Simple 3 valve stereo amplifier - peer review sought - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

davew001 · 20th Oct 2016, 11:44 pm

This might be of interest

http://dangerousprototypes.com/blog/...roject-part-1/

Radio Tech · 21st Oct 2016, 10:28 am

Hi Rob

Should you wish to build a decent simple valve amp I would suggest the Mullard 10 Watt, it is very simple to build and of course it is a recognised circuit, and the best of it, all the bits can be got from your junk box, but I will add, although the bits can be obtained from the junk box the amplifier is certainly not junk. I have details of it.

Ken

chroma04 · 23rd Oct 2016, 7:07 pm

Hi Rob,

I have recently built two mono-blocks based on the Mullard 5-10. The circuit is fairly straight-forward to construct, so long as a few basic rules are adhered to and they sound great (to me anyway!).

I used new components, but there are plenty of second-hand parts available.

Adrian.

joebog1 · 23rd Oct 2016, 11:13 pm

ohh, they do look nice

BUT, its a little further away than "A simple 3 valve stereo amplifier"

Being blinded by science is probably NOT the way to encourage a newbie.

best regards
Joe

Aerodyne · 24th Oct 2016, 10:13 am

I've built several very successful '3-3' stereo amps back in the past. I think the '3-3' is the way to go. The quality is excellent, especially with first class output transformers.
A typical radio mains transformer is unlikely to provide adequate power; you should look for something capable of supplying 120mA HT.

Tony

Restoration73 · 25th Oct 2016, 8:02 pm

As a beginner, you should consider this circuit as a starter :
https://frank.pocnet.net/other/Mulla...rd2Stereo2.pdf

bikerhifinut · 25th Oct 2016, 8:38 pm

Quote:

Originally Posted by Restoration73

As a beginner, you should consider this circuit as a starter :
https://frank.pocnet.net/other/Mulla...rd2Stereo2.pdf

Thats a neat set of instructions.
I imagine someone with the wood working skills could have a stab at the speakers too with some cheerful 8inch units from the likes of maplins for a few quid each? With those drivers I could see a decent sound output from a couple of watts?
How easy are ECL82 valves to find these days?

Diabolical Artificer · 26th Oct 2016, 6:11 am

They're all excellent suggestions but the OP hasn't been on this thread since Aug 28th, maybe they lost heart?

I'm building a single ended amplifier at the moment, more for the craic than anything. It's interesting that most two valve SE amp design's in the past used a pentode as the front end, but then the amp was intended to amplify a record players OP so needed a pentode.

With modern day gadgets line OP being around a volt or more a triode can do the job as pre amp. I had the slightly mad idea of building a multi triode OP stage perhaps using several ECC81/2's or whatever, again, just for fun and to keep the brain from turning to mush by giving it a few equations etc to do.

Andy.

Restoration73 · 26th Oct 2016, 11:25 am

The 6BM8/ECL82 may not seem to be widely available but I have found plenty of new
old stock boxed (e.g. rallies) for low cost. Some were used as the audio stages of Yaesu HF transceivers.
Obviously, the PCL82 or UCL82 circuits are the same except for heater voltage, the former being very common in tellies.

Aerodyne · 26th Oct 2016, 2:58 pm

One advantage of the neat Mullard design posted by Restoration is the low current requirement, allowing a transformer from a scrap 5 valve radio to be pressed into service. The loudspeakers would need to be fairly sensitive, so it might not suit the sealed bookshelf Hi-Fi units. Certainly well worth considering, offering valve quality at a minimal cost. Full treble and bass, too, on one of the versions.
Tony

Croozer · 6th Nov 2016, 7:14 am

Apologies all for the long delay in replying. And thanks for the continued interest amd quality responses; I never imagined that so much debate would have been sparked, and I find myself in the embarrassing position of being unable to respond individual to the points raised as there are so many. I have often looked at the Mullard 5-10 in the past, but I need to be realistic about my skills -and my maths. Inevitably there will be debugging and I should probably try to keep within a decent understanding of what I am building.

Co incidently following a previous suggestion on here, an old Sony TC 200 tape recorder came my way last month. After spending far too long chasing down a low output fault on one channel (which eventually showed to be a fault through a previous repair on the record/playback switching) (to be honest the pcb and switches were so damaged that my own repair was fairly brutal), I was disappointed to reach the end of 600ft of the first test recording to find that the mechanics were banjoed and the damxd thing was loosing speed.

