What is the bright rectangle to the left of the mains transformer in the underside view?
That clears matters up.
It did not look like it belonged in a valve amplifier.
So there is the makings of a radio hiding under the chassis.
... One thing I have come across is there looks to be two versions of the basic amp, one version uses a 2.2m for the EF86 anode load and and 820k first stage feed resistor with a 1m screen feed, the other version uses 1m, 390k & 22k in the same positions. What are your opinions on the best circuit to use?
Strange - a quick google has brought up no fewer than three different copies of the original Mullard design in various publications and all of them use 1M, 390k & 22k ...
As for the rectifier, I'd stick with the EZ80:
"It is preferable, but not essential, that a separate LT winding (6.3V) be used for the EZ80 rectifier." (My emphasis.)
The whole point of the EZ80 & EZ81 having 6.3V heaters, rather than the traditional 5V, was that the heater/cathode insulation is rated high enough for the heater to be fed from the conventionally earthed 6.3V supply.
When all else fails, read the instructions
Hi Trevor - you've missed quite few other changes as well!
That isn't a Mullard circuit - it is someone else's redrawn version!
Here's the original - note the distinctive drawing style with curves instead of corners ...
When all else fails, read the instructions
Trevor, I wonder if this might be the answer - in which case the choice is obvious!
When all else fails, read the instructions
I keep telling folks you gotta build yourself a 3 3 and it's about time I did the same and I could if those horrible LCD TVs would go away. The trouble is they help to pay the rent.
The 'phools somehow don't dig the 6BR7 or 6BS7. The characteristics are similar to the EF86. Actually some reckon the EF37A or CV358 is a better valve. Also the STC 5A/157D.
Till Eulenspiegel.
Hi Trevor,
I understand that the busbar earthing method is best approach to reducing hum. Look forward to your comments about the results.
Till Eulenspiegel.
Bus bar Earth / OV, but point to point daisy chain HT is what I would do.
Trevor, my experience of building several of these - and quite a few to the 5-10 design - is that following the original Mullard layout is the best guarantee of success!
Your time is your own, of course, but if you find yourself building it twice there is nobody to blame but yourself!
I only made that mistake once ...
When all else fails, read the instructions
... I'll ...build a ... version ... by the book, I'm intrigued though with the Busbar approach for grounding ...
The Mullard design uses a bus bar for grounding - the only connection (from memory) to the chassis is at the input socket ...
I defy you to build one that works with local grounding ...!
When all else fails, read the instructions
The Mullard prototype 3-3 amplifier has most of the components assembled on a group board. My point of view is as there are so few components in this amplifier Trevor's busbar construction method will be entirely satisfactory. The only component with a high heat dissipation is the cathode resistor of the output valve. A 7 watt resistor is required here.
This is a well proven circuit so I will not even dare to attempt to do any modifications to it. One thing that is interesting is that HT reservoir capacitor feeds the output transformer without any additional smoothing although it will be noticed that the screen grid of the EL84 is supplied from R and C smoothing.
Till Eulenspiegel.
The Mullard prototype 3-3 amplifier has most of the components assembled on a group board ...
Yes, that is correct - the same method is used for the 5-10 - but the earthing method, nevertheless, is a bus bar!
The group board is not essential to the style of construction but the bus bar is!
The full details are available on the web - I'm sure someone has posted a link to it here - take careful look at the earthing arrangements and you'll see what I mean!
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Update:
Found them!
Here you go!
http://www.r-type.org/articles/art-003h.htm
And here's the 5-10
http://www.r-type.org/articles/art-003e.htm
When all else fails, read the instructions
I think Trevor is sufficiently expert and experienced that he can safely deviate from exact Mullard layout. I agree that most people should follow the exact published layouts.
As I said, Trevor, The group board is not essential to the style of construction ...
Interestingly, from your photographs it doesn't look as there is much difference in the valve spacing compared to the original.
Also, with the small size of modern components they would probably look a bit lost on standard group board and the miniature size board, if you can still get it, would be better ...
However, whichever way you decide to build it, I'm sure you'll make a good job of it!
When all else fails, read the instructions
When I was young myself and a friend both built a 3-3 .my friend being richer used an output transformer made by hinchly ? I had to use one from an old radio gram .
both worked well but his sounded a lot better.
I always intended to build a stereo one using good transformers but never got round to it.
Rob T
Ah! Memories of following the Goodmans instructions for a pair of cabinets for a couple of Axiom 10s ...
Mahogany veneered Contiboard for the carcase - cut precisely to size by one of those massive vertical saw tables in Homebase - can you have a vertical table ...?
The rear and baffle were a problem as I didn't want to buy a full 8' x 4' sheet of 19mm chipboard but someone pointed me in the direction of an old fashioned shop that would sell me what I wanted cut to size.
This was 40 years ago, so I took the precaution of translating the metric measurements into imperial before I went.
When offered the choice, without hesitation the answer was "Oh! Imperial, please!"
So I trotted out my measurements which ended up, of course, with "by three-quarters."
"Oh, it will have to be 19mm - it's all metric now!"
Funny, he couldn't work out why I laughed ...!
When all else fails, read the instructions
I think we must all agree the chassis Trevor has chosen for his 3 - 3 looks the part for the project. It's got a real sixties look about it.
I've got a pair of loudspeakers that were used with the Ferguson 22B5 "professional series" TV set. The set has long gone but I decided to keep the speakers. The TV was based on the TX10, the audio amplifier was the Toshiba TA7227 stereo amplifier IC. 2 X 5 watts.
I have a BSR McDonald record deck. The loudspeakers and the record player need an amplifier and I'm sure the stereo 3 -3 amplifier would be ideal. The BSR record deck has an ADC magnetic pick-up so I expect the output will be too low for the 3-3 so a preamplifier will be required. Should it be valve, transistor or opamp?
Till Eulenspiegel.
I did a magnetic preamp from two language school PCBs a while ago.
It is a germanium transistor offering and worked first time with a guessed value for one added capacitor in each channel.
I checked it against a graph in my Mullard book with a signal generator and it worked a treat and with its low noise transistors proved not to be noisy in any way.
The mono blocks are the closest I could get to a Mullard 5-25 if such an amp ever existed.
When I was young myself and a friend both built a 3-3 .my friend being richer used an output transformer made by hinchly ? I had to use one from an old radio gram .
both worked well but his sounded a lot better.
I always intended to build a stereo one using good transformers but never got round to it.
Rob T
The 3-3 is a fantastic little amp. I built one into an old record player back in the 70's to replace a push-pull UCL83 job....the 3-3 sounded heaps better! For fun, last year I built one entirely out of scrap parts with a standard EL84 transformer from a small radio. It worked well although bass response was a bit restricted.
Thoroughly recommended for anyone who hasn't built one BUT take care with component layout...use the Mullard diagrams.
Rich
Well I suppose other forms of construction will be fine if care with earthing is taken. The comment was really aimed at beginners or 'home construction virgins'. The EF86 is running at high gain and sloppy wiring can cause problems. Trouble is if you do get instability, you don't always know where it's coming from. If you use the recommended layout, you can't go wrong.
Rich
P.S When I built one for fun, I deliberately 'slung' it together with little consideration for layout just to see what I could get away with. It worked but hum levels were quite high.
