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Old 29th Nov 2020, 2:29 pm   #1
Tim
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Default Mini lockdown project

Hi all.
Im having to self isolate for two & a half days between a pre-op Covid fest and the actual operation. Im spending it in our caravan on the drive.
All set with a radio to fix, 2 litres of cider and a Norman Wisdom box set.
Bliss!
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Old 29th Nov 2020, 4:47 pm   #2
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Default Re: Mini lockdown project

I can definitely think of worse things!

Regards,
Paul
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Old 29th Nov 2020, 5:24 pm   #3
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Default Re: Mini lockdown project

Best wishes Tim ! I take it you meant a Covid test, rather than a Covid fest . The latter sounds like A Very Bad Idea before an op ...

Cheers,

GJ
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Old 29th Nov 2020, 5:54 pm   #4
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Default Re: Mini lockdown project

Hi Tim,

Hope it goes OK. I had to self-isolate ahead of a Carpal Tunnel release operation last Tuesday.

My mini lock-down project was a guitar preamplifier and doing a bit more work on the EF184, EL84 and Jeffery driver mash-up bits a this amplifier whist listening to Radio Caroline.

Pinot Grigio oiled the wheels

Regards
Terry
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Old 29th Nov 2020, 5:55 pm   #5
Al (astral highway)
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Default Re: Mini lockdown project

Sounds like a great plan , Tim!

Good luck with the next 60 hours and the steps afterwards ... keep us posted !
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Old 29th Nov 2020, 7:58 pm   #6
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Default Re: Mini lockdown project

Norman Wisdom's "Trouble in store" must be his best film.
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Old 29th Nov 2020, 9:17 pm   #7
Steve G4WCS
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Default Re: Mini lockdown project

Have fun and hope the op goes ok
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Old 29th Nov 2020, 10:11 pm   #8
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Default Re: Mini lockdown project

Thanks all. Now sitting listening to it! A nice easy one. Usual wax cap replacement, and, unusually( for these) HT can was u/s. Anyway a couple of 47mF @ 450 volts cobbled in, a dodgy valve holder on the UF41 causing intermittent open circuit heater chain, and a silly wiring error by your truly. Anode resistor for the audio triode had drifted high as well so treated it to a metal film 1% which was all I had.
Not finished( they rarely are) but it’s the best I could do with the limited box of bits I brought in with me. HT caps will be tidied up and more work to do on the dropper. The grey ones always seem to go, unlike the green ones fitted to later models.
Ha ha, yes a Covid pre op TEST. Blooming spool chucker.

Yes I like “ Trouble in store”, but my favourite is “ The early Bird”( there’s a nice radio in that one too. A Ferguson I think). I am fond of “ The road menders” too.
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Old 29th Nov 2020, 10:19 pm   #9
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Default Re: Mini lockdown project

Now to investigate why the AVO8 doesn’t appear to be working on AC volts. Could have been nasty!
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Old 29th Nov 2020, 10:21 pm   #10
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Default Re: Mini lockdown project

I like the one where he builds a Tank for the War effort [and a romantic interest of course]. Favourite quiz questions. Who was the only person to win the Grand National AND the Isle of Man TT Race. Norman! What was the name of his Motor Bike? The Shuttleworth Flyer! Shuttleworth is [allegedly] a part of Ramsbottom] but not actually in the valley Dave W

Good luck with the Op.

Dave W
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Old 29th Nov 2020, 10:23 pm   #11
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Default Re: Mini lockdown project

Cuddled up with Gemma, our generic tri-colour thing! Not sure if shes into Norman Wisdom.
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Old 29th Nov 2020, 10:29 pm   #12
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Default Re: Mini lockdown project

Hi Terry, I’m in the process of “ designing”( well, scratching on the back of an envelope) a small EF80/184 EL84 amplifier for fun, and to learn about feedback etc. Been mucking about with grid curves and load lines etc.
I have set myself a challenge that it should be built entirely from bits I have acquired for free. Nearly there with the bits. Now all I need is the time to build it.
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Old 30th Nov 2020, 12:41 pm   #13
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Default Re: Mini lockdown project

Hi Tim,

I tried using the grid and ia curves etc., to arrive at some values for the EF184, however I gave up and used the curves as a very rough guide to arrive at total ball park figures for the starting values and went in an entirely non-technical method to arrive at the required values for the cathode, anode resistors the screen grid voltage.

