PCL86 Triode SE Spud Amp

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Cressy Snr
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#16 Re: PCL86 Triode SE Spud Amp

Post by Cressy Snr »

Bit more playing around with the componentry around the input stage and grid resistors on the pentode section and I've managed to randomise the residual noise at its anode. No spiky harmonics when sweeping through the time/div range on the scope. All I'm left with now is pure AC heater ripple, way down in the noise floor and barely audible with an ear to the speaker. You can just see it, hiding in the hiss on the scope screen if you look carefully, and for long enough.

Considering the gain of a PCL86 unit and the efficiency of the speakers, I think I've achieved an excellent result, though I say so myself. :)

Also, randomising the noise seems to have had a remarkable effect on the treble performance of the amplifier; very pure and liquid in characteristic, and you can hear right to the back of the recording.

I know Nick/Andrew have been sweating the small stuff like this for more years than I care to remember, but for a solder slinging hack, it is beginning to occur to me that this kind of attention, is the thing that makes the difference between a good outcome and a great one.
Last edited by Cressy Snr on Thu Jul 05, 2018 3:54 pm, edited 3 times in total.
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Nick
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#17 Re: PCL86 Triode SE Spud Amp

Post by Nick »

You can just see it, hiding in the hiss on the scope screen if you look carefully and for long enough.
Or use something that can give you a spectrum of the output. I still have a Pete Millet input box if anyone wants to use it as a sound card front end.
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#18 Re: PCL86 Triode SE Spud Amp

Post by Cressy Snr »

I've no laptop unfortunately.

I've had the scope itself for years. Mike bought a job lot of five scopes way back and I got one of them. I've used it on the odd occasion, but because of the nature of these valves, I thought it was about time I revisited some old BT skills, blew the dust off and started to use the scope properly.
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#19 Re: PCL86 Triode SE Spud Amp

Post by Nick »

Yep, the thing is, once you reach the point where the random noise is the dominant signal its hard/impossible to see the coherent signals on a scope. The ears can find them and so can a FFT of the signal. Thats where you go beyond what a simple scope can do.
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Mike H
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#20 Re: PCL86 Triode SE Spud Amp

Post by Mike H »

Nice one Steve, I've been threatening to do a "BOBA"* :D


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#21 Re: PCL86 Triode SE Spud Amp

Post by Dave the bass »

Mike H wrote: Thu Jul 05, 2018 9:50 pm Nice one Steve, I've been threatening to do a "BOBA"* :D


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<swooons> Awwwww, BOBA, he was my 1st. I was/am soooo proud.
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#22 Re: PCL86 Triode SE Spud Amp

Post by Mike H »

:thumbleft:
 
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#23 Re: PCL86 Triode SE Spud Amp

Post by Cressy Snr »

Decided to have a bit of a punt today and wired in the ultralinear taps on the 1627SE output transformers to the screens of the pentode sections.

The ECL/PCL86 works great in single ended ultralinear format. Very little difference to triode operation but it has of course, a bit more power and images are projected forward of the speakers rather than hanging back.
Not as forward and spacious as the NVA clone and less top end sparkle, but it's very nice all the same. Nice and "tubey" knowarra mean squire? :)

I really do like this valve. Once you have dealt with its tendency to go off on one if your sockets are crap and your layout is suspect, then it is a great sounding little device, deserving of ultra fabbo output transformers, though doing such a thing with what is a pretty 'budget' record player valve would seem to be on the surface of it, sheer madness.
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#24 Re: PCL86 Triode SE Spud Amp

Post by Mike H »

I might have to see if I've got any.

We got a circuit for this?
 
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#25 Re: PCL86 Triode SE Spud Amp

Post by Cressy Snr »

I'll see if I can draw one up later. :)
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#26 Re: PCL86 Triode SE Spud Amp

Post by Mike H »

Not really that complicated though is it? Going by data sheet - another thing in its favour. Like yours be all mostly metalwork. :D
 
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#27 Re: PCL86 Triode SE Spud Amp

Post by Cressy Snr »

Mike H wrote: Wed Jul 18, 2018 9:31 am Not really that complicated though is it? Going by data sheet - another thing in its favour. Like yours be all mostly metalwork. :D
No, it's not complicated at all:

255V of HT off the anode end of the OPT primary

7V cathode bias on the pentode section via a 250-270R resistor.

1K screen stoppers for the ultralinear tap or a 100R resistor between plate and screen if going triode.

10K grid stopper to pentode section.

68K series resistor and 100uF to ground, in HT line to decouple the triode section from the pentode section

100K anode resistor on the triode section

1.2V bias via a 1.2K cathode resistor.

No bypasses on either cathode.

0.47uF coupling cap.

That's it. No feedback, no fancy DC heaters.
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#28 Re: PCL86 Triode SE Spud Amp

Post by Mike H »

OK!
 
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#29 Re: PCL86 Triode SE Spud Amp

Post by Cressy Snr »

Cheers Mike :)

Hope the post didn't sound curt.

Must get on with rebuilding the thing. My 80uF Eyuda oil caps are screaming to be used with this amplifier.
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Mike H
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#30 Re: PCL86 Triode SE Spud Amp

Post by Mike H »

No I'm not upset. Image
 
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