other end of the capacitor sits at ground potential, so at 0 V). But more alarming is that because you are using solid state rectification, the full B+ (which will be higher than normal at startup because there is no current draw yet) will be at the anode of the phase splitter in the period that the filaments are still warming up and the tubes are not conducting yet. Without current there will be no voltage drop over the anode resistor of the phase splitter. I advise you to choose capacitors rated for 400 V (or atleast 350 V) at those places.

The voltage rating of the capacitor at the input of the amplifier is less critical because under normal circumstances the voltage difference over this capacitor stays low (unless there's something wrong in the source connected to the amplifier). The value of 2.2 uF is unusually high (just calculate the -3dB point).
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Last edited by Robert Gribnau; 14th Feb 2022 at 8:15 am. Reason: Corrected a typo
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Old 21st Mar 2022, 9:46 pm   #70
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Despite some health issues, my attention is back to the amplifier. On listening, the first (built) channel is louder than the newer one. This is easy to confirm by ear. The main differences between the two channels, are the brand of PCL86 (it was a random choice from the box) and the type and value of connecting capacitors between the concertina stage and the output tubes.
The 2 channels together draw 145mA. Triode anode voltages are identical, 90V for V1, and 167V for V2. The cathode voltage of the phase splitter triodes is 91V.
After disconnecting the first (louder) channel, I remeasured. The channel draws more current with V2 pentode drawing a cathode current of 40.8mA, and V1 34 mA. Screen grid current was also higher for V2: 7mA vs. 5.8 mA for V1. For the first channel, the screen grids drew 4.4 and 4.5 mA.
Using the scope and a simple signal generator and a 7.5 Ohm load (2 x 15 Ohm in parallel), a 1000 Hz input signal of 0.5-0.6 V, resulted in output of about 16 V without clipping. Scope calibration may be suboptimal, although I have used previously it in several digital circuits with reasonable accuracy.

Apart from using identical capacitors, what are your suggestions to adjust?
I have already procured suitable capacitors (value and voltage), but are not yet replaced.
Regards.
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Old 21st Mar 2022, 10:05 pm   #71
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Are you sure its 16 volts output?.
That equates to about 32watts!!! .

Just my question.

best regards

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Old 21st Mar 2022, 10:13 pm   #72
etherman
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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Are you sure its 16 volts output?.
That equates to about 32watts!!! .

Just my question.

best regards

Joe
I also questioned the results. The probe was set at 10:1 and the vertical division is 0.2 V x 10 x 8 or so divisions? Did I do something wrong? Or may be those cheapo probes are not 10:1?
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Old 21st Mar 2022, 10:41 pm   #73
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

It looks more like 18V from the picture (the trace spills over the top and bottom of the graticule).

But that's 18V pk-pk, which is about 6.4V RMS.

Into 7.5ohm that's 5.4W.

I'd be slightly surprised if it was clipping at that power. Are you sure it's not the scope that's clipping (it's best to keep the trace inside the graticule limits) ?

Cheers,

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Old 21st Mar 2022, 10:46 pm   #74
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Oops !! I didnt look at the photo of the CRO.

Thanks GJ
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Old 22nd Mar 2022, 5:50 am   #75
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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It looks more like 18V from the picture (the trace spills over the top and bottom of the graticule).

But that's 18V pk-pk, which is about 6.4V RMS.

Into 7.5ohm that's 5.4W.

I'd be slightly surprised if it was clipping at that power. Are you sure it's not the scope that's clipping (it's best to keep the trace inside the graticule limits) ?

Cheers,

GJ
I was just trying to be conservative. I have taken this picture some time ago, so I don't remember, but I will try some more measurements till clipping.
Is it better (more accurately) done with a dummy load or with the speaker?
I know speaker health won't like sine wave continuous output.
Regards.
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Old 22nd Mar 2022, 8:59 am   #76
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

People rarely make 'maximum power' measurements with speakers, partly because of possible damage to the speaker, partly because the speaker's properties may change as it heats up, but mostly because the sound level is unbearable for our ears. Some people say "But you must measure with a speaker because that is how the amp will be used". Others say "It will never be used continuously at a single frequency and maximum power".

All speakers have different impedances. Some testers have used dummy loads which have impedances more like speaker impedances. Here is one example, albeit quite a complex one https://sound-au.com/project216.htm.

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Old 27th Mar 2022, 10:04 am   #77
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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Despite some health issues, my attention is back to the amplifier. On listening, the first (built) channel is louder than the newer one. This is easy to confirm by ear.
When you say louder, do you mean greater maximum loudness (which could indeed be affected by the DC operating conditions) or louder output for the same input? The latter would imply different gain, suggesting some inadvertently different component values in the feedback system.
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Old 3rd Apr 2022, 10:10 pm   #78
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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Quote:
Originally Posted by etherman View Post
Despite some health issues, my attention is back to the amplifier. On listening, the first (built) channel is louder than the newer one. This is easy to confirm by ear.
When you say louder, do you mean greater maximum loudness (which could indeed be affected by the DC operating conditions) or louder output for the same input? The latter would imply different gain, suggesting some inadvertently different component values in the feedback system.
I meant louder output for the same input. I swapped input cables (to rule out signal level), then I swapped speakers, and the result was the same. I guess the different brand tubes need to be biased differently or one tube is grossly off spec, based on a quick observation, that one pentode is drawing more current than the other. I hope I will be able to make further tests soon and may be use a different pair of PCL86s from my stock.
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