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Old 14th Jul 2020, 6:20 pm   #1
Levente
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Default Filament wiring/ single wire upgrade

Hello Guys,

I have an old 6v6 mono amplifier. (again Ha!) Came with a single heater wiring. Attached the schema. I did my first ever heater wiring with a twisted cable. I am confused on the ground and also reading about the artificial center tap.

What I have done is so far. Filament wires soldered to two tie point on the 6x5gt rectifier. That is where i've started. One of these grounded to chassis and also this ground is sharing the "death cap" ground.

From here soldered the twisted wires to

6av6 to pins 3 and 4
12ax7 pins 4/5 tied together and to pin 9
6v6's to pins 2 and 7

My question is that and I think where I am wrong is her: I did leave the ground point for one heater wire at the rectifier tube.

Do I need to unsolder that ground point and instead, create an artificial center tap at one of the 6v6's woth two 120R resistors pin 2 and 7 to cathode pin 8?

Am I wrong here?

Please help me out

Thank you as always for this great forum and you wonderful people.

Cheers
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Old 14th Jul 2020, 6:47 pm   #2
Nuvistor
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Default Re: Filament wiring/ single wire upgrade

Is there a problem that you are rewiring the heaters?
I have not heard the term “Death Cap” which capacitor, if it is a capacitor, do you refer to.
Thanks
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Old 14th Jul 2020, 6:50 pm   #3
Skywave
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Default Re: Filament wiring/ single wire upgrade

Looking at the circuit that you have added as an attachment, I see it uses an unbalanced-to-ground heater circuit. Since its HT rectifier does not deliver its HT+ from its filament - as some rectifiers do - I would do a re-wire using a fully balanced-to-ground configuration. That way, compared to the unbalanced arrangement, there will be a reduction in hum pick-up from the heater wiring.

Al.
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Old 14th Jul 2020, 6:55 pm   #4
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Default Re: Filament wiring/ single wire upgrade

Quote:
Originally Posted by Nuvistor View Post
Is there a problem that you are rewiring the heaters?
I have not heard the term “Death Cap” which capacitor, if it is a capacitor, do you refer to.
Thanks
Hi Frank,

Tube hum and buzz at the same time. Trying to trace the source, and I was running out of options... although, I did not check the rectifier tube as I don't have another one.

Voltages are fine, nearly just like how it is displayed on the schematic.

Swapped all the tubes. New filter caps, resistors at the power stage.

There is a high buzz and lower hum at the same time when the volume control down and when I turn it up the hum appears to be much louder.

The "detah cap" sorry I was using this slang its the .047 cap from the two prong AC goes right to chassis. Thats the C13 on the schematic

I am thinking that I wired the heaters wrong, as I left one heater grounded to chassis... without artificial center tap. This transformer does not have a CT for the filaments..
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Old 14th Jul 2020, 7:00 pm   #5
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Default Re: Filament wiring/ single wire upgrade

Quote:
Originally Posted by Skywave View Post
Looking at the circuit that you have added as an attachment, I see it uses an unbalanced-to-ground heater circuit. Since its HT rectifier does not deliver its HT+ from its filament - as some rectifiers do - I would do a re-wire using a fully balanced-to-ground configuration. That way, compared to the unbalanced arrangement, there will be a reduction in hum pick-up from the heater wiring.

Al.
Thanks Al.

I did re wire this with twisted wires but I left one heater wire grounded at the transformer wire/ rectifier socket connection point. This is where I am getting confused. Should I desolder this so no ground/chassis connection and adding an artificial center tap for the heaters will reduce the hum/ buzz I am having? Or am I wrong thinking so...?

What I mean is at the rectifier pin2 is grounded. Should I lift this ground if I use an artificial CT and the two 120R resistors to cathode on one 6v6s?

Last edited by Levente; 14th Jul 2020 at 7:06 pm.
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Old 14th Jul 2020, 7:23 pm   #6
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Default Re: Filament wiring/ single wire upgrade

This is puzzling. The circuit diagram does not show any filament (6.3VAC usually) windings at all. Where did the circuit come from?

Ideally, you would connect the filaments of the amplifying valves in parallel with a tightly-twisted pair of solid-core wires. Also ideally, the rectifier valve would have its own filament supply, the reason being that the rectifier filament is physically close to the cathode, which is, of course at HT. Some rectifiers have good heater-to-cathode insulation, some don't have any. It would seem from (a) the circuit diagram and (b) the pin-out of the 6X5, that the 6X5 has good heater-to-cathode insulation and can be on the circuit with the other valves. So, remove all of the connections to earth on the filament supply wires, and put two 100R resistors between each filament wire and earth; this will create an artificial centre-tap for the filaments. The trouble would happen if someone tried to replace the rectifier with a different one which had the cathode connected to the filament.

A better solution would be to buy a dedicated filament transformer (which is capable of supplying the filament current for all the amplifier valves except the rectifier and also ideally has a real centre-tap) and wire all of the amplifier valves (but not the rectifier) to that.

