PCL86 homebrew amplifier

[Beardyman]

Yet another “Junk-Box” stereo valve amplifier.

This was started some time ago & has progressed over about a year.
There’s still some small things to address but overall it’s a worker.

It started off as the remains of a fairly generic Japanese midi-hi fi system that had a bit of a burn up.
The main PCB had two STK modules which had decided this world was no longer for them & took the PCB with them to heaven, quite a mess.
The chassis, toroidal transformer, several bits of metal work & case were salvaged then put aside for no other reason than they might come in handy & the case was an interesting design being stackable with a CD player etc.

I’d already built a cutlery box amp using AC2/PEN’s some while ago so the bug had bit as it were. I’d been looking around for some readily available audio valves at reasonable prices, bit of an oxymoron in some ways.

Whilst sorting through some stuff left to me by a now departed friend I found some PCL86’s, new & unused. These rang a dull distant bell, so I looked them up & was pleasantly surprised they could be just what I had been looking for.

I find the biggest obstacle to budget amplifier building is the HT supply and/or the output transformers. Even from China these can be very expensive by comparison to the rest of the amplifier.

The HT side was solved by a neat little SMPS boost converter, available for about a fiver, just needs a 12-36V DC supply. This provides a regulated, adjustable DC output up to a rather optimistic 500mA, I say that due to the tiny heatsink it is fitted with.
Previous experience told me it would need some shielding, said shield in this particular application doubles as a makeshift air duct to aid cooling from the internal fan, this was salvaged from the original unit & runs very quietly at a reduced voltage.
A similar buck regulator was employed for the filament supply, again these were only about £2 each & capable of up to 4A. The salvaged mains toroid was easily capable (due to multiple outputs & size) of providing all the supplies needed.
The output transformers were sourced at about £9 a piece & specifically for the PCL86, I’m assuming they were old TV pulls.
Construction began, much time was spent getting everything to fit in the space available. It underwent several mock ups before I was happy with the layout.
I decided to use a MP3/FM/BT/USB module as they were cheap (about £5) & I’d used them before so I knew they far out performed their price. Besides, this was meant to be a cheap build all round.
Having read several articles regarding the PCL86 it came across that it can be a bit prone to instability. Several people, having built their own versions, had experienced difficulty in getting the amplifier to behave. I took the opportunity to research several TV circuits that used the PCL86, they were all almost identically configured. If it was good enough for then, it is good enough for now, no point re-inventing the wheel. Plagiarism? Quite possibly!
The internal chassis being all metal provided a solid base to mount all the important stuff, mainly power supplies & readily earthed.
Being a plastic case it was easily drilled which helped a lot. As a nod to the past I used a chicken’s head knob for the volume & a 0-100 graduated scale escutcheon. In truth the volume control isn’t really needed as that can be controlled via the MP3 module remote but I find a physical control more to my liking.
I decided not to include some kind of mains indicator, if it had power then the MP3 module display provides that visual aid.
Despite checking the wiring several times I had made an error on the left channel, well, a couple actually! Nothing that would have caused any damage if power had been present, easily rectified.
Several more static checks with power off & valves out just to be certain.
With power applied the DC supplies were checked first, again with the valves out. According to the datasheet for the PCL86 it can take 230VDC in a single ended configuration. HT tweaked for a little lower at 220VDC. Similarly with the filament supply, this was adjusted to 13.3VDC, both valves running in parallel.
With both PCL86’s fitted, speakers attached & anode currents monitored individually the switch was thrown. After maybe 10-15 seconds (which seemed like an eternity) the anode currents started to climb, both levelled out at 38-40mA. All good, so far.
After an hour or two of messing around trying to find a half decent FM signal I went around various bits & pieces checking temperatures, no issues, I’m lucky enough to have a no contact IR probe. Nothing “smelt” too hot but the PCL86’s run hot anyway as do most output valves.
I know that switching regulators do not find favour amongst the audio fraternity & for good reason, some of them can induce truly fearful levels of interference. With adequate positioning & earthed shields they can, for the most part, be tamed.
Yes, this amplifier does have a tiny amount of hiss but barely audible with the volume wicked right up, that’s probably coming from the MP3 module though. Then again, it isn’t supposed to be high end.
After having listened to it for a few hours now I think it could do with some tone controls, maybe it’s the speakers, just a pair of Sony non-descript boxes. Plenty of bass, a little too much top end, good midrange but my hearing is old so no surprises there. I shall have to try the Tannoys or Mordaunt-Shorts. Regardless, it can produce enough volume to be uncomfortable.
In case you are wondering, the valve “protectors” are made up of discarded joystick mounting rings & nickel plated brass stand-offs. The rings are nylon reinforced plastic & so far haven’t shown any signs of melting etc.
The valve bases are B9A ceramics & I was a little concerned about transferred heat potentially damaging the case, so far so good! Very pleased with the outcome. I do need to find a better "prying finger" guard, I'm sure something will turn up.

