If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Interesting PM, thanks for this. I did have an idea to make a matching transformer from a recycled TV backlight output transformer as the windings on these have an impedance of 1-4k
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
Sulaiman wrote ...
I enjoyed the article, nice little amplifier, the output transformer seems key to this project,
I think having an O/P transformer of the right spec will make all the difference between very mediocre performance and something really rather good considering the simplicity of the design.
The problem with old old stock - recycled - transformers, is that those made in the greatest number for use with ECL86 in cheap radios and TVs were made to a price. There was no FM to show up the weaknesses of poor bandwidth, and people's expectations of TV sound were low. Cabinets more often than not buzzed and boomed in sympathy with loud passages of music, and audio bandwidth had to be kept below 10 kHz in 405-line TV to keep out the 10,125 Hz line frequency, a continuous whine you could hear distinctly anyway, because of magnetostriction in the LOPT core.
Well, as far as 'quality' goes, from what I've learned recently, the 'optimum' OP transformer needs large core area, thin laminations, an equal number of windings on primary and secondary. with the secondaries wired in parallel (same mass of copper in primary and secondary), maybe 10,000 turns (not sure yet), maybe a 20:1 turns ratio, so, maybe 20 layers of primary, 500 turns each, in series, and 20 layers secondary, 500 turns each, in parallel, layers alternating between primary and secondary. Basically just have to decide on core size, and work from there.
I've not 'finalized' the numbers yet, any further input/suggestions would be welcome.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
Sulaiman wrote ...
I enjoyed the article, nice little amplifier, the output transformer seems key to this project,
I think having an O/P transformer of the right spec will make all the difference between very mediocre performance and something really rather good considering the simplicity of the design.
The problem with old old stock - recycled - transformers, is that those made in the greatest number for use with ECL86 in cheap radios and TVs were made to a price. There was no FM to show up the weaknesses of poor bandwidth, and people's expectations of TV sound were low. Cabinets more often than not buzzed and boomed in sympathy with loud passages of music, and audio bandwidth had to be kept below 10 kHz in 405-line TV to keep out the 10,125 Hz line frequency, a continuous whine you could hear distinctly anyway, because of magnetostriction in the LOPT core.
Well, as far as 'quality' goes, from what I've learned recently, the 'optimum' OP transformer needs large core area, thin laminations, an equal number of windings on primary and secondary. with the secondaries wired in parallel (same mass of copper in primary and secondary), maybe 10,000 turns (not sure yet), maybe a 20:1 turns ratio, so, maybe 20 layers of primary, 500 turns each, in series, and 20 layers secondary, 500 turns each, in parallel, layers alternating between primary and secondary. Basically just have to decide on core size, and work from there.
I've not 'finalized' the numbers yet, any further input/suggestions would be welcome.
The turns ratio depends on the anode load resistance of the valve and the loudspeaker to be matched.
There must be a general formula for relating output impedance mismatch to audio crappiness, but unless the very highest standards are hoped for, a moderate degree of mismatch won't do too much harm to the perceived sound. If the theory says the impedance ratio should be 25:1, and you only have a 20:1 transformer available, it will work tolerably well, but won't be 'hi-fi'.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
In a conversation I had recently with Chris, who used a 10:1 turns ratio on his latest OP Xformers, he said he prefers 'Low impedance'. When I questioned him further, he said something about triodes have a lower impedance than pentodes, and they could also be 'run lower', which, I assume is something about running them at a lower voltage. Can anyone add to this?
He also said something about running two or more triodes in a single-ended amp in parallel, presumably to keep the voltage lower, without loss of power. Can anyone add anything to this?
Is this an 'effective' way to lower the output impedance?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
In a conversation I had recently with Chris, who used a 10:1 turns ratio on his latest OP Xformers, he said he prefers 'Low impedance'. When I questioned him further, he said something about triodes have a lower impedance than pentodes, and they could also be 'run lower', which, I assume is something about running them at a lower voltage. Can anyone add to this?
Triodes 'running lower': All thermionic valves can be made to function to some extent at voltages very much lower than the design operating parameters. The heated cathode boils off electrons and only quite a small potential difference is needed to lure some of those electrons towards the anode. While making this transit, they can be either encouraged or discouraged by the relative potential of the control grid, g1.
Aside from the loss of power inevitable on reducing anode voltage, the single biggest objection to significant anode voltage reduction is loss of linearity.
If we take the triode section of our faithful ECL86 for example, (please turn to the very last graph before the end on the datasheet here):
This graph shows the characteristic curve of the triode section. On the vertical axis on the right, we see marked off a spectrum of control grid voltages from Vg = 0V - no chrage on the grid - to -4V right down on the bottom right.
On the horizontal axis we see marked out a range of anode voltages from 0V to beyond 350V.
So we can take a change of grid voltage - say from -4V to -3V, and see how this effects anode current
You can make any permutation of grid voltage and anode current that you like, so long as it doesn't result in damage to the valve. Notice the plot marked Wa = 0.5W which defines the safe operating area. Anode watts = half a watt. Take your valve across that line at your peril. It might not fail for days or weeks, with small transgressions, but it is unlikely to provide years of faithful service if you stray outside the safe operating area.
So those are the main features of the layout of this graph.
Now back to linearity - which curves have got the longest straight sections? How can I best set anode voltage and grid voltage to keep ECL86 triode on the straight and narrow?
Now if you look at the lower part of the graph, you'll see that at low values of anode current - Ia - there are no straight lines, everything is increasingly non-linear as it heads to the bottom.
The characteristic curves of a valve are not compiled by theory, but from the actual observed behaviour of the valve in practice.
