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Old 16th Apr 2021, 8:55 pm   #21
etherman
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

My question is related to the lack of a complete schematic that I can get the parts for. I am trying to adapt existing designs to the tubes and transformers that I have. I was wondering if I should use a higher value resistor, is this going to give me smoother DC? Is it better to reduce B+ using a larger value resistor? I was wondering about the calculations done to determine the best resistor value in that position. Thank you anyway.
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Old 16th Apr 2021, 11:05 pm   #22
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

A higher value resistor will give you smoother DC, but it will also dissipate more power and lower your HT voltage. There are lots of trade-offs here. The smoothness of the HT mostly affects the output stage of the amplifier, because you will use further dropping resistors and smoothing capacitors for the HT for earlier stages. You are considering a push-pull output stage, and these can tolerate more HT ripple than single-ended stages (at least as far as background hum is concerned - the ripple will still modulate the stage gain and that will add 100Hz sideband distortion). Because you plan to use PCL86s you will have screen grids in the output valves. If you use a traditional output transformer rather than an ultralinear one then you have the option to improve the smoothing for the screen grid supply, which again can improve the overall tolerance of the output stage to HT ripple. The noise performance will depend on how well-matched your output valves are (unless you include some balancing circuitry to compensate for that).

I'm afraid the bottom line is that amplifier design is a complex subject. When valve technology was the only technology and quite a lot of people built their own amplifiers specialist designers developed, and published, standard circuits (the Williamson amp, all the Mullard amps, the amps in GEC's An Approach to Audio Frequency Amplifier Design). Ordinary people built exact versions of these, because changing the circuits could introduce a lot of unwanted problems. Otherwise they read books and educated themselves and became expert enough to design their own amplifiers. But there really isn't a short-cut to that. It takes time and effort.

As far as power supplies go, you can see what the effects of changing component values are by using the very good, and free, software psud2 available here http://www.duncanamps.com/psud2/. But choosing the best values for the components will still depend on understanding how the power supply interacts with the output stage.

Cheers,

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Last edited by GrimJosef; 16th Apr 2021 at 11:32 pm.
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Old 17th Apr 2021, 10:34 pm   #23
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

What exactly is the provenance of the OPT's? How do you know they are "25W"? At what frequency? For how much distortion? Do they have ultralinear taps? 5K, though perfectly feasible etc, is not a particularly common anode to anode impedance and it would be usual for there to be secondary taps for more than just 8Ohm.
As you are making a stereo amp the presumed use would be hi fi and in this case then the actual quality of the OPT's is the single most important factor! Many designs will not even be stable if the OPT's are of poor quality and will need the negative feedback rolling back to prevent instability which in turn will further reduce sound quality.

IF they are of decent quality and 5K then I second "GrimJosef" in that two parallel pairs of output valves should give best results. preferably EL84's but 4 ECL86 could be used and I guess you'd have more than enough left over triode sections to do the whole amp with just the ECL86's even if using a LTP as phase splitter... anode to triode section screening/separation allowing!
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Old 18th Apr 2021, 1:43 pm   #24
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Thank you for your replies; GrimJosef and Jez1234.
The output transformers are Edcor CXPP25-8-5K, their specs are available on the web and unfortunately there is only one 8 Ohm secondary winding. I thought also to connect an 8 Ohm speaker to the 4 Ohm winding (if available) so that the tubes see a 10 K load. They are rated for 25 watts. I hope I will be able to run them on a slightly lower voltage so the loads may match.
Two parallel pairs will be difficult with my existing power transformer, which I had ordered (custom wound) recently, especially the heater winding, and also the HT. If I decide on something like that I will have to order a new power transformer, which is going to be bulky.

I am (and have been) doing a lot of reading, to understand how to adjust existing designs to my resources. Previous experiments with said transformers produced excellent sound with a much lower voltage. So, learning is part of the enjoyment of the hobby for me. In this forum I am asking questions, so experts like you can help me get the answers, and probably for others to benefit, even from my failures or successes.

I have attached my oscilloscope to the output of the power supply at the first then at the second filter capacitors. The ripple is reduced fro about 25 volts at the first filter capacitor, to about 10 volts at the second, i.e., after the 100 Ohm resistor.
Is this acceptable or should I try to improve it? I can increase the size of the filter capacitor(s), and / or increase the value of the resistor? I already have a couple of 100 uF 400 volt caps, I just hope they wont blow the fuse at startup.
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Old 18th Apr 2021, 2:35 pm   #25
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Whether 10V ripple is acceptable or not depends on the design of the output stage. The classic Quad II amplifier has approaching 10V pk-pk ripple at 100Hz on the supply to its KT66 anodes (150mA drawn from a 16uF reservoir capacitor). But it gets away with this, in part at least, because the screen grids see much less ripple. This does limit the 100Hz hum, particularly if the two KT66s are well-balanced. There is still significant 100Hz sideband distortion caused by the ripple modulating the gain. Whether you think this is acceptable or not is a matter of judgement. Quad clearly did.

