I recently came accross a novel (I think !) way of battery biasing a grounded cathode stage which I believe is attributable to Thorsten.
Image 1 shows lithium bias applied conventionally in Thorsten's Toccata.
Image 2 show the alternative method applied in a TRAM linestage implementation.
Quire different and very interesting. Has anyone here tried it and compared it to the conventional method ?
Image 1 shows lithium bias applied conventionally in Thorsten's Toccata.
Image 2 show the alternative method applied in a TRAM linestage implementation.
Quire different and very interesting. Has anyone here tried it and compared it to the conventional method ?
The second one floats the whole battery with the signal. That may give some issues with the bulk capacitance of the battery to chassis or whatever. First circuit is better in that respect.
Also note that this way of biasing was in use before WWII (which was before Thorsten was born
). It went out of vogue when components for off-line power supplies became available and affordable.jd
In both setups, but mostly in figure 1, a small AC current is flowing through the battery via the grid leak resistor. As lithium batteries are primary batteries (non-rechargeable), they may show some less-than-desirable behavior in the form of LF voltage fluctuations. In figure 2, only the grid current (arguably negligible) is flowing through it, and for high frequencies it is bypassed. So I'd recommend the situation of figure two as well. A typical lithium battery is not bigger than, say, a foil coupling cap. I wouldn't worry too much about the capacitive coupling with the environment.
The pre-ww2 sets typically used a bias battery, from which the bias voltages were derived with voltage dividers. This battery was always supplying current, so it needed to be replaced periodically or the tubes would drift away from their normal bias point. The lithium battery only sees a negligible load and should last at least 10 years as it is used here.
The pre-ww2 sets typically used a bias battery, from which the bias voltages were derived with voltage dividers. This battery was always supplying current, so it needed to be replaced periodically or the tubes would drift away from their normal bias point. The lithium battery only sees a negligible load and should last at least 10 years as it is used here.
Hi Timpert,
Good points. Probably good to bypass the battery with a large electrolytic in any case?
jd
See "The EC8010 RIAA Stage" by the Dread Morgan Jones in "Valve Amplifiers." I tried this bias scheme when prototyping my phono stage and found no real advantage over LED bias.
Hi,
Neither approach is directly attributable to me.
The one used in the Toccata is from Arthur Loesch.
The second originally derived, if I remember correctly, from discussions on the Joe List where some of us recoiled in horror from the application of "battery bias" by putting nicad cells into tubes cathodes (I still think the results are poor and it is a pretty bad idea and yes, I have tried, it is nearly as bad as using LED's or Chip regulators in the cathodes to bias a tube).
The discussions resulted in the second scheme and I have been quite fond of it ever since. But I do not think I claim being the originator. This would have been in the late 90's. It is also used in the Artemis Labs Phono Stage.
I have since come up with a scheme that approximates the results, but does not rely on batteries and allows servo-adjustment of the Bias, but it is a little complex to implement and really only sensible for commercial gear.
For DIY a straight Battery is usually quite acceptable and of minimal complexity. I'd like to close by quoting "Peter Pan" from Troel Gravensen's Website:
"(This is) - not very common because people believe if it is in series with the grid; it is “in the signal path” and if it is in series with the cathode it is not “in the signal path”. So they stick a big, bad, nonlinear rechargeable Nicad Battery in the cathode, where it is exposed to a lot of varying current so this non-linearity can be injected into the input voltage loop (grid to “ground”) and amplified and they are pleased as punch with themselves for having eliminated another evil capacitor ;-)."
Ciao T
Neither approach is directly attributable to me.
The one used in the Toccata is from Arthur Loesch.
The second originally derived, if I remember correctly, from discussions on the Joe List where some of us recoiled in horror from the application of "battery bias" by putting nicad cells into tubes cathodes (I still think the results are poor and it is a pretty bad idea and yes, I have tried, it is nearly as bad as using LED's or Chip regulators in the cathodes to bias a tube).
The discussions resulted in the second scheme and I have been quite fond of it ever since. But I do not think I claim being the originator. This would have been in the late 90's. It is also used in the Artemis Labs Phono Stage.
I have since come up with a scheme that approximates the results, but does not rely on batteries and allows servo-adjustment of the Bias, but it is a little complex to implement and really only sensible for commercial gear.
For DIY a straight Battery is usually quite acceptable and of minimal complexity. I'd like to close by quoting "Peter Pan" from Troel Gravensen's Website:
"(This is) - not very common because people believe if it is in series with the grid; it is “in the signal path” and if it is in series with the cathode it is not “in the signal path”. So they stick a big, bad, nonlinear rechargeable Nicad Battery in the cathode, where it is exposed to a lot of varying current so this non-linearity can be injected into the input voltage loop (grid to “ground”) and amplified and they are pleased as punch with themselves for having eliminated another evil capacitor ;-)."
Ciao T
Hi,
They are all pretty low, depending on capacity. But batteries react quite differently between types when current flows. Many batteries show noise and non-linearity that depends on levels of DC current flow, though much can be snubbed out with big value electrolytics for bigger batteries and even just tiny cap's in something like the lithium cells referenced here.
Ciao T
They are all pretty low, depending on capacity. But batteries react quite differently between types when current flows. Many batteries show noise and non-linearity that depends on levels of DC current flow, though much can be snubbed out with big value electrolytics for bigger batteries and even just tiny cap's in something like the lithium cells referenced here.
Ciao T
Hi,
Enjoy.
Of course, next to a LED most batteries look as close to perfect as it gets on grounds of noise and linearity.
I would, based on personal experience rank LED Bias last, together with 3-Pin regulators in cathodes, in terms of subjective sound quality and last in terms of measurable linearity but second last on noise.
Fixed bias using Lithium batteries in series with the grid where the input must be ground referenced (or the active bias variant of it) and conventional fixed bias is number one. By a long stretch. All else being equal not having anything between cathode and (signal) ground beats all options of having something there.
Ciao T
Enjoy.
Of course, next to a LED most batteries look as close to perfect as it gets on grounds of noise and linearity.
I would, based on personal experience rank LED Bias last, together with 3-Pin regulators in cathodes, in terms of subjective sound quality and last in terms of measurable linearity but second last on noise.
Fixed bias using Lithium batteries in series with the grid where the input must be ground referenced (or the active bias variant of it) and conventional fixed bias is number one. By a long stretch. All else being equal not having anything between cathode and (signal) ground beats all options of having something there.
Ciao T
Hmmm, if the difference is indeed big, I'll definitely try it out on my next build...
Steve Cresswell used this battery grid bias in a PX25 DRD in 2008:
http://www.diyaudio.com/forums/tubes-valves/126088-px25-drd-power-amp.html
He eventually ended up with a different arrangement, but has some good description of his listening tests of the various arrangements on his way.
SveinB
http://www.diyaudio.com/forums/tubes-valves/126088-px25-drd-power-amp.html
He eventually ended up with a different arrangement, but has some good description of his listening tests of the various arrangements on his way.
SveinB
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