Contemplated PCL86 push-pull home brew stereo amplifier
 

Hi,
I am planning to build a hifi stereo amplifier using parts that I have accumulated over a few years. The output transformers are 25 watt Edcor PP with 5K primaries and 8 ohm secondaries, while most designs using these tubes call for 8 or 9k primaries. I have several pairs of 8 ohm speakers. I will have a power transformer custom built for this amp. So my question is what anode voltage should I use to help match the anode to transformer primary? What is the influence of anode voltage on load impedance? I also have a few PCL82 tubes, that I originally intended for this amp but opted later for the 86s because of their higher possible power output and because they were originally designed for audio purposes. I would like to hear your thoughts on this point also. Acquiring other output transformers is difficult to near impossible, where I am.
I was thinking of a transformer secondary of 250 volts at about 150 mA, to cover 4 PCL82 and may be one dula triode as preamp if needed. The other filament secondary would be 14 volts at 2 amps. I might have more questions as we proceed with the design idea.
Regards.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

PCL86 makes a very good audio amplifier.
A schematic of an amp using them is here:
http://www.dissident-audio.com/PP_ECL86/Schema.gif
That uses 245V HT.

Peter

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

What is the influence of HT voltage on load impedance, you ask?

The higher the voltage, the higher the load impedance needed - and a high voltage gives a small gain in efficiency, too.

The anode voltage swings down as the grid swings up, but the anode voltage 'bottoms' at about 25V. With a high HT voltage, this is a smaller fraction of the HT, so there is a smaller percentage of power lost.

For any HT voltage, there is a maximum anode current you can allow without exceeding anode dissipation. Obviously, the higher the voltage, the smaller the allowable current (and for good valve life, I'd restrict to no more than 85% of the manufacturer's limit).

Once you have the voltage and the current, the optimum load is given by

Rl = (HT volts - bottoming voltage - voltage across cathode resistor) / quiescent current

If you have a good transformer, you can work 'backwards' to determine HT voltage and current.

For push-pull, of course, there is the complication you can reduce the load, increase grid drive, and operate in Class AB. One valve will go well into cut-off, but non-linearities can be taken care of by negative feedback.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

Like you wrote, your 5K OPT is better suited for 2 x PCL82 pp. Looking at this Philips datasheet for the UCL82 (see the diagram on page J) the distortion with Vb = 200 V is very low compared to other power pentodes and power beam tetrodes (0,5 % at 4 Watt; 1,2 % at 6 Watt). With global negative feedback these numbers will ofcourse get even better.

https://frank.pocnet.net/sheets/030/u/UCL82.pdf

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

I've been looking at using ECL/PCL82/4/6 to knock up a SE stereo amp for someone recently,the problem I have is that Va max for all of these valves is 300v,the mains tfmr I have is over this.so you'd need a mains tfmr with a HT sec of 210 - 225v (33/1.35 or 1.414) If getting tfmr's is tricky in Cairo can you source toroidal tfmrs, something like a 100VA then pull the secondary off and wind your own?

For the heaters of of the PCL86 you might get away with a 12v SMPSU off an old laptop or similar.

Andy.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

First I would like to apologize for not responding for a few days, which was due to family health matters.
Thank you all for your responses.
Diabolical Artificer
In Cairo, it is not difficult to get a power transformer. I have ordered one which will be ready in a few days for a reasonable price: secondaries will will be 250V 150 mA, and 13V at 2A. The HT winding is not centre tapped so I plan on a diode bridge and 2 capacitors followed by a R/C filter to reduce ripple and drop the voltage to the desired voltage.
What is difficult to obtain are output transformers, so I am stuck with the ones I have, at least till I try custom winding something, may be single ended for another amp.

Robert Gribnau
Most designs for E/PCL82 and 86 push-pull amps, use 8-9K primaries, with no major differences in design except lower anode (and screen) voltage for the 82s. Distortion figures are also similar. But I am planning on trying both tubes in the initial experimental stage.

kalee20 and Electronpusher0
despite the valuable information, I am still lost. Should I go lower on anode voltage to reduce power levels to suit the lower impedance (5k) of the only transformer available to me?

Regards to all.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

If your okay with another output tube, this is a PCB i drew up today.

6P1P-EV Push pull outputs, i think these work well with your 5K iron.

ECC82 phase splitter.

Cheers,

V4LVE lover on DIYAUDIO.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

If you look at this datasheet for the ECL82, you can see that Raa gets higher when Vba goes up (and Iaa goes down). Because you wrote that you have 5K OPT's, I suggested to use PCL82 with Vba = Vbg2 = 200 V. The loss of output power is only about 1 Watt compared to the situation with Vba = 250 V (and Raa = 10K).

https://frank.pocnet.net/sheets/030/e/ECL82.pdf

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

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

There is a German eBay seller offering good quality PCBs for such an amp. If you message me, I can send you the info. I have no link with seller, but am thinking about a similar project in future.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

Thank you for the Rodenhuis book. I have a big homework to do. Despite your scientific advice, I am more inclined towards starting out with the PCL86s then try the 82s, or may be assemble one channel of each and compare the sound while slightly modifying the components. I read the "low loading" part once, but will have to go through once more as well as other parts of the book.
I picked up the power transformer after work this evening, but was too exhausted to start testing, so hopefully tomorrow. The shop is located in an ancient part of the city where traffic was jammed and extremely busy. So driving by was a real experience.
I will keep you posted.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

Late into this.
First post you say the output transformers are 25W jobs?
That suggests to me that they may be better suited to larger pentodes like EL34 or even KT88 at lower HT? No doubt better knowledge will correct me here.
For practical circuits in addition to those suggested, there are the classic Mullard ECL86 circuits available online,and I'd also have a look at something like the Rogers cadet which does work nicely with PCL86.

