Mullard 404? ECL82 amp

[woodturner-fran]

Hi All,

I've just picked up an old amp - really just for the iron to be honest.... but anyway, it uses 4 x ECL82, a ECC83 and EZ81. The guy said his father built it, and it was based on a mullard 404 design. Googling shows me diddly squat....

Anyone here heard of that? Failing that, can anyone point me in the direction of a nice schematic? Or failing that am I better selling the mullard valves that came with it and using the iron for a different project?



Fran

[dagerman]

Hi Fran,
I am probably of little help, but maybe you can get your hands on one of these Mullard Tube Circuits for Audio Amplifiers (Open Library)?
Maybe it contains the needed hints at the ominous '404' base circuit, given that you can look at the real thing and its components.
Stefan

[trobbins]

Check the Mullard 10W stereo amp in the 1962 Mullard designs booklet - that circuit I think uses EF86 PREAMP, but output stage probably similar. Is the output cathode biased, or UL. I have a UL 6BM8 PP stereo amp (ie. 4x ECL82) that uses a 12ax7 preamp per channel.

Ciao, Tim

[woodturner-fran]

Thanks fellas,

I only got the amp this evening so I have to really take a proper look at it over the next few nights. I reckon its PP at least - theres a few wires on the primary side of the output Txs, so it can't be SE.

Fran

[woodturner-fran]

I think I found it....
ECL82 PP1

[trobbins]

Yep - the mullard was the one I was refering to. There is a full article write up on that at another site. The UL for that valve is meant to be a 20% tapping, rather than the more common 40% tapping. I hadn't seen the floating paraphase version, but there were a number of small PA amps in the early 60's that used 6BM8 standard PPs.

[Soonerorlater]

Hi Fran,

Or failing that am I better selling the mullard valves that came with it and using the iron for a different project?

You might be limited by how much B+ current the power transformer can output. ( it only uses an EZ81 in this circuit.) Going solid state instead of the EZ81 might give you more B+ to play around with.The output transformers themselves may not like being pushed beyond the 7 watt schematic.

On the positive side (because it's a home build) the specs of the iron might be a lot higher than needed. With output transformers of 9k (Raa) ECL86 or 12a*7/EL84 schematics could be looked at. Yvesm's ECL86 based precurser of Gingertubes "Baby Huey" might be a contender

Personally I'm very fond of ECL82'S and think you should restore this amp and possibly exceed it's former glory. Just my opinion.........


Brgds Bill

[Knarf]

I seem to recall the original schematic with the ECL82s was called the 'Mullard 4-7' not 404, referring to 4 tubes and 7 watts of output. There is also a '4-10', which employs ECL86s and substitutes a GZ34 for the EZ81, not to mention the 3-3, 5-10 and the 5-20.

- Frank.

[woodturner-fran]

Very helpful all - and knarf - helpful to know the name so I can google it.

But good Lord, did you ever see soooo many parts? There's capacitors everywhere in this thing!!!

Fran

[woodturner-fran]

Soonerorlater,

have you any pointers towards making it even better?