I built the Mullard 2 valve preamp but whenever the power amp is on and I
try to this connect the audio cable from the preamp or the power amp (Mullard5-20) it provides a nasty shock. Mullard did not mention this; but Stern built the same preamp and clearly stated that there will be an electric shock from audio cable when the power amp is on due to return current via the audio cable shield.The return current to the power amp is via the audio cable shield. Is there another way to the send the returning current to the power amp? For example, could I use the umbilical power cord, and how may do this. I am a novice;no complicated solution please.
try to this connect the audio cable from the preamp or the power amp (Mullard5-20) it provides a nasty shock. Mullard did not mention this; but Stern built the same preamp and clearly stated that there will be an electric shock from audio cable when the power amp is on due to return current via the audio cable shield.The return current to the power amp is via the audio cable shield. Is there another way to the send the returning current to the power amp? For example, could I use the umbilical power cord, and how may do this. I am a novice;no complicated solution please.
Very common on vintage gear, you need to feed that amp from an AC isolation transformer. If you reverse the AC plug the chassis will be live with the amp off.. don't get tricked into thinking it's safe.
This is too dangerous, diy or not. Do not use it until the problem can be fixed.
There's no reason this must happen. At the minimum use an isolation transformer,
but the best way is a proper power transformer.
Early preamps derived their power supply directly from the power amp's DC supply.
Mullard never intended for builders to construct a non-isolated line power supply.
There's no reason this must happen. At the minimum use an isolation transformer,
but the best way is a proper power transformer.
Early preamps derived their power supply directly from the power amp's DC supply.
Mullard never intended for builders to construct a non-isolated line power supply.
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If the two valve/tube preamp design has stuck religiously to the old school design of Mullard then that is "normal practice " at the time as the power for the preamp is supplied via the power-amp.
I have the Mullard manual and it states only ONE earth connection should be provided and obviously connecting the audio cable with the power amp on will have the earth on the shielding and plug.
So in those old world circuits TG is right and yes I have had a large number of electric shocks in my life saved by having dry -thick skin presenting a high body resistance .
In those days advice was given to those with moist skin and fine skin to insulate themselves .
It does also say though that an "auxiliary earth" can be provided .
Do NOT use both hands as you don't want the electricity to travel across your heart .
You could of course build a separate power supply for the preamp but it would no longer be a Mullard design and selling it wouldn't be easy as most people in the USA and UK want original design - untouched equipment .
I have the Mullard manual and it states only ONE earth connection should be provided and obviously connecting the audio cable with the power amp on will have the earth on the shielding and plug.
So in those old world circuits TG is right and yes I have had a large number of electric shocks in my life saved by having dry -thick skin presenting a high body resistance .
In those days advice was given to those with moist skin and fine skin to insulate themselves .
It does also say though that an "auxiliary earth" can be provided .
Do NOT use both hands as you don't want the electricity to travel across your heart .
You could of course build a separate power supply for the preamp but it would no longer be a Mullard design and selling it wouldn't be easy as most people in the USA and UK want original design - untouched equipment .
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Anyone building this and then selling it commercially could be liable for injuries and damages.
It isn't 1950 anymore.
It isn't 1950 anymore.
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Then tell the world to stop building Mullard designs .
Have you any idea of the influence on those involved in audio worldwide regarding Mullard stuff ?
If you want to go that far there are plenty of very old USA designs that might be looked on as "dicey " as regards non mains isolated designs .
They are so popular Amazon /eBay and many audio websites promote them.
Have you any idea of the influence on those involved in audio worldwide regarding Mullard stuff ?
If you want to go that far there are plenty of very old USA designs that might be looked on as "dicey " as regards non mains isolated designs .
They are so popular Amazon /eBay and many audio websites promote them.
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OK, I built the mullard 5-20 power amp in 2015.It has a solid EARTH ground to chassis and it is working fine. I built the preamp in 2016 and it is working fine. The chassis of
the power amp or the preamp is NOT hot. I have a 100 watt power transformer in the
power amp. The problem is the return current from preamp via the audio cable that
is causing the shock. If you watch this video from vintage audio work shop; at the 34.23 minute it addressed this very issue:
the power amp or the preamp is NOT hot. I have a 100 watt power transformer in the
power amp. The problem is the return current from preamp via the audio cable that
is causing the shock. If you watch this video from vintage audio work shop; at the 34.23 minute it addressed this very issue:
A bad design is a bad design, no matter who is responsible for it.
What is this nationalism stuff about, and what does it have to do with bad design?
Are you an engineer, or a politician?
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Qpiter read my post I said its not the power amplifier but the preamp which gets its power via the power amp unless you haven't built the preamp to the ORIGINAL design but have added a power supply in the preamp-----have you ?