Cheers,

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Old 18th Apr 2021, 3:19 pm   #26
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

I have finally succeeded in attaching the images from the scope for your reference. I had some difficulty in the past hour or so.
The first scope screen shows the ripple at the first capacitor at a height of 2.5 to three vertical divisions, each division is 1x10 volts (probe set to x 10). The last photo is the ripple at the second capacitor, about one vertical division.
I will probably try to improve the filter components. Increasing the resistor will reduce the ripple (hopefully), and will also reduced B+ to around 250 volts, which is what I initially intended. I may later increase one or both capacitor values.
Greetings.
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Old 19th Apr 2021, 12:25 am   #27
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

I continued with power supply. I wanted to increase the resistor between the two filter electrolytics to around 200 Ohms and see its effect on ripple voltage. I lacked a suitable powered resistor (10 W). The closest fit was two 620 Ohm 5 W resistors which I connected in parallel. I started by soldering a 47 uF 400 volt electrolytic parallel to the second filter capacitor. Ripple voltage became 5 volts.
I replaced the 100 Ohm with 310. Ripple was reduced to about 2.5 volts (first attached photo). However, the voltage drop was large. The two resistors got very hot. I think I calculated a power loss (as heat), about 9-10 watts based on the voltage drop across the 310 Ohms.
I went back to 100 Ohms and kept the additional 47uF cap, this resulted in a ripple of 5 volts (second photo). The 100 Ohm resistor, which could be seen in the previous post bolted to a small aluminium heat sink, together with the bridge rectifier, got really hot. Local temperature was 40 degrees C today. Both the heat sink and the power transformer got hot during the test, unlike a couple of days ago. I will have to consider temperature management for the amp.
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Old 19th Apr 2021, 7:09 am   #28
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

You need a choke really,resistors as you've found drop too much voltage. For a 3dB (half) attenuation you need a 3k3 resistor (1/6.28 x 0.000047) = 3387 ohms. That'd drop 660v @ 0.2A ( 4x 0.050A) !! : ) On my amps I use at least 350u choke 350u to get about 10mv of ripple. I use something like 220u or 470u as decoupling for each stage. Caps are relatively cheap.

For a rough guide your PSU needs to be capable 1.5 times quiescent current,there's a program here that helps design - http://www.duncanamps.com/psud2/index.html also see - http://www.valvewizard.co.uk/psu.html & http://www.valvewizard.co.uk/smoothing.html & http://www.turneraudio.com.au/powersupplies.html

Andy.
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Old 19th Apr 2021, 8:40 am   #29
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Both Leak and Radford made amps which avoided choke smoothing.

The Leak TL/10 used 8uF+8uF with a 1k resistor, and when I worked on a pair recently the 10W-rated resistor did smoke at switch-on. The Stereo 20 uses 32uF+32uF and 100ohms. This doesn't smoke but it does run very hot.

The Radford STA100 (details here https://www.radfordrevival.co.uk/wor...uctions-P3.jpg) uses a choke for its low-voltage supply. But the high-voltage one, which powers the stereo ultralinear 100W output stages, uses a solid-state bridge rectifier and what the British military used to refer to as a single CFE (Capacitor, Flippin' Enormous). That said, the amp does hum a bit. I reworked one for a customer and I fitted a smaller reservoir capacitor and a choke before the CFE and, as I recall, the hum fell by 20dB !

Cheers,

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Old 21st Apr 2021, 2:55 pm   #30
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Quote:
Originally Posted by Diabolical Artificer View Post
You need a choke really,resistors as you've found drop too much voltage. For a 3dB (half) attenuation you need a 3k3 resistor (1/6.28 x 0.000047) = 3387 ohms. That'd drop 660v @ 0.2A ( 4x 0.050A) !! : ) On my amps I use at least 350u choke 350u to get about 10mv of ripple. I use something like 220u or 470u as decoupling for each stage. Caps are relatively cheap.

