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10th May 2020, 6:08 pm | #1 |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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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, can’t 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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10th May 2020, 7:44 pm | #2 | |
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Re: My PCL86 SE amp
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10th May 2020, 8:02 pm | #3 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,737
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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. |
11th May 2020, 6:51 am | #4 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
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Re: My PCL86 SE amp
Nice work, like the chassis, good to see all the homebrew amplifiers being built.
Andy.
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11th May 2020, 2:19 pm | #5 |
Pentode
Join Date: Jul 2014
Location: Bangalore, India
Posts: 132
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Re: My PCL86 SE amp
Nice job! The photos make me nostalgic!
- Nandu.
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11th May 2020, 5:11 pm | #6 |
Nonode
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 2,034
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Re: My PCL86 SE amp
Very nice work. Love the orange.
Aub
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11th May 2020, 6:55 pm | #7 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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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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11th May 2020, 8:29 pm | #8 |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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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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12th May 2020, 2:25 am | #9 |
Heptode
Join Date: Mar 2014
Location: Meath, Ireland
Posts: 547
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Re: My PCL86 SE amp
Beautifully put together and a great use of plentiful valves that would usually be overlooked.
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12th May 2020, 11:53 am | #10 |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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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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12th May 2020, 12:29 pm | #11 | |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: My PCL86 SE amp
Quote:
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12th May 2020, 4:58 pm | #12 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: My PCL86 SE amp
Quote:
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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13th May 2020, 11:31 am | #13 |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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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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13th May 2020, 3:42 pm | #14 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: My PCL86 SE amp
Quote:
I can use a transformer from a scrap solid state amplifier for the heaters. Dave, USradcoll1, |
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13th May 2020, 5:56 pm | #15 |
Guest
Posts: n/a
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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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13th May 2020, 8:21 pm | #16 |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
Posts: 903
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Re: My PCL86 SE amp
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 |
13th May 2020, 9:52 pm | #17 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
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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 |
14th May 2020, 10:16 am | #18 | |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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Re: My PCL86 SE amp
Quote:
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14th May 2020, 9:53 pm | #19 |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
Posts: 903
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Re: My PCL86 SE amp
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. Last edited by tri-comp; 14th May 2020 at 10:13 pm. Reason: Added Revox manual |
14th May 2020, 10:12 pm | #20 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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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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