• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Poll..anyone interested in an Aikido linestage PCB group buy?

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Bas Horneman

Bas Horneman

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Elevated voltage for referenceing heater supplies

As mentioned earlier in this thread for the Aikido it is a necessity to reference the heater supplies up.

But it can also be beneficial to the sound of your pre-amp.

Here is a post by our diyAudio member AW (Allen Wright)
Posted by Allen Wright ( M ) on August 19, 2002 at 14:54:33
In Reply to: + Biasing the heater voltages... posted by Bas Horneman on August 19, 2002 at 03:34:57:


I have found by long experience that raising the heater voltage of all indirectly heated signal tubes above their cathode voltages helps a preamp or poweramp to sound better.
I try to get all heaters about 30V more positive than their cathodes. If some tubes have their cathodes well up above gnd - like in a CF, or the upper tube in a SRPP - then a second heater supply is needed to have these tubes run the same way.

I don't really know why it sounds better - I guess that it stops a 'mini tube' being created between the heater and cathode structure - but it sure can be the difference between something sounding good and something sounding magic!

First saw it used in a Lux CL35 preamp - vintage 1975. Not too many new things under the sun...

Allen
Click to expand...

And a reply by Mark Kelly confirming Allen's idea and giving us a reason.

Allen
My understanding of this is the same as yours. Basically the actual heater element is usually run hot enough to achieve a low level of thermionic emission even without the semiconductor effect of the oxide coating. Obviously this is much worse if there is any contamination from the coating on the element itself.

If the element is at ground potential and the cathode sleeve at above ground this low level of emission will create a forward biased diode between the two and it's probably going to be highly non-linear and could only have a detrimental effect on performance.

The cure is to reverse bias the diode into cutoff, so the element needs to be positive WRT the cathode. Again I agree with you that the ideal value is 30 to 40 V positive.
Click to expand...

And finally Paul "Braniac" Joppa gives the "real" reason?
Posted by Paul Joppa ( P ) on August 19, 2002 at 18:53:06
In Reply to: Unintentional diodes posted by Mark Kelly on August 19, 2002 at 17:43:23:


The way I read it (some 50-year-old article), the diode can go either way, i.e. the heater or cathode can act as either anode or cathode. But it's not a very good cathode, so it is easily saturated with 20-30v of either polarity. Once saturated, the current is constant and nonlinearity does not come into play. In theory... :^)
Click to expand...


For the whole thread : http://db.audioasylum.com/cgi/m.mpl...ht=plus+biasing+the+heater+supply&r=&session=

Or just buy the book Valve Amplifiers 3rd edition by Morgan Jones :D He also describes this phenomena...Can't find my 2nd edition right now..(Probably somewhere in the bathroom:cannotbe: )
 
leadbelly

leadbelly

Member
Joined 2002
Re: Elevated voltage for referenceing heater supplies

Bas Horneman said:
As mentioned earlier in this thread for the Aikido it is a necessity to reference the heater supplies up.

But it can also be beneficial to the sound of your pre-amp.

Here is a post by our diyAudio member AW (Allen Wright)
Click to expand...

Indeed, AW's comments (I don't know if this is what you were getting to) bring to my mind the question of whether an improvement could be heard if the 1st and 2nd stages were both built with matched tubes, the top triodes in the circuit being in the same tube with one heater bias, and likewise the bottom triodes in the circuit with a different heater bias, such that both could be above the cathode voltage.
 
Bas Horneman

Bas Horneman

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basaudio.wordpress.com/
Ok ...so how do we connect the heater/filament supply to our reference?

Let's start with AC supply (which is what I would advise for the Aikido) My preferred method is to get a filament transformer that is center tapped and has an electrostatic screen.

The center tap is simply wired to the heater/filament supply reference. And the 2 outer voltages 3,15 and the other 3,15 go to the heater. Like so :

An externally hosted image should be here but it was not working when we last tested it.


