A few weeks ago I won a very scruffy, ancient Mullard 5-10 in a raffle. I decided to clean it up a bit and see if I could get it to play, after finding most of the right valves in my second-hand spares box.
I tested the power transformer and got 320-0-320, 3.15-3.15 and 2.5-2.5. I then changed the rectifier back to an EZ80 (I had to make and fit a small base plate for the 9-pin valve socket after removing the big 8-pin socket) and configured the circuit for “low loading”.
I replaced all the capacitors and a number of the resistors - some were literally crumbling.
When it did not produce sound at the end of this exercise, I tested the output transformer and found that it was broken. I removed it, and fitted a temporary output transformer (a beautiful old Partridge I have) and tested again.
This time it worked, and made beautiful music.
This is probably where I would have left it, if I had not been tidying up the next day, and found quite unexpectedly that I had another, identical power transformer on the shelf. I started thinking, and reading up on the Mullard 5-10.
Further scratching around unearthed a pair of 8K output transformers that a friend salvaged from an old broken amp some time ago. How would this sound?
I promptly removed the partridge from Scruffy and fitted one of the “new” pair of output transformers. It worked well, and sounded great!
So now I knew that I had two identical power transformers and two identical output transformers that were suited to a “new” build of a pair of Mullard 5-10 amplifiers. To get things going, I ordered all the caps, resistors, pots, knobs, switches, lamps, fuse holders etc. from Mantech. For the 2 x 50Uf 350V caps I intended using 2 x 32+32uF 450V caps which I had on hand, and which measured spot on (well, +-10%).
But then the real issue - the chassis. I do not have the funds to have these commercially made, so I walked around looking and scratching my head… until I found two baking trays (R19 each) at Cash Converters and decided to give them a try!
I marked out and cut the first chassis, then primed and spray painted it. After that the parts were fitted, the components assembled and fitted and the whole build tested piece by piece.
Come final switch on time there was an almighty howl, and after “phoning a friend” was quickly advised to swop the 2 x EL84 anode connections around. I did that, and the problem was solved. And, beyond my greatest expectation, there was virtually no hum or noise!
I then started on the second amp, which I “designed” to be a mirror image of the first one (I call them M510A and M510B). Scruffy now had to offer up her power transformer, and after cutting and spray painting the second chassis I started assembling the unit this week.
This morning I finished it off, tested it in mono (i.e. M510A at rest) and then connected up both M510A and M510B and, wow, how good these two sound in stereo!
Interestingly, I build both with tone controls (no particular reason) and, for now, having separate volume controls for L and R is rather nice! I will probably soon go back in and fit a “bypass” for the tone controls to allow for connecting the pair to a pre-amp as pure power amps.
Pictures in my next post. I have really enjoyed this build, and have, once again, learned a lot. And after all these years, the sound produced by the Mullard 5-10 valve amplifier is among the best I have ever heard (for my type of listening).
I tested the power transformer and got 320-0-320, 3.15-3.15 and 2.5-2.5. I then changed the rectifier back to an EZ80 (I had to make and fit a small base plate for the 9-pin valve socket after removing the big 8-pin socket) and configured the circuit for “low loading”.
I replaced all the capacitors and a number of the resistors - some were literally crumbling.
When it did not produce sound at the end of this exercise, I tested the output transformer and found that it was broken. I removed it, and fitted a temporary output transformer (a beautiful old Partridge I have) and tested again.
This time it worked, and made beautiful music.
This is probably where I would have left it, if I had not been tidying up the next day, and found quite unexpectedly that I had another, identical power transformer on the shelf. I started thinking, and reading up on the Mullard 5-10.
Further scratching around unearthed a pair of 8K output transformers that a friend salvaged from an old broken amp some time ago. How would this sound?
I promptly removed the partridge from Scruffy and fitted one of the “new” pair of output transformers. It worked well, and sounded great!
So now I knew that I had two identical power transformers and two identical output transformers that were suited to a “new” build of a pair of Mullard 5-10 amplifiers. To get things going, I ordered all the caps, resistors, pots, knobs, switches, lamps, fuse holders etc. from Mantech. For the 2 x 50Uf 350V caps I intended using 2 x 32+32uF 450V caps which I had on hand, and which measured spot on (well, +-10%).
But then the real issue - the chassis. I do not have the funds to have these commercially made, so I walked around looking and scratching my head… until I found two baking trays (R19 each) at Cash Converters and decided to give them a try!
I marked out and cut the first chassis, then primed and spray painted it. After that the parts were fitted, the components assembled and fitted and the whole build tested piece by piece.
Come final switch on time there was an almighty howl, and after “phoning a friend” was quickly advised to swop the 2 x EL84 anode connections around. I did that, and the problem was solved. And, beyond my greatest expectation, there was virtually no hum or noise!
I then started on the second amp, which I “designed” to be a mirror image of the first one (I call them M510A and M510B). Scruffy now had to offer up her power transformer, and after cutting and spray painting the second chassis I started assembling the unit this week.
This morning I finished it off, tested it in mono (i.e. M510A at rest) and then connected up both M510A and M510B and, wow, how good these two sound in stereo!
Interestingly, I build both with tone controls (no particular reason) and, for now, having separate volume controls for L and R is rather nice! I will probably soon go back in and fit a “bypass” for the tone controls to allow for connecting the pair to a pre-amp as pure power amps.
Pictures in my next post. I have really enjoyed this build, and have, once again, learned a lot. And after all these years, the sound produced by the Mullard 5-10 valve amplifier is among the best I have ever heard (for my type of listening).