I built a pile of wires hook-up with a range of anode resistors and used a variable resistor in the cathode to set the bias and varied the screen grid voltage using a pot. All very non-scientific, but I arrived at workable values.

I fed the pile of wires hook-up with a signal generator and monitored the output using a Scope and VVM. I had the luxury of a THD analyser so I could measure the THD.

I looked for symmetrical clipping with the maximum output voltage with the minimum of THD.

I swapped out valves to make sure its works with a selection of valves from the toy box.

Ive noted the EF184 can be microphonic causing some problems, but with using Brimar valves (they appear less microphonic than the Mullards) and using some salvaged shock mount bases the microphonics can be tamed.

Regards
Terry
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Old 30th Nov 2020, 9:01 pm   #14
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Default Re: Mini lockdown project

Quote:
Originally Posted by Valvepower View Post

My mini lock-down project was a guitar preamplifier and doing a bit more work on the EF184, EL84 and Jeffery driver mash-up bits a this amplifier whist listening to Radio Caroline.
Hi Terry, I have a pile of EF80/EF184 valves, I've seen them used in preamps, I was wondering if I could also use a couple in push/pull for a low watt output stage (it's for one of my sons to use in Uni digs so needs to be not too loud). I've no idea what ratios the OT would work.

Doug
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Old 1st Dec 2020, 10:13 am   #15
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Default Re: Mini lockdown project

Hi Doug,

Off the top of my head I suppose my first port of call would the EF91 Audio Amplifier on the National Valve Museum site, which originally appeared in Practical Wireless, February 1964. Broadly speaking the EF91 and EF184/80 are kind a similar, and this article would give an idea of where to start.

http://www.r-type.org/articles/art-149.htm

The EF184 has a maximum anode dissipation of 2.5W and Va of 250V which gives a max Ia of 10mA. Looking at the EF91 circuit you could take these values and build this circuit using a push pull output transformer with the same a-to-a load, however, I do wonder if a higher a-to-a load would be better and keeping the Va nearer 200V to stop the valve over dissipating. Maybe finding a multi ratio (Radiospares?) output transformer to experiment with different a-to-a loads would help.

I can only offer a suck it and see approach, however, this could give you an idea of where to start.

Regards
Terry
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Old 1st Dec 2020, 11:23 am   #16
Tim
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Default Re: Mini lockdown project

Hooray! Fixed the Avo. Now works on AC volts (and current which also didn't work)
Fault proved to intermittent solder connection to meter cut out.
Unfortunately the coil resistor(670 ohms) across the output of the bridge rectifier gave up the ghost during the re-soldering operation. Replaced with 240 and 430 ohm 1% metal film resistors in series. Compared against a cheapo DMM, and it seems accurate enough!
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Old 1st Dec 2020, 12:01 pm   #17
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Default Re: Mini lockdown project

Quote:
Originally Posted by Valvepower View Post
Hi Doug,

Off the top of my head I suppose my first port of call would the EF91 Audio Amplifier on the National Valve Museum site, which originally appeared in Practical Wireless, February 1964. Broadly speaking the EF91 and EF184/80 are kind a similar, and this article would give an idea of where to start.

http://www.r-type.org/articles/art-149.htm

The EF184 has a maximum anode dissipation of 2.5W and Va of 250V which gives a max Ia of 10mA. Looking at the EF91 circuit you could take these values and build this circuit using a push pull output transformer with the same a-to-a load, however, I do wonder if a higher a-to-a load would be better and keeping the Va nearer 200V to stop the valve over dissipating. Maybe finding a multi ratio (Radiospares?) output transformer to experiment with different a-to-a loads would help.