(Picture has been taken from Lynn Olson's Amity schematic. ) Lynn's website is http://www.nutshellhifi.com one of the very best tube sites on the net.
 
Bas Horneman

Bas Horneman

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Reason for the electrostatic screen between primary and sec.

From Rod Elliot's site (excellent tutorials)
Transformers - The Details Section 2
http://sound.westhost.com/xfmr2.htm

It is also very simple to add an electrostatic shield between the windings - a flat plate of thin metal is cut so that it can be slipped over the bobbin, and the ends are insulated so that it does not create a shorted turn. This is connected to earth, and prevents noise from being capacitively coupled between windings. The shield would logically be placed on the secondary side of the bobbin divider for safety.
Click to expand...

I can tell you that in a quick little test I could not hear the difference between an unconnected and a connected electrostatic shield (to earth) on my Aikido preamp. But I can tell you it made quite a difference in a DHT amp I put on the scope, I could actually see the rubbish become less on the scope.

So even if you can't hear it I like to spec an electrostatic screen on a filament transformer. (There used to be a claim on Lynn Olson's site that it actually reduced noise coupling by 40dB) But I can't find that claim anymore so it was probably less...:D

But I did find this quote on the AA
Matt Kamna did a lot of measurements of primary-to-secondary and secondary-to-secondary interwinding capacitance. Matt was the one that mentioned that electrostatic screening would reduce this capacitance by ten to fifty times ... no small improvement, and the decrease in HF response of the power transformation is an additional bonus
Click to expand...

http://www.audioasylum.com/forums/tubediy/messages/2454.html
 
Bas Horneman

Bas Horneman

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basaudio.wordpress.com/
Indeed, AW's comments (I don't know if this is what you were getting to) bring to my mind the question of whether an improvement could be heard if the 1st and 2nd stages were both built with matched tubes, the top triodes in the circuit being in the same tube with one heater bias, and likewise the bottom triodes in the circuit with a different heater bias, such that both could be above the cathode voltage
Click to expand...

That would be interesting to try in some all tricked out Aikido :) . The current Aikido PCB does not allow for that though.
 
leadbelly

leadbelly

Member
Joined 2002
For the PS PCB, what I would like to see is that the big resistor locations also have pads for the TO220 type of power resistors. Ideally also pads for stand up heat sinks as well, but that is less important. If anyone is actually going to go ahead and try to build a balanced Aikido (I am) they will need some big units for the heat. Plus some people prefer those kind and always use them.
 
J

jleaman

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Joined 2002
plus.google.com/photos/104656656065604242634/albums/5725366675475046257?banner=pwa&gpsrc
leadbelly said:
For the PS PCB, what I would like to see is that the big resistor locations also have pads for the TO220 type of power resistors. Ideally also pads for stand up heat sinks as well, but that is less important. If anyone is actually going to go ahead and try to build a balanced Aikido (I am) they will need some big units for the heat. Plus some people prefer those kind and always use them.
Click to expand...


Do this board do balanced ? That was one of my questions i was going to ask lastnight.. It would be good if it did.
 
Bas Horneman

Bas Horneman

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Joined 2002
basaudio.wordpress.com/
For the PS PCB, what I would like to see is that the big resistor locations also have pads for the TO220 type of power resistors. Ideally also pads for stand up heat sinks as well, but that is less important. If anyone is actually going to go ahead and try to build a balanced Aikido (I am) they will need some big units for the heat. Plus some people prefer those kind and always use them.
Click to expand...
I'll ask bonny_kjs...

The board obviously makes it possible to use a choke instead of R1 as well.
 
Bas Horneman

Bas Horneman

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basaudio.wordpress.com/
MoreAikido4.gif


Do this board do balanced ? That was one of my questions i was going to ask lastnight.. It would be good if it did.
Click to expand...

Could do! You'd have to share one pcb's 0,47uF 100k -> 100k reference. Leave out RV2...junction it and use the unused pad to join to the other RV2 pad.
 
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