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2nd Oct 2020, 12:30 pm | #1 |
Hexode
Join Date: Feb 2008
Location: Wrexham, North Wales, UK.
Posts: 465
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Odds and Ends amplifier
I wanted to build a small amplifier to attach to my laptop for watching films etc. I had the transformers already, though I suspect the alleged output transformer was intended as a heater transformer as the secondary is centre tapped!
The only new parts were a piece of Tufnol and some turrets to make the main component board. Otherwise everything was from stock, some used and some unused. I used Pen4A output valves as the power transformer has a centre tapped 4 volt winding. The rectifier is an American type 80 and the other valves EF37A and 6SN7. It's a bit of a conglomeration, but it works well! The additional knob is for a feedback control - it gives the options of feedback from the O/P transformer as shown in the circuit, no feedback or a capacitor bypassing the EF37A cathode resistor for maximum gain. The case was made from a sheet of MDF that I happened to have. I'd originally intended to veneer it, but since I had some sticky-back plastic in stock I used that instead. It looks quite good from a distance... |
2nd Oct 2020, 12:40 pm | #2 |
Octode
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 1,566
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Re: Odds and Ends amplifier
Anything built from odds and ends is a good thing. Congrats.
Adrian |
2nd Oct 2020, 2:29 pm | #3 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
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Re: Odds and Ends amplifier
Made from odds and ends, sticky black plastic..... All it needs is a Blue Peter badge
David
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2nd Oct 2020, 4:17 pm | #4 |
Nonode
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 2,033
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Re: Odds and Ends amplifier
Tone controls as well! Very good.
Cheers Aub
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2nd Oct 2020, 4:25 pm | #5 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,636
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Re: Odds and Ends amplifier
Nice work and an interesting circuit. Two things, that's an interesting tone control circuit, did you design it yourself? Not being 100% up this subject why the different value caps? The typical circuits I've seen have equal value filter caps. How many dB loss? Looks lossy hence high gain IP stage and 100k anode resistors on the PS, it's usual to use 47k.
Second the PS is interesting too, it's not a classic LTP, no big tail - 1k and why VR4? AC balance? But note no AC decoupling on the PS 6SN7 second triode grid. Andy.
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2nd Oct 2020, 4:58 pm | #6 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Odds and Ends amplifier
It's not an LTP phase splitter at all Andy. The common cathode resistor is AC bypassed, so there's negligible AC there. Instead the second triode's grid is fed from the output of the first triode, via the VR4 attenuator, which does indeed give you the option of AC balancing the push-pull.
Cheers, GJ
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2nd Oct 2020, 5:37 pm | #7 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,314
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Re: Odds and Ends amplifier
Very fine! I've always admired tag board layouts. Is there some knack or formula for laying them out? I want to do it for my preamplifier rebuild but haven't dared drill the holes yet for fear of not having the layout right.
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2nd Oct 2020, 5:42 pm | #8 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
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Re: Odds and Ends amplifier
problem with the feedback vol and tone controls shown inside the feedback loop!
Never right the feedback will be acting against the controls Other than that cracking amp Trev |
2nd Oct 2020, 6:38 pm | #9 |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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Re: Odds and Ends amplifier
Ab Fab project. Love it.
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2nd Oct 2020, 9:58 pm | #10 | |
Hexode
Join Date: Feb 2008
Location: Wrexham, North Wales, UK.
Posts: 465
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Re: Odds and Ends amplifier
Quote:
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2nd Oct 2020, 10:01 pm | #11 | |
Hexode
Join Date: Feb 2008
Location: Wrexham, North Wales, UK.
Posts: 465
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Re: Odds and Ends amplifier
Quote:
No, sorry, the tone control design was not my own, I lifted it from a Mullard design. For my next amp I have some ideas... |
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2nd Oct 2020, 10:21 pm | #12 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Odds and Ends amplifier
Quote:
I sometimes think it should be possible to knock up a box containing a decent 1kHz oscillator and a 2kHz bandpass filter with a detector on the end of it for doing this job on amps with a drive balance adjuster. Cheers, GJ
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3rd Oct 2020, 10:17 pm | #13 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
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Re: Odds and Ends amplifier
Can't get my head round the EF37A anode volts. Its cathode current looks like about 1.6mA- even if 1.5mA of this is anode current, the anode should be at about 284-16(R5 drop)-150(R2 drop) or about 120V.
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5th Oct 2020, 7:23 pm | #14 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,420
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Re: Odds and Ends amplifier
Hi.
I like the phase splitter in the circuit. Well done on the project. Regards. Trevor |