Having lost patience with what was never more than a mediocre machine, I have followed suggestions from here to cannabilise for parts. Two matched o/p transformers, and even a mains transformer ( addressing the safety concerns I had overlooked!) It also comes with four valves with odd (American?) Numbers that I don't recognise. . Two 6267s which appear to be EF86 (joy !l) and three of something called a 30A5 (low volts on the anode but 30v heaters- maybe for a 110v AA5 with no mains TX?). Am in the process of doing the arithmetic and plagerising from the circuit diagram to try to come up with a design based on these parts as it seems a shame to waste them.

Again, thanks for all the replies. Will keep posted with progress when I have something concrete to report.

Croozer · 6th Nov 2016, 9:33 pm

Andy, have had the chance to read your constructional thread which brings experience that I don't have to a similar task and has provoked me to action.

I have reworked (again) based on components to hand together with £3- worth of new passive components from Maplin this afternoon. (Slightly off topic, but Maplin staff in Edinburgh deserve commendation. On entry I was pounced on with the words 'a customer with a schematic' (which appears to be a bit of a novelty) and the assistant spent nearly an hour enthusiastically assembling a long list of components totalling less than a fiver).

This will be my first scratch construction project for nearly 20 years, so I doubt the quality will be anything like Adrian's pics, but hopefully the components are so few and the design so basic that it can be made to work. Circuit attached. (Sony designers work from 1964 acknowledged! !)

bikerhifinut · 6th Nov 2016, 9:52 pm

I'd be inclined to use a full wave silicon bridge rectifier rather than the half wave single diode shown in your circuit diagram. Gives the mains transformer an easier time.
It looks a fairly conventional circuit so should work ok, but it's an odd combination with the EF86 with 6.3V Heater and the output valve which I guess is of a type normally used with series heater strings? Just watch that series dropper resistor, it's gonna get hot!
Looking forward to seeing how you are getting on with it.

The other Andy.

6th Nov 2016, 9:55 pm

Nice to see (upside down!) a bit of negative feedback, this does make the less than optimal transformer work a lot better. I rather like your 'Harry Beck' (of Underground map fame) way of drawing a circuit, it shows the equipotential points clearly.

Diabolical Artificer · 9th Nov 2016, 7:09 am

Glad to have inspired you Croozer. Did you design this yourself? Well done, nice work.

Yep, use a bridge, good practice to use a 0.1u cap to decouple each diode. Everything else looks ok, you can adjust component value's as you go.

There's more info here on my build - http://golbornevintageradio.co.uk/fo...d.php?tid=5964
that may help.

I learnt more in a few days by building/designing my own amp than I did in years of reading. Keep up the good work, Andy.

Radio Wrangler · 9th Nov 2016, 8:24 am

Another vote for a bridge rectifier:

The transformer gets an easier life
The reservoir capacitor gets a much easier life
There will be much less background hum.

David

wd40addict · 9th Nov 2016, 2:11 pm

There's currently no ground connection shown on the output transformer secondary so the feedback won't do anything!

Croozer · 9th Nov 2016, 10:38 pm

Thanks for the comments guys. Will replace the diode with a bridge package - and earth the midpoint of the o/p tx 2nry. (Oops!!)

Diabolical Artificer · 11th Nov 2016, 6:03 am

Watch which mains tfmr sec's you connect to with the bridge. If the tfmr has 300v 0v 300v as for use with a valve rectifier and you use the two 300v taps, you'll end up with nearly 1kv when unloaded.

Your probably know that anyway, just thought I'd mention it as someone I know did this, much to his surprise. : )

Andy.

TonyDuell · 11th Nov 2016, 6:40 am

In that case, I'd use a pair of diodes, connected as the valve rectifier would be. Or use a bridge rectifier module, but ignore half of it. In other words, connect the AC terminals of the bridge to the 300V taps. Earth the centre tap of the transformer winding. Connect the +ve terminal of the bridge to the smoothing network. And ignore (but insulate well) the -ve terminal of the bridge. That uses 2 of the diodes in the bridge as a biphase full wave rectifier (same electrically as using the rectifier valve) and ignores the other 2.

20th Oct 2016, 11:44 pm	#21
davew001 Retired Dormant Member Join Date: Oct 2016 Location: Leicester, Leicestershire, UK. Posts: 12	Re: Simple 3 valve stereo amplifier - peer review sought This might be of interest http://dangerousprototypes.com/blog/...roject-part-1/

21st Oct 2016, 10:28 am	#22
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Simple 3 valve stereo amplifier - peer review sought Hi Rob Should you wish to build a decent simple valve amp I would suggest the Mullard 10 Watt, it is very simple to build and of course it is a recognised circuit, and the best of it, all the bits can be got from your junk box, but I will add, although the bits can be obtained from the junk box the amplifier is certainly not junk. I have details of it. Ken