Duncan2, Morgan Jones, in Valve Amplifiers, wrote of three CLASSIC amplifiers:Quad,
Williamson and Mullard. In fact, he went over the three schematics and only lower the screen resistor on the Mullard. So, what is this "bad design" you are talking about. Can you provide me with a credible source if Morgan Jones is not. My understanding is that
many vintage tube amps have the same design for preamp but they connected the audio cable differently. I am a novice builder; I can't argue with your opinion. I just need help.
Williamson and Mullard. In fact, he went over the three schematics and only lower the screen resistor on the Mullard. So, what is this "bad design" you are talking about. Can you provide me with a credible source if Morgan Jones is not. My understanding is that
many vintage tube amps have the same design for preamp but they connected the audio cable differently. I am a novice builder; I can't argue with your opinion. I just need help.
I would say a bad design is one which will shock you by handling the audio cable.
You need a properly designed power supply, properly installed into the preamp.
Any other option is dangerous, to say the least. Have a technician or engineer
that you trust look at your set up and make a recommendation to resolve the problem.
You need a properly designed power supply, properly installed into the preamp.
Any other option is dangerous, to say the least. Have a technician or engineer
that you trust look at your set up and make a recommendation to resolve the problem.
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Qpiter -WHY ?? are you addressing this to me cant you read my posts correctly ?
Its Rayma you should be addressing this to.
All I have done is answer your original thread post of - Shocking !
I stated facts I was not condemning the circuits as an audio design ---don't you understand that ? Its being condemned due to modern safety Standards -- The UK & the US have Standard Boards who decide appliances safety factors and build quality before it should be allowed in the public domain.
JLH had to amend his 80Watt Mosfet power amp design of the 1980,s by adding an earth whereas he quite rightly pointed out that earths introduce noise of varying sorts but he had to conform to those Standards before it could be sold by entrepreneurs in the UK .
The EU also has a Standard Board .
Didnt you read my post where I said its extremely popular all over the world , and you still haven't addressed my question --
Have you built it to the ORIGINAL design or not ?
If Yes then this "shocking" business is actually part of the design .
IF you have added a power supply to the PREAMP then we are talking about something else -- do you understand this ?
Please say which it is .
You don't have to mention Morgan Jones he appeared in many articles in EW I have known about him for about 40 years.
Its Rayma you should be addressing this to.
All I have done is answer your original thread post of - Shocking !
I stated facts I was not condemning the circuits as an audio design ---don't you understand that ? Its being condemned due to modern safety Standards -- The UK & the US have Standard Boards who decide appliances safety factors and build quality before it should be allowed in the public domain.
JLH had to amend his 80Watt Mosfet power amp design of the 1980,s by adding an earth whereas he quite rightly pointed out that earths introduce noise of varying sorts but he had to conform to those Standards before it could be sold by entrepreneurs in the UK .
The EU also has a Standard Board .
Didnt you read my post where I said its extremely popular all over the world , and you still haven't addressed my question --
Have you built it to the ORIGINAL design or not ?
If Yes then this "shocking" business is actually part of the design .
IF you have added a power supply to the PREAMP then we are talking about something else -- do you understand this ?
Please say which it is .
You don't have to mention Morgan Jones he appeared in many articles in EW I have known about him for about 40 years.
Quite correct, many of the pioneers both over here, and t'other side of the pond designed somewhat unsafe designs, which noone should be happy to use, without modification.
I didnt see any nationalism?
But, if we want to talk facts, and not hyperbole - As a brit, and working in power generation, I too will get a bit short with respect to electrical safety "discussion".
The UK lead the world with Electrical regulation and safety, and we still do. (That's why EU was so glad to harmonise our regulations into their BS SN system)
The US, from my experience, sadly, still has a way to go
meanwhile, back to the op's problem.
An as-built schematic would be useful.
Qpiter, what do YOU think is happening?
An as-built schematic would be useful.
Qpiter, what do YOU think is happening?
Back on the subject.
Without pictures or the actual connection arrangement (the video link is not present by the way) I assume the reason for the 'shock' is that the negative HT (B+ return) uses the screen of the 'audio' in/out cable? Then the HT+ and heaters are continuously connected to the pre amp? So if the audio cable is disconnected the pre amp floats to the power amp HT+ voltage. Not clever. As TG said switching the cables with the system on was never intended...
The solution is simple, safer and is more or less standard practice today.
Connect the heaters, HT+ (B+) and HT -ve (return) permanently from the 5-20 to the pre amp. Use an umbilical or what ever suits you.
Now the audio cable screen does not carry the HT negative return. You can unplug / plug it as you wish.