For a rough guide your PSU needs to be capable 1.5 times quiescent current,there's a program here that helps design - http://www.duncanamps.com/psud2/index.html also see - http://www.valvewizard.co.uk/psu.html & http://www.valvewizard.co.uk/smoothing.html & http://www.turneraudio.com.au/powersupplies.html

Andy.
Thank you Andy.
I am definitely considering the choke. I will have to wait till the middle of May to have it custom wound, because the one shop that I know is busy.
What is the ideal impedance or number of turns?
I will probably order it a bit oversized on the current side (wire thickness), just in case I use it for higher current draw tube combination, e.g.; if I would add a phono preamp.
But till then I will test it with the present arrangement as a starting point.
I did not find too many PCL86 push pull schematics. For the output stage with automatic bias there is great variability in cathode resistor value. Some use a single common resistor, some use two separate resistors, +/- a balancing pot.
Which would be easier to manage?
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Old 21st Apr 2021, 3:00 pm   #31
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Quote:
Originally Posted by GrimJosef View Post
Both Leak and Radford made amps which avoided choke smoothing.

The Leak TL/10 used 8uF+8uF with a 1k resistor, and when I worked on a pair recently the 10W-rated resistor did smoke at switch-on. The Stereo 20 uses 32uF+32uF and 100ohms. This doesn't smoke but it does run very hot.

The Radford STA100 (details here https://www.radfordrevival.co.uk/wor...uctions-P3.jpg) uses a choke for its low-voltage supply. But the high-voltage one, which powers the stereo ultralinear 100W output stages, uses a solid-state bridge rectifier and what the British military used to refer to as a single CFE (Capacitor, Flippin' Enormous). That said, the amp does hum a bit. I reworked one for a customer and I fitted a smaller reservoir capacitor and a choke before the CFE and, as I recall, the hum fell by 20dB !

Cheers,

GJ
Thank you GJ for the info.
Why did you fit a "smaller" capacitor? Was there a single capacitor initially?
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Old 21st Apr 2021, 9:59 pm   #32
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Yes, originally there just one C (large). I added a choke and another C (a small one) to make

C(small)-L-C(large).

Cheers,

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Old 22nd Apr 2021, 7:32 am   #33
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

The bigger the Z the better but high z means more turns, something like 10H should do the job, but the Z of your choke depends on the C of your 2nd cap, so you'd need less inductance if you have more capacitance, you'd have top do the sums. Together the L & C form a high pass filter. Ordering a choke with a higher current capability is a good idea, keep your design flexible.

As regards cathode bypass resistors the PCL86 datasheet doesn't give figures for Class AB1 PP operation, but the ECL86 does, see attached. I worked out you'd need either two 800r or two 330r cathode resistors for a bias of -12v @ 0.015A & -10v @ 0.030A respectively, that's for a Z pri of 9k and 300v. These are rough approximate values and a lot higher values than those given on the datasheet, but those values are for a single resistor, still,my values are higher.I pulled the ECL86 dynamic graph and drew loadlines on the screen of the monitor. Biasing at -10v is biasing the valve too hot though, so better to go somewhere in the middle of those two, EG -11v but you'd need to draw a loadline really and experiment on the bench.

That said when prototyping the amp pop something like an 820r on the cathodes with something like a 22u bypass cap as a start, then take DC readings then test with an IP signal.

Andy.
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Old 11th Jan 2022, 7:47 pm   #34
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Thread reopened by request.
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Old 11th Jan 2022, 9:49 pm   #35
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Originally Posted by paulsherwin View Post
Thread reopened by request.
Thank you for the prompt response.
I restarted experimenting after a long distraction.
I decided to assemble one channel on sort of a breadboard, inspired mainly by the construction techniques of forum member Robert Gribnau demonstrated by photos on his blog: http://otl800.blogspot.com/. I will attach photos of my construction, though much less elegant.
The circuit is based on the Dansette HIFI mk2 discussed a few times on the forum (e.g. https://www.vintage-radio.net/forum/...d.php?t=144533).

The power supply is based on the previous components but with one more resistor/capacitor stage to reduce B+ voltage and ripple. The output transformers are the only ones I have: 5k primary, 8 Ohm secondary, 25 W Edcor. The ultralinear taps were connected to G2 via 47 Ohm resistors, I added these in order to measure screen current, and because I saw them in another ECL86 amp. Pentode cathodes were fitted with 330 Ohm resistors bypassed by 100uF electrolytics. The output was connected to one of my HiFi speakers ( a big Mordaunt Short from the 1990s), after initial testing using a old driver to avoid accidents with my valued speaker.
I have tried to take accurate readings while I tweaked the values of resistors in the power supply and later the cathodes of the output pentodes.
The first trials showed a filament voltage of 13 volts with only 2 tubes connected (one channel). I hope it won't drop much when I connect the second channel. B+ was much higher than expected. A/C before rectifier was 271 volts. DC after the rectifier bridge was 357 volts. After increasing the values of power resistors: 316 volts following the first ^20 Ohm, and 300-303 volts after the following 220 Ohm.
Cathode voltage was 9.6 volts on both tubes. G2 current was 3.5 mA (below values in the datasheet, don't know why).
Increasing input signal to increase volume, resulted in audible distortion. I have used either my old iPhone or an old CD player as music sources.

