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Old 10th May 2020, 6:08 pm   #1
ValvoStef
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Default My PCL86 SE amp

Hi fellows,

Finally I finished my PCL86 SE amp. The valve line up is 2 x PY802, 2 x PCL86, 1 x PABC80 and 1 x PM84. Bit chaotic wiring and I had to use two transformers in the power supply section, one for the heaters and on for the anodes. The strange valve line up is used to match the heater voltages to the transformer output. To be honest, the amp, despite the simple output transformer, does sound good. There is a faint mains hum in the speaker, barely noticeable, probably a result of the chaotic layout and wiring. Annoyingly is the fact that on power up, both PCL86 heaters flash up in a worryingly white light. Both are connected in series to 28V AC, all the other valves are behaving fine, heaters in series to the same transformer 2 x 15V withe a few drop down resistors. All the needed parts I found in my stash, canít remember where I got that orange enclosure from. The PM84 acts as an output level indicator (obviously), the PABC80 amplifies and rectifies the audio signal for the indicator tube. For a single ended PCL86 stage, the amp is loud enough for my needs. I have not done any measurement whilst in operation, I guess the output should be around 2W. Pleasant to listen to music and speech.
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Old 10th May 2020, 7:44 pm   #2
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Default Re: My PCL86 SE amp

Quote:
Annoyingly is the fact that on power up, both PCL86 heaters flash up in a worryingly white light.
Some valves always did that, don't worry about it.
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Old 10th May 2020, 8:02 pm   #3
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Default Re: My PCL86 SE amp

Superb ! what a way to put those unwanted valves to good use, I must have discarded loads of tTV valves in the past .

I have a mind to build an amplifier using EL506's some day maybe.

Mick.
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Old 11th May 2020, 6:51 am   #4
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Default Re: My PCL86 SE amp

Nice work, like the chassis, good to see all the homebrew amplifiers being built.

Andy.
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Old 11th May 2020, 2:19 pm   #5
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Default Re: My PCL86 SE amp

Nice job! The photos make me nostalgic!

- Nandu.
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Old 11th May 2020, 5:11 pm   #6
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Default Re: My PCL86 SE amp

Very nice work. Love the orange.

Aub
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Old 11th May 2020, 6:55 pm   #7
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Default Re: My PCL86 SE amp

Orange box (not that sort of orange box) looks like the case from a Clare Insulation Tester.

Is there a model number on the rear panel label?
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Old 11th May 2020, 8:29 pm   #8
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Default Re: My PCL86 SE amp

Indeed, Herald, Clare A203C. Well spotted, I seem to remember there were no components, I probably fished the enclosure out of a skip. Never imagined it as an insulation tester. Learned something new. That is the beauty of this forum, there is always someone who spots the overlooked details.
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Old 12th May 2020, 2:25 am   #9
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Default Re: My PCL86 SE amp

Beautifully put together and a great use of plentiful valves that would usually be overlooked.
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Old 12th May 2020, 11:53 am   #10
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Default Re: My PCL86 SE amp

The amp is based on the attached design I found somewhere on the net. Unusual volume control. This morning I connected the amp to a Kef speaker and it sounds so much better.
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Old 12th May 2020, 12:29 pm   #11
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Default Re: My PCL86 SE amp

Quote:
Originally Posted by ValvoStef View Post
Indeed, Herald, Clare A203C. Well spotted, I seem to remember there were no components, I probably fished the enclosure out of a skip. Never imagined it as an insulation tester. Learned something new. That is the beauty of this forum, there is always someone who spots the overlooked details.
Glad to help. I used one of these occasionally at work along with a similarly cased (different colour?) earth continuity tester. They were routinely used in production test for safety checks on each newly built bit of kit.
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Old 12th May 2020, 4:58 pm   #12
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Default Re: My PCL86 SE amp

Quote:
Originally Posted by ValvoStef View Post
The amp is based on the attached design I found somewhere on the net. Unusual volume control. This morning I connected the amp to a Kef speaker and it sounds so much better.
I was following this thread with a bit of interest.
What's the second PCL86 used for? I see the need for the PABC80 and the PM84!
I like the use of what call "Damper Diodes" as rectifiers.
Dave, USradcoll1, at 75, still working on homebrew projects.
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Old 13th May 2020, 11:31 am   #13
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Default Re: My PCL86 SE amp

Hi Dave, I needed another triode for the pre amp section. Because of the heater voltage I used another PCL86 and wasted the pentode. I could have used a ECC83 however I have plenty of PCL86 and only a few ECC83.

