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Hi,

A newbie here, I see that this forum concerns a clone of the Mullard 5-20.

I've recently started to resurrect an original, that appears to be a homebrew dating from the late 50's.

I thought you may enjoy seeing some 'before' pictures. I have since replaced various components and changed a few things back to original spec, and can report that it now performing very well,









Cheers Clanger

Clanger wrote:Hi,

A newbie here, I see that this forum concerns a clone of the Mullard 5-20.

I've recently started to resurrect an original, that appears to be a homebrew dating from the late 50's.

Where are you, Clanger? (I'm guessing the Netherlands?) Looks like that was dipped in a polder for a few years, surprising that the transformer insulation survived...

Uncle Ned wrote:Where are you, Clanger? (I'm guessing the Netherlands?) Looks like that was dipped in a polder for a few years, surprising that the transformer insulation survived...

Ned, rather I believe this brill find was salvaged from a dustbin or boot fair.

Clanger wrote:I have since replaced various components and changed a few things back to original spec, and can report that it now performing very well.

Hi Clanger - so you've got this gem humming again already? You'll have to post those pics, too. Be sure to check out my Rustiest ST70 thread. It had rested on an alligator poachers porch in the Florida Everglades. :o

Hi guys,

I got it cheap from flea-bay! Had no idea what it was (neither did the seller) , but it was cheap enough to take a punt and asking around I found what it was and got a couple of schematics. All the filter caps reformed ok under variac control, however the TCC metalpak decouplers were leaky as was the wrong spec treacle Wima, and a couple of others.

Interestingly there was some crazyness occuring with I think the screen grid resistor on the EF86. One schematic I have says 100k and another says 360k IIRC. Anyway 1meg was actually in there! and on the other end of the 50nf cap there was another 1meg to earth! This resulted in a subtle and nasty distortion on transients. I clipped the earthing 1meg and compromised on 270k for the screen grid and it sounds great now. - Playing sam moore through it the voice is absolutely crystal clear!

Next I guess I should try and purty it up a bit.

The OPT seems the same (Partridge) as in an old publicity shot for the circuit.

I'd be grateful for advice on the 100k/360k circuit conundrum, and I'll post some newer shots. Thought you guys would like it.

Cheers Clanger

Hi All,

My drawing of the 5-20 shows the screen resistor as 390K, but a couple of Genelex schematics--very similar to the Mullard--show 1 Meg.

Always happy to further muddy the waters . . .

Hi Eric,

This project has languished a bit.

Actually the other schematic I have does actually say 390K not 360k, so the same as you've seen

I've realised the bass response is a bit on the light side, so I'm going to experiment with the feedback bypass cap to see what effect that has.

Cheers Clanger

Hi All,

The project is pretty much there now. Working fine and sounding very sweet. I ended up at 370k on the contentious plate resistor, and just made everything else was as per the original design schematic.



Hmm, the image linking doesn't seem to be working. Any ideas anyone?

Cheers Clanger