Mullard 5-20

[JDeV]

While looking for components to build a valve amp, I came across a guy here in Cape Town, actually not even 1km form me, who got about the biggest collection of valves in Africa, I would say. He used to own his electronic shop but work from his house now.
(Ton Sistilan)

Anyway, while asking him about what he got and what he can suggest to me as a 1st project, he showed me what looks like 2 x Mullard 5-20 amps that he is in the process of restoring. When I asked him about them, he offered them to me, as is, for about $130 a piece without the EL34 valves, off which he can sell me "Electro Harmonics" EL34's for $20 a piece , brand new 1's.
All components are already replaced with high quality new 1's except for transformers, which are still O.K. He did not check the bias on any of them yet or worked on the chasis.

1) Is this a good amp?
2) Is this a good price for the amps?
3) Are these Electro Harmonics valves good and are their price good?
4) Will I be able to use them with 8ohm, 89dB speakers?
5) Will I be able to use them with a high quality pre-amp (XBoSoZ)?
6) Does anybody have a better picture of a Mullard 5-20?

Thanx for all answers.

Here is an old pic I found that looks almost exactly like these 1's.

[bournville]

Originally posted by JDeV

1) Is this a good amp?

The Mullard 5-20 is quite an old design, dating from about the mid-1950s, but had quite an impressive performance - 20w output at less than 0.4% distortion (1% at total output of 27w). The pic you show is similar the the originals I have seen, so the ones you've been offered appear to be old, so perhaps they are collectors items!

The amp was quite innovative at its time, offering direct coupling between the first stage and phase splitter, ultra linear output (what Mullard called 'distributed loading'), 30db of feedback, and requiring an input of 220mV for full output (high imp input into 1 meg). Frequency response is fundamentally flat from 30hz to 30khz.

Factory produced units offered a choice of 3, 8 or 15 ohm outputs, and UL tappings at 20% or 43%. You need to check with your friend these details.

I have always had a fondness for these old Mullard designs, but my opinions may be biased by nostalgia - after all, it is an almost 50 year old design! In terms of updating the amp you can increase feedback by another 10db or so before instability sets in, but otherwise major circuit changes would be needed (such as converting the EL34 output stages for normal loading - greater output but greater distortion maybe).

[Bas Horneman]

Wish I could have met this guy when I lived in Durbanville.

Anyway

1) Yes
2) Yes
3) Yes..and Yes (Svetlana are rated higher by most)
4) Yes
5) Don't know

Good luck..

To give you an idea on prices...getting just the output transformers for the Mullard these days will set you back 56 pounds a piece.

If you don't want them. I'll buy em from you and pick em up next year when I am on holiday in SA. (I am serious)

What else does the guy have..6SN7's? GZ34?
;-)

Cheers.

[EC8010]

Oh yes, oh yes! A little work will need doing, primarily replacing capacitors, but you have a lot of good iron here. The driver circuitry could be improved, but it is still a very fine amplifier.

It will make an excellent first project.

Full circuit and constructional details are in the Mullard book reprinted by Audio Amateur and titled "Tube circuits for audio amplifiers." Oddly the photograph of the 5-20 in the reprint looks like you photograph, whereas the one in my original copy has unshrouded transformers

[Tube_Dude]

Hi folks

It uses a EF86 input stage a choice not much comum this days...but that i like! Nowdays the triode imput stage is much more in fashion.
Interestingly the Hiraga designed Lectron amp also use a pentode input stage...in this case a RCA 6SJ7(metal case).

Another positif point in my view, in this design, is the tube rectifier wich is the GZ 34... i still have same waiting for a project (NOS metal base mullards)! The sound of a vacum tube rectifier is much better that of a solid state one...in my opinion....

And also i like that amp for sentimental reasons...but that's is another story!

regards

Jorge

[Bas Horneman]

Mullard 5-20 schematic.

[Bas Horneman]

If you can find any more of those treasures ...I'd like to deps them.

[Brett]

Originally posted by JDeV
1) Is this a good amp?
2) Is this a good price for the amps?
3) Are these Electro Harmonics valves good and are their price good?
4) Will I be able to use them with 8ohm, 89dB speakers?
5) Will I be able to use them with a high quality pre-amp (XBoSoZ)?
6) Does anybody have a better picture of a Mullard 5-20?

1) Yes
2) Yes
3) I haven't used the EH's personally, but they get some good press. The Svets are really excellent. I can usually find better prices, but if you have a source within walking distance who'll warranty them, then the price is fine.
4) Probably. Depends on how big the room, how loud you like to play and how close you sit, but will likely be OK.
5) It's a single ended input, so you'll only be using one side of the pre. Gain overall might still be too high. Stepdown transformers at the inputs of the Mullard would solve some of these issues.
6) Not me.

I'd buy them.

Recently I picked up a set of Lowther LL26 poweramps in ugly cosmetic condition, which use a similar topology, but with a 13D3 for the splitter with a 6BJ6 under it as a constant current source. I originally bought it for the Partridge OPTs, but I think I might restore them, after hearing how good they can (potentially) sound.

[JDeV]

Originally posted by Bas Horneman
What else does the guy have..6SN7's? GZ34?

He most probaply does. He showed me tubes there that only 12000 was made of in 1944 - he got 11 - (I can not remember the name number now) used in army radios. I can find out for you about these.

Thanx for all other info, I'ill buy them today.

[EC8010]

Originally posted by Brett
Recently I picked up a set of Lowther LL26 poweramps in ugly cosmetic condition, which use a similar topology, but with a 13D3 for the splitter with a 6BJ6 under it as a constant current source. I originally bought it for the Partridge OPTs, but I think I might restore them, after hearing how good they can (potentially) sound.

Brett, do you have a diagram for the Lowther? I don't think I've seen it anywhere.