Stef.
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Old 13th May 2020, 3:42 pm   #14
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Default Re: My PCL86 SE amp

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Originally Posted by ValvoStef View Post
Hi Dave, I needed another triode for the pre amp section. Because of the heater voltage I used another PCL86 and wasted the pentode. I could have used a ECC83 however I have plenty of PCL86 and only a few ECC83.

Stef.
I looked up the cost and availability of the PCL86. It's priced very reasonable at a mail order tube dealer. It's a 14 volt version of the ECL86, which is a 6GW8.
I can use a transformer from a scrap solid state amplifier for the heaters.
Dave, USradcoll1,
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Old 13th May 2020, 5:56 pm   #15
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Default Re: My PCL86 SE amp

The odd volume control looks like it has an automatic "loundness" function. Could be handy if one can't find a tapped pot, which did much the same thing.
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Old 13th May 2020, 8:21 pm   #16
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Default Re: My PCL86 SE amp

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Originally Posted by ValvoStef View Post
I used another PCL86 and wasted the pentode.
Stef.
It's there, it's already heated. Why don't you connect it in parallel with the other and see what happens ? The output transformer I guess has a random primary-impedance anyway so it can't hurt (much). You're not saying if you use auto-bias or fixed bias but if it's auto, just drop the cathode-resistor to half value.

rgds,

/Torben
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Old 13th May 2020, 9:52 pm   #17
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Default Re: My PCL86 SE amp

Hi Gents, these U/P/E valves of the same nominal type often had a constant wattage heater, so very easy to make up a tiny auto transformer and fit under the chassis to convert between types

Ed
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Old 14th May 2020, 10:16 am   #18
ValvoStef
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Default Re: My PCL86 SE amp

Quote:
Originally Posted by tri-comp View Post
Quote:
Originally Posted by ValvoStef View Post
I used another PCL86 and wasted the pentode.
Stef.
It's there, it's already heated. Why don't you connect it in parallel with the other and see what happens ? The output transformer I guess has a random primary-impedance anyway so it can't hurt (much). You're not saying if you use auto-bias or fixed bias but if it's auto, just drop the cathode-resistor to half value.

rgds,

/Torben
Good idea, Torben. Iíll give it a go, see what happens. The transformer is from an old radio chassis, probably designed for EL84.
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Old 14th May 2020, 9:53 pm   #19
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Default Re: My PCL86 SE amp

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Originally Posted by ValvoStef View Post
probably designed for EL84.
In that case you'd expect the primary-impedance to be around 5K-Ohm.

A PCL86 in class-A, single-ended mode will need around twice that.
Two in parallel would be perfect for the transformer you've used.
If your high-voltage power-supply is up to it and you check that the screen-grid voltage is not sagging because of the second valve (reduce series resistor value or give each valve it's own), you should be expecting a neat rise in output power.
My guess would be output around 7-8Watt, obviously depending on what anode-supply voltage you are feeding the output pentodes. 300V should be the absolute max. if you want a reasonable long life from the valves. (Something the ECL86 variant ISN'T known to have in any case, possibly because it was usually run HOT and to the limit in many sets.) The ECL86 will easily develop Grid-1 current causing runaway cathode-current. I really don't know if that the case with the PCL86 as I don't have much experience with these. There should be no other difference than the filament configuration but who knows..?


Rgds,

/Torben

Edit:
Re. HOT and to the limit, see the attached.
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Last edited by tri-comp; 14th May 2020 at 10:13 pm. Reason: Added Revox manual
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Old 14th May 2020, 10:12 pm   #20
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Default Re: My PCL86 SE amp

Indeed, the datasheet for the EL84 http://www.r-type.org/pdfs/el84.pdf indicates that with a 250V HT supply it needs around a 5kohm transformer primary load for single-ended operation.

But the datasheet for the PCL86 http://www.r-type.org/pdfs/pcl86.pdf also indicates around a 5kohm load for the pentode section, albeit with a 230V HT and auto-bias.

I always understood that the ECL86/PCL86 were, approximately, an EL84 and half of an ECC83 built into a single glass envelope. So I'm not surprised to see that their specified anode loads are similar.

Cheers,

GJ
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