Why do people now want to run valves at relatively low voltages? Justifiable fear of electric shock and fire, difficulty of obtaining valve HT transformers and other parts of adequate voltage rating at a reasonable cost, and heat disposal problems associated with small chassis and cabinet designs that people now expect, must be among the principal reasons underpinning the sad decline of anode voltage standards in many of today's hobbyist designs.
Ash Small wrote ...
He also said something about running two or more triodes in a single-ended amp in parallel, presumably to keep the voltage lower, without loss of power. Can anyone add anything to this?
Two or more valves may be operated in parallel, and often were, though care had to be taken to maintain stability across the safe operating area. Aside from the doubling of total grid to cathode and grid to anode capacitance we also have double the Miller capacitance (in a triode) which is an open invitation to uncontrolled feedback, that is, oscillation. We also have double the amount of potentially mischievous stray capacitances and objectionable interactions with other wiring to contend with, all usually quite straightforward though a good deal of trial and error at the layout stage may be needed to ensure stable operation.
Instability: when you look at the circuit diagram of a simple two stage AF amplifier such as the triode-output pentode ECL86, the diagram itself gives no clue at all about the layout needed to ensure stability, the layout that will keep hum to a minimum, and so on. The very cost effective triode-output pentodes are especially demanding on layout, because of the very high gain that must be achieved in a two-step process from triode to pentode. Instability is always a risk when very high gain is demanded. With very high gain, only a mere whiff of feedback from anode to grid, and the valve will start oscillating for all it's worth. The space around the nine tags on the B9A 'noval' valve socket under the chassis is very limited. The golden rule to keep anode circuits as far away from grid circuits as is reasonably possible is hard to put into practice around the cluttered B9A socket tags of a triode-output pentode.
ECL86 was a mass-produced cost-effective two-valves-in-one envelope designed for mass-produced consumer audio applications, so while the layout may have been very fiddly and time-consuming at the protoyping stage, once the optimum layout had been established, it was all mass production from then on. The home experimenter does not have this economy of scale!
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
Why do people now want to run valves at relatively low voltages? Justifiable fear of electric shock and fire, difficulty of obtaining valve HT transformers and other parts of adequate voltage rating at a reasonable cost, and heat disposal problems associated with small chassis and cabinet designs that people now expect, must be among the principal reasons underpinning the sad decline of anode voltage standards in many of today's hobbyist designs.
I think this is a fear people have not to be able to have the same quality in power supplies, while there are plenty of solutions for small PFC forward converters or such, with current limit and good reactivity
something like a FAN7530 or another PFC converter would do the trick for an anode supply, for what I know, without enormous coils or big oil capacitors
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Shrad wrote ...
Why do people now want to run valves at relatively low voltages? Justifiable fear of electric shock and fire, difficulty of obtaining valve HT transformers and other parts of adequate voltage rating at a reasonable cost, and heat disposal problems associated with small chassis and cabinet designs that people now expect, must be among the principal reasons underpinning the sad decline of anode voltage standards in many of today's hobbyist designs.
I think this is a fear people have not to be able to have the same quality in power supplies, while there are plenty of solutions for small PFC forward converters or such, with current limit and good reactivity
something like a FAN7530 or another PFC converter would do the trick for an anode supply, for what I know, without enormous coils or big oil capacitors
or am I completely wrong?
You got it! Valves don't care where their HT+ comes from so long as there's enough of it. Decisions have to be made about acceptable ripple voltage, about noise, about the power supply's output impedance and regulation ("stiffness" and "sag") and voltage drift over time and temperature, as with making a power supply for anything else.
In the Thermionic Age, consumer electronics like radios generally avoided the use of expensive, heavy, space-consuming HT transformers by using the so-called 'live chassis technique' - the mains Neutral line was wired to the chassis, and the mains Live line went straight to a half-wave rectifier (via a fuse). A small transformer was then used to supply LT - generally 6.3V - to the valve heaters. In TVs, even the need for a heater transformer was done away with by connecting all the heaters in a series string and connecting this directly across the mains via a big fat hot wire-wound dropping resistor and a fuse. When one heater failed, or the dropping resistor developed a smoking hot spot and went open circuit, all the heaters in the string went out and the cabinet went dark inside.
Only more expensive equipment had HT transformers.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
Instability: when you look at the circuit diagram of a simple two stage AF amplifier such as the triode-output pentode ECL86, the diagram itself gives no clue at all about the layout needed to ensure stability, the layout that will keep hum to a minimum, and so on. The very cost effective triode-output pentodes are especially demanding on layout, because of the very high gain that must be achieved in a two-step process from triode to pentode. Instability is always a risk when very high gain is demanded. With very high gain, only a mere whiff of feedback from anode to grid, and the valve will start oscillating for all it's worth. The space around the nine tags on the B9A 'noval' valve socket under the chassis is very limited. The golden rule to keep anode circuits as far away from grid circuits as is reasonably possible is hard to put into practice around the cluttered B9A socket tags of a triode-output pentode.
ECL86 was a mass-produced cost-effective two-valves-in-one envelope designed for mass-produced consumer audio applications, so while the layout may have been very fiddly and time-consuming at the protoyping stage, once the optimum layout had been established, it was all mass production from then on. The home experimenter does not have this economy of scale!
Chris definitely said he only used triodes for this, as pentodes aren't suitable (higher impedance?). I will ask him some more questions about this when I next see him, but I was listening to his latest 'Mighty Atom' amp on Saturday, and it is probably the best amp I've heard. He was running 480mW per channel, less than a watt of power, but he was running it through some 1920's horn speakers from a theatre somewhere. It just sounds superb.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Output Transformer questions