I can only offer a suck it and see approach, however, this could give you an idea of where to start.

Regards
Terry
Thanks Terry, nice find, and an excellent place to start, also I'll be teaching my son tube amp builds while he's back for the long xmas holidays, a bit of trial and error should get us there.

There are lots builds for low watters out there using low mu dual triodes but I'd like to find a use these old 50's/60's junk box pulls.

I have a couple of small PP OTs in a 10 + 10 watt stereo amp with DAB and Bluetooth that I built I could try.
It uses a pair of the exotic rare bird valve ECLL800, basically 2 x EL95 and a triode phase splitter in one bottle, add a triode section for the preamp for the DAB/BT and you get a compact stereo push pull amp with only 3 valves. The EL95 likes to see 10k in class AB so I could start there.
A while back I measured the ratio on the PP OT, 25:1 to the ct, it only has a single secondary. I'm running the amp into a pair of bookshelf 8 ohm Tannoys.
I could try it out and then wind a copy of it and maybe add some extra taps on the primary and secondary sides for some more options.

As it's for a small guitar amp some output stage distortion is a nice to have.

I'll start a new thread in the homebrew section with my progress...

Doug
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Old 1st Dec 2020, 2:45 pm   #18
Tim
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Default Re: Mini lockdown project

I was/am thinking of building that "small amp" EF91/EL91single ender on The Valve Museum too.

http://www.r-type.org/articles/art-122.htm

I have loads of EF91/CV138's but no EL91's as far as I can remember. I might adapt it for 6J6/6V6. It will probably depend on the output transformer.
Everybody seems to use the ECC83/6sn7 so I wanted to do something a bit different.


I'll be off for a few weeks after the op, and I expect SWMBO has plenty for me to do but hopefully I can get a chassis marked out and drilled for a couple of sockets.

Look forward to reading of your exploits, Doug.
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Old 1st Dec 2020, 3:27 pm   #19
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Default Re: Mini lockdown project

Quote:
Originally Posted by Tim View Post
I was/am thinking of building that "small amp" EF91/EL91single ender on The Valve Museum too.

http://www.r-type.org/articles/art-122.htm

I have loads of EF91/CV138's but no EL91's as far as I can remember. I might adapt it for 6J6/6V6. It will probably depend on the output transformer.
Everybody seems to use the ECC83/6sn7 so I wanted to do something a bit different.


I'll be off for a few weeks after the op, and I expect SWMBO has plenty for me to do but hopefully I can get a chassis marked out and drilled for a couple of sockets.

Look forward to reading of your exploits, Doug.
Hi Tim, it's more rewarding to try something a wee bit different. The OT T1 spec is given in the EL91 se amp at least tho "70:1 or 80:1" into 3 ohms may be hard to find, you can always run an 8 or 16 ohms load to reduce the ratio/reflected impedance.

I've installed 2 rubber mounted B9A sockets in my chassis to use for the pre amp/phase splitter and a couple of normal B9A McMurdo sockets for the output section and a lenght of tag strip today so the ball is rolling...

I've checked my junk box stuff and found 2 Mullard EL95 and a Telefunken one too with matching tiny little OTs too, I'll have to post a pic of the EL95 OT beside my 120watt quad EL34 monster OT!

Lang may yer soldering iron reek
Doug
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Old 1st Dec 2020, 4:02 pm   #20
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Default Re: Mini lockdown project

Here's the poor wee beastie but it should easily deliver the 2-3 watts from an EL95.
The 2 Mullard EL95s and the OT came from a Grundig TK24 which was inside the dead radiogram (tho the gram was missing and sadly someone brute forced the seized tape transport controls stripping the alloy metal to shreds) I repurposed the stereo 10 + 10 watt ECLL800 based amp except for the dead power transformer, all 8 valves it powered were ok, there was also a spare ECLL800. Not bad haul for just the cost of the petrol.

Doug
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