UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 2nd Oct 2020, 12:30 pm   #1
kestrelmusic
Hexode
 
kestrelmusic's Avatar
 
Join Date: Feb 2008
Location: Wrexham, North Wales, UK.
Posts: 465
Default 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...
Attached Thumbnails
Click image for larger version

Name:	Amp (0).jpg
Views:	291
Size:	43.7 KB
ID:	216834   Click image for larger version

Name:	Amp (4).jpg
Views:	224
Size:	50.5 KB
ID:	216835   Click image for larger version

Name:	Amp (3).jpg
Views:	292
Size:	78.9 KB
ID:	216836   Click image for larger version

Name:	Amp (2).jpg
Views:	276
Size:	89.7 KB
ID:	216837   Click image for larger version

Name:	Amp (1).jpg
Views:	234
Size:	90.3 KB
ID:	216838  

kestrelmusic is offline  
Old 2nd Oct 2020, 12:40 pm   #2
AdrianH
Octode
 
AdrianH's Avatar
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 1,566
Default Re: Odds and Ends amplifier

Anything built from odds and ends is a good thing. Congrats.

Adrian
__________________
Asking questions and learning, or trying to!
Youtube EF91Valve
AdrianH is online now  
Old 2nd Oct 2020, 2:29 pm   #3
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
Default Re: Odds and Ends amplifier

Made from odds and ends, sticky black plastic..... All it needs is a Blue Peter badge

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 2nd Oct 2020, 4:17 pm   #4
Aub
Nonode
 
Aub's Avatar
 
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 2,033
Default Re: Odds and Ends amplifier

Tone controls as well! Very good.

Cheers

Aub
__________________
Life's a long song, but the tune ends too soon for us all.
Aub is online now  
Old 2nd Oct 2020, 4:25 pm   #5
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,636
Default 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.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is online now  
Old 2nd Oct 2020, 4:58 pm   #6
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default 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
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 2nd Oct 2020, 5:37 pm   #7
Uncle Bulgaria
Nonode
 
Uncle Bulgaria's Avatar
 
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,314
Default 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.
Uncle Bulgaria is online now  
Old 2nd Oct 2020, 5:42 pm   #8
Trevor
Octode
 
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
Default 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
Trevor is offline  
Old 2nd Oct 2020, 6:38 pm   #9
ValvoStef
Hexode
 
ValvoStef's Avatar
 
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
Default Re: Odds and Ends amplifier

Ab Fab project. Love it.
__________________
Failing to prepare is preparing to fail ...
ValvoStef is offline  
Old 2nd Oct 2020, 9:58 pm   #10
kestrelmusic
Hexode
 
kestrelmusic's Avatar
 
Join Date: Feb 2008
Location: Wrexham, North Wales, UK.
Posts: 465
Default Re: Odds and Ends amplifier

Quote:
Originally Posted by GrimJosef View Post
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
Yes - the idea of VR4 is to balance the two halves of the output stage. It allows for differences in the two halves of V2 and/or differences between V3 and V4. Easy to set up - connect the two inputs of a dual trace 'scope (via suitable caps) to the anodes of V3 and V4, feed a signal to the grid of V2a and adjust VR4 until the voltage swing at the anodes is the same. Rough and ready, but it works!
kestrelmusic is offline  
Old 2nd Oct 2020, 10:01 pm   #11
kestrelmusic
Hexode
 
kestrelmusic's Avatar
 
Join Date: Feb 2008
Location: Wrexham, North Wales, UK.
Posts: 465
Default Re: Odds and Ends amplifier

Quote:
Originally Posted by Trevor View Post
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
Yes - I worked this out too late! Though the tone controls still have some effect even with the feedback in place. With it switched out, they are very effective.

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...
kestrelmusic is offline  
Old 2nd Oct 2020, 10:21 pm   #12
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Odds and Ends amplifier

Quote:
Originally Posted by kestrelmusic View Post
... Easy to set up - connect the two inputs of a dual trace 'scope (via suitable caps) to the anodes of V3 and V4, feed a signal to the grid of V2a and adjust VR4 until the voltage swing at the anodes is the same ...
Or if you have an audio spectrum analyser feed a good quality sine wave in and use the pot to null the even order harmonic distortion out.

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
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 3rd Oct 2020, 10:17 pm   #13
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default 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.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 5th Oct 2020, 7:23 pm   #14
murphyv310
Dekatron
 
murphyv310's Avatar
 
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,420
Default Re: Odds and Ends amplifier

Hi.
I like the phase splitter in the circuit.
Well done on the project.

Regards.
Trevor
murphyv310 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:04 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.