23rd Oct 2016, 7:07 pm	#23
chroma04 Pentode Join Date: Jul 2008 Location: Doncaster, South Yorkshire, UK. Posts: 116	Re: Simple 3 valve stereo amplifier - peer review sought Hi Rob, I have recently built two mono-blocks based on the Mullard 5-10. The circuit is fairly straight-forward to construct, so long as a few basic rules are adhered to and they sound great (to me anyway!). I used new components, but there are plenty of second-hand parts available. Adrian. Attached Thumbnails __________________ Chroma 04

23rd Oct 2016, 11:13 pm	#24
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Simple 3 valve stereo amplifier - peer review sought ohh, they do look nice BUT, its a little further away than "A simple 3 valve stereo amplifier" Being blinded by science is probably NOT the way to encourage a newbie. best regards Joe

24th Oct 2016, 10:13 am	#25
Aerodyne Octode Join Date: Jun 2004 Location: Hampton Vale, Peterborough, UK. Posts: 1,698	Re: Simple 3 valve stereo amplifier - peer review sought I've built several very successful '3-3' stereo amps back in the past. I think the '3-3' is the way to go. The quality is excellent, especially with first class output transformers. A typical radio mains transformer is unlikely to provide adequate power; you should look for something capable of supplying 120mA HT. Tony

25th Oct 2016, 8:02 pm	#26
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Simple 3 valve stereo amplifier - peer review sought As a beginner, you should consider this circuit as a starter : https://frank.pocnet.net/other/Mulla...rd2Stereo2.pdf

26th Oct 2016, 6:11 am	#28
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: Simple 3 valve stereo amplifier - peer review sought They're all excellent suggestions but the OP hasn't been on this thread since Aug 28th, maybe they lost heart? I'm building a single ended amplifier at the moment, more for the craic than anything. It's interesting that most two valve SE amp design's in the past used a pentode as the front end, but then the amp was intended to amplify a record players OP so needed a pentode. With modern day gadgets line OP being around a volt or more a triode can do the job as pre amp. I had the slightly mad idea of building a multi triode OP stage perhaps using several ECC81/2's or whatever, again, just for fun and to keep the brain from turning to mush by giving it a few equations etc to do. Andy. __________________ Curiosity hasn't killed this cat...so far.

26th Oct 2016, 11:25 am	#29
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Simple 3 valve stereo amplifier - peer review sought The 6BM8/ECL82 may not seem to be widely available but I have found plenty of new old stock boxed (e.g. rallies) for low cost. Some were used as the audio stages of Yaesu HF transceivers. Obviously, the PCL82 or UCL82 circuits are the same except for heater voltage, the former being very common in tellies.

26th Oct 2016, 2:58 pm	#30
Aerodyne Octode Join Date: Jun 2004 Location: Hampton Vale, Peterborough, UK. Posts: 1,698	Re: Simple 3 valve stereo amplifier - peer review sought One advantage of the neat Mullard design posted by Restoration is the low current requirement, allowing a transformer from a scrap 5 valve radio to be pressed into service. The loudspeakers would need to be fairly sensitive, so it might not suit the sealed bookshelf Hi-Fi units. Certainly well worth considering, offering valve quality at a minimal cost. Full treble and bass, too, on one of the versions. Tony

6th Nov 2016, 7:14 am	#31
Croozer Pentode Join Date: Jul 2016 Location: Edinburgh, Scotland, UK. Posts: 110	Re: Simple 3 valve stereo amplifier - peer review sought Apologies all for the long delay in replying. And thanks for the continued interest amd quality responses; I never imagined that so much debate would have been sparked, and I find myself in the embarrassing position of being unable to respond individual to the points raised as there are so many. I have often looked at the Mullard 5-10 in the past, but I need to be realistic about my skills -and my maths. Inevitably there will be debugging and I should probably try to keep within a decent understanding of what I am building. Co incidently following a previous suggestion on here, an old Sony TC 200 tape recorder came my way last month. After spending far too long chasing down a low output fault on one channel (which eventually showed to be a fault through a previous repair on the record/playback switching) (to be honest the pcb and switches were so damaged that my own repair was fairly brutal), I was disappointed to reach the end of 600ft of the first test recording to find that the mechanics were banjoed and the damxd thing was loosing speed. Having lost patience with what was never more than a mediocre machine, I have followed suggestions from here to cannabilise for parts. Two matched o/p transformers, and even a mains transformer ( addressing the safety concerns I had overlooked!) It also comes with four valves with odd (American?) Numbers that I don't recognise. . Two 6267s which appear to be EF86 (joy !l) and three of something called a 30A5 (low volts on the anode but 30v heaters- maybe for a 110v AA5 with no mains TX?). Am in the process of doing the arithmetic and plagerising from the circuit diagram to try to come up with a design based on these parts as it seems a shame to waste them. Again, thanks for all the replies. Will keep posted with progress when I have something concrete to report.