Having re-read a previous post in this thread by Robert Gribnau:

"Running a pair of PCL86's push-pull using a 5K OPT would mean that you have to lower Vba (and higher Ia) compared to the situation at Vba = 250 V. I doubt if you would get much better results from a pair of PCL86's running at Vba = about 230 V (and Raa = 5K) than from the suggested pair of PCL82's.

You could search on the internet for what is called "low loading" (mostly in combination with a pair of EL84's in push-pull). Perhaps this could work well for a pair of PCL86's in push-pull too.

Addition: See page 51 of this book.

http://www.introni.it/pdf/Rodenhuis%...20Circuits.pdf"

I started tweaking for "low loading". For that I increased resistor values in the power supply to reduce B+, and later increased cathode resistors. Increasing the latter from 330 to 470 Ohms resulted in much lower volume output, and higher B+ voltage due to reduced cathode (and anode) current.
Lowering B+ to 200 volts was associated with low anode overall current.
After a few back and forth resistor swapping and limited by my small collection of 5 and 10 watt resistors, I settled on a B+ of 230 volts and cathode resistors are back to 330 Ohms. Distortion is reduced, but also maximum volume.
The positive part: there was no audible hum from the speaker, even with no signal.
What I didn't do (yet):
measure gain using an oscilloscope,
tweak the cathode bypass capacitor value,
tweak the value and type of the two capacitors carrying signal from the concertina phase splitter to the grids of the pentodes.
I will also try to improve my breadboard to make it easier to swap components.
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Old 12th Jan 2022, 1:51 am   #36
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Breadboard, Meccano and a rat's nest all in one.



The fun starts when you go for the tidy "production" version.
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Old 12th Jan 2022, 7:24 am   #37
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Nice work, like the spiders web breadboard.You can use a standard bread board, just solder on some "legs" (resistor lead cut-offs) onto the base and carefully seat into the BB, your method works though, that's all that matters.

Have you checked your DC conditions yet? Next is to stick a signal in one end have a bit of a test and a listen, keep up the good work, Andy.
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Old 12th Jan 2022, 10:46 am   #38
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

Hi,

Now that’s what I call a pukka hook-up!

Whilst talking PCL86, I recently converted a Cadet 2 power amp to PCL86 using a voltage doubler from the 6.3V winding to give the 12.6V for the heaters. The extra current drain of the voltage doubler on the 6.3V winding came at the expense of not using a preamplifier.

Regards
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Old 12th Jan 2022, 11:06 am   #39
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Default Re: Contemplated PCL86 push-pull home brew stereo amplifier

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.
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Old 12th Jan 2022, 6:41 pm   #40
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Breadboard, Meccano and a rat's nest all in one.



The fun starts when you go for the tidy "production" version.
Thank you Herald, Meccano was a childhood fascination, I was able to acquire an incomplete box a couple of years ago. Changing parts in the rat's nest is not that easy. I was thinking of migrating to a semi-permanent construction, with an aluminium chassis and copper clad boards (instead of tag boards, which are difficult to come by here). The aluminium chassis was made at a local store by bending inexpensive 2 mm thick sheet at a local shop. However I made major mistakes when drilling holes for the tube / valve sockets using a stepped

Quote:
Originally Posted by Diabolical Artificer View Post
Nice work, like the spiders web breadboard.You can use a standard bread board, just solder on some "legs" (resistor lead cut-offs) onto the base and carefully seat into the BB, your method works though, that's all that matters.

Have you checked your DC conditions yet? Next is to stick a signal in one end have a bit of a test and a listen, keep up the good work, Andy.
Thank you.
I have checked some values but not in a comprehensive manner. I was more eager to listen rather than measure.
I would like to point out that for safety's sake, each time I wanted to change the site where I hooked-up my DMM probes, I had to de-energize the circuit, to avoid an electric shock.

Quote:
Originally Posted by Valvepower View Post
Hi,

Now that’s what I call a pukka hook-up!

Whilst talking PCL86, I recently converted a Cadet 2 power amp to PCL86 using a voltage doubler from the 6.3V winding to give the 12.6V for the heaters. The extra current drain of the voltage doubler on the 6.3V winding came at the expense of not using a preamplifier.

Regards
Terry
Thank you Terry for the complements, although I couldn't find out what pukka hook-up is. Is it a sort of knitting needle?
12.6 volts seem low for PCL86. The heaters are quoted 13.3, 13.5 and 14 volts from various sources.
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