Incidentally, I cannot see a "death cap" in this circuit. It is a capacitor which can be switched to connect either of the two mains wires to earth. Since they originated in the US, where the two mains wires are not designated as "line" and "neutral" as in the UK and were only rated for the 110VAC US mains, they often blow up when used on guitar amplifiers connected to 240VAC. They should be removed.
Colin

Good Luck, Colin.
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Old 14th Jul 2020, 7:31 pm   #7
Levente
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Default Re: Filament wiring/ single wire upgrade

Thanks Colin, and guys I do apologize, the bottom of the circuit has been chopped off for some reason. Stupid me I did not realize that... here it is again.

It is a US built amp, using 110 VAC. This so called "death cap" now is visible I hope with the filament windings.

Can I use a 120R resistors instead of the 100? OR would that be overkill?

An these resistors should go to ground/ chassis?

Thanks for your help
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Last edited by Levente; 14th Jul 2020 at 7:40 pm.
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Old 14th Jul 2020, 11:04 pm   #8
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Arrow Re: Filament wiring/ single wire upgrade

You do not want any earth / chassis / 0v. connections to anywhere on the heater circuit. Fit 100 ohm 1/2-watt resistors (120 ohm will be O.K.) to each 6.3 v. output pin of the mains transformer. Join the 'free ends' of those resistors together and wire that join to 0v. / earth / chassis.
That way you get a 'balanced-to-earth' heater supply with an artificial centre tap on the 6.3 v. source. That 'tap' enhances the balance and gives the 6.3 v. a 'reference', rather than allowing it to 'float', relative to earth / chassis / 0v.

Finally, there is the question of the actual layout of the heater winding. When the tightly-twisted heater wires are joined to the pins of the first valve - say pins 4 & 5 - make sure that the wiring to the next valve heater does not form a loop under that first valve: a common mistake and is easily done. In other words, if the incoming heater wiring to valve 1 is from the right, going left, make sure that the outgoing heater wiring to the next valve starts by running to the right - then curves away from the first valve - and around to the second valve - etc., etc. A bit difficult to explain in words; much easier with a picture.

And finally, that capacitor wired between one pin of the primary of the mains transformer and earth / chassis / 0v. is not wanted: remove it. A fuse in series with incoming mains supply to the transformer would be a good idea, too. Make sure it has insulated connections: heat-shrink may be necessary.

Al.

Last edited by Skywave; 14th Jul 2020 at 11:12 pm. Reason: Add last para.
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Old 15th Jul 2020, 2:58 pm   #9
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Default Re: Filament wiring/ single wire upgrade

Quote:
Originally Posted by Skywave View Post
You do not want any earth / chassis / 0v. connections to anywhere on the heater circuit. Fit 100 ohm 1/2-watt resistors (120 ohm will be O.K.) to each 6.3 v. output pin of the mains transformer. Join the 'free ends' of those resistors together and wire that join to 0v. / earth / chassis.
That way you get a 'balanced-to-earth' heater supply with an artificial centre tap on the 6.3 v. source. That 'tap' enhances the balance and gives the 6.3 v. a 'reference', rather than allowing it to 'float', relative to earth / chassis / 0v.

Finally, there is the question of the actual layout of the heater winding. When the tightly-twisted heater wires are joined to the pins of the first valve - say pins 4 & 5 - make sure that the wiring to the next valve heater does not form a loop under that first valve: a common mistake and is easily done. In other words, if the incoming heater wiring to valve 1 is from the right, going left, make sure that the outgoing heater wiring to the next valve starts by running to the right - then curves away from the first valve - and around to the second valve - etc., etc. A bit difficult to explain in words; much easier with a picture.

And finally, that capacitor wired between one pin of the primary of the mains transformer and earth / chassis / 0v. is not wanted: remove it. A fuse in series with incoming mains supply to the transformer would be a good idea, too. Make sure it has insulated connections: heat-shrink may be necessary.

Al.
Hello Al,

Thank you so much for this. Well, I have done it it was a nightmare. This amp is little and as you see on the attachments the space is so limited. I have ended up putting these resistors at the last 6v6s as simply did not have space for this at the rectifier tube.

This was my first ever heater wiring and I am not tech guy, hopefully will do the job. (haven't tested it yet)

One thing is kinda odd to me. I am crossing the heater wire at the 6av6 input tube. These are also twisted wires between the input tube and the control potentiometer. I am very curious if this will be responsible for any sort of further hum.

Will do a quick test.

Thanks again and have a great day !

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Old 16th Jul 2020, 11:14 pm   #10
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Question Re: Filament wiring/ single wire upgrade

Hi!
Just out of interest, in your second photo, what is the twisted pair of wires, red and yellow?

Al.
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Old 17th Jul 2020, 1:31 am   #11
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Default Re: Filament wiring/ single wire upgrade

Hi!

The twisted red/yellow pair are annotated on the circuit and are used to be connect slider and earthy end of the volume control to pins 1 & 2 respectively of the EBC90/6AV6 first valve, the red lead is the earthy end of the volume control to the cathode, pin 2, and the yellow wire from the slider to V1 (EBC90/6AV6) grid pin 1!