6th Nov 2016, 9:33 pm	#32
Croozer Pentode Join Date: Jul 2016 Location: Edinburgh, Scotland, UK. Posts: 110	Re: Simple 3 valve stereo amplifier - peer review sought Andy, have had the chance to read your constructional thread which brings experience that I don't have to a similar task and has provoked me to action. I have reworked (again) based on components to hand together with £3- worth of new passive components from Maplin this afternoon. (Slightly off topic, but Maplin staff in Edinburgh deserve commendation. On entry I was pounced on with the words 'a customer with a schematic' (which appears to be a bit of a novelty) and the assistant spent nearly an hour enthusiastically assembling a long list of components totalling less than a fiver). This will be my first scratch construction project for nearly 20 years, so I doubt the quality will be anything like Adrian's pics, but hopefully the components are so few and the design so basic that it can be made to work. Circuit attached. (Sony designers work from 1964 acknowledged! !) Attached Thumbnails Last edited by Croozer; 6th Nov 2016 at 9:35 pm. Reason: typo

6th Nov 2016, 9:52 pm	#33
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: Simple 3 valve stereo amplifier - peer review sought I'd be inclined to use a full wave silicon bridge rectifier rather than the half wave single diode shown in your circuit diagram. Gives the mains transformer an easier time. It looks a fairly conventional circuit so should work ok, but it's an odd combination with the EF86 with 6.3V Heater and the output valve which I guess is of a type normally used with series heater strings? Just watch that series dropper resistor, it's gonna get hot! Looking forward to seeing how you are getting on with it. The other Andy.

6th Nov 2016, 9:55 pm	#34
Guest Guest Posts: n/a	Re: Simple 3 valve stereo amplifier - peer review sought Nice to see (upside down!) a bit of negative feedback, this does make the less than optimal transformer work a lot better. I rather like your 'Harry Beck' (of Underground map fame) way of drawing a circuit, it shows the equipotential points clearly.

9th Nov 2016, 7:09 am	#35
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: Simple 3 valve stereo amplifier - peer review sought Glad to have inspired you Croozer. Did you design this yourself? Well done, nice work. Yep, use a bridge, good practice to use a 0.1u cap to decouple each diode. Everything else looks ok, you can adjust component value's as you go. There's more info here on my build - http://golbornevintageradio.co.uk/fo...d.php?tid=5964 that may help. I learnt more in a few days by building/designing my own amp than I did in years of reading. Keep up the good work, Andy. __________________ Curiosity hasn't killed this cat...so far.

9th Nov 2016, 8:24 am	#36
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Simple 3 valve stereo amplifier - peer review sought Another vote for a bridge rectifier: The transformer gets an easier life The reservoir capacitor gets a much easier life There will be much less background hum. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

9th Nov 2016, 2:11 pm	#37
wd40addict Octode Join Date: May 2009 Location: Stevenage, Herts. UK. Posts: 1,518	Re: Simple 3 valve stereo amplifier - peer review sought There's currently no ground connection shown on the output transformer secondary so the feedback won't do anything!

9th Nov 2016, 10:38 pm	#38
Croozer Pentode Join Date: Jul 2016 Location: Edinburgh, Scotland, UK. Posts: 110	Re: Simple 3 valve stereo amplifier - peer review sought Thanks for the comments guys. Will replace the diode with a bridge package - and earth the midpoint of the o/p tx 2nry. (Oops!!)

11th Nov 2016, 6:03 am	#39
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: Simple 3 valve stereo amplifier - peer review sought Watch which mains tfmr sec's you connect to with the bridge. If the tfmr has 300v 0v 300v as for use with a valve rectifier and you use the two 300v taps, you'll end up with nearly 1kv when unloaded. Your probably know that anyway, just thought I'd mention it as someone I know did this, much to his surprise. : ) Andy. __________________ Curiosity hasn't killed this cat...so far.

11th Nov 2016, 6:40 am	#40
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: Simple 3 valve stereo amplifier - peer review sought In that case, I'd use a pair of diodes, connected as the valve rectifier would be. Or use a bridge rectifier module, but ignore half of it. In other words, connect the AC terminals of the bridge to the 300V taps. Earth the centre tap of the transformer winding. Connect the +ve terminal of the bridge to the smoothing network. And ignore (but insulate well) the -ve terminal of the bridge. That uses 2 of the diodes in the bridge as a biphase full wave rectifier (same electrically as using the rectifier valve) and ignores the other 2.