[Sideband]

It's got that 'valve audiophile' look about it. I've seen ads for Chinese amps that look similar and sell for megabucks. You could probably flog it for a couple of hundred......

[paulsherwin]

Yes, nice construction job.

[dazzlevision]

The best colour TV PCL86 audio amp and speakers setup that I have ever come across is the one in the Swedish designed and made Philips K70 chassis. Excellent sound from the two Philips speakers, large output transformer and comprehensive tone controls - sounded really good.

[kalee20]

Nice job!

The PCL86 (and ECL86) packs a lot of punch in a small envelope, as a result it runs stinking hot. I'd have been inclined to derate the output section - however, you've got first-class cooling to the bottles, so you should get a good long life!

Incidentally, if you look carefully, you'll see that the output section is a beam tetrode (no suppressor grid is visible, but beam-forming plates are).

The switching regulators - if well shielded, you might get away with them. It's the RFI that they can generate which makes me eschew them - though I have got several battery AM superhets running with switching power supplies providing LT and HT - with 3-stage filtering on input and output and in screened boxes.

And as others have said - it looks super! I hope you enjoy many happy hours of listening.

[Reelman]

Very impressive and a first class example of re(up?)cycling.

Peter

[Beardyman]

Many thanks for all your kind comments, much appreciated.
It was one of those back burner projects, all the bits & pieces I already had pretty much.
I suppose the next one ought to be a push-pull setup, I shall see.
Yes, the PCL86's do run very hot, the IR thermometer had them at about 120C idling!?!
Then again, it can give odd readings on shiny surfaces but I wouldn't be surprised if it was about right.
It's been running now for a total of 60 odd hours without fault. I thought I might have to balance the channels as valves rarely have the same gain & these two were random NOS from my friends collection. I did try a pair of Mordaunt-Shorts (old & well broken in!) & a pair of Tannoy Eclipse Mini's. Went back to the Sony's! The Mordaunt-Shorts sounded a bit muffled & the Tannoy were far too bright! No pleasing some people is there!

[Vintage Engr]

That is absolutely classy!
The ultimate in recycling.

David.

[kalee20]

Originally Posted by Beardyman

Yes, the PCL86's do run very hot, the IR thermometer had them at about 120C idling!?!

That's hot - but at least it's the worst-case condition!

With a signal applied, the anode dissipation in the PCL86 will drop. Probably not by very much with typical speech or music - you'd need continuous sine-wave drive to be significant - but at least you know it won't go up.

[Beardyman]

True kalee20, I shall have to thrash it a bit more!
In the interests of reliability of course!

[Richardgr]

There is something very satisfying about taking some consumer electronics hifi that has expired and breathing new life into it by imbuing it with a bit of 60s electronic engineering practice.

[Chris55000]

. . .One small point. . .!

Incidentally, if you look carefully, you'll see that the output section is a beam tetrode (no suppressor grid is visible, but beam-forming plates are).

. . .It isn't mandatory for the suppressor–grid of a pentode valve to consist of a wound grid – open plates will serve the same function ! (The ECL85/PCL85 are also valves with pentode characteristics with open metal plate suppressors!)

. . .The E/P/UCL82 and the ECL86/PCL86 valves are all true pentode characteristics despite their appearance because the control and screen grids are wound with progressively more open windings, so the electron streams are NOT formed into "beams" as in the true beam–tetrode type of valve, which has the wires for G1 and G2 in line ! This is why they are always shown as pentodes in valve data books and maker's circuit diagrams!

. . .The ECL83, PCL83 and UCL83 are true pentodes with wound wire suppressor grids tho, the EL41 and EL84 likewise!

. . .The PCL82, PCL84, PCL85/805 and PCL86 (and their 6.3 V/100 mA equivalents) were all constructed in this way with open metal plate suppressors because the available space for the third grid in the output section of the B9A envelope precluded a wound wire suppressor grid, whereas the PCL83, etc., has a smaller output electrode assembly that it was possible to make with a wound suppressor, the EL41, EL42 and EL84 are sufficiently large to use wound suppressor grids as well!

. . .Some of the Mazda triode/output valve combinations ARE beam tetrodes and correctly drawn as such – the 30PL1 is one such type!

Chris Williams

[paulsherwin]

Originally Posted by Beardyman

I did try a pair of Mordaunt-Shorts (old & well broken in!) & a pair of Tannoy Eclipse Mini's. Went back to the Sony's! The Mordaunt-Shorts sounded a bit muffled & the Tannoy were far too bright! No pleasing some people is there!

Those Sony speakers look very like the ones that come with the CMT CP1 microsystem. If that's what they are, they're surprisingly good - much better than typical microsystem speakers.

https://www.radiomuseum.org/r/sony_compact_disk_deck_receiver_hcd_cp1.html