Chris Williams
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Old 17th Jul 2020, 6:09 am   #12
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Default Re: Filament wiring/ single wire upgrade

Some useful info here - http://www.valvewizard.co.uk/heater.html

Andy.
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Old 17th Jul 2020, 10:48 am   #13
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Default Re: Filament wiring/ single wire upgrade

Hi Looking at the second pic if the phono socket is signal input ,I think you have got the twisted pair too close see if you can re route them away from the signal path . Mick.
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Old 17th Jul 2020, 12:41 pm   #14
Skywave
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Default Re: Filament wiring/ single wire upgrade

Reference post 11.
Chris: you've explained that in detail and I can now spot that arrangement on the cct. diag. - which here is hard to read since it is very small.

I've never met that arrangement in this application before. Personally, I don't like it. It's unconventional, looks like a cheap-'n'-nasty way of making those connections and could easily be the source of the hum. I would suggest a re-wire, replacing the grid wire with screened cable, one end of the screening joined to chassis.

However, in the light of what we have discovered about this amplifier to date, if this item was mine, I'd be tempted to strip all components and wiring from the chassis and re-wire it in total along conventional lines and making a much neater job of the under-chassis wiring.

Al.
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Old 17th Jul 2020, 3:13 pm   #15
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Default Re: Filament wiring/ single wire upgrade

Quote:
Originally Posted by Skywave View Post
However, in the light of what we have discovered about this amplifier to date, if this item was mine, I'd be tempted to strip all components and wiring from the chassis and re-wire it in total along conventional lines and making a much neater job of the under-chassis wiring.

Al.
I am inclined to agree with Al here, but that is entailing a lot of work for what appears to be a pretty simple amp.

Looking at the input circuitry, I was clearly wrong in assuming that this is a guitar amplifier (originally). A phono input is decidedly not likely for a guitar input. There are also some oddities there (apart from the twisted-pair connection), including what looks like an indication of a tape input and a weird tapped volume potentiometer. I wonder what this amplifier was originally designed for?

Incidentally, the "death cap" designation really only applies to guitar amplifiers. Originally, it was switched between either of the mains wires; the choice was made for minimum noise and hum. The trouble came when that meant that one side of the guitar cable was at a different voltage to the metalwork of any microphones being used at the same time with different amplifiers. If a guitarist, holding the "earthed" strings of his guitar in one hand, happened to grab the metal parts of the microphone, s/he would then experience the possibility of having full mains voltage between his/her hands and therefore across their heart. Possibly lethal - and it did happen that some guitarists doing this expired.

Colin.
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Old 17th Jul 2020, 6:00 pm   #16
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Default Re: Filament wiring/ single wire upgrade

Arh hum,
It's a Stromberg Carlson AF-501 console...

Radio museum. https://www.radiomuseum.org/r/stromb...al_af_501.html
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Old 17th Jul 2020, 8:05 pm   #17
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Default Re: Filament wiring/ single wire upgrade

Guys, thank you so much for all of the input and suggestions. Really really appreciate this and again, as always here, I am learning a lot from you all which is why I love this forum and all your inputs mean a lot to me.

Started to check everything from scratch. Checked all the soldering, removed those ceramic caps, re soldered old soldering with new ones. Re wired that twisted cable where the input socket is.

Turned the amp on upside down and decided to go through with the chopstick again. Accidentally, I did move away one lead from coming in from the PT high AC. As soon as I moved away from the R9 resistor and the control's, the hum decreased. I did move that wire as far as I could and clipped all the PT wires together away from basically any components. The hum went down nearly fully. I only had a tiny tiny buzz which again disappeared after re wiring the 12ax7 socket from scratch.

So that to me is happiness already I never thought that I will have a tube amplifier with nearly zero hum ever.

I would only need a new cap for the PI plates as the leg of the cap broke off and I did not have the same value...i did out there a 5nf cap but that is way to much I think as the sound is distorting badly but the hum has gone.

This amp is a Carlson and I think it was a built in amp for a turnable / speaker.

Thank you so much again for all your input.. I will order this missing cap of 100pf for the plates and I hope everything will be OK after that.

Have a wonderful weekend.
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Old 18th Jul 2020, 11:47 am   #18
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Arrow Re: Filament wiring/ single wire upgrade

Quote:
Originally Posted by Levente View Post
Guys, thank you so much for all of the input and suggestions. Really really appreciate this and again, as always here, I am learning a lot from you all which is why I love this forum and all your inputs mean a lot to me.
W.r.t. the above: I can only speak for myself. It was many decades ago when I was a raw beginner and had to deal with similar problems of which I had very little understanding - but the enthusiasm was strong! Sources of help etc. then were almost non-existent, so I understand your situation. Your problem required 'fix suggestions' of which I felt confident of making sensible and appropriate comments. Unfortunately, there are occasions when my enthusiasm to help / contribute leads me make statements that are simply wrong: here, such errors are usually quickly pointed out; never too old to learn! Hopefully, my inputs to this thread aren't in that category.
Have fun; stay safe.

Al.
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