Millenium 4-20 (Maplin Electronics)

[a91gti]

My mistake folks, it seems this is not a Millenium.

[GoofysNewquay]

Dug this out of the shed the other night...



... was listening to spotify through half a channel (could only find one set of valves), and to my surprise music came out of the other end!

... until one of the capacitors went pop



... same happened when I tried the other channel.

Other than that, not looking too bad (considering I built it 20 years ago)



When I started trawling Google for a schematic, I came across this thread, Mike H's website, and this updated/upgraded schematic

http://www.audio-talk.co.uk/phpBB2/v...?p=46226#46226

My plan of action is:

1. get some cheapy replacement capacitors so that I can check everything works (there's an intermittent fault on the heater supply on one channel)
2. get a full set of EL34's, and some cheapy ECC83/EF86's and see if it will make some noise for any length of time
3. re-home the thing is something more attractive (saw somewhere an amp in an old wine crate - might give that a go)
4. replace some of the critical components with their modern equivalents (might give Mike H's updated design a go)
5. replace the coils

any tips/suggestions?

[Greg.]

My recommendation would be to contact Mike directly through his website as he knows absolutely all the 'onions'.

http://livinginthepast-audioweb.co.uk

[Mike H]

That cap failure looks exactly the same as previous page, pretty sure it's one of the EL34 cathode bypass caps, EL34 cathodes should only be about 30 - 35V each so something deffo wrong somewhere

[Mike H]

PS: check cathode resistors are correct 470R, and that their wiring up is OK, one of these goes O.C. or high resistance then *pop!* goes the cap. Ditto if you lose grid bias e.g. bias resistor goes O.C., also if pos current is leaking through the coupling cap(s) from ECC83 anode(s), that will raise grid Voltage which raises cathode Voltage which then blow the cap(s).

HTH

[Mike H]

PS #2: it's remotely possible EL34's have got poisoned grids or something so anode current is too high even though grid bias is correct. But I'm guessing wildly.....

[Richard]

Hello and welcome, yes replace the old electrolytics and valves and see how it goes. The heaters are all AC from a common winding I think so if just one or two aren't working it should be just a bad connection or valve.

When Mike put the amp together 20 years ago there was no web and very little valve info around. I have the original article in the Maplin Project Book and he talks of doing much of the design before finding a genuine 5-20 circuit.

You could rebuild yours as 4-20, assuming it used to work well, or try the original Mullard 5-20 circuit. Mike's new design in that thread (ECC82 first valve followed by ECC81 splitter) is a spice model I think (unless he's built it now) so bear in mind it may need work to stabilise etc.

It looks good to me apart from the dust It could be spray painted if you wipe it over well with thinners, scotch-brite it, then use acid etch primer such as Upol #8 from Halfords (otherwise the paint will drop off after a few months).

[Mike H]

^ What he said, I was trying to make something work based on just data sheets, but wouldn't work, then someone gave me a photocopy of the 5-20 schematic so we thought sod it just do it like that. Mostly.

Yes internet and e-mail as we know it didn't exist, and even then just mostly used by universities, but was beginning to be used by businesses, Maplin had an e-mail account which was pretty forward thinking in 1993, us ordinary plebs just didn't.

Who remembers the 'dot coms' rows of the mid - late '90's, where the educational organisations objected to the use of the Internet for selling stuff, must only be used for the free exchange of information.

[GoofysNewquay]

Thanks for the pointers - got my multimeter out today - checked voltages everywhere - you were bang on the money...

... three out of four 470R's were open circuit - hence the three blown capacitors (80v across a 63v component isn't good)!

Got a full set of valves on order, and picked up some 70's Sony speakers - might need to tweak the crossovers, but I think I'll wait until my ears re-calibrate to hearing all the instruments!

[Victor]

I think the Amp would look good with some felt round it. Some people usually build a wooden frame round the millenium kit to make it a little stronger framed.

Can`t think of anything else to say...

One question for everyone!

Can you still get the metal covers for the transformers? I need one for the PSU transformer.