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
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
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
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
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
Ciao, Tim
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
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
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.
Hi Fran,
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
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
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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.
- Frank.
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
But good Lord, did you ever see soooo many parts? There's capacitors everywhere in this thing!!!
Fran
I would start with the usual stuff just to bring it back to it's original spec- replace all the old electrolytic caps in the power supply in case they're starting to leak, upgrade decoupling caps c3,10,11 if necessary, check the value of the resistors haven't strayed too far outside their tolerance. Replace any small ceramic caps that are in the signal path with silver mica such as might have been used for c5 or the cap bypassing the feedback resistor. Some people think it's worthwhile to upgrade the pots to Alps or suchlike.
As for the Mullard design - many if not most would have a better idea than me as to how to go about it.
However....😀
I think experimenting with a ccs on the ECL82 triode ltp and /or ccs on the pentodes might be a bit of fun.
A lot might depend on what you intend to use the amp for, ie are you going play vinyl, do you really need tone controls, perhaps you don't need the gain of the ECC83?
I have a quad of NOS Mullard ECL82'S and all the necessary iron, hence my interest. All I really need is a good schematic and this one looks like the best one I've seen so far.
Good luck whichever way you go.........🙂
As for the Mullard design - many if not most would have a better idea than me as to how to go about it.
However....😀
I think experimenting with a ccs on the ECL82 triode ltp and /or ccs on the pentodes might be a bit of fun.
A lot might depend on what you intend to use the amp for, ie are you going play vinyl, do you really need tone controls, perhaps you don't need the gain of the ECC83?
I have a quad of NOS Mullard ECL82'S and all the necessary iron, hence my interest. All I really need is a good schematic and this one looks like the best one I've seen so far.
Good luck whichever way you go.........🙂
Ta kindly...
I hope to give the amp a once over this afternoon and maybe even fire her up. Then I can have a listen. I don't need tone controls, but a volume control would be handy. It'll probably be in the workshop for a while I think. I remember shoog did a ECL82 headphone amp that sounded really good, so I might get on to him for the sch. I think he used parafeed with some stepdown transformers ..... but it was ages ago.
I also have an ecl86 amp too and it sounds pretty good too.....
Fran
ps. ZIBI - thanks for that....
I hope to give the amp a once over this afternoon and maybe even fire her up. Then I can have a listen. I don't need tone controls, but a volume control would be handy. It'll probably be in the workshop for a while I think. I remember shoog did a ECL82 headphone amp that sounded really good, so I might get on to him for the sch. I think he used parafeed with some stepdown transformers ..... but it was ages ago.
I also have an ecl86 amp too and it sounds pretty good too.....
Fran
ps. ZIBI - thanks for that....
Always good to read Shoogs posts and would also be interested in a good schematic for an ECL82 based pp amp.
Lucky man.........😀
Lucky man.........😀
Well, I took her out to the workshop today to give her a good look over. Nothing much out of place it seemed, nothing burnt etc. The whole thing was built on a linear tag board - with the caps and resistors across the tags. The power cap had been changed more recently it seems, with a twin cap in one can type.
So I turn it upside down on the bench, take a look at the fuse.... just as well, it was a piece of wire!! So fuse replaced and suspicions aroused, I thought I better take it handy....
So I wired her to the mains and used a screwdriver to flick on the switch. I could see the filaments start to glow. Mmmm no fuse blow anyway. Might not be too bad I thought. So next I decided to see what voltage I had on the power caps. 420VDC on the first cap, but only 2V on the second section. Whats going on here I wondered. Ok, lets look at the outputs..... whats this? 410 on one of the secondaries, i.e. after the Tx.
Then the horror of the thing actually dawned on me...... There was 410VDC on the chassis of the bloody amp. Sweet Lord J***** I could have been fried by this thing!!!
Cue a deep intake of breath, power off and gently backing away, DMM left in position so I could see the voltage drop. Which took about 10 mins to drop below 50V.
Let this be a warning to anyone poking around inside an old amp!!!!
*****************************************
So I decided to gut the thing, rather than spend time chasing down faults in it. The chassis was rubbish anyway, so no big loss. Turns out the iron was OK though, and managed to get the windings more or less figured out. I just need to work out the output Tx secondaries phasing.
I have one question though on the power transformer. The transformer is a nice beefy unit, with 300V CT HT winding, 6.3V and 3.15-0-3.15V heaters. This BTW, is exactly as shown in the schematic from mullard..... Anyway, I've identified all those wires. However, there is another wire from the power Tx. Its a bare stranded wire, no insulation, coming from the inside of the windings (opened the bell caps to have a look). Its bare all the way back right into the Tx. Is this some kind of electrostatic screen do you think? It was connected to ground in the old amp.
Thanks
Fran
(soonerorlater... the ecl86 is a very early series 300 from audio innovations, I met him at a show recently and he could tell me that it was one of the first made and he reckoned it had the best transformers of the lot, partridge I think he said?) I have telefunken eCL86 in it and some EI ecc83 and ecc82 in there.
So I turn it upside down on the bench, take a look at the fuse.... just as well, it was a piece of wire!! So fuse replaced and suspicions aroused, I thought I better take it handy....
So I wired her to the mains and used a screwdriver to flick on the switch. I could see the filaments start to glow. Mmmm no fuse blow anyway. Might not be too bad I thought. So next I decided to see what voltage I had on the power caps. 420VDC on the first cap, but only 2V on the second section. Whats going on here I wondered. Ok, lets look at the outputs..... whats this? 410 on one of the secondaries, i.e. after the Tx.
Then the horror of the thing actually dawned on me...... There was 410VDC on the chassis of the bloody amp. Sweet Lord J***** I could have been fried by this thing!!!
Cue a deep intake of breath, power off and gently backing away, DMM left in position so I could see the voltage drop. Which took about 10 mins to drop below 50V.
Let this be a warning to anyone poking around inside an old amp!!!!
*****************************************
So I decided to gut the thing, rather than spend time chasing down faults in it. The chassis was rubbish anyway, so no big loss. Turns out the iron was OK though, and managed to get the windings more or less figured out. I just need to work out the output Tx secondaries phasing.
I have one question though on the power transformer. The transformer is a nice beefy unit, with 300V CT HT winding, 6.3V and 3.15-0-3.15V heaters. This BTW, is exactly as shown in the schematic from mullard..... Anyway, I've identified all those wires. However, there is another wire from the power Tx. Its a bare stranded wire, no insulation, coming from the inside of the windings (opened the bell caps to have a look). Its bare all the way back right into the Tx. Is this some kind of electrostatic screen do you think? It was connected to ground in the old amp.
Thanks
Fran
(soonerorlater... the ecl86 is a very early series 300 from audio innovations, I met him at a show recently and he could tell me that it was one of the first made and he reckoned it had the best transformers of the lot, partridge I think he said?) I have telefunken eCL86 in it and some EI ecc83 and ecc82 in there.
Partridge - mmmmm I can almost smell the brown varnish. Valves are nice too, EI are starting to fetch good prices now on Ebay, I think some of the early valves were supposed to have been made with ex Telefunken factory tooling.
Once again..... Lucky man........😀
I should spend less time on Ebay and more time mooching around local Auctions. Guess fortune not only favours the brave but those that are willing to do some leg work.
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