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Old 5th Apr 2014, 5:29 pm   #1
ValvoStef
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Default Re: JVC HRD140 (Thorn 8947, Ferguson 3v44/45 etc) cam

Hello all,

I completed today my latest project

This started as a flying lead project whist experimenting with some used old TV valves. The result is a PL504 head phone amp. I build the circuit then into an old junction box from the yard. The remarkable thing is the amp works with a dual 30 volt toroidal. Both winding in series for the Anode voltage and one winding for the DC heater. Despite the valves seem to be heavily used it sounds very good with my AKG 300 ohms headphones.

A picture with the amp powered up ...

Inside view ...

And the circuit diagram ...

For some strange reason I can only attach one photo per post with my Ipad
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Old 5th Apr 2014, 10:12 pm   #2
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Default Re: Rainy day with some TV valves

Love that well done. Ive built a few TRF radio sets using only TV valves just for the fun and challange of it. However that looks like a good usable headphone amp you have there. I have literaly crates full of used TV valves so may just have to have a go at this myself.

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Old 5th Apr 2014, 11:45 pm   #3
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Default Re: Rainy day with some TV valves

push-pull amplifier on PL500 it's a easy way to get over 25 W
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Old 6th Apr 2014, 7:06 am   #4
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Default Re: Rainy day with some TV valves

Probably this needs special output transformers which need to be made to spec.
The idea sounds very seducing, I have boxes full of 50s and 60s tv valves which are just waiting to be used ...

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Originally Posted by jay_oldstuff View Post
Love that well done. Ive built a few TRF radio sets using only TV valves just for the fun and challange of it.
A while ago I found a schematic on the net for a headphone amp using PCL84 and 85, I am sure almost any type valve be used for something interesting.
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Old 6th Apr 2014, 8:38 am   #5
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Default Re: Rainy day with some TV valves

Interesting build.

You can make an OTL with Pl509.

Where did you get the power Tx?
I have a couple of PL504 looking at me under this table..

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Old 6th Apr 2014, 9:36 am   #6
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Default Re: Rainy day with some TV valves

Hi tubeglow,

The power transformer is a standard 2 x 30v transformer, salvaged from scrap electronic equipment, the 'hood' is a small terminal box from the scrap bin at work as well. Cheap and cheerful as it should be.

Sure you can make OTL with the 509 as well.

Interesting thing with the PL504 is they are in linear range at low anode voltages. I haven't looked at the 509 data sheet but I would expect the same.
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Old 6th Apr 2014, 9:49 am   #7
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Default Re: Rainy day with some TV valves

Regards the 509's I'm talking futterman topology not low volts. (driving speakers)..

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Old 6th Apr 2014, 10:59 am   #8
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Default Re: Rainy day with some TV valves

Thirty odd years ago, I built a trio of amplifiers. One used bipolar output devices, one used Mosfets and the third was entirely valve. These were built to each produce over 100W per channel into 8 Ohms, with the best bandwidth and lowest distortion possible.

The valve job used loads of PL519s paralleled in a totem-pole output-transformerless structure. Without a transformer, the limitation on loop gain and bandwidth goes away and distortion and intermodulation products could be driven down to 100dB below the fundamentals.

It lost the 'valve sound' and I had three amps which couldn't be told apart by listening at normal living room levels - which was the purpose of the experiment. The valve one could certainly be told apart by the heat output. I never got a bias control system going that I was happy with, so it needed a bit of manual driving. It and the bipolar one got dismantled for bits afterwards. The Mosfet one is in the living room.

I rather like that headphone amp. The TV valves avoid cult prices and allow you the freedom to enjoy music without a feeling of being exploited

David
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Old 6th Apr 2014, 11:45 am   #9
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Default Re: Rainy day with some TV valves

Output transformer from factory amp on PL500
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Old 6th Apr 2014, 12:02 pm   #10
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Default Re: Rainy day with some TV valves

Nice one
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Old 6th Apr 2014, 12:57 pm   #11
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Default Re: Rainy day with some TV valves

I'm looking at this amp using PL36 valves and some ordinary 100v line transformers for 17W ultra-linear output!

Richard
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Old 6th Apr 2014, 1:50 pm   #12
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Default Re: Rainy day with some TV valves

Several posts edited to remove excessive nested quotes.
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Old 6th Apr 2014, 2:15 pm   #13
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Default Re: Rainy day with some TV valves

Quote:
Originally Posted by k_yller View Post
push-pull amplifier on PL500 it's a easy way to get over 25 W
TV "sweep tubes" are fine for audio - I recall some years ago having to fix an early-1970s trawler-transmitter that used a pair of PL519s in push-pull to 100% anode-modulate a parallel-pair of TT21 running 100 watts or so of carrier - meaning the 519s were producing at least 50 watts of audio (probably more like 60 when you consider the transformer-loss and the audio wasted across the screen-grid feed-resistor).

The problem with it was that at certain levels of the audio-cycle the 519s would oscillate at a supersonic frequency - somewhere around 50-70KHz - causing horrible audio, a drop in output-power, and occasional flashover in the TT21s. Apparently it had been a problem from new... If you're using TV Line/Frame output valves at audio frequencies you need to include stopper resistors in the grid and anode circuits to tame them.
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Old 6th Apr 2014, 2:44 pm   #14
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Default Re: Rainy day with some TV valves

You do indeed!

In service as a modulator the first thing seen after the modulation transformer is an RF decoupler shunting mod HT to make the PA anode choke work. This means that the PL519 anodes saw the leakage inductance of the modulation transformer. The PL519 is barely stable at the best of times, so anode and grid stoppers are essential.

Fortunately the headphone amp above has a cathode-follower output, and Stef has been generous with his grid stopper values so it looks good.

David
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Old 6th Apr 2014, 3:32 pm   #15
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Default Re: Rainy day with some TV valves

I have a toroid..

I have an itchy feeling coming on.. and it is as said a cathode follower.. no voltage across your head..
I guess you could fuse the cathode.(Before the output capacitor).

Does anyone know about the pin position for horizontal mounting PL504? (Any limitations?)
http://www.r-type.org/pdfs/pl504.pdf


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Old 6th Apr 2014, 3:52 pm   #16
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Default Re: Rainy day with some TV valves

Quote:
Originally Posted by Tubeglow View Post
Does anyone know about the pin position for horizontal mounting PL504? (Any limitations?)
PL5xx line-output valves were mounted at all sorts of orientations - bottom-up, bottom-down, horizontal, oblique angled - in TVs for a couple of decades, depending on the space available. Forget which way up they are, concentrate more on maximising free airflow [the dissipation limit is bulb-temperature not anode-temperature] and make sure there are no shiny heat-reflective surfaces facing them.

The use (and abuse) of PL5x9/EL5x9 valves is well-understood in the amateur radio world - see: http://pa0fri.home.xs4all.nl/Lineair...near400eng.htm
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Old 6th Apr 2014, 4:01 pm   #17
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Default Re: Rainy day with some TV valves

Thanks for the link,

Yes it would seem they can be used in any orientation.
Some tubes specify pin alignment for heater sag.

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Old 6th Apr 2014, 4:06 pm   #18
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Default Re: Rainy day with some TV valves

Tubes are normally mounted with the phosphor screen facing the viewer.
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Old 6th Apr 2014, 4:09 pm   #19
Tubeglow
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Default Re: Rainy day with some TV valves

LOL..OK
Valve orientation is different.

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Old 6th Apr 2014, 4:16 pm   #20
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Default Re: Rainy day with some TV valves

Hi Usual orientation when valves are mounted horizontally is to have the electrode supports at top and bottom (6 & 12 o'clock).

Ed
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