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.
(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.
Attachments
JDeV said:
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).
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.
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.
2) Is this a good price for the amps?
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
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
Mullard 20
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
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
JDeV said:
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.
Re: Lowther
I can't seen to shrink it enough to post it, so have a look here for a poor copy. I have a better one I can email if you want.
Here's a link to the best info I've found on them, and below is a pic of one in nicer condition than mine.
<img src=http://home.mira.net/~kiewavly/Lowther/LL26Front.JPG>
Indeed I do old chap. I'd be interested in your opinion.EC8010 said:
I can't seen to shrink it enough to post it, so have a look here for a poor copy. I have a better one I can email if you want.
Here's a link to the best info I've found on them, and below is a pic of one in nicer condition than mine.
<img src=http://home.mira.net/~kiewavly/Lowther/LL26Front.JPG>
Bias on Mullard 5-20
I got my Mullard 5-20's today and want to know the following:
1) I measure 6.9V for the heater supply, is this still o.k. or to much?
2) How can I bring it down to a safe level?
3) How do I measure the bias?
4) What must it be?
5) How do I adjust it?
Thanx for all your help.
I got my Mullard 5-20's today and want to know the following:
1) I measure 6.9V for the heater supply, is this still o.k. or to much?
2) How can I bring it down to a safe level?
3) How do I measure the bias?
4) What must it be?
5) How do I adjust it?
Thanx for all your help.
The great Mullard!
Hi JDeV!
First of all congratulations for a great purchase!
Busines now!
1-It's ok...its + 10% trusting in the acuracy of your multimeter..
2-If you care about this 10 %, you need a serie resistor with the filaments.
3-Measure the voltage across the 470 Ohms resistor at the catode of the output stage (this voltage must be 32 V).After calculate the current with the Ohms law....this design use the european standard catode bias..so there are no adjustment.
4-aprox. 68mA
5-See 3..
Regards
Jorge
Hi JDeV!
First of all congratulations for a great purchase!
Busines now!
1-It's ok...its + 10% trusting in the acuracy of your multimeter..
2-If you care about this 10 %, you need a serie resistor with the filaments.
3-Measure the voltage across the 470 Ohms resistor at the catode of the output stage (this voltage must be 32 V).After calculate the current with the Ohms law....this design use the european standard catode bias..so there are no adjustment.
4-aprox. 68mA
5-See 3..
Regards
Jorge
MULLIES.
Hi,
6.9V is the value one usually measures without the tubes in place.
In case you had the tubes in their sockets it is not O.K although I would lower that to 6V.
Why? Well the tubes sound better and last longer that way.
Tolerance is usually +/- 10%.
As an example: just put a 6DJ8 on a tester with 7V on the heaters and see its' tranconductance go down the "tubes" within the hour.
Cheers,
Hi,
6.9V is the value one usually measures without the tubes in place.
In case you had the tubes in their sockets it is not O.K although I would lower that to 6V.
Why? Well the tubes sound better and last longer that way.
Tolerance is usually +/- 10%.
As an example: just put a 6DJ8 on a tester with 7V on the heaters and see its' tranconductance go down the "tubes" within the hour.
Cheers,
Heater tolerance and true RMS
Heater voltage ought to be as close as possible. Mullard specified that it should be within +/-5%. High heater voltage will quickly damage valves, as Frank comments. Mains transformers of that time tended to have comprehensive primary tappings, so all you need to do is to adjust the primary slightly until you get as close as you can to 6.3V.
I would suggest that you make the measurement and adjust the settings after 23.00 unless you have a true RMS voltmeter. I kid you not! If, as is quite common, your mains is distorted, perhaps due to people flagrantly using computers, TVs, or Hi-Fis, an ordinary meter will measure incorrectly. But your precious valve heaters respond to RMS voltage. If you measure when the mains is cleaner, an ordinary meter will mesure correctly, and allow you to set your mains transformer correctly. Of course, if you have a true RMS meter, you can do it any time you like.
Heater voltage ought to be as close as possible. Mullard specified that it should be within +/-5%. High heater voltage will quickly damage valves, as Frank comments. Mains transformers of that time tended to have comprehensive primary tappings, so all you need to do is to adjust the primary slightly until you get as close as you can to 6.3V.
I would suggest that you make the measurement and adjust the settings after 23.00 unless you have a true RMS voltmeter. I kid you not! If, as is quite common, your mains is distorted, perhaps due to people flagrantly using computers, TVs, or Hi-Fis, an ordinary meter will measure incorrectly. But your precious valve heaters respond to RMS voltage. If you measure when the mains is cleaner, an ordinary meter will mesure correctly, and allow you to set your mains transformer correctly. Of course, if you have a true RMS meter, you can do it any time you like.
hifi said:
Quit right, because the previous owner was not finish with his restoration process, last worked on it more then 3 years ago, I 1st tested all voltages without any tubes fitted. When tubes inplace, Vh=6.35V
Now for some other questions: This seems to be not an org. Mullard but rather a diy copy.
1St it got 2 silicone diodes for bridge and also 2 x Output transformers in parallel.
1st OT is quit bulky with only centre tap.
R24 and 25 goes direct to centre tap.
2nd OT , smaller with various tappings.
On 1 amp. anodes of EL34's are connected with series RC.
Rest of circuit is same as Mulllard 5-20 (as circuit on page 1 of this thread)
At moment I only got 1 problem. With or without input signal, I get 10Vpp sine output. With input , output is same , does not change amplitude when changing input.
On ECC83: (Actually 12AX7)
A1=300v, A2=230v
G1=62v, G2=58v
On El34's:
K1=28v, K2=24v
What can be wrong, where do I start looking?
The Mullard faked!
Well as you say ,your Mullard seem a Mullard clone, but not to similar!
You talk about two output transformers in paralel...rc networks in the anodes of the EL34...nothing of it exist in the original mullard amplifier...so...we are talking of another animal...
See if across the feedback resistor exist a capacitor of 100pF...if not ...put one...and keep as informed!
Good luck!
Jorge
Ps. It could be also bes tubes...bad capacitors...bad grunding.....bad........................................................
Well as you say ,your Mullard seem a Mullard clone, but not to similar!
You talk about two output transformers in paralel...rc networks in the anodes of the EL34...nothing of it exist in the original mullard amplifier...so...we are talking of another animal...
See if across the feedback resistor exist a capacitor of 100pF...if not ...put one...and keep as informed!
Good luck!
Jorge
Ps. It could be also bes tubes...bad capacitors...bad grunding.....bad........................................................
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