You may however get some hum, this is why the instructions say to use only one -ve connection between the power and pre amps.
If this does happen, then you need to insulate the pre amp output connector from the pre amp chassis, so the audio cable screen only connects at the power amp side, not to the pre amp as well. (Or you can make a 'special' audio lead with the screen only connected at one end and is insulated from the other.)
Just found this, Chapter 4 of the Mullard book:
''To maintain a low level of hum when two or more units are used
together, the chassis of the units should be connected
by one lead only. If the negative return line is conveyed from one unit
to the other by way of the power cable, the connection between
chassis should not be duplicated in the a.f. coaxial lead, and one end
only of the outer conductor of this lead should be earthed.''
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It is an interesting dilemma where we have purists that insists on having all original equipment, when all original is actually deadly. So how can the seller be held liable for the death of a collector that knows full well what he is buying? This is no different than in the vintage car market. A 1960's muscle car is a death trap compared to a modern vehicle.
Everyone is willing to take the risk until it kills them or a family member, then it's somebody's fault and the lawyers move in for the kill. If you are selling unsafe equipment because that is what the buyer wants, make sure you get them to sign a waiver, and I'm not sure even that will even help you. Justice is really vengeance, the sooner you realize that the better off you will be.
Everyone is willing to take the risk until it kills them or a family member, then it's somebody's fault and the lawyers move in for the kill. If you are selling unsafe equipment because that is what the buyer wants, make sure you get them to sign a waiver, and I'm not sure even that will even help you. Justice is really vengeance, the sooner you realize that the better off you will be.
Shock! Mullard Preamp
Duncan2, I am very sorry addressing my response directly to you. I am not very savvy with this chat thing. May I ask everyone to take a look at alan4411
post, because he right on target. First let me repeat: I am a novice, an older
dude that grow up tube amp;meaning played them but never pull screw on one.In 2015, I counted the number of parts on a schematic and told myself that I should be able to put them together and I look at the Mullard 5-20 and wired it point to point.Then I decided I want the preamp and wired it point to point.They work.There is a little noise from the preamp, but nothing I can"t live with.The shocking part with the audio cable:Like someone said, all I have to do is turn the power amp off before unplugging the preamp. I also put heat shrink on the end of the cable. I know the word SHOCK in this hobby is,wow! Frankly,I am more afraid to drive on 95.Don't get me wrong. I kept reminding myself of the 500 volt lurking at the first filter cap. But like you guys I want to learn. I am studing Alan4411 post carefully.Remember I am a NOVICE. At 34.30 minute on the video the cable issue is addressed Valve Amplifier Study 027: Heathkit Pre-Amplifiers Models UMC-1 & USC-1 Part 1 (EF86 x 2, ECC83) - YouTube
Duncan2, I am very sorry addressing my response directly to you. I am not very savvy with this chat thing. May I ask everyone to take a look at alan4411
post, because he right on target. First let me repeat: I am a novice, an older
dude that grow up tube amp;meaning played them but never pull screw on one.In 2015, I counted the number of parts on a schematic and told myself that I should be able to put them together and I look at the Mullard 5-20 and wired it point to point.Then I decided I want the preamp and wired it point to point.They work.There is a little noise from the preamp, but nothing I can"t live with.The shocking part with the audio cable:Like someone said, all I have to do is turn the power amp off before unplugging the preamp. I also put heat shrink on the end of the cable. I know the word SHOCK in this hobby is,wow! Frankly,I am more afraid to drive on 95.Don't get me wrong. I kept reminding myself of the 500 volt lurking at the first filter cap. But like you guys I want to learn. I am studing Alan4411 post carefully.Remember I am a NOVICE. At 34.30 minute on the video the cable issue is addressed Valve Amplifier Study 027: Heathkit Pre-Amplifiers Models UMC-1 & USC-1 Part 1 (EF86 x 2, ECC83) - YouTube
The EF86 is a UK low noise tube specially designed for Mullard circuits and I take it if you built it to the original design that's the tube that was recommended by them at the time.
I too have the original Mullard tube amplifier design book and I did say one earth was recommended .
I too have the original Mullard tube amplifier design book and I did say one earth was recommended .
Shock! Mullard
I thank everyone, but a special thank you to Alan4411. I did exactly what
you recommended. The amazing thing about it is that voltages fell right in line and the Plate voltages are spot-on. Above all, the sound, the sound, the sound!
Thank you guys.
I thank everyone, but a special thank you to Alan4411. I did exactly what
you recommended. The amazing thing about it is that voltages fell right in line and the Plate voltages are spot-on. Above all, the sound, the sound, the sound!
Thank you guys.