Apologies if I have misunderstood any part of the post.

Andy

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

It is impossible to locate those valves (KT88, EL34) around here. I will have to buy them in another country. However I should give ebay a try and find out if it works, as some selleres do not ship here, or it is expensive to an insane level.
I did preliminary tests on my new power transformer. I connected a 40 W incandescent light bulb to the HT secondary through a 100 ohm power resistor. It read 267 volts, with the lamp bright. I removed the lamp and it read 280. The voltage drop across the 100 ohm resistor was about 20 volts. The output was higher than what I ordered. I thought this was due to imprecise turn count. But when I measured the mains voltage it was about 240 volts (239-242 using different cheapo digital multimeters). The mains supply is supposed to be 220 V. I suspect the present reading is due to less consumption, since the weather is cool and people did not start using their air conditioners yet. When summer sets in, I expect to see lower voltages but I don't know how much.
The heater winding read 13.4 volts before and after connecting a 22 ohm power resistor. The center tap read 6.7 volts on both halves.
Any comments on the HT readings?

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

One option for making full use of the transformers' power-handling capabilities would be to run four smaller valves in parallel push-pull. This would allow you to use more readily available valves and could also give you a better impedance match (if one pair of PCL86s needs 9k then two pairs in parallel would need 4.5k).

Cheers,

GJ

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

Hi, a mains transformer is exactly that. It transforms what you put into a different level
So if input is high, output will also be high.
To compound that the transformer windings have resistance, and an I*R drop.If this is constant it can be compensated for by adding extra turns.
In decreasing magnitude there is also the supply impedance and the loading on the supply in your neighbourhood etc, etc.

Ed

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

During my stays in Ghana I had some older equipment like cd-players dying on me and refrigerators were cleary struggling (locals often attach a small fan on their fridge, blowing on the heat-exchange pipes at the back).

It took me a while before I thought about measuring the mains voltage. That turned out to be way too low. At best, in the early mornings, it was about 200 V. In the early evenings this sometimes dropped to about 175 V).

Since then I use voltage stabilizers in Ghana.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

After some delay I started assembling a simple power supply from the parts I have in a temporary, experimental fashion. The circuit is sketched below. It was based on multiple schematics and past experience. Before the bridge rectifier, there's a 250mA fuse. The small capacitors are 155 nF, more than 400V. I used a 1k Ohm, 2 W resistor between the positive poles of the two 400V electrolytics. There is a 100k bleeder resistor, and a 22k resistor from B+ to the anode of a red LED. As a load I used the same 40 W incandescent lamp, with a 100 Ohm series resistor, as used before as AC load for testing. I connected a DMM to measure output voltage under load.

When I switched on the power, I measured only 179 volts. Then I saw smoke. I switched off, but couldn't find the source initially. I thought it was the bleeder so increased its value to 220k. It still smoked when switched on. Then Off and on again, I spotted the 1k resistor glowing, but it was too late. The resistor went open. I removed it and called it a night.
The wattage of the resistor was not suitable for the amount of heat dissipated. Tomorrow I hope I will work on it again.

My question is: how do you determine the value of the resistor between the two smoothing electrolytics? It seems my chosen value was large compared to the current draw of the test lamp/resistor rig. I did not have the time to measure the current (voltage drop across the 100 ohm resistor, but my estimate is it was about 250 mA, which was large for the secondary winding of the transformer.

Here are photos of the test. (Please forgive the chaos).

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

If you really want to be able to do the calculations yourself, you should learn Ohm's Law and the formulas for power. If you do not want to, than you should use a proven schematic.

A value of 1K is way too high for your the amplifier you want to build (a lot of voltage loss over the resistor & poor regulation for a class AB amplifier). To give you some idea: If the current really was 250 mA (= 0,25 A), than the voltage loss over the 1K resistor would be: V = I x R = 0,25 x 1000 = 250 V. The power dissipated in the 1K resistor would than be: P = I x V = 0,25 x 250 = 62,5 Watt.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

Thank you Robert,
I am very familiar with Ohm's law and power calculations. I even use Ohm's law in my practice as anesthesiologist, since the factors governing blood flow could be simplified to an analogy of voltage (blood pressure) and resistance, although this is an over simplification.
This mistake comes from copying what I placed in a previous power supply that I made to power smaller B+ currents, for a regen receiver.
I have already corrected things. I used a 100 Ohm 5 W resistor between the two electrolytics, and used two 47 ohm 5W resistors in series with the 40 W light bulb. This produced 300 volts at the second capacitor. I measured a 20 volt drop across the 100 Ohm resistor, so it is drawing 200 mA. So far so good for the transformer, but I could not keep it on for a long time since the light bulb is receiving 275 volts.
I changed the 22 K resistor supplying the anode of the LED to a larger wattage (it started to smoke).
In the middle of all this I got called to work. :-[
I will keep you posted.

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

Robert Gribnau,
But my question remains, how do we determine the proper value of the resistor between the 2 capacitors in a smoothing network for the power supply? I understand that this resistor acts as a cheap substitute to the classic choke, in this position. Am I right?
I hope that later tonight I will be able to look at the output of the power supply using my vintage Soviet oscilloscope C1-94.
Regards.
Gamal

Re: Contemplated PCL86 push-pull home brew stereo amplifier
 

My answers to your questions so far did not seem to help you. I find it difficult to understand (the context of) your questions. Because of this I prefer to leave this thread. Good luck with your project.